Compositional interpretation of PFS/MEx and TES/MGS thermal infrared spectra of Phobos

• Published in: Planetary and space science Vol. 59; no. 13; pp. 1308 - 1325
• Main Authors: Giuranna, M., Roush, T.L., Duxbury, T., Hogan, R.C., Carli, C., Geminale, A., Formisano, V.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.10.2011
• Subjects: Composition; Mars; Mineralogy; Phobos; Satellite; Mars; Satellite; Composition; Mineralogy; Phobos
• ISSN: 0032-0633; 1873-5088
• DOI: 10.1016/j.pss.2011.01.019

The origin of the Martian satellites presents a puzzle of long standing. Addressing the composition of Phobos will help constrain theories of its formation. Visible and near-infrared spectra of Phobos lack deep absorption features, making the compositional interpretation a tricky task. PFS/MEx and TES/MGS observations in the thermal infrared show several spectral features that can be used to investigate the composition of the surface. Our results show that the majority of the spectra are consistent with the presence of phyllosilicates, particularly in the area northeast of Stickney. This area corresponds to the “blue” region as defined by Murchie et al. (1999). Analysis of PFS and TES observations in the “red” region defined by Murchie et al. (1999) are consistent with tectosilicates, especially feldspars/feldspathoids. We discuss several physical and chemical mechanisms that can act to eliminate or reduce the strength of bands in the VIS/NIR spectra, with possibly little or no effect in the mid-IR. Comparison of the TES and PFS data to the meteorites shows that no class of chondritic meteorites provide significant agreement with the spectral features observed. The lack of consistency of the PFS and TES spectra to analogs of ultraprimitive materials (organic residues) suggests that an origin via capture of a transneptunian object is not supported by these observations, although it cannot be completely ruled out. Derived surface temperatures from PFS and TES data are in very good agreement with brightness temperatures derived from Viking orbiter measurements, Earth-based observations, and values predicted by numerical models. Our results show that the surface temperature of Phobos varies with solar incidence angle and heliocentric distance, reconciling the different results. We collect and summarize the compositional clues for the origin of Phobos discussed in this paper, including our results. Currently, the most likely scenario is the in-situ formation of Phobos, although a capture of achrondrite-like meteorites is not ruled out. ► We use thermal infrared spectra to investigate the composition of Phobos' surface. ► No class of chondritic meteorites is in agreement with the observed features. ► We detected phyllosilicates, particularly in the area northeast of Stickney. ► Our derived surface temperatures are consistent with previous results. ► Currently, the most likely scenario is the in-situ formation of Phobos.