|The spectral properties of Phobos have been previously investigated at both visible/near-infrared and mid-infrared wavelengths. These works have shown that Phobos spectra include a broad 0.65 micron feature that may be attributable to Fe-bearing phyllosilicates or Rayleigh scattering by nanophase metallic Fe particles. VNIR spectra of Phobos also include a 2.8 micron metal-OH feature that may be diagnostic of desiccated phyllosilicates or solar wind-induced hydroxylation of the Phobos regolith surface. At mid-IR wavelengths, Phobos displays a variety of spectral classes, which are consistent with tectosilicates, such as feldspars and mixtures of phyllosilicate minerals.
In this work, we re-evaluate MIR spectra of Phobos acquired by the Mars Global Surveyor (MGS) Thermal Emission Spectrometer (TES) experiment. We gathered TES spectra from four early mission TES aerobraking orbits (orbits 476, 501, 526, and 551) and culled the data to include only the highest temperature daytime observations of the Phobos surface. For the surface of Phobos, subpixel temperature mixing is clearly an issue. Therefore, we modeled the radiance of the surface as a linear combination of blackbodies of many different temperatures. The resulting emissivity spectra occasionally have emissivity values greater than unity, but are not affected by strong slopes due to surface anisothermality.
Surface emissivity spectra of Phobos display a variety of spectral shapes while having some features in common. Most spectra display a strong drop in emissivity shortward of the Christiansen Feature (CF), typical of finely particulate silicates. Average spectra all have modeled CFs in the 8.34 to 8.4 micron range, indicating an olivine/pyroxene-dominated surface if the surface is optically immature (devoid of space weathering). It has been shown for the Moon, however, that optically mature surfaces have CF positions that tend to be shifted by ~0.2 microns to longer wavelengths compared to optically immature surfaces of the same composition. Assuming the average surface of Phobos is optically mature, corrected CF positions of 8.14 to 8.2 microns indicate a more feldspar-rich surface. This is consistent with VNIR observations, which lack strong 1 and 2 micron Fe2+ absorption features and longer wavelength MIR transparency features that are consistent with finely particulate tectosilicates such as feldspar.
Nearly all spectra display emissivity maxima at ~6 microns, consistent with varying levels of surface hydration. This hydration feature may be due to water bound in minerals or transient water formed through interaction of the solar wind with the Phobos surface. The relatively strong 6 micron features found in the TES data are somewhat surprising due to the lack of a strong 3 micron water band in VNIR observations of Phobos. Finally, some spectra display emissivity maxima at ~6.7 microns, which has been shown to be consistent with small amounts of carbonates intimately mixed with silicates. This observation may support the hypothesis that Phobos is a captured D-type asteroid, similar in composition to the Tagish Lake meteorite, which is carbonate-bearing.