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 $\sim100 \mu$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.

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

The Martian moons, Phobos and Deimos, have long presented a conundrum regarding their origins. The traditional perspective that they were captured asteroids fails to provide a convincing explanation for their nearly circular orbits and equatorial alignment. However, recent models suggesting formation through accretion within an impact-generated disk offer a compelling alternative.

The study by Hyodo et al. employs high-resolution Smoothed Particle Hydrodynamics (SPH) simulations to explore the thermodynamic and physical properties of the disk resulting from a giant impact on Mars—the same impact theorized to have created the Borealis basin. These simulations ascertain that the disk material is significantly heated, achieving temperatures around 2000 K with an entropy increase of approximately 1500 J K(^{-1}) kg(^{-1}). As a result, the majority of the disk material is predicted to be molten, although a minor fraction, less than 5%, may undergo vaporization.

A pivotal finding of this research is the initial size of molten disk material—estimated to form meter-sized droplets due to fragmentation governed by shear velocities and surface tension. Over time, these solidify and participate in high-velocity collisions (between 3-5 km s(^{-1})), leading to further fragmentation down to particles as small as 100 micrometers. A small proportion of the vaporized material condenses into even smaller grains, approximately 0.1 micrometers in size. Thus, the edifices of Phobos and Deimos are likely constituted by a diverse array of particle sizes.

The study provides a quantitative composition analysis, with at least 35% of the moons’ material originating from Martian sources, potentially encompassing material from the Martian mantle depths of 50-150 km. Such detailed insights have profound implications for planned space missions, particularly the Japan Aerospace Exploration Agency's (JAXA) Martian Moons eXploration (MMX) mission set to perform lunar sample collection.

This research not only elucidates the formation pathways of Phobos and Deimos but also underscores the utility of advanced simulation techniques in planetary science. Future studies will benefit from expanding on the variable impact conditions to further refine the parameters influencing disk formation and examine the role of additional elements or compounds potentially ejected during the impact event.

In conclusion, Hyodo et al.'s work substantiates the impact-driven accretion model as a coherent explanation for the Martian moons’ origins, while also providing valuable predictive insights for forthcoming exploratory missions. As space exploration and computational capabilities progress, there will be ample opportunity to bolster these findings, ultimately advancing our comprehensive understanding of satellite formation in the solar system.