Can Phobos and Deimos be used to estimate the impact rate at Mars?
Assuming that Phobos and Deimos are captured asteroids:
The orbital positions of Phobos and Deimos through time are roughly known.
The amount of coherent ejecta from impacts reaching escape velocity is roughly known, and the amount of that which stays near Mars is roughly known.
The spectra for Mars is known.
The spectra of Phobos and Deimos: we have MRO and ME in Mars orbit, in addition to other ground and space assets.
Subtract Mars spectra, until you arrive at an initial spectra for Phobos and Deimos.