Introduction. Phobos represents a unique target for observations. The moons of Mars are the only other terrestrial planet moons beside our own, and may have an origin as captured carbonaceous asteroids. Mars-observing spacecraft have the opportunity to study Phobos from Mars orbit (e.g. Mars Reconnaissance Orbiter, 2001 Mars Odyssey, Mars Express, Viking Orbiters) and as they enter low orbit (e.g. Mars Global Surveyor (MGS)). These missions have produced a long record of observations using the same instruments that study the surface of the planet below. These observations, however, are generally few and far between, and are acquired rarely over each mission.
Using these observations, we can begin to characterize Phobos’s surface, which is blanketed in a fine layer of regolith. The properties of this regolith determine how heat is absorbed, transferred, and reradiated to space. Thermophysical modeling allows us to simulate these processes and predict, for a given set of assumed parameters, how the observed thermal infrared spectra will appear. By comparing models to observations, we can constrain the properties of the regolith, such as grain size, density, conductivity, and how each of these varies with depth, and regionally across the surface. These constraints are key to understanding how Phobos formed and evolved over time, which in turn informs us about the environment and processes that shaped the solar system as a whole.
Thermophysical Model. We have developed a thermophysical model of Phobos adapted from a model used for unresolved observations of asteroids. This model will incorporate the effects of temperature-dependent conductivity, changes in physical properties with depth, and uses the complete shape model to define the facets that make up the modeled surface.
Several modifications were made to retool our asteroid model for use with Phobos. Unlike sun-orbiting asteroids, Phobos receives significant scattered, reflected, and thermal radiation from the Martian surface. Phobos is also eclipsed by Mars. Another significant change is the linking of the model to resolved observations from Mars-orbiting spacecraft. In this case, the thermal infrared flux must be integrated across the portion of the Phobos surface where each observation was made. We verify our model’s results using observations gathered by MGS’s Thermal Emission Spectrometer (TES).
Thermal Infrared Spacecraft Observations. The mapping of TES footprints to the Phobos surface has not previously been undertaken, and must consider spacecraft orientation, TES mirror angle and Phobos/MGS position (using SPICE) for four orbits (OCKS 476, 501, 526, 551). Approximately 50 resolved observations of Phobos are available that cover a subset of the Phobos surface. While far from complete coverage, these data, when combined with the newly adapted thermal model, will reveal variations of thermophysical parameters across the surface.
Future Work. To date, the primary focus of this work has been the development and adaptation of the thermal model. We will compare our model to TES observations. Insights gleaned from a better understanding of the Phobos surface could be applied to future Phobos-centered spacecraft missions. We also hope to apply our refined thermophysical model to asteroids, whether spacecraft- or telescopically observed.