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Home , Asteroid capture, Astroecology, Phobos (moon)

Review of on and

Y.A. Takagi1, SETI Institute, 189 N Bernardo Ave, Mountain View, California 94043

Abstract Phobos and Deimos, the of , represent unique bodies within the . 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 or , 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 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 (Burns, 1978; Singer, 2002) (see Table 1). No dedicated Phobos or Deimos missions have been successfully launched to date, with the Soviet mission coming the closest. It failed on route (Duxbury et al., 2014). Several spacecraft including the (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 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’ [a] 1.0756◦ 1.7878◦ Period of revolution[a] 7h 39’ 19.47” 30h 18’ 1.36” Near-surface bulk [b] < 1.6 ± 0.3 g/cm3 < 1.1 ± 0.3 g/cm3 Surface temperature[c] 130 - 353 K 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 ’s own in terms of Δv expended (Lewis et al., 1993). Their low gravity well and equatorial 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 , would increase greatly if they were found to contain raw resources: in particular water, which could be used for fuel production, shielding, and 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 spectra resemble that of D- type asteroids more common to the 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 μm 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 μm have also not been detected (Fraeman et al., 2014). However, weak features at 2.8 μm 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 . 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 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 . 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.

6 Figure 3: Ice depth over time at indicated for a porosity of 0.1 and pore radius of 1 micron, assuming bond A = 0.02, emissivity  = 1.0, thermal conductivity K = 1.67 × 105 ergs/cm s K, density of Phobos 3 3 ςs = 2.0 g/cm , density of ice ςi = 1.0 g/cm , density of non-volatile solids 3 ςd = 2.6 g/cm , and tortuosity τ = 2.0. From Fanale and Salvail(1989).

3.2. Constraints imposed by bulk density The low bulk of Phobos and Deimos (see Figure4) require either a substantial amount of frozen volatiles, or void-space. Some have argued that water ice could explain the observed densities (Rosenblatt, 2011; Fanale and Salvail, 1990, 1989), while others argue that they are due to void (Craddock, 2011; Andert et al., 2010; Britt et al., 2002; Consolmagno et al., 2008). It is unknown whether the interior of Phobos is homogeneous or not (P¨atzoldet al., 2014). It is possible to constrain the ice water component based on porosity and rock density, or vice versa (Rosenblatt, 2011; P¨atzoldet al., 2014). Rosenblatt(2011) presents an equation describing the relationship between the rock density, porosity, and possible ice content:

ρb (ρg − ρice) Φb = 1 − − ficeρb (1) ρg ρgρice where Φb is the volume fraction of bulk porosity, fice is the mass fraction of water ice, ρb is the observed bulk density of the moon, ρg is the grain density 3 of the rocky component, and ρice (0.97 g/cm ) is the density of ice water.

7 Figure 4: Bulk density of Phobos and Deimos, of low-albedo carbonaceous asteroids and of carbonaceous meteoritic samples ( and densities are from Britt et al.(2002)). Figure taken from Rosenblatt(2011).

A mass fraction of water ice can be converted to a volume fraction:

ρb Φice = fice (2) ρice Thus: ρb (ρg − ρice) Φb = 1 − − Φice (3) ρg ρg It is important to note that Equations1-3 do not consider other frozen volatiles, such as dioxide ice. Thus they represent the hypothetical case when no other volatiles are present and should only be taken as an upper limit to potential water ice content.

4. Origin hypotheses and their implications for the existence of water

The formation and origins of Phobos and Deimos are highly disputed topics (Craddock, 2011; Rosenblatt, 2011; Witasse et al., 2014; Oberst et al., 2014; Andert et al., 2010; Pieters et al., 2014; Rosenblatt and Lee, 2015). It is unclear whether they even share the same origin scenario (Singer, 2002). There are pros and cons to all of the various proposed scenarios, however, these are widely

8 discussed in the literature and thus will not be a subject of speculation here. For a comprehensive discussion of the likelihood of the various scenarios, see Rosenblatt(2011). In the following sections, different origin hypotheses will be presented, and their implications for past and present water content will be discussed.

4.1. from material In this scenario, Phobos and/or Deimos accreted from material in the pro- toplanetary disk of Mars (Safronov, 1986). This would suggest a very porous and dry composition similar to that of average bulk mars material (Wanke and Dreibus, 1988), reflecting the composition of the protoplanetary disk (Witasse et al., 2014; Murchie et al., 2014). In any accretion scenario, Phobos and Deimos would resemble loosely aggregated rubble piles with no volatiles and plenty of voidspace (Craddock, 2011). If this formation scenario is correct, it is unlikely that the bodies would contain any water.

4.2. Accretion from giant impact ejecta In this scenario, Phobos and/or Deimos accreted from material that was ejected into martian orbit after a large impactor hit the surface (Craddock, 2011). This is the accepted hypotheses for the origin of the earth’s own moon (Bell et al., 1993; Benz et al., 1986). The expected composition for the moons in this scenario is similar to the mars mantle and crustal material (Witasse et al., 2014; McSween et al., 2009), note that the crustal layer on mars is thin allowing the mantle to contribute significantly. It is likely that the impactor struck at a high enough velocity to vaporize rock (Craddock, 2011), meaning that not only would ice water be obliterated, any water of hydration would also be lost. If this formation scenario is correct, it is not expected that the bodies would contain any water.

4.2.1. Shallow impact angle This modification to the giant impact scenario allows for impactor material to make up a portion of the ejecta. For most impactor trajectories, it is not expected that the resulting moons would contain any water. However, if the impactor angle was very shallow, significant proportions of the impactor mate- rial could also be included in the ejecta (cit?). If the impactor was made up of carbonaceous or otherwise water-rich material, some of this may be preserved in Phobos and/or Deimos. Due to the fact that the material is broken up, and exposed to heating from the impact, and extended exposure to space, before accreting to form the moons, it is unlikely that water ice would survive the pro- cess. However, water could survive locked in the structures of hydrous minerals (cit?). See section 4.3.1 for a discussion on the possible water of hydration com- ponent of the moons. Note that in this scenario, the moons would be composed of a mixture of Mars and impactor material so the water of hydration can be expected to scale with the ratio of the two materials.

9 4.3. Capture of small bodies In this scenario, Phobos and/or Deimos are captured small bodies (Hart- mann, 1990; Burns, 1978; Pascu et al., 2014). Their water content would depend on the water content of the body pre-capture, their ability to retain water, and their age in orbit (Fanale and Salvail, 1989, 1990).

4.3.1. Capture of CM asteroids Some spectral analysis suggests that Phobos and Deimos are CM type ob- jects (Fraeman et al., 2014). Consolmagno et al.(2008) measured seven different CM and calculated an average grain density of 2.82 ± 0.13 g/cm3. It is uncertain how closely this value reflects asteroid bodies in space. We can constrain the moons’ water ice component using Equations1&3, the CM grain density found by Consolmagno et al.(2008), and density values presented in Table1. It has been found that meteorites can have as much as ∼10 wt.% water of hydration (Beck et al., 2010). If the rock to ice ratio is taken to be 2:1 by weight (Consolmagno et al., 2008), then the fraction of water of hydration to rock Chydr becomes ∼15 wt.%. The total weight fraction of water locked in hydrous minerals can be calculated as:

fhydr = Chydr(1 − fice) (4)

See Table2 for maximum possible water content for Phobos and Deimos based on values discussed in this section. Maximum ice fractions were calculated assuming a porosity of zero. This cannot be true because surface ice would have sublimated according to the model developed by Fanale and Salvail(1989). However, these values can still serve as a generous upper limit.

Phobos Deimos Bulk porosity Φb−max 0.34 ± 0.02 0.47 ± 0.10 Ice fraction by mass fice−max 0.26 ± 0.08 0.47 ± 0.23 Ice fraction by volume Φice−max 0.51 ± 0.08 0.72 ± 0.24 Water of hydration fraction by mass fhydr−max ∼ 0.15 ∼ 0.15 Total water fraction by mass fwater−max ∼ 0.37 ∼ 0.55 15 14 Total ice content Mice−max 2.77 ± 0.86 × 10 kg 7.10 ± 3.61 × 10 kg 15 14 Total water of hydration Mhydr−max ∼ 1.6 × 10 kg ∼ 2.3 × 10 kg 15 14 Total water Mwater−max ∼ 3.9 × 10 kg ∼ 8.3 × 10 kg

Table 2: Maximum water content estimates for CM scenario, based on Equations 1-4 and literature values presented in Table1 and Section 4.3.1. Note that the maximum values for water of hydration occur when no water ice is present, therefore the maximum possible total water content is less than the sum of the maximum ice and maximum water of hydration content.

Bell et al.(1993) propose that Phobos and Deimos are composed of an anhydrous silicate material such as olivine or pyroxine, mixed with water ice, resembling the parent material of CM2 meteorites. They call this class CM3.

10 This material would not have been sufficiently heated to experience metamor- phosis, thus resulting in a lack of hydrated minerals. However, after capture, they would have lost surface ice in accordance with the model by Fanale and Salvail(1989) described in Section 3.1, leaving substantial pore-space on the surface, but a permafrost core containing frozen volatiles.

4.3.2. Capture of D-type asteroid D-type asteroids are common amongst the Jupiter and Hilda popu- lations (DeMeo et al., 2014). D-type asteroids are not very well characterized. There is only one known meteorite analog, the Tagish lake meteorite (Howard et al., 2015), which suggests that D-type objects are among the most primitive solar system bodies (Brown et al., 2000). It is believed that D-type objects may be even more rich in water than the CM asteroids (Brown et al., 2000). Much of the water in D-class asteroids would exist in the form of minerally locked water of hydration (cit?). While there are insufficient meteorite samples to do a mathematical treatment like that done for the CM asteroid scenario in Section 4.3.1, especially given that the Tagish Lake sample may not be representative of D-type asteroids in general (Vernazza et al., 2013), one can expect values in a similarly broad range.

4.3.3. Capture of D-type It is possible that D-type objects in the near earth object (NEO) population do not originate in the , but are in fact comets from the or beyond (Whitman et al., 2006). It is unknown whether these objects have a different composition from D-type asteroids (DeMeo et al., 2014). The discovery of D-type comets in the NEO population lends credence to this sce- nario (Mommert et al., 2014). If Phobos and/or Deimos had cometary origins, it is expected that they would have initially contained substantial amounts of frozen volatiles.

4.3.4. Capture, breakup, and reaccretion scenario This is a significant modification to the capture scenario, in which gravi- tational stresses tear apart the objects during the capture process, forming a disk of debris. This would eventually reaccrete, forming Phobos and/or Deimos (P¨atzoldet al., 2014; Rosenblatt, 2011). This scenario is similar to the shal- low angle giant impact hypothesis described in Section 4.2.1 in that frozen ice volatiles would be lost, but mineral water could be retained.

4.3.5. Collision of asteroid with moon In this scenario, the current moons may have formed after an asteroid or comet collided with a native moon already in orbit, that had formed from the protoplanetary nebula (Andert et al., 2010; Peale, 2007). The resulting body would contain material from both bodies and could contain water according to the origin/composition of each.

11 5. Discussion and conclusions Clearly more data is required to determine the water contents of Phobos and Deimos. The strongest available observational data apply only to the very surface layers and apply mostly to Phobos. While spectral data suggests a com- plete lack of water, and a definite lack of large quantities of water, there are confounding issues and alternative interpretations within the scientific commu- nity. However, it is safe to assume that the surfaces of Phobos and Deimos do not contain water. Analytical models suggest no reason for there not to be water on the moons at greater depths. However, these results depend largely on the initial condi- tions. If the bodies contained significant amounts of water originally, then they could still contain water today. The original water content depends heavily on their origin/formation sce- nario. Various origin scenarios have been proposed with little consensus amongst the community. Accretional formation scenarios do not allow the possibility of frozen volatiles (i.e. water ice). Whether they contain hydrous minerals depends on the accreting material. Mars material would have no water of hydration, but asteroidal or cometary material could potentially contain significant amounts of hydrous minerals, so long as they were not heated excessively. If the moons originated through the capture of asteroidal or cometary material without dis- ruption, then they are likely to contain ice water and/or hydrous minerals. The determination of water contents on Phobos and/or Deimos would tell us a lot about the origins of the two bodies and early solar system history. As the larger moon, Phobos is somewhat better characterized than Deimos. However, there is no reason to believe Deimos could not contain as much or more water than Phobos. Large quantities of water would be an important resource for human explo- ration of Mars and the outer solar system. The most generous estimates yield ice content as high as 50% by volume, but even a fraction of this would be an invaluable asset to future missions. Even if no water ice is found, water locked in hydrous minerals, or even hydroxyl groups bound to the mineral structure could be worth recovering for use. The future of human space exploration would have to take very different paths depending on the water content of these bod- ies. Therefor it is important to better characterize Phobos and Deimos, and specifically address the issue of water in the near future, in preparation for the next generation of space exploration.

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