  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.
