Assessing the likelihood of Phobos and Deimos being captured fragments of a disrupted planetary embryo

It has been suggested that Phobos and Deimos are fragments of a common progenitor—likely a planetary embryo—that was disrupted either through a collision with Mars or with another planetary embryo (Canup & Salmon 2018; Bagheri et al., 2021). We tested the viability of this scenario by calculating the probability of its occurrence during the formation of terrestrial planets. Using extensive numerical simulations of the late stage of terrestrial planet formation, where planetary embryos undergo giant impacts while submerged in an ocean of small planetesimals, we found that within simulations producing Mars-analogs, the probability of the required Mars-embryo or embryo-embryo collisions is less < 3%. Within these systems, the probability of producing fragments with the specific size characteristics of Mars’ satellites is even smaller. Conversely, our simulations reveal that Saturn’s secular resonance efficiently scatter a large quantity of planetesimals into the Martian region where they can be potentially captured. Given this strong scattering effect and the low probability of a common progenitor disruption, we argue that the capture of asteroid-belt objects represents a more viable pathway for the origin of Mars’ satellites. We detail our simulations framework and discuss the implications of their results.