Review of Water on Phobos and Deimos Y.A. Takagi1, SETI Institute, 189 N Bernardo Ave, Mountain View, California 94043 Abstract Phobos and Deimos, the moons of Mars, represent unique bodies within the solar system. They are ideally positioned as a command point for human ex- ploration of the solar system, and would be invaluable refueling stations if they were found to contain significant fractions of water, either in the form of frozen water ice, or minerally locked water of hydration. Due to the lack of dedicated spacecraft missions to date, direct observation has yielded inconclusive results regarding the potential water content of the two bodies. Spectroscopic data sug- gests that there is little to no water on the surface of Phobos. Analytical models are in agreement with this observation, but allow for substantial subsurface wa- ter content depending on the original water content of the bodies. The original water content depends entirely on the origin/formation of the moons, a highly disputed topic amongst the scientific community. If Phobos or Deimos are cap- tured asteroids or comets, they are likely to still contain significant amounts of water today. If they formed through accretionary processes, then they are not expected to contain any ice water, but could still have water of hydration depending on the origin of the accreting material. Further spacecraft missions are necessary to determine the water content of the moons in order to deter- mine their origins and inform on the history of the early solar system, and to determine their value as a resource for future human space exploration. Keywords: Phobos, Deimos, water 1. Introduction Phobos and Deimos are the two known natural satellites of mars. They both have near-circular, near-equatorial orbits (Burns, 1978; Singer, 2002) (see Table 1). No dedicated Phobos or Deimos missions have been successfully launched to date, with the Soviet Phobos 2 mission coming the closest. It failed on route (Duxbury et al., 2014). Several spacecraft including the Mars Express (MEx) mission have opportunistically observed Phobos (Duxbury et al., 2014; Witasse et al., 2014). Because of the limited data on these two bodies, it is unknown how much water they contain, if any. This review presents the current state of knowledge regarding the subject. Phobos and Deimos are often considered as necessary intermediate targets for human exploration of Mars and the outer solar system (Murchie et al., 2014; Preprint submitted to Icarus August 20, 2015 Phobos Deimos Semimajor axis[a] 9375.0 km (2.76 R ) 23458 km (6.90 R ) [a] Eccentricity 0.01511♂ 0.00024 ♂ Inclination to Mars' equator[a] 1.0756◦ 1.7878◦ Period of revolution[a] 7h 39' 19.47" 30h 18' 1.36" Near-surface bulk density[b] < 1:6 ± 0:3 g=cm3 < 1:1 ± 0:3 g=cm3 Surface temperature[c] 130 - 353 K Mass 1:0668 ± 0:003 × 1016 kg[d] 1:51 ± 0:03 × 1015 kg[e] Volume 5689:8 ± 60 km3[f] 1017 ± 130 km3[g] Bulk density 1:8749 ± 0:025 g=cm3 1:48 ± 0:22 g=cm3 [a] From Jacobson(2010) [b] From Busch et al.(2007) [c] From Giuranna et al.(2011) [d] From Andert et al.(2010) [e] From Jacobson(2010) [f] From Willner et al.(2010) [g] From Thomas(1993) Table 1: Values for orbital and physical properties of Phobos and Deimos. O'Leary, 1992; Oberst et al., 2014). This is due to their ease of accessibility. In fact, they are more readily accessible than even the earth's own moon in terms of Dv expended (Lewis et al., 1993). Their low gravity well and equatorial orbit around mars places them in an ideal position to serve as a command base for robotic mars missions, in preparation for human missions (Lewis et al., 1993; O'Leary, 1992). Their value in this intermediate stage of human space exploration, would increase greatly if they were found to contain raw resources: in particular water, which could be used for fuel production, radiation shielding, and life support systems (O'Leary, 1992; Lewis et al., 1993; Drake, 2009). Water could be recovered using solar or nuclear furnaces in an economically viable manner (O'Leary, 1992). Mautner(2014) suggests that Phobos has sufficient raw materials to sustain an outpost with a small crew compliment effectively indefinitely (on the order of 109 yrs). 2. Results of direct detection methods for water Due to limited dedicated spacecraft observations of Phobos and Deimos, there are few experiments with the capacity to directly detect water. Direct observations thus far largely suggest a lack of water near the surface, but some controversy exists in the interpretation of data. 2.1. Spectroscopic data While the optical spectra of Phobos and Deimos resemble that of C-type asteroids common to the main belt, their infrared spectra resemble that of D- type asteroids more common to the Jupiter family asteroids (Bell et al., 1993). 2 Some solar system models suggest that hydrated mineral bands would not be expected in primitive asteroids because their water would be stored entirely as ice (Bell et al., 1993). Phobos has two slightly different spectral units, a bluer unit and a redder unit. The spectrum of Deimos matches that of Phobos' redder unit (Murchie et al., 1991) (see Figure1). Some have suggested that the interior of Phobos is a heterogenous mix of sections of both types (Basilevsky et al., 2014), while others suggest that the underlying bulk of Phobos is composed of the bluer material, and the redder material forms a thin veneer of dust donated by Deimos (cit?) (see Figure2). Infrared spectroscopy does not indicate the presence of hydrated minerals on Phobos (Bell et al., 1993). Hydrated minerals should have strong spectral features in the 3 mm range which are absent from the spectra of Phobos (Rivkin et al., 2002; Murchie et al., 1991). Gendrin et al.(2005) however, have described a possible weak hydrated mineral feature in this area that may have gone unde- tected in previous studies. Space weathering is also known to subdue hydration bands in this range (Rosenblatt, 2011; Clark et al., 2002). Molecular water and OH absorption bands expected at 1.9 mm have also not been detected (Fraeman et al., 2014). However, weak features at 2.8 mm on both Phobos and Deimos could suggest the presence of metal-OH (Fraeman et al., 2014). Bell et al.(1993) obtained a spectrum of Deimos and compared it with spectra of hydrous and anhydrous asteroids, showing that its spectrum more closely resembled that of an anhydrous chondrite. Spectroscopic data should be taken with caution, as they can only tell us with confidence about the surface layer (Murchie et al., 2014). Especially given that spectra of inner solar system objects are known to evolve due to space weathering (Hapke, 2001; Pieters et al., 2000). The spectra of surface material may not represent the composition of the interior bulk material (Rosenblatt, 2011). 3 Figure 1: Top: Phobos. Bottom: Deimos. Left: Images from the Mars Reconnaissance Orbiter's (MRO) High-Resolution Imaging Science Experiment (HiRISE) presented in Thomas et al.(2011). Right: false-colour representation of the color ratios. Relatively blue regions are shown as blue or black. Yellow and red regions are more strongly reddened with respect to a solar spectrum. A colour bar is shown. The scale bar is 5 km. 4 Figure 2: Distribution of `red' and `blue' spectral units of Phobos. Case 1 shows a scenario where the body is made up of `blue' material that is covered by a veneer of `red' material. Case 2 shows a scenario where the entire body is made up of relatively large chunks of `blue' or `red' material in a heterogenous manner. From Basilevsky et al.(2014). 5 2.2. Magnetospheric data Phobos 2 magnetospheric data suggests that Phobos is outgassing water vapor (Bogdanov, 1981; Dubinin et al., 1991; Bell et al., 1993). 2.3. Geological data Impacts have significantly reworked the surface of Phobos making it highly unlikely that any frozen volatiles exist near the surface (Basilevsky et al., 2014). It has been suggested that some crater ejecta deposits on Phobos exhibit features similar to Martian rampart craters with fluidized ejecta, suggesting the presence of a subsurface layer of water ice (Basilevsky et al., 2014). 3. Models for water on Phobos and Deimos If water exists on Phobos and/or Deimos, it could be locked in the mineral structure as water of hydration, or exist independently as ice water. Water of hydration is more robust to sublimation processes (cit?). However, it may prove more technically difficult to extract (cit?). Ice water is subject to sublimation processes and is easily lost (cit?), but would be most ideal for human use, although considerations such as purity and accessibility must be taken into account (Mautner, 2014). Furthermore, some water could be chemically altered to form metal-OH (Fraeman et al., 2014), which is not subject to sublimation processes. This hydroxyl may also be recoverable for use (cit?). 3.1. Water retention over time The capacity for Phobos and Deimos to retain water depends on a number of parameters, most importantly their porosity, pore-size, and temperature. Based on crater counting techniques, the age of Phobos is estimated to be between 3.5 and 4.3 Ga (Schmedemann et al., 2014). Fanale and Salvail(1989) created an analytical model showing that Phobos could retain water ice in its core using reasonable parameter values. Their results indicate that if Phobos was initially water ice rich, it could potentially still contain substantial amounts of water ice in its core. These results are shown in Figure3.