On the Impact Origin of Phobos and Deimos IV: Volatile Depletion

TL;DR

This study investigates how volatile elements could have been lost from the building blocks of Martian moons Phobos and Deimos due to hydrodynamic escape and radiation pressure shortly after their formation from a giant impact, explaining their volatile depletion.

Contribution

The paper provides a hydrodynamic and radiation pressure model showing significant volatile loss from Phobos and Deimos' precursors immediately after impact, advancing understanding of their composition.

Findings

Over 10% of vapor with T > 1000 K could escape hydrodynamically.

Moderately volatile dusts could be removed by radiation pressure at 700-2000 K.

Bulk Phobos and Deimos are likely depleted in volatile elements.

Abstract

Recent works have shown that Martian moons Phobos and Deimos may have accreted within a giant impact-generated disk whose composition is about an equal mixture of Martian material and impactor material. Just after the giant impact, the Martian surface is heated up to $\sim3000-6000$ K and the building blocks of moons, including volatile-rich vapor, are heated up to $\sim2000$ K. In this paper, we investigate the volatile loss from the building blocks of Phobos and Deimos by hydrodynamic escape of vapor and radiation pressure on condensed particles. We show that a non-negligible amount of volatiles ($> 10\%$ of the vapor with temperature $> 1000$ K via hydrodynamic escape, and moderately volatile dusts that condense at $\sim700-2000$ K via radiation pressure) could be removed just after the impact during their first signle orbit from their pericenters to apocenters. Our results indicate that bulk Phobos and Deimos are depleted in volatile elements. Together with future explorations such as JAXA's MMX (Martian Moons eXploration) mission, our results would be used to constrain the origin of Phobos and Deimos.