The MMX mission, led by JAXA, is a sample return mission whose primary goal is to test whether the Martian moons, Phobos and Deimos, were formed during a giant impact or were captured. One of the main observations to test these scenarios will be whether Phobos and Deimos have lost volatile elements. If Phobos formed in a giant impact, simulations show that the impact was much less energetic than the Moon-forming impact, with peak temperatures as low as 2000K. We present here a quantification of the volatile loss in anticipation of the MMX mission, assuming that Phobos’ building blocks were made of bulk silicate Mars material. We investigate the cooling of Phobos in two end-member scenarios : a convective case (relevant for an initially fully molten proto-Phobos) and a conductive case (relevant for an assemblage of 10 m building blocks). A homogeneous evaporation model is used for the convective case, and a diffusion-limited evaporation model is used for the conductive case. In both cases, we find that the cooling time is about 1-10 years in the absence of external heating sources (but the Sun). This leaves little time for evaporation: the most volatile elements, Na and K, may be depleted by 10% for the case of a fully molten and convective proto-Phobos. If Phobos is rather an assemblage of 10 m building blocks that cool conductively, the loss of Na and K would be limited to the first 10 cm below the blocks’ surface (by about 4%) representing about 0.1% loss in averaged bulk composition.

If external sources of heating were present (such as a hot radiating Mars or a hot surrounding disk), and the body was kept at T1400K (our assumed rheological transition temperature) for more than 10 years, a larger loss of Na and K is found. If degassing lasted more than 100 years with exterior temperature1400K, then all Na and K may have been lost for the convective case, and more than 50% for the conductive case. Furthermore, a significant fraction of the refractory elements may also have been lost in both cases. K abundance will be measured from space by the MEGANE instrument onboard the MMX mission. If low K content is measured by the MEGANE instrument, this would favor the giant impact formation scenario and would imply either a long cooling time of the proto-Phobos ( 100 years), or degassing prior to the assembling of the proto-Phobos. If MEGANE does not measure K depletion, this could mean either that Phobos was not formed in a giant impact, or that it formed in a giant impact but experienced a short cooling time ( 10 years). In that case, laboratory analysis of the returned sample will be crucial in deciphering the origin of Phobos by focusing on various volatile elements and constraining their isotopic ratios.