  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.
