Modelling Martian moons’ surface temperature: Surface temperature variations and surface thermal emission removal from future MMX/MIRS data

Context. The mission Martian Moons eXploration (MMX) from the Japanese space exploration agency (JAXA) will study the Martian system for three years starting from 2027. The spectro-imager MMX InfraRed Spectrometer (MIRS) will observe the surface of Phobos and Deimos in the near-infrared between 0.9 and 3.6 μm. At the surface of the Martian moons, thermal emission starts to contribute around 2.5 μm and will modify the future MIRS radiance measurements. Aims. We propose a physical method to remove the surface thermal emission contribution from the future measured spectra from MIRS data. Methods. The method simulates the moons’ surface temperatures locally and computes their thermal emission accounting for ephemerides and topography. We therefore developed a simulation of the absorbed flux at the moons’ surface that allows their sub-surface temperature to be computed using a 1D thermal model. We used this to estimate the emitted thermal flux in the near-infrared domain. Results. We computed the surface temperature on the complete surface of Phobos at a lateral resolution of 3° × 3°. We found that surface temperatures on Phobos vary in the range [100, 320] K depending on the thermal and textural properties of the surface, in good agreement with previous estimations. Our results indicate that the reflected solar illumination and thermal emission from Mars increase the annual average temperature of the sub-Mars hemisphere on Phobos by 7 K. This underscores the importance of accounting for the Mars effect when modelling the thermal tail observed by MIRS. We tested the sensitivity of the method to the presence of absorption features in the spectrum and found that the method is stable as long as the absorptions are located before the thermal emission range.