The Icy Origins of the Martian Moons

Courteney Summer Monchinski, Shigeru Ida, Hidenori Genda

The origins of the Martian moons, Phobos and Deimos, are still heavily debated. There are currently two leading theories surrounding their origin: giant impact or asteroid capture. Asteroid capture mainly explains the moons’ asteroid-like shape, C-asteroid-like albedo, and low porosity (~15%; suggesting hydrated carbonaceous material) but cannot explain the moons’ low orbital inclination and near-circular orbital eccentricity (Rosenblatt, 2011). The giant impact scenario explains the orbital characteristics of the moons but cannot rectify their spectral observations, shape, or density (Rosenblatt, 2011). However, it is extremely difficult to capture the moons in the current orbits, and their possible internal structure does not provide enough tidal dissipation. Previous giant impact studies can create an impact-generated disk from an undifferentiated basalt body large enough to recreate the moons in their current positions, but this disk also creates many massive moons within Phobos’ orbit, which later reaccrete to Mars, for which we have not seen definitive evidence (Craddock, 2011; Rosenblatt and Charnoz, 2012; Citron et al., 2015). Therefore, it is important, along with reproducing the compositional characteristics, to understand how the moons can be formed without this extra mass. This study proposes that the extra disk mass could be abolished by an impactor containing mostly ice, allowing some mass to vaporize on impact and escape the system (Ida et al., 2020). For this study, SPH simulations of giant impacts with impactors of varying ice content were performed to create an impact-generated disk massive enough to form both Phobos and Deimos. We start with an impactor ~3% the mass of Mars, ~104 SPH particles, and an impact angle of 45 degrees. From the SPH data, we then perform an iterative analysis to determine which particles are part of the planet, disk, or escaped from the system, which yields the final post-impact parameters.