The purpose of this paper is to describe the goals of the MEGANE science investigation as well as document the conceptual design of the MEGANE instrumentation. Section 2 describes the MEGANE science goals in the context of the MMX mission; section 3 describes the MEGANE instrumentation; section 4 describes the expected baseline and additional measurements that will be carried out by MEGANE instrumentation. Finally, section 5 describes the current status of the MEGANE development and summarizes the paper.

A key measurement objective of the MMX mission is to remotely determine the elemental composition of Phobos. One of the standard remote sensing techniques for measuring planetary elemental compositions is planetary nuclear spectroscopy. This technique uses measurements of gamma‐ray and neutron emissions from airless or nearly airless planetary surfaces to determine elemental concentrations within the top meter of the surface. The gamma rays and neutrons, most of which are generated by nuclear interactions initiated by galactic cosmic rays (GCRs), can be measured either from orbiting and/or landed spacecraft. Successful composition measurements have been carried out on a variety of planetary bodies, including the Moon, Mars, Mercury, Venus, and multiple asteroids (Boynton et al., 2007 ; Feldman et al., 1998 , 2002 ; Lawrence, Feldman, et al., 2013 ; Peplowski et al., 2011 ; Prettyman et al., 2006 , 2012 ). The MMX mission will carry a gamma‐ray and neutron spectrometer (GRNS) called Mars‐moon Exploration with GAmma rays and NEutrons (MEGANE). The MEGANE instrumentation is funded by NASA and is being designed and developed by The Johns Hopkins University Applied Physics Laboratory in collaboration with Lawrence Livermore National Laboratory.

The fundamental question regarding Phobos and Deimos concerns how they came to be in orbit about Mars; namely are they captured asteroids (Higuchi & Ida, 2017 ), or were they formed via an impact of a larger body into Mars (Rosenblatt et al., 2016 )? One of the limiting factors in addressing these questions is the lack of chemical composition information for these moons. The Japan Aerospace Exploration Agency is planning the Martian Moons eXploration (MMX) mission to answer this and other key questions regarding Phobos and Deimos (Kuramoto et al., 2018 ). MMX will accomplish its objectives by making comprehensive remote sensing measurements of Phobos and Deimos and then returning regolith samples of Phobos to Earth for detailed chemical analyses.

The inner solar system contains three moons, two of which – Phobos and Deimos – orbit Mars. Compared to what we currently know about Earth's Moon, which is believed to have formed following a giant impact on Earth, our understanding of Phobos and Deimos is relatively sparse. Their shape and spectral reflectance properties resemble outer solar system objects, suggesting an origin as captured asteroids. Yet dynamical accretion theories suggest they formed in the vicinity of Mars. A comprehensive exploration of both bodies has still not been accomplished and promises to significantly add to our understanding of early solar system evolution, in particular the processes that lead to the formation of moons around terrestrial planets (Murchie et al., 2015 ).

2 The MMX Mission and MEGANE Science Goals and Objectives

The MEGANE science investigation was designed to address MMX science goals related to Phobos surface composition measurements, as well as overall science goals for NASA planetary science investigations. The MMX mission goals and design are summarized by (Kuramoto et al., 2018). MMX has two primary science goals: 1) Reveal the origin of Mars' moons and gain a better understanding of planetary formation and material transport in the solar system; and 2) Observe processes that have affected the Mars system and Mars surface environment. MMX mission goals lead to four science objectives: 1) To determine whether the origin of Phobos is captured asteroid or giant impact; 2) to obtain a basic picture of surface processes acting on small airless bodies in orbit around Mars; 3) to gain new insight on Mars surface environment evolution; and 4) to better understand behavior of the Mars air‐ground system and the water cycle dynamics. To accomplish its science goals and objectives, the MMX mission will carry out comprehensive remote sensing measurements aimed at understanding the geology, geophysics, and elemental composition of Mars' moons, as well as the environment around Mars. In addition to these remote sensing measurements, a primary objective of the mission is to collect and deliver to Earth 10 g or more of Phobos material for detailed characterization using Earth‐based laboratory instrumentation.

The MEGANE investigation has three science goals, two of which were adopted from MMX science objectives, with a third that supports the MMX Phobos sample acquisition and interpretation (Table 1). MEGANE science goals are 1) determine whether Phobos is a captured asteroid or the result of a giant impact; 2) study surface processes on airless bodies in Mars orbit; and 3) support the MMX Phobos sample‐return objective. The science goals will be accomplished via science objectives, as detailed in Table 1.

Table 1. MEGANE Science Goals, Objectives, and Measurements MMX Objectives = MEGANE Science Goals MEGANE Science Objectives MEGANE Measurements 1. Determine whether Phobos is a captured asteroid or the result of a giant impact 1. Determine whether Phobos has a chondritic or achondritic (Mars‐like) composition Characterize the bulk concentrations of major, minor, and trace radioactive elements in Phobos' regolith 2. Determine if Phobos' surface materials are depleted in volatile elements Measure the K/Th ratio of Phobos' near‐surface material Measure the H content of Phobos' regolith 2. Study surface processes on airless bodies in Mars orbit 3. Characterize variations in the elemental composition of Phobos' surface Make spatially resolved measurements of the Si, K, Fe, and Th content of Phobos' regolith Make spatially resolved measurements of H, Σ a , and <A> on Phobos' surface 4. Characterize horizontal (surface) and vertical (subsurface) variations in the H content of Phobos' near‐surface (depth < 30 cm) materials Measure the H content and H layering in the top 30 cm of the regolith 3. Support the MMX Phobos sample‐return objective 5. Assist with sample site selection Provide rapid, spatially resolved assessment of the concentrations of H, Si, K, Fe, and Th, and the bulk composition parameters Σ a and <A> on Phobos' surface 6. Document compositional context of returned samples

2.1 MEGANE Science Goal 1: Determine Whether Phobos is a Captured Asteroid or the Result of a Giant Impact Theories for Phobos' and Deimos' origins (Table 2) fall into two broad categories: gravitational/orbital capture of primitive solar system objects, and in situ formation near Mars. Capture hypotheses attempt to explain the characteristics that Phobos and Deimos share with C‐ and D‐type asteroids, which have low densities, low albedo, and irregular shapes (Burns, 1978; Burns, 1992; Fraeman et al., 2012, 2014; Hartmann, 1990; Murchie & Erard, 1996; Pajola et al., 2013; Rivkin et al., 2002). Materials that can explain Phobos' D‐type spectrum include both volatile‐rich and volatile‐poor lithologies, similar to primitive carbonaceous chondrite meteorites (Table 2). Solar system dynamical models, e.g., the “Nice model” (Gomes et al., 2005; Tsiganis et al., 2005), predict scattering of material from the outer to the inner solar system, providing a means for D‐type bodies to reach the vicinity of Mars. Alternatively, Phobos and Deimos may be captured inner solar system (S‐type) objects, which have been associated with ordinary chondrite materials (Nakamura et al., 2011; Peplowski, Bazell, et al., 2015; Yurimoto et al., 2011); however, this would require darkening and reddening, perhaps via space weathering, to match Phobos' D‐type spectrum (Fraeman et al., 2012, 2014). Table 2. Phobos and Deimos Formation Theories Origin hypothesis Composition predicted Expected MEGANE Compositional Measurements Asteroid capture Capture of organic‐ and volatile‐rich outer solar system (D‐type) body Ultraprimitive (carbonaceous chondrite) composition, for example, the Tagish Lake meteorite (Brown et al., 2000 Volatile‐rich carbonaceous chondrites (e.g., Tagish Lake, CI, CM; Figure 1) Capture of organic and volatile‐poor outer solar system (D‐type) body Carbonaceous chondrite composition with anhydrous silicates plus elemental C (Emery & Brown, 2004 Volatile‐poor carbonaceous chondrites (e.g., CR, CK, CO; Figure 1) Capture of inner solar system (S‐type) body Composition like common meteorites (e.g., ordinary chondrites) (Brearley & Jones, 1998 Ordinary Chondrites (Figure 1) In situ formation Giant impact on Mars Mixture of impactor and silicate‐Mars‐like materials (McSween et al., 2009 Intermediate between Mars basaltic meteorites and impactor, with low K/Th (Figure 4) Coaccretion with Mars Composition like bulk Mars Ordinary chrondrites with elemental ratios consistent with values derived for bulk Mars (Figure 3) Phobos' and Deimos' near‐circular, near‐equatorial orbits are difficult to explain dynamically by capture, which typically results in objects with inclined, highly elliptical orbits. In situ formation models more easily predict the current orbits of Phobos and Deimos. In situ formation resulting from a giant impact on Mars follows from our understanding of the formation of Earth's moon. Here, a large impact produces a disk of materials from which Phobos and Deimos accreted (Craddock, 2011; Rosenblatt & Charnoz, 2012). The material is predicted to be composed of >50% devolatilized material derived from the impactor, with the remainder from Mars (Pignatale et al., 2018; Rosenblatt et al., 2016). Phobos and Deimos would spectrally resemble D‐type bodies if a large amount of the material was from a D‐type impactor, as the spectral properties of mixtures of bright and dark materials are dominated by the dark materials. However, the admixture would be depleted in volatile components due to heating during the impact event (Craddock, 2011; Hyodo et al., 2018; Nakajima & Canup, 2017), as opposed to the capture model, which would result in a nonvolatile depleted D‐type composition. In situ formation by coaccretion with Mars (Safronov et al., 1986) predicts that Phobos and Deimos accreted from the same material that formed Mars, which has been modeled to be similar to ordinary chondritic materials (Wanke & Dreibus, 1988), though the bulk composition of any planet is necessarily model‐dependent. However, this scenario could be distinguished from the capture of an inner solar system object via sample analysis of the returned MMX sample, as it predicts that Mars and Phobos would have similar isotopic ratios for oxygen and other elements, isotopic signatures that are highly diagnostic of Martian materials. The diversity of formation scenarios for Phobos and Deimos (Table 2) and the wide‐ranging interpretations of the existing spectral reflectance data (e.g., Fraeman et al., 2012, 2014; Giuranna et al., 2011; Glotch et al., 2015; Yamamoto et al., 2018) illustrate a point: the origin of the Martian moons is an open issue for which spectral data are inconclusive and whose resolution requires knowledge of the elemental composition of the surface (Figure 1). To accomplish MEGANE science Goal 1, the MEGANE investigation will carry out two focused science objectives: 1) determine whether Phobos has a chondritic or achondritic (Mars‐like) composition; and 2) determine if Phobos' surface materials are depleted in volatile elements. Figure 1 Open in figure viewer PowerPoint 1998 1981 2000 1981 1988 1998 MEGANE's suite of elemental measurements provides multiple, redundant means to discriminate between Phobos origin hypotheses (Table 2 ). Curves and squares are reference compositions representing the mean class of materials. MEGANE's element‐by‐element precision (Table 4 ) is sufficient to differentiate between the four compositional groups. Data for the Mars basaltic meteorite compositions are compiled from McSween and Treiman (); data for the volatile‐rich carbonaceous chondrite compositions are compiled from Kallemeyn and Wasson () and Brown et al. (); data for the volatile‐poor carbonaceous chondrite compositions are compiled from Kallemeyn and Wasson (); data for the ordinary chondrite compositions are compiled from Wasson and Kallemeyn () and Brearley and Jones (). 2.1.1 MEGANE Science Objective 1: Determine Whether Phobos has a Chondritic or Achondritic (Mars‐Like) Composition MEGANE global‐average values of H, O, Na, Mg, Si, K, Cl, Ca, Fe, Th, and U, derived from gamma‐ray measurements, as well as H concentrations derived from neutron measurements, provide a robust dataset that can be used to discriminate between the Phobos and Deimos formation models (Figure 1). Thermal and epithermal neutrons also provide complementary information regarding Phobos' origin (Figure 2) (Elphic et al., 2016). Figure 2 Open in figure viewer PowerPoint 2016 Breakdown of compositional end members (Figure 1 ) in neutron measurement space, showing that NS measurements also contribute to Phobos origin science (Elphic et al.,). Data points show simulated neutron count rates for a MEGANE‐NS‐type measurement with an eight‐day measurement and a nominal orbit altitude (see Section 4 ). CC stands for carbonaceous chondrites; OC stands for ordinary chondrites. The diagnostic Fe/Si versus Fe/O ratio (Figure 3), which segregates materials according to chemical fractionation via differentiation, is one example of the use of MEGANE‐derived element composition information to discriminate between chondritic (primitive) and achondritic (Mars‐originating) compositions. Ordinary chondrites cluster at high Fe/Si versus Fe/O values due to the presence of both Fe‐bearing silicates (Fe + Si + O) and FeNi metal (Fe). More volatile‐rich carbonaceous chondrites are separated from the ordinary chondrites by greater O abundance, a result of containing higher abundances of clays, alteration minerals, and other volatile‐rich phases. Volatile‐depleted carbonaceous chondrites (higher Fe/O than volatile‐rich carbonaceous chondrites) overlap with the ordinary chondrites, but are separated via Mg, Fe, H, and Cl measurements (Figure 1). Finally, achondritic material clusters at low Fe/Si versus Fe/O values, due to fractionation of FeNi metal to the core and core/mantle boundary during core formation (removal of Fe from the bulk) and the resulting formation of lithic crustal material (leftover Fe + Si + O), from which achondritic material is derived. Because Mars‐originating exogenous material on Phobos is expected to be scarce (2–500 ppm) (Chappaz et al., 2013; Ramsley & Head, 2013), observation of chondritic versus achondritic material with MEGANE is diagnostic of Phobos' native composition, not later surface modification. Figure 3 Open in figure viewer PowerPoint 2015 Fe/Si versus Fe/O measurements separate achondritic and chondritic meteorite groups, direct analogs to compositions predicted for each origin hypothesis. Color‐coded values for each meteorite are plotted in Peplowski, Bazell, et al. (). The overlap of the carbonaceous chondrites with the ordinary chondrites can be separated via Mg, Fe, H, and Cl measurements. 2.1.2 MEGANE Science Objective 2: Determine if Phobos' Surface Materials are Depleted in Volatile Elements Giant impact formation models of Phobos and Deimos predict surfaces depleted in highly and moderately volatile elements (Craddock, 2011; Hyodo et al., 2018; Nakajima & Canup, 2017), as heating during the event removes volatiles from the debris disk. Our Moon, whose giant impact origin is evident in its depletion of such volatiles relative to the Earth, is a good example of this type of process. One specific means of testing the giant impact hypotheses is K/Th measurements (Peplowski et al., 2011; Prettyman et al., 2015). MEGANE will provide robust measurements of K and Th, naturally radioactive elements that can be measured at trace amounts (~ppm to ppb). As incompatible lithophile elements, their ratio is generally preserved after melting and recrystallization. Because K is moderately volatile, whereas Th is refractory, K/Th decreases when a material is heated sufficiently in a chemically open system, that is, in a disk of ejected material after a giant impact. CI chondrites provide the primordial solar system K/Th ratio (~20,000; Figure 4). The terrestrial planets have moderate K/Th values (~3,000 to ~8,000), a consequence of volatile loss due to heating (solar and accretion). The Moon has the lowest K/Th value (~300), reflecting volatile loss during/following the Moon‐forming giant impact. A chondritic K/Th value (~20,000) for Phobos supports captured asteroid and coaccretion hypotheses; a lower value supports the giant impact hypothesis. Figure 4 Open in figure viewer PowerPoint K/Th ratio for a number of planetary bodies versus solar distance, along with CI chondrites. The Moon's low K/Th ratio is believed to be due to its formation from a giant impact event.

2.2 MEGANE Science Goal 2: Study Surface Processes on Airless Bodies in Mars Orbit Chemical variations on planetary surfaces offer insights into the processes that shaped those surfaces (Table 3). Phobos' surface, which is geologically diverse (Basilevsky et al., 2014; Veverka & Duxbury, 1977) shows evidence for compositional diversity via two distinct spectral units (Figure 5). The “blue unit” is located around Stickney crater, and the “red unit” (Murchie & Erard, 1996) is found on the remainder of Phobos' surface, as well as Deimos' entire surface. Here “red” and “blue” refer to the relative slope of the reflectance spectra, as both moons are dark and colorless in visible wavelengths. Table 3. Range of Surface Processes Possibly Operating on Phobos Surface process Effects Examples Spatial compositional signature Dust accumulation Deimos‐originating dust accumulates on Phobos as it spirals in toward Mars. Models suggest rates of 1–5 kg/year, accumulating in a highly asymmetric distribution (Horanyi, 2015 Longitudinal spatial variations that match the predicted Deimos dust accumulation model Excavation of distinct subsurface materials The red and blue units represent distinct compositions that are present in Phobos. Geologic evidence that red and blue unit materials form large blocks comprising the interior of Phobos (Basilevsky et al., 2014 Variations in the compositions of multiple elements that correspond with the red and blue units (Figure 6) Space weathering Exogenous, chondritic infalling material accumulates. Contamination of Vesta's basaltic equatorial regions with carbonaceous chondrite‐like materials (Prettyman et al., 2012 Comparison of MEGANE‐ and sample‐derived elemental composition measurements, including end‐member (achondritic, chondritic) admixing ratios and depth profiles as inferred from MEGANE and sample acquisition depth Thermal modification depletes uppermost surface of volatile elements (Na and K). S depletion on Eros (Kracher & Sears, 2005 Figure 5 Open in figure viewer PowerPoint (A) High Resolution Imaging Science Experiment Phobos color map (based on data from Thomas et al., 2011 2018 A) High Resolution Imaging Science Experiment Phobos color map (based on data from Thomas et al.,), showing the blue and red spectral units, projected onto a Phobos shape model using the Applied Physics Laboratory's Small Body Mapping Tool (Ernst et al.,). (B) MEGANE's spatial footprint at an altitude of one‐Phobos radius, with the colors corresponding to the fractional contribution each surface location yields to the MEGANE signal. At one‐body radius, MEGANE's footprint enables a measurement dominated by Phobos' blue unit. (C) MEGANE footprints at lower altitudes, such as may be achieved during MMX descent operations. Lower altitudes enable MEGANE to measure more localized areas and also increase the strength of the surface signals. The red and blue units may or may not be fundamental chemical units, as chemical and spectral units are not always correlated on planetary surfaces (e.g., Weider et al., 2015). On Mercury, much of the spectral reflectance variations are controlled by minor differences in graphite and is disconnected from variations in major element chemistry (Peplowski, Lawrence, et al., 2015). Phobos' spectral units have only subtle spectral features (Fraeman et al., 2012, 2014) to provide hints to the origin of their color difference. The red unit has absorption near 0.65 μm, attributed to Fe‐phyllosilicates or a mixture of grain sizes of metallic Fe, which is lacking from the blue unit. Both units exhibit absorption due to OH near 2.8 μm, but the absorption is stronger in the red unit. This difference could be a signature of variable H content in the units, or alternatively, the blue unit may be enhanced in C, which would flatten its spectrum and subdue its mineralogic absorptions. Red and blue materials may be exogenous and endogenous materials, respectively. The blue unit is associated with Stickney ejecta, but not all ejecta are blue, suggesting that the blue unit may be exposed material from a subsurface reservoir of chemically distinct material (Basilevsky et al., 2014). Due to Phobos' location in Mars' orbit, Phobos can accumulate material from Mars and Deimos. Impacts on Deimos will produce dust that will spiral into Mars, coating Phobos in the process (Horanyi, 2015). Similarly, Mars‐originating material, lofted into orbit by Mars impactors, places dust in orbit (e.g., Andersson et al., 2015; Soter, 1971) that will coat Phobos; however, estimates are that just 2–500 ppm of Phobos' surface is Mars‐originating material (Chappaz et al., 2013; Ramsley & Head, 2013). Space weathering (Hapke, 2001) results from the combination of meteorite and micrometeorite, solar wind, and high‐energy particle bombardment, and for volatile‐rich bodies, desiccation and alteration due to thermal processing. Space weathering creates nanophase metallic Fe and Fe sulfides, which darken and redden the surface and obscure absorptions at visible to near‐infrared wavelengths (Noble et al., 2001; Noguchi et al., 2011; Pieters et al., 2000). For C‐rich bodies, formation of nanophase C may also be important (Trang et al., 2017). Mars' moons (1.5 AU) are located between the Moon (1 AU) and Vesta (2.4 AU), two well‐studied objects that have played a large role in our understanding of space weathering. Phobos' surface compositions may be richer in C and volatiles than those bodies, so the relative importance of the various space weathering processes for the Martian moons is unknown. MEGANE's science goal to study surface processes on airless bodies in Mars orbit is thus broad in scope, as it is inclusive of impact excavation processes, chemical processing, and space weathering. The two science objectives that address this science goal are 1) characterize variations in the elemental composition of Phobos' surface; and 2) characterize horizontal (surface) and vertical (subsurface) variations in the hydrogen content of Phobos' near‐surface (depth < 30 cm) materials. 2.2.1 Science Objective 3: Characterize Variations in the Elemental Composition of Phobos' Surface MEGANE elemental (H, Si, K, Fe, and Th) and neutron composition maps will characterize the compositional variability of Phobos' surface. Systematic differences in composition between the leading and trailing hemispheres would be evidence for the presence of exogenous material from Mars or Deimos (Horanyi, 2015). Correlations between elemental composition and spectral units would reveal underlying differences between the red and blue units. For instance, differences in the H content of the units have been invoked to explain their differences. MEGANE samples H content to depths of tens of cm, well below the upper few microns that is measured by reflectance spectroscopy. The upper centimeter has been repeatedly heated to 340 K in vacuum (Giuranna et al., 2011; Kuhrt & Giese, 1989) due to diurnal illumination, which would have removed all but the most tightly bonded H species. Thus, knowledge of subsurface volatile content is essential for characterizing Phobos' native volatile content and its heterogeneity among Phobos' red and blue units. Neutron‐derived composition parameters will also be used to characterize elemental abundance variations across Phobos' surface. Macroscopic thermal neutron absorption, Σ a , which is sensitive to variations in elements with large or small neutron capture cross sections, has been used to derive compositional information on the Moon (Elphic et al., 2000; Feldman et al., 2000; Peplowski, Klima, et al., 2016), Vesta (Prettyman et al., 2013), and Mercury (Peplowski, Beck, & Lawrence, 2016). On Mercury, Σ a , Fe, and albedo data were used to infer C concentrations (Peplowski, Klima, et al., 2016). Carbon concentrations could be similarly derived on Phobos, thus providing additional information toward meeting the science objectives (Figure 1). Fast neutrons measure average atomic mass, <A> (Gasnault et al., 2001), which supplies additional constraints on elemental variations (e.g., Lawrence et al., 2017). 2.2.2 Science Objective 4: Characterize the Surface and Subsurface Volatile Content of Phobos' Near‐Surface (Depth < 30 cm) Materials The MMX mission will use Phobos' elemental composition as a window from which to understand volatile delivery to the inner solar system. This objective requires the acquisition of a pristine sample of Phobos' surface material, particularly with respect to its H content. For airless solar system objects, the upper centimeters of the surface are subject to chemical alteration (e.g., Peplowski, Bazell, et al., 2015, and references therein). The MMX mission seeks to avoid this issue by acquiring a sample from a depth of >2 cm. MEGANE neutron data provide a means to confirm that the returned sample of Phobos is representative of the near subsurface, as neutron data measure H content, including discriminating between upper and lower layers with differing H content to a depth of ~30 cm (Lawrence et al., 2006; Lawrence, Feldman, et al., 2013; Maurice et al., 2011).