It is strongly recommended that steps be taken towards the definition and implementation of a European Astrobiology Platform (or Institute) to streamline and optimize the scientific return by using a coordinated infrastructure and funding system. Key Words: Astrobiology roadmap—Europe—Origin and evolution of life—Habitability—Life detection—Life in extreme environments. Astrobiology 16, 201–243.

The European AstRoMap project (supported by the European Commission Seventh Framework Programme) surveyed the state of the art of astrobiology in Europe and beyond and produced the first European roadmap for astrobiology research. In the context of this roadmap, astrobiology is understood as the study of the origin, evolution, and distribution of life in the context of cosmic evolution; this includes habitability in the Solar System and beyond . The AstRoMap Roadmap identifies five research topics, specifies several key scientific objectives for each topic, and suggests ways to achieve all the objectives. The five AstRoMap Research Topics are

1. Introduction

1.1. The project AstRoMap within the Framework Programme for Research and Innovation (FP7) of the European Union

Astrobiology is a broad research domain that encompasses wide areas of the scientific landscape. This maturing field cuts across many disciplines ranging from prebiotic chemistry to geomicrobiology, atmospheric sciences, and astronomy. Astrobiology is not only diverse in terms of disciplines. It also traverses a very wide spectrum of spatial and temporal scales: from the molecular level to ecosystems and planetary systems, at scales ranging from Earth's (sub)surface to planetary objects detected thousands of light-years away, and from understanding the origins of life to its future evolution and destiny. By nature, astrobiology touches on some of the more fundamental societal questions: What is life? How did it start? Is it present beyond Earth? It is also relevant to more earthbound concerns such as the evolution of ecosystems under growing environmental pressure or the development of new and innovative (bio)technologies.

Active for the period 2013–2015 and supported by the European Union through its Seventh Framework Programme for Research and Innovation (FP7), the AstRoMap project has undertaken the task of developing a roadmap for European research in astrobiology, taking into consideration its multidisciplinary nature and putting forward overarching priority research topics and key scientific objectives in a structured and coherent way. The AstRoMap partner organizations were the Instituto Nacional de Técnica Aeroespacial – Centro de Astrobiología (INTA-CAB, Spain), the European Science Foundation (ESF, France), Deutsches Zentrum für Luft- und Raumfahrt (DLR, Germany), the Belgian User Support and Operations Centre (B-USOC, Belgium), the Istituto Nazionale di Astrofisica (INAF, Italy) and the European Astrobiology Network Association (EANA, Europe). In the context of the AstRoMap Roadmap, astrobiology is understood as the study of the origin, evolution, and distribution of life in the context of cosmic evolution; this includes habitability in the Solar System and beyond (see http://www.astromap.eu).

This document presents the outcome of this effort. It results from the integration of inputs from the scientific community collected via expert workshops as well as online consultations. Overall, more than 45 European experts were involved in the definition of the AstRoMap Roadmap.

The AstRoMap European Astrobiology Roadmap is structured around five core research topics that represent interrelated roadmap building blocks. Each research topic is broken down into several key objectives (and sub-objectives) that put forward more detailed priority areas to be addressed in the future. For each research topic, the roadmap also suggests potential achievements in the short (within the next decade), medium (within the next two decades), and long (beyond 20 years) term; it also addresses the specificities of the European landscape in terms of strengths and needs relevant to each topic.

1.2. The European astrobiology environment and landscape in Europe

1.2.1. European Space Agency (ESA)

At ESA, astrobiology research themes are covered by the Directorate of Science and Robotic Exploration (D/SRE) and the Directorate of Human Spaceflight and Operations (D/HSO). ESA's mandatory space science program funds the development and construction of the spacecraft and the launch and operation of space missions but not the development and construction of scientific instrumentation nor the exploitation of scientific data or ground-based laboratory research. Support for the latter needs to be obtained from national organizations. ESA's optional space research program includes work on robotic and human exploration in the context of three potential destinations: low-Earth orbit (LEO), the Moon, and Mars.

Astrobiology (originally termed “exobiology”) was recognized by ESA as a future research area as early as 1996. A study group (the ESA exobiology team) was established to survey the state of research in exobiology and related fields and to make recommendations to ESA on the nature of future “Search for Life” actions elsewhere in the Solar System (Brack et al., 1999a, 1999b). This first European initiative to embrace astrobiology resulted in a comprehensive exobiological view of the Solar System by exploring the four following topics:

• Chemical evolution in the Solar System;

• Limits of life under extreme conditions;

• Morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues;

• Potential sites for extraterrestrial life.

This far-seeing initiative strongly influenced the definition and setup of the Aurora program as an optional program, ESA's strategic framework for space exploration. Aurora commenced in January 2002 by setting out a strategy over the next 30 years for Europe's robotic and human exploration of Mars, the Moon, and even beyond to asteroids. ExoMars, the first European Mars mission dedicated to astrobiology, was one of the program's flagship missions.

The very ambitious plans of the program also envisaged two Mars sample return missions, a robotic outpost on Mars, and a possible human mission to the Moon and a human mission to Mars in the horizon 2025–2030. However, the Aurora program was not sufficiently backed by high-level political commitment and was later terminated.

Although the Aurora program was cancelled, its first flagship mission, ExoMars, is in the process of being implemented. The ExoMars mission is structured in two elements: the first element, launched on March 14, 2016, includes the ExoMars Trace Gas Orbiter (TGO, atmospheric gas analysis, in particular methane mapping) and the Schiaparelli lander (Entry, Descent, and Landing Demonstrator Module and meteorological payload). The 2018 element will carry the ExoMars rover with its suite of astrobiology experiments (the Pasteur payload) intended to assess the habitability of past and present Mars and detect signatures of extinct life (Vago et al., 2006, 2015; Parnell et al., 2007; Vago and Kminek, 2007).

In the early 2000s, in addition to its exploration strategy and to follow up on the previous plan (Horizon 2000 plus), ESA developed the framework and the backbone of its mandatory science program for the period 2015–2025. The Cosmic Vision program was issued in October 2005 (ESA, 2005). Among the scientific questions subdivided into topics where important progress was expected, the first theme of the program is strongly related to astrobiology, although the term astrobiology is not mentioned: What are the conditions for planet formation and the emergence of life? This theme is divided into three topics:

• From gas and dust to stars and planets. Place the Solar System into the overall context of planet formation, aiming at comparative planetology. Map the birth of stars and planets by peering into the highly obscured cocoons where they form.

• From exoplanets to biomarkers. Search for planets around stars other than the Sun, looking for biomarkers in their atmospheres, and image them.

• Life and habitability in the Solar System. Explore in situ the surface and subsurface of the solid bodies in the Solar System most likely to host—or have hosted—life. Explore the environmental conditions that make life possible.

The Cosmic Vision program was used to delineate the boundary conditions of the subsequent call for scientific missions open to the community. As of mid-2015 six missions have been selected and are in various stages of development; half of those missions have relevance to astrobiology. Two missions, CHaracterising ExOPlanet Satellite (CHEOPS, launch planned in 2017) and PLAnetary Transits and Oscillations of stars (PLATO 2.0, launch planned in 2024), are dedicated to the detection of exoplanets; and one large mission, JUpiter ICy moons Explorer (JUICE, launch planned in 2028), will explore the jovian system, flying by three of its icy moons (Callisto, Europa, and Ganymede) before orbiting Jupiter.

In 2013, ESA's High-level Science Policy Advisory Committee (HISPAC) published a report (HISPAC, 2013) in which four major science themes were recommended as scientific challenges that should be tackled by the agency in the next decades, beyond the already defined scientific objectives being currently implemented in the various programs. In this report, the third theme specifically addresses “Life in the Universe,” and “Astrobiology” is outlined as a case study.

In addition to the ExoMars and Cosmic Vision program, astrobiology is mentioned and fully integrated in ESA's optional European Life and Physical Sciences in Space program (ELIPS). Started in 2001, ELIPS allows research teams to have access to the European Columbus laboratory on the International Space Station (ISS) as well as to other ISS platforms (including exposure facilities) and ground-based platforms and facilities (for example, drop towers, sounding rockets, radiation facilities). ELIPS allowed the development and use of the EXPOSE facility; this facility, mounted on the outside of the ISS, allows exposure of biological samples and organic compounds to a combination of space environmental factors (full spectrum of extraterrestrial solar electromagnetic radiation, cosmic radiation, vacuum, freezing/thawing, and microgravity) to investigate their resistance and adaptation to extreme space and (simulated) planetary conditions. The EXPOSE facility was deployed on three occasions:

• in 2008–2009 for the EXPOSE-E mission: seven experiments—18 months of exposure (EXPOSE-E, 2012)

• in 2009–2011 for the EXPOSE-R mission: nine experiments—22 months of exposure (EXPOSE-R, 2015)

• in 2014–2016 for the EXPOSE-R2 mission: four experiments—planned for 18 months of exposure.

1.2.2. Astrobiology in the context of the European Union

One of the main research and technology development programs of the European Union is Horizon 2020; this program succeeds the Seventh Framework Programme (FP7) and covers the period 2014–2020. Horizon 2020 addresses all aspects of science and technology development and is structured around biannual work programs and annual calls.

It is expected that Horizon 2020 will dedicate approximately 1.4 B€ to space activities, 5% of which will target space sciences. In effect, most Horizon 2020 space-related call topics are targeted towards developing appropriate space technologies and services and fostering European industry competitiveness. In fact, space is positioned under the heading “Leadership in enabling and industrial technologies” of the program.

In the first call of Horizon 2020 (2014), the topic Space exploration and science (with a total budget of 4 M€) focused on Mars data evaluation and on the definition of a European sample curation facility for sample return missions (see http://www.euro-cares.com), in addition to dedicated technology developments. In the second call (2015), this same topic focused on scientific exploitation of astrophysics, comets, and planetary data with a budget of 1.5 M€, as well as a topic addressing international cooperation in the context of planetary protection with 1 M€ budget.

1.2.3. The European astrobiology community

In addition to the programmatic arena, the European astrobiology community benefits from networking and interaction platforms at national, European, and international levels. At the European level, the European Astrobiology Network Association (EANA) was established in 2001 and provides a platform and a forum for the astrobiology community in Europe (19 European countries are represented) and beyond (see eana-net.eu). For 15 years, EANA has organized an annual workshop on astrobiology, providing a tangible forum for interaction and collaboration, and is very active in the area of education through the ABC-Net lecture courses, a live teleteaching program performed in cooperation with ESA, interconnecting several European universities (Horneck and Rettberg, 2007) (see http://eana-net.eu/education.html).

Launched in 2014 for a period of 4 years, the Cooperation in Science and Technology (COST) Action ORIGINS (TD1308) is a European network involving researchers from 29 European countries and focuses on scientific questions related to the origins and evolution of life on Earth and habitability of other planets. It provides a common platform for interdisciplinary interactions and coordination of nationally funded investigations (see http://life-origins.com).

Building on the success of two previous European Union–funded projects and structured around a platform gathering together more than 60 research institutions (all signatories to the Europlanet Memorandum of Understanding), the European Planetary Network (Europlanet) has developed and is implementing the 2020 Research Infrastructure project (EPN2020-RI) supported by the Horizon 2020 Research Infrastructure program. EPN2020-RI is a 4-year, 10 M€ initiative that started in September 2015 (see http://www.europlanet-2020-ri.eu) and brings together 33 partners from 18 European countries. Its main objective is to provide a pan–European Union infrastructure dedicated to planetary sciences. EPN2020-RI will (i) network state-of-the-art research facilities and provide access to them; (ii) organize access to planetary analog sites on Earth; and (iii) set up a “virtual observatory” for planetary science, making high-level data accessible to the community. EPN2020-RI will provide the community with a platform that will catalyze and facilitate multidisciplinary research for European planetary scientists.

Other ongoing significant initiatives include the European Astrobiology Campus (EAC—an Erasmus+ strategic partnership) that will be active for the period 2014–2016 (see http://astrobiology-campus.eu/about-eac-2). The EAC brings together 12 European universities and science education organizations. This initiative will provide multidisciplinary education in astrobiology to students as well as training to lecturers. Another objective of the EAC is to raise awareness of astrobiology by delivering information, material, and tools to the general public.

European early-career astrobiologists are also organizing their networking through the Astrobiology Graduates in Europe (AbGradE) association (see http://eana-net.eu/AbGradE/about.html). Since 2014, this association organizes symposia and workshops during which master classes, keynote lectures, and scientific sessions are provided and held.

In 2013, the AstRoMap project organized a survey of the European astrobiology science community; this survey was later published in Acta Astronautica (Horneck et al., 2015). Compilation of the data provided by 105 European investigators allowed for the creation of a detailed profile of the community involved in astrobiology in Europe. This community is mostly composed of planetologists interested in habitability and life detection, as well as biologists interested in extremophiles on Earth. It also involves astrophysicists, physicists, chemists, geologists, and astronomers. The community makes use of laboratory studies and simulation facilities and includes direct investigations in Earth extreme environments and in space. Connected to astrobiology, this community is also interested in earthbound issues, in particular global change, biosphere sustainability, and humanities and social sciences.

Astrobiology is perceived in different ways by the various communities involved, and the national astrobiology landscapes differ significantly from one country to another. Nevertheless, building on the first European astrobiology experiments supported through ELIPS, on the creation and activities of EANA in the early 2000s, and on the various national astrobiology societies, the burgeoning implication and strength of the European astrobiologists' community is apparent. The pan-European initiatives presented above are new and innovative. They support this community in a coherent manner at various career stages and provide visibility and stability to its researchers. This demonstrates that the European astrobiology community has reached a level of maturity that enables ongoing projects to be built up, data from past projects to be exploited, and researchers across the board to engage in better coordination and consolidation of astrobiological projects. Thus, it is very timely to put forward and suggest a number of scientific priorities that should be addressed at the European level.

1.3. Setting the scene: timeline and astrobiology concepts

1.3.1. Timeline—from formation of stellar systems to life

As one of the main questions of astrobiology, the origin and evolution of life on Earth has attracted great interest from many different fields of expertise. The presence of life has changed the panorama of primitive Earth. Indeed, one of the difficulties concerning study of primitive Earth, upon which life arose, comes from the fact that life itself changed its environment. Life is a physicochemical process that takes inputs from the exterior in order to auto-organize its interior, modifying the environment in the process. Several crucial events took place along the timeline of Earth's history up to the period when environmental conditions allowed an explosion in biodiversity resulting in the multitude of species colonizing all habitable environments of Earth, as we know it today (Fig. 1). Earth's timeline starts about 4600 million years ago, when the planet formed from the accretion disc revolving around a young star. Even during this very early phase, complex organic molecules necessary for later life may already have formed in the protoplanetary disc of gas and dust grains. The formation of the Moon had important consequences from an astrobiological point of view. Very soon after its consolidation, 4500 million years (or 4.5 billion years) ago, and according to the giant impact hypothesis, the Moon was formed when planet Earth and another hypothesized planet collided (Hartmann and Davis, 1975). It has been suggested that the newly formed Moon stabilized Earth's fluctuating axis of rotation due to its gravitational pull and, hence, the environmental conditions in which life formed. Meanwhile, several other events took place during the formation of the gas giants and rocky planets.

FIG. 1. Timeline in million years from the formation of the Solar System to the appearance and evolution of life on Earth. [1] Cloud, 1948; [2] Grosberg and Strathmann, 2008; [3] Knoll, 2011; [4] Mills et al., 2014; [5] Javaux, 2007; [6] Parfrey et al., 2011; [7] Melezhik, 2006; [8] Holland, 2006; [9] Nisbet and Wilks, 1988; [10] Altermann and Wotherspoon, 1995; [11] Dhuime et al., 2015; [12] Byerly et al., 1986; [13] Hofmann et al., 1999; [14] Tice and Lowe, 2004; [15] Westall and Southam, 2006; [16] Walsh, 1992; [17] Westall, 2011; [18] Ohtomo et al., 2013; [19] Tera et al., 1974; [20] Holland, 1984; [21] Van Kranendonk et al., 2015; [22] Westall, 2012; [23] Elkins-Tanton, 2012; [24] Wilde et al., 2001; [25] Lebrun et al., 2013; [26] Zahnle et al., 1988; [27] Kleine et al., 2005; [28] Dalrymple, 2001; [29] Sagan and Mullen, 1972; [30] Gough, 1981; [31] Feulner, 2012; [32] Walsh et al., 2011; [33] Baker et al., 2005.

Owing to lack of evidence (all rocks older than about 4 Ga have been recycled), there is much debate about the geology of early Earth, but it is known that primitive continents (like oceanic plateaus) formed during the Hadean between about 4.4 and 4.0 Ga. True continents did not appear until much later, about 3 Ga (Dhuime et al., 2015). The earliest direct signatures of primitive life date back to about 3.5 Ga (Walsh, 1992; Westall, 2011), although life must have arisen much earlier because by 3.5 Ga it was already diversified and included anoxygenic photosynthesizers (Byerly et al., 1986; Hofmann et al., 1999; Tice and Lowe, 2004; Westall and Southam, 2006). Moreover, there is a tantalizing indication of its presence at 3.8 Ga (Ohtomo et al., 2013). However, it is with the rise of photosynthesis and, especially, oxygenic photosynthesis that life started to modify the environment. This absolutely critical evolution has not yet been precisely dated but occurred probably after 3.0 Ga. Its signatures in the form of giant stromatolites were already in place around the world by 2.7–2.6 Ga (Nisbet and Wilks, 1988; Eriksson and Altermann, 1998), but it is only as of 2.4 Ga with the “Great Oxidation Event” (Holland, 2006) that there was sufficient oxygen in the environment to make significant global changes—leading, for instance, to the first global glaciation (the Huronian or Makganyene glaciation, 2.4–2.1 Ga, Melezhik, 2006). The modification of the environment through the atmospheric oxygen rise allowed other organisms (with other metabolic pathways) to appear and different forms of life to evolve.

1.3.2. Approaching habitability

The concept of habitability is one of the fundamental issues for our understanding of the origin, diversity, and extent of life on Earth and our ability to identify extraterrestrial environments that are, or were, able to support life or may even support life in the future. These may include environments that allow life to emerge and enable its continued existence as well as evolution towards a higher complexity (Gershenson and Lenaerts, 2008). Therefore, in the simplest sense, a habitable environment can be identified as one that can allow the development, maintenance, and evolution of life. This ability relates either to a given time and location (“instantaneous habitability”) or to the maintenance of habitable conditions over geological time (“continuous habitability”). While most habitable environments on Earth are typically characterized by the presence of life, the presence of biological activity is not a prerequisite for habitability. A habitable environment, therefore, may be either inhabited or uninhabited (e.g., Cockell et al., 2012; Harrison et al., 2013; Westall et al., 2013).

Despite the diversity of life on Earth, all organisms explored so far share several requirements, such as the availability of elements (in particular C, H, N, O, P, S, trace metals, etc.) needed for macromolecular synthesis, liquid water, energy, and appropriate physical-chemical conditions (such as temperature). The ability of an environment to sustain life can be measured in several ways, and as a result the exact definition of habitability varies between scientific fields. Today, habitability is classified into two general categories: “planetary habitability” (the ability of a planetary body to develop and sustain life) and the “habitable zone” (the zone around a star where liquid water is stable at the surface of a planetary body) (Kasting et al., 1993). Investigations of the former rely on our knowledge of extremophilic organisms and the physical-chemical limits to biological processes on Earth, providing a detailed starting point for understanding the habitability of other planetary bodies such as Mars, the icy moons of outer planets, or exoplanets. Research into the limits for life and its evolutionary potential on Earth relies on the characterization of two key parameters: (i) physiological mechanisms that underpin stress tolerance, measurements of cell division, metabolic activity, dispersal, and survival in response to different physical-chemical extremes; and (ii) evolutionary ecological principles that serve as driving forces for the coevolution of life and its environment, thereby constantly reshaping the boundaries of habitability and, thus, the conditions for the continuity and evolution of life.

Investigations of habitability at the level of entire planetary systems are based on the concept of the habitable zone. The presence of liquid water, therefore, plays a key role in investigating the habitability of extraterrestrial environments. For very distant objects, such as exoplanets, the identification of potentially habitable conditions depends on numerous factors including planetary mass, orbit, type of atmosphere, and the properties of the central star. However, the question is whether these few parameters are sufficient. The AstRoMap Roadmap intends to identify key research topics that will broaden our knowledge of habitability on all levels of organization, ranging from planetary bodies within our solar system to other planetary systems in our galaxy.

2. The Astrobiology Roadmap for Europe

Based on the inputs from the experts involved in AstRoMap, a roadmap has been constructed for astrobiology in Europe, which consists of five interconnected research topics to be addressed in parallel:

• Research Topic 1: Origin and Evolution of Planetary Systems

• Research Topic 2: Origins of Organic Compounds in Space

• Research Topic 3: Rock-Water-Carbon Interactions, Organic Synthesis on Earth, and Steps to Life

• Research Topic 4: Life and Habitability

• Research Topic 5: Biosignatures as Facilitating Life Detection

These five research topics reflect essential steps towards reaching the final goal: a better understanding of life and its origin and evolution within the context of cosmic evolution (Fig. 2), that is, regarding life as a cosmic phenomenon (De Duve, 1996).

FIG. 2. The AstRoMap Roadmap: An astrobiology roadmap for Europe, consisting of five research topics to be addressed in parallel towards reaching the final goal—a better understanding of life within the context of cosmic evolution. Background picture: M51 Hubble Remix (http://apod.nasa.gov/apod/ap080614.html). Credit: S. Beckwith (STScI), Hubble Heritage Team (STScI/AURA), ESA, NASA. Additional Processing: Robert Gendler. Graphic Design: Kerstin Kopp, DLR.

Based on the scientific experience and technical capabilities available in Europe, three to five key objectives have been determined for each research topic, and approaches have been suggested as to how these objectives will be reached within the next 10 or 20 years or after that period (Chapters 3–7 and Figs. 3–7).

FIG. 3. AstRoMap Roadmap, approaches to reach the key objectives of Research Topic 1 “Origin and Evolution of Planetary Systems” within the next 10, 20, or follow-on years.

FIG. 4. AstRoMap Roadmap, approaches to reach the key objectives of Research Topic 2 “Origins of Organic Compounds in Space” within the next 10, 20, or follow-on years.

FIG. 5. AstRoMap Roadmap, approaches to reach the key objectives of Research Topic 3 “Rock-Water-Carbon Interactions, Organic Synthesis on Earth, and Steps to Life” within the next 10, 20, or follow-on years.

FIG. 6. AstRoMap Roadmap, approaches to reach the key objectives of Research Topic 4 “Life and Habitability” within the next 10, 20, or follow-on years.

FIG. 7. AstRoMap Roadmap, approaches to reach the key objectives of Research Topic 5 “Biosignatures as Facilitating Life Detection” within the next 10, 20, or follow-on years.

It is strongly recommended that this AstRoMap Roadmap be adopted by the European Union as a challenge to enhance Europe's standing as an attractive partner for international partnerships in space science and exploration. The Roadmap should be supported by cross-disciplinary research in the five Research Topics listed above and described below. This requires the establishment of a pan-European astrobiology coordination platform or European virtual astrobiology institute under the auspices of a pan-European funding organization, for example, the European Union, the European Space Agency, or the European Science Foundation (see section 8.2).

3. Research Topic 1: Origin and Evolution of Planetary Systems

3.1. State of the art

Our solar system is the planetary system that is best known to us, and so far it is the only system known to host life. A wealth of data, collected over many decades through observations, space missions, and field and laboratory studies, have been used to formulate our ever-evolving theories of how Earth, the other planets, and their satellites formed and how the system has dynamically evolved towards its current state.

In the context of astrobiology, understanding the origin of water and other materials essential for life, on Earth and on other, possibly habitable environments in the Solar System, is of profound importance. The chemical composition of the first planetesimals—the building blocks of planets—and how they evolved as a function of distance from the Sun can be inferred by studying collected extraterrestrial materials and by analyzing remote sensing data of comets and asteroids. New amazing results from the ESA Rosetta space mission demonstrate the presence of organic material on the nucleus of the Jupiter-family comet 67P/Churiumov-Gerasimenko, probably formed by the interaction of ices (H 2 O, CO 2 , CO) with solar radiation (Capaccioni et al., 2015). The measured D/H ratio of the comet 67P was found to be larger than that previously measured in comets, and this finding opens up new questions about the formation mechanisms of comets and the chemical models of the protoplanetary solar nebula (Altwegg et al., 2015). Models are necessary to explain the present Solar System architecture. These models are continuously refined by feedback both from observations of our own and other planetary systems and from laboratory simulation experiments. Nevertheless, many gaps remain. These observations should involve protoplanetary discs for which the only known species are CO, CN, HCN, or HCO+, and future observations are needed to build up more robust chemistry models (Beuther et al., 2014, and references therein; Dutrey et al., 2014).

Since the observation of the first exoplanet orbiting a Sun-like star (Mayor and Queloz, 1995), astrophysicists have realized that planet formation should include orbit migration involving chaotic phases in planetary systems, which makes it difficult to reconstruct the initial configuration of the protoplanets.

Stars and planets are the end products of a complex chain of mechanisms, starting in the galactic interstellar medium where molecular cloud contraction and collapse define the initial conditions for the formation of stars and their planetary systems. Our understanding of how planets form around stars is far from complete and increasingly challenged by new discoveries. Recent space missions, such as COnvection ROtation and planetary Transits (CoRoT) and the US-led counterpart Kepler, have provided valuable new information on this topic and helped develop new concepts of missions such as CHEOPS and PLATO 2.0, which are slated to be launched by ESA in 2017 and 2024, respectively. As of July 2015 more than 1500 confirmed planets have been observed, both by missions and by ground-based facilities (see www.exoplanets.org).

Planets form in protoplanetary discs that surround young stars. The formation process of discs, their physical, chemical, and dynamical evolution, as well as the various steps of planetary growth and subsequent evolution, obviously lead to the wide variety of observed planetary environments. The orbital architecture of exoplanetary systems is also highly diverse: “hot” giant planets, highly compact multiplanet systems, planets around binary stars, highly eccentric orbits, and even inclined planetary systems. The mechanisms responsible for these turbulent events, their strength and duration, as well as the final dynamical configuration could be paramount for determining whether a system may host habitable planets or not.

Much progress has recently been made concerning the basic steps of planet formation as well as the basic astrophysical and geochemical timescale constraints. Nearer the star, the “core accretion” mechanism (Pollack et al., 1996; Ida and Lin, 2004; Alibert et al., 2005) involves collisional growth from planetesimals up to planet-sized objects. Farther away from the star, the “gravitational instability” mechanism (Safronov, 1969; Goldreich and Ward, 1973) involves “clumps” that arise from disc fragmentation. Recent advances in modeling capabilities have revealed the progression of different dynamical evolutionary stages that led to the present Solar System architecture: planet migration, followed by large-scale transport and redistribution of material, giant collisions, such as the Moon-forming event, and subsequent intense bombardment episodes (Tsiganis et al., 2005). At the same time, a deeper understanding of the post-formation geological, geochemical, and atmospheric evolution of planets and their satellites has been achieved.

On the other hand, the effects of mutual collisions on the chemical evolution of the forming planetary embryos, the relative contribution of bodies originating in different regions of the Solar System, and the relative significance of the successive formation stages in the final water budget of Earth are not well constrained. Understanding the sequence of events for Earth and other planetary bodies in our solar system will enable us to probe also the habitability of exoplanets.

3.2. Key objectives

Key Objective 1. To assess the elemental and chemical picture of protoplanetary stellar discs

An important aspect in planet formation theories is the evolution of protoplanetary discs in relation to their host stars. So far we have very limited data on protoplanetary discs. Observations are still scarce, but prospective Atacama Large Millimeter/submillimeter Array (ALMA) observations should extend our knowledge and help compile more information about the distribution of different chemical species in discs and other parameters that are necessary for refining numerical models of protoplanetary disc evolution and early phases of planet formation. Hydrodynamic codes that are generally used to model such processes only allow simulations over comparatively short timescales, as the equations involved are quite complex. Hence, an improvement of existing computing capabilities is needed.

Sub-objective 1. To understand the metallicity of stars

The chemical composition of planetary systems is linked to the elementary composition of the star around which they form. Therefore the “metallicity” of stars is a crucial factor in the development of the “Solar System” and has been a major project in the Kepler study. In turn, metallicity is linked to stellar synthesis, which is a topic in its own right.

Sub-objective 2. To improve chemical models of protoplanetary disc formation and evolution

Models should take into account complex environmental (variable luminosity of the young star, stellar environment) and internal (radiative transfer, magnetohydrodynamic instabilities and turbulence, viscosity variations, etc.) processes in an effort to better constrain key parameters (e.g., solids-to-gas ratio, snowline position) that are important for deriving the correct formation timescale, size distribution, and composition of the first planetesimals, as functions of heliocentric distance.

Sub-objective 3. To improve our understanding of the evolution of circumstellar discs, in relation to their host stars

Long-term evolution of a protoplanetary disc can only be modeled by implicit hydrodynamic codes that include radiative transfer, where the physical properties are resolved to the desired level. Incorporating chemical evolution inside the disc is a necessary but difficult task—commonly used numerical codes do not include this aspect. Coupling the dynamics with the chemical evolution of the disc will constitute a great improvement.

Sub-objective 4. To determine the chemical history of key molecules (such as water, oxygen) in the evolution from molecular clouds to star-planet(s) system

Some key elements can help us reconstruct the chemical history of the evolution that, starting from the original molecular nebula, ends in the formation of the star-planet(s) system. One of the most important compounds is the water molecule. The presence of water in liquid state on a planet's surface is generally accepted as essential for its potential habitability (Kasting et al., 1993). Water in gaseous form acts as a coolant that allows interstellar gas clouds to collapse to form stars, while water ice facilitates the adhesion of small dust particles that eventually grow to planetesimals and planets. The development of life requires liquid water, and even the most primitive cellular life on Earth consists primarily of water. Water assists many chemical reactions that lead to complexity by acting as an effective solvent. It shapes the geology and climate on rocky planets, helps maintain plate tectonics, and is a major or primary constituent of the solid bodies of the outer Solar System. Thanks to a number of recent space missions, culminating with the Herschel Space Observatory, an enormous step forward has been made in our understanding of where water is formed in space, what its abundance is in various physical environments, and how it is transported from collapsing clouds to forming planetary systems. At the same time, new results are emerging on the water content of bodies in our own solar system and in the atmospheres of known exoplanets.

Another particularly useful investigative tool is the study of deuterium fractionation, namely, the process that enriches the amount of deuterium with respect to hydrogen in molecules. Deuterium fractionation initiates at the very early stages of the evolution of the protosolar nebula. Therefore, analyzing the D/H ratio in different objects, which will eventually form new stars, and in comets, meteorites, and small bodies of the Solar System will provide insight into the very first steps of the Solar System's formation.

Sub-objective 5. To interconnect chemistry with disc hydrodynamics and structure

When studying the protoplanetary disc, a static disc-model is used; this is mainly concerned with the mass and the structure of the disc. One significant challenge has already arisen from observations by ALMA of discs around young stars. These have revealed distinct asymmetries in the dust continuum emission, which has led to the development of new models to explain the observations (Flock et al., 2015). Moreover, ALMA detected features that are most probably the result of young protoplanetary bodies forming in the disc around the young Sun-like star HL Tau, an unexpected observation at this stage of stellar system formation (ALMA, 2014).

Key Objective 2. To better understand our solar system: planet formation, dynamical evolution, and water/organics delivery to Earth and to the other planets/satellites

Addressing this second key objective should allow us to reach a robust theory of Solar System formation (planet formation and dynamical evolution of the early system), characterized by a well-defined sequence of all major system-changing events and calibrated with available data (e.g., geochemical) where possible. The generic mechanisms pertaining to planet formation and dynamical evolution have to be defined and studied in a wider context, while key conditions and parameters that lead to the “particular solution” of our solar system have to be identified. Our knowledge of Earth-specific events (e.g., Moon formation, a possible late heavy bombardment) has to be refined and placed in the overall context of Solar System formation. Such a theory would constitute the basic “input” for testing different astrobiological scenarios for Earth and other, possibly habitable environments in the Solar System and beyond.

Sub-objective 1. To better understand the transition from planetesimals to planets and satellites (end to end)

High-performance computing resources with increased-resolution simulations are required to address this task, taking into account the evolving disc environment and more refined collisional models (size distribution, accretive vs. non-accretive collisions). The goal is to resolve important issues, such as understanding the short accretion timescale for the cores of the giant planets (which should be smaller than the lifetime of the gas disc) and better matching the formation timescales for the terrestrial planets and the Moon with geochemical data. At present, no numerical code of planet formation is able to perform fully self-consistent simulations, starting with micron-sized solid grains (i.e., chondrule-like) and ending up with fully formed planets. Hence, the development of necessary technologies to address this issue has to be supported.

Sub-objective 2. To better understand the dynamical evolution of the “young” Solar System

Emphasis should be placed on resolving ambiguities related mainly to deciding between different scenarios proposed up to now for the gas- and planetesimal-driven migration of the giant planets. This has important repercussions for understanding the large-scale transport of asteroidal and cometary material throughout the Solar System, the evolution of the small-body reservoirs, the accretion of the terrestrial planets, and the sequence of bombardment episodes on the geologically evolving planets and satellites at various epochs.

Collisions between objects of different sizes (geologically differentiated or not) are an integral part of the formation process, both during the planetary accretion phase and after. Material that is essential for the development of prebiotic chemistry (water, organics) can be delivered to forming (or already formed) planets by so-called “catastrophic” events.

Sub-objective 3. To improve models on conditions for survival and/or generation of essential molecules during impacts

Information can be obtained by simulating the coupled thermodynamic and chemical evolution of the hot plasmas generated during high-velocity impacts between various types of bodies and by testing with gun impact simulation facilities (e.g., Fraunhofer Institute for High-Speed Dynamics, Ernst Mach Institute, EMI, Freiburg, Germany; and the two-stage light gas gun at the University of Kent, UK).

Sub-objective 4. To identify dynamical processes that can redistribute essential material throughout a system

The conditions under which generic dynamical mechanisms enhanced the transport of essential material throughout the Solar System should be identified. This information is also important for other planetary systems (see Key Objective 3), which may contain various types of planets in various orbital architectures.

Sub-objective 5. To better understand the effects of postformation bombardment episodes on Earth and other planetary bodies generally assumed to have been important for the development of life

Questions about whether such events bring necessary “fresh” material (water/organics), supply “energy” (heat, radiation), create “friendly” habitats (site resurfacing, exposure of subsurface material) and whether these are critical or not for the development of life have to be properly addressed (see also Research Topics 2 and 3).

To thoroughly test theoretical models, we need to have better knowledge of the distribution and physical properties of Solar System objects; for example, the outer Solar System, whose structure may provide important constraints for dynamical evolution models, is largely unexplored. More ground- and space-based observations are needed (including occultation surveys). We also need to improve our knowledge of the composition of primordial objects; on a decade-long plan, this means enhancing our efforts to collect and analyze a wide variety of meteorite samples (including extending fireball observations networks), the only low-cost way of collecting primordial material. Finally, important constraints for impact modeling could be derived by further enhancing laboratory research (impact experiments).

Sub-objective 6. To better define the timeline of the formation of the Solar System and water/organic delivery on Earth

In all the scenarios proposed for the early evolution of the Solar System (i.e., the first ∼10 million years), the presence of water and volatile elements inside the water-ice condensation line appears to be a natural by-product of the appearance of the giant planets. It is presently unclear, however, how much of these volatile elements would actually survive the formation process of the terrestrial planets. The same holds true for the organic material originally present in the ice-rich planetesimals: did these organics, that were incorporated in the growing terrestrial planets, survive the impacts, or were they destroyed?

Sub-objective 7. To interpret the temporal link between Solar System evolution and the rise of life on Earth

Earth and the Moon completed their formation about 4.5 billion years ago (i.e., about 30–100 million years after the formation of the Sun and the first solids, calcium-aluminum-rich inclusions, in the Solar System), yet the oldest proven evidence for life on Earth is no older than 3.5 Ga, although it is likely that life had already appeared before this time. Further, due to recycling of the crust, older, well-preserved rocks have not survived tectonic recycling of the crust. Although life most likely appeared well before the event known as the Lunar Cataclysm or the proposed Late Heavy Bombardment (LHB), which should have ended about 3.8 billion years ago (Westall, 2012), this event must certainly have had an important impact on Earth, probably contributing to the destruction of the crust and erasure of much information about the oldest events on our planet. The exact nature of this correlation is, however, uncertain. Did the LHB play a role in resupplying the terrestrial planets with organic material and water that possibly got lost during their formation process? Did the LHB accelerate chemical evolution by providing, for example, surface restructuring and/or energy on small (local) scales? Or was the LHB simply the final event in the long-lasting, violent process of terrestrial planet formation that did not contribute directly to the appearance of life on Earth but only marked the onset of a more quiescent era, characterized by a life-friendly environment? (See also Fig. 1 and Research Topics 2 and 3.)

Key Objective 3. To better understand the diversity of exoplanetary systems and the development of habitable environments

Detailed information from observed star-disc configurations will enable us to constrain a general planet formation theory, which is the main task to be addressed. Different properties of discs (solid-to-gas ratio, density and viscosity profile), host stars (luminosity, spectral type, metallicity, etc.), and their environment (gravitational torques and radiation from neighboring stars) should be considered.

Sub-objective 1. To better understand the dynamical mechanisms that lead to the observed diversity of exoplanetary architecture, and assess how they affect habitability

Various dynamical mechanisms have been proposed to explain the observed exoplanetary architecture. This sub-objective's goal is to identify the key generic mechanisms responsible for the onset of dynamical instabilities during different stages of formation, and their relationship to fundamental characteristics of the initial system. Orbital dynamics are frequently used as an indicator of the habitability of a planet but are not sufficient (see Section 1.3.2). This should enable us to constrain dynamical evolutionary pathways (including water delivery and bombardment episodes) and final dynamical configurations of planetary systems that are compatible with the development of prebiotic chemistry and, possibly, life.

Sub-objective 2. To identify biomarkers and promising methods of detection

Observations of atmospheres of different types of planets (Earth-like, warm and hot super-Earths, “hot” Neptunes, etc.) are crucial for understanding their postformation evolution in relation to different parent stars. For planets labeled as “habitable,” spectrally resolving their atmosphere is currently the only foreseeable means of detecting possible signatures of life. However, a theoretical effort will have to be executed to decide which characteristics constitute real “biomarkers,” to properly model them (see Research Topic 5), and to discern what new technologies will need to be developed so as to observe them. This work will constitute an important asset for designing future missions.

Sub-objective 3. To find out how the study of exoplanets can help fill the gaps in our understanding of the formation of our own solar system

Observations by Spitzer (NASA) and Herschel (ESA) suggest that debris discs are at least as common around nearby stars as planets. A study of the nearest G-type stars by Wyatt et al. (2012) indicates a correlation between low-mass planets and observed debris discs, concluding that systems with only low-mass planets are preferentially dusty. High-resolution observations as provided by ALMA are needed to understand this early phase in the formation process, which defines the initial chemical distribution for the material of which terrestrial planets are formed. With observations of different ages of a disc, we could improve the existing models also for the Solar System.

3.3. Approach to achieve the key objectives

Next ten years

Observations

More and better-quality observations from ground and space are needed. The last generation of millimeter and submillimeter interferometers (ALMA and the Northern Extended Millimetre Array, NOEMA) has the required sensitivity and spatial resolution to provide new and strong observational constraints on chemical abundance of molecular species to chemical models for all the phenomena that play a key role in planetary system formation, namely, prestellar cores, protostars, young stellar objects, and protoplanetary discs. Because of these two important observing facilities, major advancements are expected in the following:

• Observations and detection of complex molecules;

• 3-D disc models including chemistry;

• Combination of hydrodynamics and chemistry;

• Comparison with astrochemistry laboratory experiments.

Protostellar nebular (PSN) observations made by ALMA and the upcoming James Webb Space Telescope (JWST) are needed to obtain necessary information on early stage of formation (of planetesimals, embryos, and protoplanets) from which new models can be developed to bridge the gaps in our understanding of the formation process.

However, because of the high abundance of water in Earth's atmosphere that can partially obscure the observations from ground-based facilities, the bulk of the data will come from space observatories. Except for in situ mass spectroscopy in planetary and cometary atmospheres, all information about interstellar and Solar System water comes from spectroscopic data obtained with IR telescopes [the Infrared Space Observatory (ISO), Herschel]. The validation of chemical models with the Herschel observations is ongoing work.

An improvement of observational and technical facilities should also be pursued. The European Extremely Large Telescope (E-ELT) and PLATO 2.0 will open a new era for the European astronomical community. The results from various observational campaigns will offer increased added value for the theory, if collaboration between observers and theorists is optimized in the framework of a larger network and follow-up programs supported by the European Union.

Modeling

Our ability to develop sophisticated theoretical models depends critically upon our knowledge of the relevant physical and chemical processes, as well as on our computational capabilities. In particular, the early stages of planet formation are still not well understood. The developed models for the growth of planetesimals from dust grains by subsequent sticking collisions have revealed obstacles to dust growth, such as bouncing, fragmentation, and radial drift barriers (referred to as the “meter-size barrier”).

Such knowledge gaps on specific key points prevent us from formulating a robust theory; there are, however, several solutions that have been proposed to overcome these gaps. Therefore, a robust, system-independent, theoretical framework has to be developed, which could successfully be applied to explain the diversity of observed (and others that cannot yet be observed) planetary systems and place our singular life-bearing example—the Solar System—in context. Improving our theoretical tool kit should be based on supporting basic research in planetary science and further promoting collaboration on a European scale, by further developing interdisciplinary networks. However, special provisions have to be made in future research programs to exploit and safeguard important expertise (currently dispersed in, for example, small groups, working in small institutes and/or in less-favored regions in Europe).

At present, testing theoretical hypotheses is heavily based on accurate, high-resolution computer simulations. Our current capabilities, however, do not suffice. Hence, research groups and institutes should be supported so as to invest in new technologies (hardware, parallel computing protocols) and key scientific software development.

The refinement of theoretical models is based also on the availability of data from space missions. For example, results from impact experiments need to be calibrated on large scales, which are inaccessible in laboratories. Hence, an impact-experiment mission like Asteroid Impact and Deflection Assessment (AIDA, a joint ESA/NASA mission) should produce extremely important results. We note that the European part of AIDA [called Asteroid Impact Mission (AIM)] is currently in Phase A/B1 study by ESA.

We expect that a robust theory of formation for our solar system can be developed within the next decade, or at least the main uncertainties will be sufficiently studied, to significantly narrow the admissible set of solutions. In particular, we expect significant progress towards resolving the main challenges related to the following:

• Formation of planetesimals and planet cores,

• Dynamical evolution scenarios,

• Impact history of early Earth.

This would provide a well-constrained time line of the main events that sets the picture for the development of life on Earth.

Furthermore, it is a central aim during the next decade to improve our theories to the point where the key parameters that affect the observed diversity of planets and their orbital architecture will be elucidated. This will enhance the theoretical background for PLATO 2.0, which is expected to bring Europe into a new era in the field of exoplanetary science. Also, we expect to have achieved a better understanding of habitable conditions and the associated biomarkers that we should be looking for in the atmospheres of habitable planets.

Interdisciplinary collaboration

Progressing on this research topic requires input from many different areas of expertise; interdisciplinary research groups have to be linked together in order to tackle the various key scientific objectives adequately. To develop knowledge of exoplanets and habitability in particular, it is important to create and strengthen interactions and collaborations with the nuclear community that explores stellar synthesis. Such collaborations were apparent in European Science Foundation programs GREAT (Gaia Research for European Astronomy Training), EuroGENESIS (Origin of the Elements and Nuclear History of the Universe), CompStar (the New Physics of Compact Stars), and COSLAB (Cosmology in the Laboratory).

Ten to twenty years from now

Advanced physicochemical models

The role of magnetic fields, the generation of turbulence by fluid instabilities, and their relation to the distribution of different chemical species are of profound importance for understanding the condensation sequence; the evolution of the density and temperature profile; the formation of the first solids; and the dynamical evolution of grains, pebbles, and planetesimals in protoplanetary discs. This is the most fundamental step in planet formation. Hence, advanced models that combine all major physical and chemical processes (gravity, magnetohydrodynamics, collisions, thermodynamics, chemistry) in a self-consistent manner should be developed. Statistical analyses of their range of solutions will then enable us to understand how the observed diversity of planetary systems is produced and how our solar system fits in. This requires advances not only in theory but also in technology (computer platforms and software).

Candidate list for habitable exoplanets

Building on the results of the next decade, we should be able to develop a system-independent theory that will enable us to understand the details of planet formation around different stellar classes (from F to M). We will have considerably expanded our database of Earth-like planets in habitable zones around different stellar classes. Improvements and new awareness due to PLATO 2.0 and E-ELT observations should enable us to compile a candidate list for habitable exoplanets.

Solar System missions

Theoretical models should be further refined that will enable us to understand more of the fine details of Solar System formation. This, however, requires additional data from space missions, such as dedicated sample return missions from small bodies (primitive-type asteroids, comets, main-belt transition objects) that will allow us to access samples of primordial material. Sample return missions from Mars will, of course, be of utmost importance for astrobiology. Also, missions to satellites of the outer planets, which have been identified as additional potential candidates for hosting life, should be pursued (e.g., Europa, Titan). The development of the required observational technology to achieve these objectives needs to be promoted.

Sample return missions would greatly benefit from the development of a European extraterrestrial sample-receiving and curation facility, the requirements of which are presently compiled and defined in the European Curation of Astromaterials Returned from the Exploration of Space (EURO-CARES) project (supported by the European Union Horizon 2020 program). At the same time, sample return missions raise planetary protection issues that need to be properly addressed.

After twenty years

Solar System missions

Solar System exploration will enable an increasing quantity of pristine material to be accessed, which will allow, in parallel with improved computing and analytical capacity, additional insight into the formation of planetary systems. Figure 3 gives an integrated view of Research Topic 1 together with its timeline.

3.4. European strengths and needs

Europe has been a leader in numerous fields of exoplanetary research, and many world-renowned experts on observations and theory of planetary science work for European institutes, performing high-quality research. Different countries in Europe (such as Germany, Austria, and Switzerland) have already established national and transnational networking groups on this research topic, and these actions should be further supported.

Upcoming European space missions like CHEOPS and PLATO 2.0 as well as new ground-based telescopes (e.g., E-ELT) will certainly improve the quality of observations and provide exciting new data. Europe has ongoing programs (e.g., CARMENES: Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs) in place to study planets orbiting cooler (M dwarf) stars, which are considered favorable targets for finding habitable, Earth-like planets (see Research Topic 5).

European astronomers have taken a leading role in the development of dynamical models to simulate the evolution of planetary systems, for example, the Nice model for the Solar System (Tsiganis et al., 2005). For the European planetary science community, the European Planetary Network Europlanet was founded in 2005 with the aim of acting as a platform for the exchange of ideas, personnel, tools, and data (see http://www.europlanet-eu.org). Meanwhile, more than 60 institutions are members of Europlanet. In addition, in recent years many European countries (e.g., Germany, Switzerland, Austria, Belgium, France) have launched national networking programs on planetary science, which have increased the capacity of front-line European teams by promoting collaboration between scientists of different backgrounds (geology, astrophysics, celestial mechanics). Strong observational capabilities exist (e.g., European Southern Observatory, ESO), and significant experience has been gained by European teams in designing, executing, and exploiting data from space missions, within the framework of collaboration between national space agencies and ESA. An example is the ESA space mission Rosetta that allows for the study of the dynamics of a Jupiter-family comet (Altwegg et al., 2015).

However, European missions to outer Solar System bodies are significantly limited by technological challenges, such as energy (unavailability of radioisotope thermoelectric generators) and data transmission rate. Also, the scientific return from such missions could be greatly enhanced if provisions were made to perform flybys of targets of opportunity in the asteroid belt during cruise phase. European and national space agencies should consider such options, in relation with the scientific community, whenever opportunities arise.

As for the various other astrobiology research activities (see Section 8.2), European investigation of the origin and evolution of planetary systems needs to be better coordinated, for example, within a large scientific network (or Virtual Institute). Such networking activities would promote interactions of groups from different research fields, an important necessity for this interdisciplinary research topic. A first step in this direction has already been achieved by the networking activities of Europlanet, COST ORIGINS, and EANA.

4. Research Topic 2: Origins of Organic Compounds in Space

4.1. State of the art

Almost 200 organic molecules have been detected as gaseous species in space (http://astrochymist.org/astrochymist_mole.html). They are formed in cold molecular clouds and in the early phases of star formation. Gas-phase chemistry cannot explain the abundances observed for most of them. It is therefore thought that most of them form on cold dust grains on which atoms and molecules accrete to form icy mantles. As demonstrated by a large number of laboratory studies (e.g., Allodi et al., 2013), surface reactions and/or energetic processing (by UV photons, electrons, atoms, and ions) induce the formation of a solid complex organic refractory material (often referred to as “organic residue”) and of a plethora of molecules that, once released by thermal (e.g., around a forming star) or nonthermal (e.g., by sputtering in shocks) processes, are observed in the gas phase. Circumstellar dust grains are the building material of planetesimals and, in turn, of larger planetary objects (see Research Topic 1). Thus, there is a link between the composition of preplanetary materials and that of planetesimals, comets being assumed to be their direct remnants. This scenario has been recently confirmed and extended to protoplanetary discs by Öberg et al. (2015). Those authors showed that complex organics accompany simpler volatiles in protoplanetary discs, and that the rich organic chemistry of our solar nebula is not unique. It is also widely accepted that comets and other small bodies, such as asteroids, meteorites, and interplanetary dust particles, have delivered huge amounts of molecules (including water and organics) to Earth (and possibly on exoplanets), especially during its early history when the flux of extraterrestrial materials was much higher (e.g., Schmitt-Kopplin et al., 2010; Westall, 2012). This scenario demonstrates the evident role of meteorites in this field. Evidence for catalytic properties of meteorites regarding the synthesis of organics was recently provided by investigating thermal processes of widely diffused chemical precursors (e.g., formamide, HCN) at relatively high temperatures (333 and 413 K) (Saladino et al., 2013) or at the most extreme temperature conditions during impacts on Earth's atmosphere or surface (Ferus et al., 2014). Structurally complex nucleosides (as well as a variety of other biologically relevant molecules) have been synthesized from formamide by proton irradiation at 170 MeV, mimicking the effect of the solar wind (Saladino et al., 2015).

These results have been obtained by observational, experimental, and theoretical studies. The advent of a new generation of telescopes, including the interferometer ALMA, the Square Kilometer Array (SKA), and JWST, will enable scientists to study the distribution of abiotic organic molecules in star-forming regions to a much greater extent and so learn more about the relationship of these molecules to exogenous delivery on Earth or potentially habitable exoplanets.

4.2. Key objectives

Key Objective 1. To promote our understanding of the diversity and the complexity of abiotic organics

The interaction between electromagnetic radiation and organic molecules in heterogeneous environments is a prebiotically relevant process. Minerals may have played a pivotal role in the prebiotic evolution of complex chemical systems, mediating the effects of electromagnetic radiation, influencing the photostability of organic molecules, catalyzing important chemical reactions, and/or protecting molecules against degradation. In particular, nucleobases are relevant biomolecules to investigate in the prebiotic context, because they code components of nucleic acids, and from the standpoint of the survival of biological systems in space conditions (Fornaro et al., 2013; Vergne et al., 2015). Several studies on the photodynamics of nucleobases suggest that their structure could have been naturally selected for their ability to dissipate electronic energy through ultrafast photophysical decay.

Sub-objective 1. To study the mechanisms for the formation of organics and their evolution under space conditions

Considering the putative involvement of minerals in prebiotic chemistry, it is necessary to study the photostability of nucleobases under space conditions in the presence of mineral matrices in order to investigate both the prebiotic processes that might have had a role in the development of the first living entities on Earth and the physical and chemical processes occurring in extraterrestrial environments. As mentioned above, thermal equilibrium chemistry, surface chemistry, and energetic processing of interstellar dust produce several complex organic molecules observed in space and eventually delivered to Earth. It is not yet clear how complex these molecules are. Molecular abundance decreases with increasing molecular complexity, and it becomes virtually impossible to detect them in either laboratory experiments or in space by astronomical observations. Of course, the limit of observability changes as the detection techniques become more sensitive.

Sub-objective 2. To better understand the role of catalysts in the formation processes of organics

An additional issue that is not fully explored is the role of specific solid substrates, such as catalysts (see also Research Topic 3). As an example, it is very important to increase knowledge of the reactivity and the catalytic role of meteorites (and cosmic dust particles that are collected on Earth and/or in space) in the synthesis of organic molecules of high structural complexity (possibly intermediates of both the genetic and metabolic mechanisms). For example, meteorites of different origin and composition, available from international scientific collections, can be studied and compared for their reactivity under different flux and energy conditions (keV, MeV ions or UV photons) using the most simple and widely diffused chemical precursors (e.g., one-carbon-containing compounds such as formamide, HCN, isocyanate, and others). Different temperature conditions should be tested depending on the nature of the energy source applied. In a similar way, cosmic dust analogues that are prepared in the laboratory by different deposition procedures with a specific and controlled elemental composition can be evaluated for their catalytic properties. Data about yields, complexity, and (eventual) selectivity should be correlated to elemental composition and mineralogical origin to better define the effect of metal composition.

Key Objective 2. To better understand the molecular evolution of abiotic organics present in Solar System objects, including early Earth, under the combined role of physical agents such as thermal variations, high-energy particles, photons, and solar wind irradiation

Complex organic materials exist in the protoplanetary solar nebula either in the solid state (refractory organic material) or as gaseous molecules. Such materials are completely destroyed near the young Sun but can be at least partially preserved in objects formed far from the central star (e.g., comets and some classes of asteroids). These objects and their debris (meteorites and dust particles) are in turn subjected to an extreme variety of temperature conditions, to energetic photons, and to ionizing radiation of galactic and solar origin that modifies the organic material. Ad hoc experiments and observations are necessary, aimed at reproducing different environments that are potentially relevant to astrobiology. These clearly include Mars, Titan, the icy moons of the giant planets as well as comets (both from the Oort cloud and the Kuiper belt) and some classes of asteroids and their debris (meteorites and dust particles). Within the Solar System, the surfaces of those objects are exposed to the electromagnetic solar spectrum and to ionizing radiation mostly generated by the Sun (solar wind and solar cosmic rays, SCR, primarily protons accelerated by flares and coronal mass ejections to energies typically of tens to hundreds of megaelectronvolts). Also relevant are high-energy (i.e., more penetrating) particles coming from the Galaxy (galactic cosmic rays, GCR). SCR and GCR are dominated by protons that exhibit different energy spectra (see, e.g., Bennett et al., 2013). The flux of energetic ions and electrons in the jovian magnetosphere that irradiate the icy surfaces of satellites and rings is particularly intense. Such a process induces a continuous exchange of atomic and molecular species with their environments (e.g., the tenuous atmospheres of the satellites and the gas surrounding the particles in the rings). Chemical species are continuously expelled from surfaces by exogenic processes, such as sputtering caused by the high fluxes of bombarding energetic ions and electrons from the jovian magnetosphere (e.g., Johnson et al., 2008). This is a well-studied phenomenon, and data concerning sputtering yields exist for a wide range of combinations of projectile energy and target composition. Sputtered species include mostly neutral atoms and molecules as well as ionized species (the latter include clusters and deserve further study). The released species can be lost to space or can populate the exospheres of the icy satellites (e.g., Plainaki et al., 2015). In the coming years, it will be important to collect further data on the nature of the species released by ion irradiation of realistic ice mixtures, as well as their yields and energy and angular distribution. Additional ice-gas interactions are due to endogenic geological processes, such as geyserlike activity that may produce plumes as those observed at the icy moon Europa (Roth et al., 2013). Chemical compounds present in the plumes could originate in the putative oceans underneath the surface, and once released in the atmosphere and observed by JUICE (the long-term ESA mission to the jovian system) instruments, they could give information on the (bio?)chemistry occurring in the underlying oceans.

The interaction of organic chemical precursors with the surrounding material in the form of silicates (e.g., in meteorites) or ices (e.g., on a dust aggregate expelled from a comet) modifies their excitation states, producing energetically favorable conditions for the generation of reactive radical species able to be further transformed into very complex organics (Adhikari et al., 2000). A key question is, can the interaction between energetic protons (and/or other ions) and organics on meteorites (used as template for materials coming from space) offer a benign environment for the formation of prebiotic molecules during their journey through space, during their impact, and during their lifetime on a planetary surface? Some experimental evidence to address this issue has already been produced. As an example, Simakov has described the possibility of prebiotic syntheses under experimental conditions that simulate the energetic processing of the mineral surface of asteroids and meteorites. In particular, mixtures of nucleosides (ribose and 2′-deoxyribose derivatives) and inorganic phosphate (NaH 2 PO 4 ) were exposed to space irradiation on Bio-Sputnik to yield the corresponding nucleoside derivatives (Simakov, 2008). An additional important contribution comes from the results of the EXPOSE-E and EXPOSE-R chemical experiments on prebiotic photochemistry, as it occurs in the interstellar medium or in the clouds of Saturn's moon Titan, and on the stability of organic compounds and microorganisms under simulated martian surface conditions (EXPOSE-E, 2012; EXPOSE-R, 2015). Organics in space can be delivered to all objects in the Solar System, including Earth, where they can be in contact not only with rocky materials but also with water both in the liquid and in the solid phase (e.g., early Earth, icy moons of the outer Solar System or planetesimals formed beyond the snow line). While aqueous alteration of extraterrestrial organic matter is well studied (see Research Topic 3), less information exists on the interaction promoted by energetic penetrating radiation (e.g., energetic protons) at the interface of water (liquid or frozen) and solid (rock and/or organic).

Key Objective 3. To understand the role of spontaneous inorganic (organic) self-organization processes in molecular evolution

Self-organization processes that spontaneously occur in both inorganic and organic systems can facilitate the synthesis of biomolecules that act as catalysts, protective environments, and template facilities. For example, π-π interactions (HOMO-LUMO) between the aromatic moieties of purine nucleobases in cyclic nucleotide monophosphates favor, in solid or liquid phases, the spontaneous formation of pillared columns of molecules that are very organized and react to spontaneously yield oligonucleotides with the appropriate regiochemistry (which proceeds via so-called “click-like oligomerization” without a template) (Di Mauro et al., 2015). A similar process can occur in inorganic systems. For example, inorganic membranes can spontaneously grow from silica solution and traces of metal salts under experimental conditions intended to model primordial planetary conditions (including pristine Earth) (García-Ruiz, 2009). These membranes can act as catalysts in the prebiotic synthesis of biomolecules, protecting the novel products from possible degradation processes.

Sub-objective 1. To identify and structurally characterize novel spontaneous self-organized inorganic and organic systems

Sub-objective 2. To determine the mechanism of spontaneous self-organized systems and their role in the prebiotic synthesis of biomolecules

4.3. Approach to achieve the key objectives

Next ten years

Laboratory studies

Laboratory work simulating organic chemistry in space has already been well developed in Europe, although it has spread over a large number of relatively small facilities. It is commonly thought that many more relevant molecules are still to be revealed in laboratory experiments and possibly confirmed by astronomical observations. In fact, the detection techniques used up to now, mostly based on UV-vis-IR spectroscopy, are sensitive only to those synthesized species whose abundances are larger by a factor of several thousand compared to the astronomically observed values. At present, there are ongoing efforts in some laboratories distributed worldwide to build up and use experimental apparatus based on new techniques that are different from the traditional, widely used approaches. These techniques include novel methods (e.g., nanoscale secondary ion mass spectrometry and two-step laser ablation and ionization mass spectrometry) that are more sensitive and can better demonstrate the formation of additional (complex) molecules and/or fragments that could be of primary relevance (see, e.g., Allodi et al., 2013). With increased support of simulation work, a much more complete inventory of molecules produced by surface chemistry and energetic processing of ices should be obtained by the end of the next decade.

The interaction between ice and rocky materials with organics, present in the different environments promoted by the (synergic?) effects induced by ionizing radiation and thermal excursions, has to be investigated with particular effort placed in fully controlled experimental facilities. Of course, much data already exists, both from observations (e.g., the plethora of cometary molecules observed in cometary comae, see http://astrochymist.org/astrochymist_comet.html) and from laboratory experiments. These experiments have to be continued in a coordinated way and should be the basis of a “comparative astrobiology,” the focus of which would be to better understand the similarities and specificities of the (bio)chemistry possible in the different environments. Such information would be essential to understanding whether life exists or existed in other objects in the Solar System and would contribute to our understanding of the origin of life on Earth and possibly on exoplanets.

Spontaneous organic and inorganic self-organized systems have been simulated in European laboratories. Novel systems should be designed and fully characterized. Their connection with the prebiotic synthesis of biomolecules of relevance for the origin of life needs to be studied in more detail, with particular attention to the selectivity and region-chemistry of the processes (especially in the case of polymerization reactions), as well as to catalysis phenomena. In spacelike conditions, more attention should be paid to chemical transformations that occur under high-energy particle or photon irradiations, thus providing information for better modeling of prebiotic processes in icy media or primitive planetary conditions.

Exposure experiments in space

Currently, the analysis of exposed materials in the EXPOSE experiments on board the ISS is performed exclusively on the ground after retrieval of the samples. It would be a relevant step forward for the next 10 years if at least some analysis could be performed in situ, that is, directly on board space platforms. A first step in this direction was achieved during the Organism/Organic Exposure to Orbital Stresses (O/OREOS) mission where UV/vis/near-IR transmission spectra were retrieved by telemetry from the experiments on board an Earth-orbiting nanosatellite (Cook et al., 2014).

Theoretical studies

The already well-developed theoretical studies on organic chemistry in space should be further elaborated, as, for example, those investigating the energy profile of excitation states for some of the main chemical precursors in the synthesis of organics (e.g., formamide, HCN, isocyanate, and others) under high-energy conditions and estimations on the effect due to surface interaction and metal complexation processes.

Theoretical studies on spontaneous inorganic (organic) self-organization processes are also required to fully characterize the structure of supramolecular aggregates, as well as to understand possible relationships between inorganic (organic) self-organization systems and the emergence of complex biomolecules (peptides, oligonucleotides, polysaccharides).

Ten to twenty years from now

Exposure experiments in space

It would be beneficial to have permanent facilities that allow direct exposure (and in situ analysis) of materials to space environments. This could be done in an ad hoc laboratory in a space station and/or on the Moon.

Specific space experiments should be designed in which the self-organization process is connected to self-catalysis for the synthesis of biomolecules under different energy conditions.

Experiments mimicking conditions in space and on terrestrial planets

It is essential to develop an understanding of the transformations that prebiotic molecules may undergo over their real time spans and in their real environments by performing laboratory experiments under conditions that mimic space and terrestrial planet environments. The interaction between the dynamic environment on early Earth (or other planetary surfaces) and prebiotic monomers might have played a prime role regarding the appearance of self-sustaining and replicating entities (see Research Topic 3). Hence, the quantification of processes occurring in space can provide additional insights into the organic inventory of a planet.

Curation facility for samples returned to Earth

It would be relevant to have a fully operative receiving, curation, and analysis facility (for meteorites, interplanetary dust particles, and returned samples from space missions) that could be combined with the most sophisticated experimental techniques in a European Laboratory for Astrobiology (see Research Topics 1 and 5). At present, there are several small-to-medium-sized European laboratories that are doing an excellent job. In the future, however, it will be necessary to have larger facilities available with breakthrough instrumentation.

Interdisciplinary collaboration

Projects should be implemented that allow scientific crossover of information between the different science communities (chemistry, geology, astrophysics, biology, etc.).

After twenty years

Fully equipped and functioning laboratories on Earth and possibly on the Moon, along with results from astronomical observations, would enable a more complete picture of abiotic evolution. We should have the capacity to start from simple molecules and synthesize, in fully controlled experiments, their evolution towards those molecules that are at the interface between abiotic and biotic evolution.

Life affects its environment, and at the same time the environment affects life. This give-and-take is often expressed in feedbacks within planetary systems, that is, responses to change that either resist or enhance the perturbation and tend to stabilize an environment at a particular state, transfer it to different stable states, or send it into a runaway state. The nature, extent, and prevailing directions of these feedbacks are poorly understood. We should pursue more informed answers to questions regarding the existence of life in other environments in the Solar System and better understand the role that organics from space play in the origin of life on Earth (see also Research Topic 3). Figure 4 gives an integrated view of Research Topic 2 together with its timeline.

4.4. European strengths and needs

Europe plays a leading role in many relevant subfields. This is due to the coordinated efforts of many groups across Europe. As an example, it is relevant to outline the role played by the European COST Actions that have been, and are, very important in promoting collaborations between European groups with different backgrounds. Presently, there are two COST Actions particularly relevant in the field: Astro-Chemical History (CM1401; chair: Laurent Wiesenfeld) and Origins and evolution of life on Earth and in the Universe (ORIGINS) (TD1308; chair: Muriel Gargaud).

Recently, some of the Europlanet network members have been successfully involved in the European Union Horizon 2020 program, in particular through the EURO-CARES project, which was launched in January 2015 and has as its primary objective the definition of a sample curation facility to allow the analysis and preservation of extraterrestrial materials.

Also relevant is the role that Europe has in experiments undertaken by exposing organic and biological material to solar electromagnetic and particle radiation. Experiments have been performed on board Mir, EURECA, Biopan, EXPOSE-E, EXPOSE-R, and presently EXPOSE-R2 on the ISS (e.g., Rabbow et al., 2015), in addition to ground-based research with heavy ion accelerators.

5. Research Topic 3: Rock-Water-Carbon Interactions, Organic Synthesis on Earth, and Steps to Life

5.1. State of the art

Geology and geochemistry provide the boundary conditions for our understanding of life on Earth—the best-studied planet so far and an example for more distant systems. Central to this understanding are rock-water-carbon interactions between carbon and the environment, which have been too long neglected and/or unrecognized by relevant neighboring disciplines.

Rock-water-carbon interactions dissipate energy at a planetary level. At a microscopic level, the energy produced by these interactions is expended by microbial metabolisms. Thus, the “rocks” in microbial cells are represented by catalytic Fe-Ni-S clusters in enzymes of the core carbon and energy metabolism (Russell and Hall, 1997; Wächtershäuser, 2006). On the planetary scale, the main process of energy dissipation on metal-rich planets such as Earth is serpentinization. During serpentinization, water circulates through hydrothermal systems and chemically reacts with rocks (Shock et al., 1998). Electrons from the inexhaustible reservoirs of reduced iron in Earth's crust are transferred to water, generating H 2 (Holm et al., 2015) and, to CO 2 , generating methane and other reduced carbon compounds (Schrenk et al., 2013; McDermott et al., 2015). Methane in fluid inclusions in plutonic rocks could also be released by circulating hydrothermal fluids (McDermott et al., 2015). These products of serpentinization become dissolved in the hydrothermal fluids and are discharged at vents into the ambient seawater (McCollom and Seewald, 2013). The significance and ubiquity of rock-carbon-water interactions represent a paradigm-changing concept that has already had tangible impact on microbiological aspects of life's origin, including the most widely read undergraduate textbooks (Madigan et al., 2014). Geologists and biologists have begun to work together on this topic to provide new, very explicit and testable theories for life's origin in a realistic geological context on Earth and on other planets (Martin et al., 2008; Arndt and Nisbet, 2012; Stüeken et al., 2013; Westall et al., 2013; Baross and Martin, 2015).

The main ingredients of these essential planetary chemical reactions are carbon of extraterrestrial (see Research Topic 2) or terrestrial (atmospheric, crustal) origin or from CO 2 , liquid water, transition metals (e.g., Fe, Ni, Mo) and, in essential supporting roles but probably not as participants in the core exergonic reaction, nitrogen and sulfur. Central to microbial activities are energy-releasing (exergonic) redox reactions that are coupled to carbon chemistry, which generates the substance of cells (Fuchs, 2011; Schuchmann and Müller, 2014). Energy conserved from the core exergonic reaction also powers monomer polymerization and growth. Of all naturally occurring geochemical reactions currently known, only the process of serpentinization involves exergonic redox reactions that emulate the core bioenergetic reactions of some modern microbial cells (Buckel and Thauer, 2013). Moreover, organisms that use such primordial reactions to harness carbon and energy inhabit the rocky interior of our planet today (Chapelle et al., 2002; Lever et al., 2010) and formed an important part of microbial biomass on primitive Earth (Westall et al., 2015a, 2015b).

5.2. Key objectives

Key Objective 1. To better characterize and understand the dynamic redox interactions of rock, water, and carbon in their geological context on planets and moons

On wet rocky planets and moons, planetary thermodynamics requires that heat generated from within the planet has to be radiated to space. This generates convective currents in which surface water is circulated through the upper crust. During the early part of planet formation, the heat flux from the mantle is high, gradually decaying as the core and mantle cool. For instance, the temperature of the mantle on early Earth was ≥300°C higher than today (Herzberg et al., 2007), with consequent effects on hydrothermal and geochemical circulation (Arndt, 1994). Thus, understanding the internal structure of planets and their dynamics and evolution is an important component of addressing the flux of energy available from the mantle (as well as from natural radiogenic decay) over the lifetime of the target body. The mere possibility that life might have had chemolithoautotrophic origins on Earth also impacts our view of habitable zones. For example, the recent discovery of hydrothermal activity and evidence for serpentinization processes on Saturn's moon Enceladus (Hsu et al., 2015) demonstrates that the chemical prerequisites for rock-water-carbon interactions that are far from equilibrium can exist without energy input from solar radiation.

During the process of serpentinization, water circulating through Earth's crust reacts with iron- and nickel-containing minerals at depths on the order of a few kilometers. In that process, electrons change hands, leaving their native iron and nickel source, being transferred to water to generate H 2 , a powerful source of energy and mobile, accessible electrons. Carbon dissolved in the circulating water reacts in much the same manner to generate reduced carbon compounds. Serpentinization is the main energy-releasing reaction on young planets; it is similar to—and possibly the precursor of—the core energy-releasing reactions that are essential to microbial life. The contribution of serpentinization to organic synthesis on Earth is newly recognized (Proskurowski et al., 2008). This process clearly needs to be more intensely investigated and better understood.

Key Objective 2. To better characterize and understand transition metals as electron sources and catalysts in geo-organic chemistry

The chemistry of the serpentinization reaction of magnesium-iron silicate and water to serpentinite and magnetite and hydrogen was summarized by Bach et al. (2006) as follows:

The above reactions represent an abundant source of geological reducing power. These reactions occur at depths of roughly 2–8 km under the ocean floor and at temperatures between ca. 80°C and 200°C. They provide copious amounts of molecular hydrogen for organic synthesis, and they bring dissolved carbon compounds in contact with reduced transition metals.

In serpentinization, the electrons that generate H 2 and reduced carbon stem from the transition metals iron and nickel. It is highly likely that serpentinization has been taking place on Earth since there has been liquid water on the planet, and the same will be true for other wet, rocky planets (McCollom and Seewald, 2013; Schrenk et al., 2013). This process is observed in nature but has not been systematically characterized in the laboratory. Initial studies are very encouraging, with formate, methanol, acetate, and pyruvate having recently been synthesized from CO 2 under hydrothermal conditions using FeS mineral catalysts (Roldan et al., 2015). More comprehensive laboratory investigation is a critical element of this key objective.

Key Objective 3. To better characterize and understand carbon reduction in modern serpentinizing hydrothermal vents

The importance of reduced carbon compound synthesis in hydrothermal systems—the prerequisite to generating the building blocks of life—is becoming an increasingly recognized property of these systems (Russell and Hall, 1997; Shock et al., 1998). A key approach to this understanding will involve laboratory experiments to simulate the organic-synthetic ability in early Earth environments (Barge et al., 2014; Herschy et al., 2014). Note that the kinds of hydrothermal systems currently in the foreground of scientific investigations are not primarily the ca. 350°C hot, “black smoker” kinds of vents, with life spans on the order of decades. The vents of interest are, rather, the more recently discovered, cooler (ca. 70°C) and geologically more stable kinds of vents, with life spans on orders of 104 to 105 years. These would have been very widespread on early Earth, where they played an important role in sequestering and modifying reduced carbon species. The early terrestrial rocks document the importance of hydrothermal activity on early Earth (Hofmann and Harris, 2008; Westall et al., 2015a, 2015b).

Of the many suggestions for organic synthesis on early Earth, the only one that we can see in action, in measurable amounts, is serpentinization-dependent CO 2 reduction, generating methane and short hydrocarbons in those submarine and terrestrial systems studied so far (McCollom and Seewald, 2013; Schrenk et al., 2013). On early Earth, there was more hydrothermal activity than today (Hofmann and Harris, 2008; Westall et al., 2015a); consequently, there was also much more H 2 and CO 2 (and methane). Hence, the capacity for geo-organic synthesis was also much greater than today. There is a need to investigate through experimentation, observation, and modeling the magnitude of this process in an early Earth context.

Key Objective 4. To better characterize and understand hydrothermal modification of carbon delivered to Earth from space

Based on the measured flux of extraterrestrial organic carbon in micrometeorites that reach the surface of present-day Earth, Maurette (2006) calculated that a huge amount of ∼5 × 1024 g reached Earth's surface in the 300 million years following the Moon-forming impact. Interaction of that carbon with serpentinizing systems was unavoidable. This is a promising and, to date, unexplored avenue of pursuit (see also Research Topic 2).

The far-reaching consequences of chemical interactions between carbon from space and rock-water-carbon interactions on early Earth for the generation of life's building blocks have not yet been explored. This, and the broader characterization of organic synthesis in laboratory-scale simulations of hydrothermal systems, is a very high priority for experimental investigation.

Key Objective 5. To better understand the role of molecular self-organization, higher-order organization, and cellular organization in the origin of life

A property of matter crucial to our understanding of life's origin is its ability to undergo self-organization into higher aggregation states under suitable conditions. The organization of one-carbon and one-nitrogen species, for example amino acids and bases, in the basic building blocks of cells is increasingly well understood in that a variety of conditions deliver convergent results. Whether starting from methane, formamide, or atoms in space, the carbon atoms are combined into amino acids and bases (Saladino et al., 2012), as predicated by chemical thermodynamics.

Higher orders of spontaneous self-organization can be observed at the experimental level, for example, phase separation and stacking forces that lead to spontaneous, noncovalent fiber formation of nucleobaselike organics (Cafferty et al., 2013). At the theoretical level, a very rich body of work on self-organization exists (Eigen, 1971; Kauffman, 1993) that is only beginning to be tested in laboratory experiments (Vaidya et al., 2012). Prior to the advent of molecules with genetic properties (selectable inheritance), chemical organization is best described by spontaneous reactions under kinetic and thermodynamic control. Understanding energy flux through cells and the kinds of energy that run life processes (Martin et al., 2014) is salient to issues concerning the innate tendency of living matter to attain more stable states. To understand the transition from collections of complex organic molecules to more structured states with novel and possibly emergent properties, a better knowledge of spontaneous self-organization is needed.

5.3. Approach to achieve the key objectives

There is currently rapid convergence between geochemistry and microbiology in the context of hydrothermal origins. This is a new and unexpected development in understanding life's origins, the nature and distribution of primitive life, as well as the possibilities and requirements for its origin elsewhere. The means chosen to meet these scientific objectives should capitalize upon this convergence.

Next ten years

Forging major progress in understanding rock-water-carbon interactions in an early Earth–early life context is a goal that can be achieved within the next decade. Accordingly, 10 years from now we can expect substantial closure of the gaps that impair our understanding of the energy-releasing processes that originate in Earth's core and are at the center of terrestrial life's origin.

Study and model hydrothermal systems

Geothermal activity requires convection, hence, heat flux. The source of heat from the core can stem from radioactivity or gravity. Better understanding of hydrothermal activity requires more knowledge of processes on wet rocky planets that generate internal heat and, as a consequence, reduced carbon compounds. Hydrothermal activity had an enormous influence on rocks and fluids on early Earth, and in the process, it had an enormous influence on geo-(organo)-chemical reactions and the habitats for early life (Westall et al., 2015a, 2015b). It has only recently been discovered that serpentinizing systems generate reduced carbon compounds (Proskurowski et al., 2008). The volume of the modern oceans circulates about once every 100,000 years (on early Earth, much faster) through hydrothermal vents (Fisher, 2005). Hydrothermal systems have therefore been altering the state of carbon—all of it and all the time—since there has been water on Earth. That will continue until there is no water left; and for other wet rocky planets, there is no reason to assume that the situation is, or ever has been, different. Therefore, it is imperative to study and model the flux of ocean water through the crust via hydrothermal cycling and its effects on the cycling and fate of carbon speciation through geological time, recognizing the grad