Spectral Constraints on the Internal Characteristics of Phobos and Deimos
Abstract
The origin of the two Martian moons Phobos and Deimos is controversial. Leading hypotheses include capture and accretion in Martian orbit during planet formation or from ejecta from one or more large impact basins. Until landed measurements or returned samples provide definitive evidence, key observational constraints on the moons' composition and origin will be derived from remote spectral measurements and bulk density. The highest spatial resolution spectral measurements were acquired in 2007 by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on the Mars Reconnaissance Orbiter (MRO). CRISM data cover the spectral range 0.4-3.9 μm at 6.55 nm/channel, and sample the surface of Phobos at 350 m/pixel and Deimos at 1.4 km/pixel. Other key constraints also come from the Observatoire pour la Mineralogie, l'Eau, les Glaces et l'Activite (OMEGA) spectrometer and radio science investigation Mars Express, the Thermal Emission Spectrometer on Mars Global Surveyor, Viking Orbiter imaging, and several investigations on Phobos 2. Both moons have red-sloped, smooth spectra lacking strong mineralogic absorptions, with reflectances at 0.55 microns (corrected to i=30, e=0, and g=30 degrees) near 0.03. Two distinct materials are identified. The "bluer unit" excavated by Phobos' large crater Stickney is brighter at visible wavelengths, and has a lesser spectral slope than does the areally dominant "redder unit". The "bluer unit" is only observed on Phobos and may represent large parts of the moon's interior. The "redder unit" exhibits a weak absorption near 0.65 microns consistent with the presence of graphite or phyllosilicate. The "redder unit" dominates Deimos and is exposed in craters that excavate shallower depths on Phobos than Stickney does. Neither unit exhibits spectral signatures of molecular water, hydroxyl, organics, or mafic minerals. These properties are inconsistent with even the most space-weathered mafic mineral assemblages that may originate from Mars. However the low albedos and weak spectral features are consistent with low-grade carbonaceous chondrite meteorites such as CM types or the ultra-primitive Tagish Lake, with bound water absorptions subdued by desiccation and space weathering of surface regolith. These compositions would imply condensation outside the Mars system. Given previously published satellite bulk densities, these compositions are consistent with total porosities of 20-45%, comparable to observed microporosities of relevant meteorite types plus perhaps ~15% additional macroporosity resulting from fractures.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.P34A..02M
- Keywords:
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- 6008 PLANETARY SCIENCES: COMETS AND SMALL BODIES / Composition;
- 6230 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS / Martian satellites;
- 6024 PLANETARY SCIENCES: COMETS AND SMALL BODIES / Interiors