Using data and images from the United Arab Emirates Al-Amal Mars orbiter, scientists have obtained good imagery of large areas of the Martian moon Deimos previously unphotographed, and have used that data to compile maps of the crater and boulder distribution across most of its surface.

That data is shown to the right, Figure 1 of their paper. The top map shows the crater distribution. The bottom map shows the boulder distribution in the area of the top map outlined in green.What they learned was that the distribution was uneven, but contrary to theoretical predictions. From the paper:

Previous studies suggest that the Mars-facing side of the Martian moons may experience lower impact rates than the anti-Mars side due to the shielding effect of Mars (Christou et al., 2014). However, our results show the opposite trend: the Mars-facing side of Deimos exhibits a higher crater density than the anti-Mars side.

Moreover, much of the moon’s surface was smoother than expected. From the paper’s abstract:

Our results show that Deimos’s surface has not reached a steady state where old craters are steadily replaced by new ones, which would happen if it had been exposed to asteroid impacts for a long time. This suggests that the surface was renewed relatively recently. We also found small differences in crater spatial density depending on location, suggesting that Deimos may have shifted its orientation over time or that dust in the Martian system collected unevenly on its surface.

If the surface was renewed, they believe this was due to a recent impact that caused the surface to be reshaped, either from melting or a simple shake-out that smoothed things. A depression near the south pole is a candidate for that impact.

That impact might have also shifted the moon’s orientation, with the scientists proposing two possibilities, either a 90 degree or 180 degree flip. Both flips would also explain the contradiction between the distribution and theory.

Finally, they consider the possibility that Martian dust might be filling in craters, thus hiding them. This theory posits the existence of a dust torus around the moon, coming from the planet below. At present however this is only a theory, as no evidence of such a dust torus as been obtained.