The Habitability of Titan and its Ocean

Saturn’s largest moon, Titan, hides a subsurface ocean that potentially could support life. Image credit: NASA/JPL–Caltech/Space Science Institute.

Saturn’s largest moon, Titan, is a hotbed of organic molecules, harboring a soup of complex hydrocarbons similar to that thought to have existed over four billion years ago on the primordial Earth. Titan’s surface, however, is in a deep freeze at –179 degrees Celsius (–290 degrees Fahrenheit, or 94 kelvin). Life as we know it cannot exist on the moon’s frigid surface. Deep underground, however, is a different matter. Gravity measurements made during fly-bys by NASA’s Cassini spacecraft revealed that Titan contains an ocean beneath its ice shell, and within this ocean, conditions are potentially suitable for life.

A cross-section of what the interior of Titan might look like, with organic chemistry in the atmosphere and on the surface, above a crust of ice that encases a global ocean, which in turn may lie on top of another ice layer surrounding a rocky core. Image credit: A. D. Fortes/UCL/STFC.

An NAI-funded team led by researchers at NASA’s Jet Propulsion Laboratory is seeking to better understand the potential for life in Titan’s ocean, and its possible relationship with the organic molecules in the moon’s atmosphere and on its surface. Titan’s rich diversity of organic molecules is a product of ultraviolet light from the Sun initiating chemical reactions with the dominant gases in Titan’s atmosphere – hydrogen, methane and nitrogen. The resulting complex hydrocarbons could be the building blocks of life, or provide chemical nutrients for life, and within its ocean Titan harbors a potential habitat for that life.

The formation of organic compounds in Titan’s atmosphere, which contribute to the hazy that obscures the surface. Image credit: ESA/ATG Medialab.

A schematic showing the creation, precipitation and transport over the surface of organic compounds. Image credit: ESA.

This latter query has yielded a surprising possibility. One of the main results from the project so far is a paper by Kelly Miller, Hunter Waite and NAI team-member Christopher Glein of the Southwest Research Institute in Texas, which proposes that Titan’s nitrogen atmosphere originates from organic molecules that were trapped inside Titan when the moon formed, and the subsequent heating of these gases released nitrogen that seeped up to the surface. For the purpose of the NAI project, it suggests that there are already organics inside Titan that could enter into the ocean from below, so even if organics cannot reach the ocean from the surface, the ocean could still contain life’s building blocks. “These organics may actually be able to percolate up through cryovolcanism,” says Lopes, creating a possible origin too for some of the organics on Titan’s surface. Objective 2: Habitability If pathways exist for organics to pass through the ice shell from the surface to the ocean below, then the next step is to figure out whether the ocean, or anywhere in the ice on the journey to the ocean, is potentially habitable. This is where the biologists on the team, studying high-pressure, cold-tolerant organisms, come into play. Before that can be done, more needs to be known about the ocean. Although Cassini confirmed that the ocean exists via gravity measurements, “What we don’t know is the exact composition of the ocean, its density, its thermal profile, the overall structure of the icy crust on top of it,” says Malaska. To better understand the ocean and its potentially habitability, researchers on the team start off with several possible compositions that could reasonably be expected to exist, and work backwards, developing theoretical models. Although it may be impossible to ever directly explore the deep subsurface or ocean of Titan, the NAI team intend to use both theoretical modeling and laboratory experiments to simulate the possible conditions, to better understand the interface between the ice shell and the ocean, and the ocean with the rocky core, and the flow of oxidants and reductants at these interfaces that could support microbes. Objective 3: Life For life to be able to exist in or near Titan’s ocean, there must be a source of chemical energy to metabolize. Building on the work done in Objectives 1 and 2 relating to what organics reach the ocean and what the environment of the ocean is like, the team will then be able to construct theoretical models of how much energy is available in the ocean, as well as possible metabolisms that could exist in those conditions, to gauge the likelihood that life could survive there. Assuming the ocean is habitable, with sources of chemical energy and a healthy supply of organics, the high pressure and low temperature environment may constrain the variety of lifeforms that could exist there. However, one terrestrial organism that the team are considering as a suitable example is Pelobacter acetylenicus, which can survive on acetylene as its only source of metabolic energy and carbon. “Our goal is to think of Pelobacter acetylenicus as the model organism, something that could exist in the deep sub-surface on Titan,” says Malaska. Laboratory experiments will be conducted, placing microbes such as Pelobacter acetylenicus in simulated environments described by the aforementioned theoretical modeling to see if the microbes can thrive in them, to learn how they adapt in order to survive, and what new types of biomolecules might result from these adaptations. These biomolecules may then leave behind biosignatures – molecular traces of life. However, while the possible existence of life in the ocean of Titan is all well and good, we also need to be able to detect that life via biosignatures. Understanding what biomarkers life could leave is therefore the second part of Objective 3, and a database of potential biosignatures will be produced, including isotopes of carbon, nitrogen and oxygen, as well as biological structures such as the lipids in cell membranes. Objective 4: Detection Of course, if the biosignatures remain in the ocean, they will be impossible to detect from orbit or on the surface. Therefore, the final objective is to seek means by which those biosignatures can be transported to the surface – the inverse of the part of Objective 1 that explored ways that organics could reach the ocean from the surface. The principal means of transport are likely to be either convective (i.e. warmer, slushy) ice rising upwards, or perhaps cryovolcanism. “Methane in the atmosphere is destroyed by ultraviolet light, so there has to be some replenishment,” points out Lopes. “And there may still be outgassing happening.” Although no active cryovolcanism has been detected on Titan yet, several features on the surface have been identified as potentially cryovolcanic. “We’re already studying theoretical ways that cryovolcanism can transport material,” says Lopes, in anticipation for when the results of objective 3 are available.

A false-color, 3D representation of radar data from Cassini showing a feature on Titan called Sotra Facula, which appears to be an inactive cryovolcano. Image credit: NASA/JPL–Caltech/USGS/University of Arizona.