THE THERMOPHYSICAL PROPERTIES OF PHOBOS FROM TES AND THEMIS OBSERVATIONS

Over the past decades a suite of datasets have been acquired of the Martian moon Phobos. These datasets include compositional, morphological, and thermophysical information and they have been leveraged in a variety of studies to address outstanding formation hypotheses and the evolutionary history of Phobos. Recently, the Thermal Emission Imaging System (THEMIS) onboard NASA’s 2001 Mars Odyssey spacecraft has acquired a systematic set of thermal infrared observations of Phobos. A total of eight observations have been acquired to date, resulting in ~270 m/pixel sampling that cover nearly all local times, including two eclipses, and a zero-phase angle observation of the Mars-facing surface of Phobos. These disk-resolved data allow us to address a wide range of questions related to the thermal inertia, surface roughness/block abundance and composition. In general, Mars Global Surveyor Thermal Emission Spectrometer (TES) and THEMIS data, much like early Viking data, indicate that the surface has a relatively uniform, low thermal inertia (TI, ~20-80 J K^-1 m^-2 s^-1/2), though the higher-resolution THEMIS data may reveal further subtleties not observed by other, lower resolution datasets. THEMIS daytime observations show a clear correlation of spectral slope (indicating surface anisothermality) with solar incidence angle (zero-phase angle having the lowest spectral slopes), which indicates there is significant fine-scale surface roughness. THEMIS nighttime observations show spectral slopes consistent with a small fraction of blocks (~5%) present on the surface. The afternoon cooling trends place a strong constraint on the derived TI and values from both TES and THEMIS data are consistent with those previously reported (~40 J K^-1 m^-2 s^-1/2).

This work was some of the last that the late Dr. Joshua L. Bandfield carried out. Unfortunately, he did not get the chance to see the campaign he started through to its conclusion. Here, we present the status of TES and THEMIS observations related to Phobos, future plans for analyzing the full suite of Phobos THEMIS data and detail the constraints these measurements provide to understand the origins and evolutions of this enigmatic Martian moon. The early results presented here, initiated by Dr. Bandfield, continue to be refined by a dedicated team of some of his close colleagues.