How thick is the dust layer on Phobos? Is it pretty much constant or are there significant extremes?
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Nobody knows! We've never done radar sounding of Phobos, and we have no idea what lies beneath its layer of dust. We can surmise minimum depth by looking at craters - the estimate range is 5 - 100 meters. There does appear to be significant variation, likely because of the Stickney impact, which laid many (can't remember number) meters of ejecta, and because dust from Deimos predominantly falls on Phobos's trailing edge (which I believe to be the explanation for the lack of grooves there).
Two links with details on regolith thickness:
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1$\begingroup$ Do you have a source that states the dust from deimos predominantly falls on Phobos's trailing edge? $\endgroup$ Jun 1, 2017 at 15:01
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2$\begingroup$ @MichelTouw Yes. Presentation with numbers available here: phobos-deimos.arc.nasa.gov/on-demand ; scroll down to "Mihaly Horanyi - Dust Transport between the Martian Moons Phobos and Deimos". It's obvious in retrospect - dust passed from Deimos to Phobos has a higher orbital velocity than Phobos and will impact it from the rear. $\endgroup$ Jun 1, 2017 at 15:31