MGS‐TES Spectra Suggest a Basaltic Component in the Regolith of Phobos

Abstract

The origins of the Martian moons Phobos and Deimos have been the subjects of considerable debate. Visible and near‐infrared spectra of these bodies are dark and nearly featureless, with red slopes of varying degrees. These spectra are generally consistent with those of carbonaceous asteroids, leading to the hypothesis that Phobos and Deimos are captured carbonaceous asteroids. The shapes and inclinations of the orbits of Phobos and Deimos present problems for the asteroid capture hypothesis. This had led researchers to suggest that Phobos and Deimos coaccreted with Mars or that they are the result of an impact with Mars or in the Mars vicinity. In this work, we reexamine Mars Global Surveyor Thermal Emission Spectrometer (MGS‐TES) data of Phobos and compare spectra of the Phobos surface to mid‐IR spectra of the ungrouped C2 meteorite Tagish Lake (a suggested analog for D‐class asteroids) and particulate basalt and phyllosilicate samples (mixed with carbon black to reduce their visible albedos) acquired in a simulated airless body environment. We find that Tagish Lake is a poor mid‐IR spectral analog to Phobos and that major spectral features in the Phobos spectrum are best matched by a silicate transparency feature similar to that found for finely particulate basalt. Other features in the spectrum are likely best explained by a phyllosilicate component. We suggest that these results indicate that at a portion of the Phobos surface regolith is derived from the Martian crust.

Particle Size Effects on Mid‐Infrared Spectra of Lunar Analog Minerals in a Simulated Lunar Environment

“…SLE conditions are achieved by pumping the chamber down to a pressure of <10 −5 mbar, heating the samples from below to 350 ± 0.5 K, heating from above via quartz halogen lamp connected to a tunable power source, and cooling the chamber to <150 K with variation within ±2 K during a measurement, as measured at the cold shield. Samples are allowed to heat under the lamp for >30 min until the spectral maximum stabilizes (between 600 and 800 cm −1 ), and the calculated brightness temperature at the CF is near 350 K with accuracy within a mineral set of 10 K and between minerals of ~15 K, temperature ranges shown to produce less than ~0.003 variation in emissivity (Glotch et al, ). A temperature of 350 K was chosen as the target brightness temperature as it is a reasonable surface temperature but not the most extreme reached at equatorial noon of ~400 K (Williams et al, ), especially since PARSEC has a relatively high incidence angle (55°), which generally corresponds to midlatitude nonnoon measurements.…”

Introduction to Science and Exploration of the Moon, Near‐Earth Asteroids, and Moons of Mars

“…This and similar studies provide the bases for remote thermal IR spectral analyses of airless bodies. For example, Glotch et al (2018) Glotch et al (2018) suggest that these results support an impact, rather than an asteroid capture origin for Phobos.…”

Machine Learning Mid‐Infrared Spectral Models for Predicting Modal Mineralogy of CI/CM Chondritic Asteroids and Bennu

“…This results in smaller differences between ambient and SAE spectra than is typically seen for lunar or lunar analog samples. Previous SAE measurements of the low albedo Tagish Lake meteorite did note some differences between the ambient and SAE spectra, especially for fine particulates (Glotch et al, 2018). Donaldson Hanna et al (2019) examined mineral mixtures and chondritic meteorites (CM, CI, CV, CR, and L5) under ambient and SAE conditions.…”