9:15 AM - 9:30 AM
[PPS06-02] Effect of Water Ice on Impacts in the Martian System and Implications for the Formation of the Martian Moons
Keywords:Mars, Martian moons, Water-ice, Giant impact
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. While asteroid capture theory can straightforwardly explain their observed spectral features, the giant impact theory can straightforwardly explain the moons’ orbital characteristics. However, it is extremely dicult to capture two objects into the orbits that the moons are currently in, and there is not enough tidal dissipation to move them into their current orbits (Rosenblatt, 2011). Previous giant impact studies can create an impact-generated disk large enough to recreate the moons in their current positions, but this large disk also creates a massive moon within Phobos’ orbit, which later would need to fall back to Mars (Craddock, 2011; Rosenblatt and Charnoz, 2012; Citron et al., 2015). These studies also create disks whose temperatures are around 2000K, meaning that if there were any primitive materials that match the moons’ spectra in the impactor, they would be completely melted and altered (Hyodo et al., 2017a). This study proposes the use of an impactor containing mostly ice for the following three reasons: (1) that the extra disk mass could be abolished by in ice-dominated impactor, allowing some mass to vaporize on impact and escape the system (Ida et al., 2020). (2) The moons’ compositions, density, and possible porosity can result by adding ice to the system, as the vaporization of water will also help to protect carbonaceous materials that partly form the moons from being altered during impact, as well as bring water to the Martian system. (3) The water would also be key for forming Deimos beyond the synchronous orbit, as the viscous interaction between rock grains and vapor would help extend the impact-generated disk (Ida et al., 2020; Woo et al., 2022). For this study, Smoothed Particle Hydrodynamic (SPH) simulations of giant impacts with impactors of varying ice content were performed to create an impact-generated disk, from which Phobos and Deimos would form. We used the Tillotson Equations of State to model both the iron-rock Mars and the water-ice and basalt impactor. Initially using previous studies as a framework, we started with an impactor with 3% of the mass of Mars, 3x10^5 total SPH particles, impacting at 1.4 times escape velocity at an angle of 45 degrees, with compositions of the impactor varying from 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 fraction of water-ice mantle to basalt core. From the SPH simulation data, we determined each par- ticle type (planet, disk, or escape) based on orbital and energy parameters. The final disk masses were compared to understand the effect of the impactor’s water-ice content on the system. We found that, compared to previous studies, the disk mass produced by an impactor with any amount of ice was slightly larger. We also found that an impactor of 0.3 and larger ratio of water-ice to basalt formed disks containing more than 50% water, which will be important when forming Deimos. This water content was found to decrease disk temperatures, allowing for temperature changes before and after impact to be less than the melting temperature for silicates (>1200°C) for impactors containing more than 30% ice, and below the dehydration temperature for hydrated minerals (>500°C; King et al., 2021) for impactors containing more than 70% ice. This suggests that primitive, chondrite-like materials, of which the moons are suspected to be composed of, can survive a giant impact event.