Fraeman, Abigail - Compositions of Phobos and Deimos: The View from Visible to Near Infrared Spectroscopy

The compositions of Mars’ moons, Phobos and Deimos, are a direct indicator of the mechanism that formed them.  One of the longstanding questions of planetary science is whether the moons formed in situ around Mars, either through co-accretion or giant impact, or if they are captured asteroids originating from elsewhere in the solar system [1].  The key to unlocking this mystery will be to determine whether the moons are composed of materials native to the Martian system or if they are made of something that could only have arrived from another location [2]. 

Disk-resolved, visible to near infrared hyperspectral observations of Phobos acquired by OMEGA at a range of lighting and viewing geometries are fit with a Hapke photometric function to solve for the single particle phase function.  This knowledge is used to derive single scattering albedos of CRISM and OMEGA Phobos and CRISM Deimos observations, which can be projected to any viewing geometry for direct comparison with laboratory spectra.  Fe electronic absorptions diagnostic of olivine and pyroxene are not detected.  A broad absorption centered on 0.65 µm within the red spectral units of both moons is seen, and this feature is also evident in telescopic, Pathfinder, and Phobos-2 observations of Phobos.  A 2.8 μm metal-OH combination absorption on both moons is also detected, and this absorption is shallower in the Phobos blue unit than in the Phobos red unit and Deimos.  The strength, position, and shape of both features are similar to absorptions seen on low-albedo primitive asteroids.  Two end-member hypotheses could explain these spectral features: the presence of highly desiccated Fe-phyllosilicate minerals indigenous to the bodies, or Rayleigh scattering and absorption of small iron particles formed by exogenic space weathering processing, coupled with implantation of H from solar wind [3].  Both end-member hypotheses may play a role, and in situ exploration will be needed to ultimately determine the underlying causes for this pair of spectral features. Phobos' and Deimos' low reflectances, lack of mafic absorption features, and red spectral slopes are incompatible with even highly space weathered chondritic or basaltic compositions. These results, coupled with similarities to laboratory spectra of Tagish Lake (possible D-type asteroid analog) and CM carbonaceous chondrite meteorites, show that Phobos and Deimos have primitive, carbonaceous-chondrite like compositions.  If the moons formed in situ rather than by capture of primitive bodies, primitive materials must have been added to the Martian system during accretion or a late stage impact [4]. 

References: [1] Burns, 1992, Mars, Univ. Arizona Press, [2] Murchie et al., 2014, Acta Astronautica, 93, 475-482, [3] Fraeman et al., 2014, Icarus, 229, 196-205, [4] Fraeman et al., 2012, JGR, 117.