On the Impact Origin of Phobos and Deimos I: Thermodynamic and Physical Aspects

Abstract

Phobos and Deimos are the two small moons of Mars. Recent works have shown that they can accrete within an impact-generated disk. However, the detailed structure and initial thermodynamic properties of the disk are poorly understood. In this paper, we perform high-resolution SPH simulations of the Martian moon-forming giant impact that can also form the Borealis basin. This giant impact heats up the disk material (around $\sim 2000$ K in temperature) with an entropy increase of $\sim 1500$ J K${}^{-1}$ kg${}^{-1}$. Thus, the disk material should be mostly molten, though a tiny fraction of disk material ($<5\%$) would even experience vaporization. Typically, a piece of molten disk material is estimated to be meter sized due to the fragmentation regulated by their shear velocity and surface tension during the impact process. The disk materials initially have highly eccentric orbits ($e\sim 0.6-0.9$) and successive collisions between meter-sized fragments at high impact velocity ($\sim 3-5$ km s${}^{-1}$) can grind them down to $\sim 100\mu$m-sized particles. On the other hand, a tiny amount of vaporized disk material condenses into $\sim 0.1\mu$m-sized grains. Thus, the building blocks of the Martian moons are expected to be a mixture of these different sized particles from meter-sized down to $\sim 100\mu$m-sized particles and $\sim 0.1\mu$m-sized grains. Our simulations also suggest that the building blocks of Phobos and Deimos contain both impactor and Martian materials (at least 35%), most of which come from the Martian mantle (50-150 km in depth; at least 50%). Our results will give useful information for planning a future sample return mission to Martian moons, such as JAXA's MMX (Martian Moons eXploration) mission.

11 pages, 6 figures. Accepted for publication in ApJ

References in corpus (7)

• Equilibration in the Aftermath of the Lunar-Forming Giant Impact
• Melting and Mixing States of the Earth's Mantle after the Moon-Forming Impact
• Formation of Phobos and Deimos via a Giant Impact
• Ring Formation around Giant Planets by Tidal Disruption of a Single Passing Large Kuiper Belt Object
• Numerical aspects of Giant Impact simulations
• Formation of Multiple-Satellite Systems From Low-Mass Circumplanetary Particle Disks
• Reconceiling the orbital and physical properties of the martian moons

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