Mars impact ejecta in the regolith of Phobos: Bulk concentration and distribution

Abstract The gravity of Mars and the observation of a thick Phobos regolith suggest that nearly all Phobos ejecta returns to Phobos and re-impacts on Phobos. Primary ejecta from Mars typically impacts Phobos at velocities of ~2–3 km/s and due to the low escape velocity from Phobos of ~4–10 m/s, ~95–99% of the secondary ejecta from Phobos is inserted into temporary orbits around Mars. Most Phobos ejecta fragments remain trapped in orbits around Mars for several days to several hundred years until they re-impact with Phobos and produce new generations of ejecta. Mars-orbiting Phobos ejecta fragments typically re-impact on opposite hemispheres of Phobos from their previous impact sites, and when combined with the typical conical dispersion pattern of impact ejecta, this suggests that just two or three generations of re-impacts on Phobos are sufficient to uniformly disperse Mars ejecta fragments globally across the geographic surface of Phobos. Due to the energy conversion inefficiencies of impact processes, subsequent re-impacts produce subsequent generations of ejecta with lower launch velocities, and a higher proportion of Phobos ejecta remains on Phobos. Once no new ejecta fragments are launched from Phobos at velocities that are greater than the local escape velocity from Phobos, no new Mars-orbiting ejecta are produced. While in orbit around Mars, particles of ejecta are perturbed by martian gravity and solar photon forces that combine to produce an increase in the orbital eccentricities, which preferentially alters the orbits of the smallest fragments. Dust fragments ≲300 µm are typically de-orbited to the atmosphere of Mars or to solar orbits within several years, whereas fragments ≳300 mm tend to remain in orbit until they re-impact with Phobos. The rapid removal of dust fragments ≲300 µm places a severe limit on their opportunities for a re-impact with Phobos and suggests a deficiency of dust grains ≲300 µm in the regolith of Phobos. For the present-day altitude of Phobos, we calculate that the flux from solar system projectiles that impacts Phobos is ~200× greater than the flux from primary Mars ejecta. Based on lunar regolith observations, the bulk concentration of solar system projectiles in the regolith of Phobos is likely to be ~3%, which, when adjusted for solar system flux in the vicinity of Mars, suggests a bulk concentration of Mars ejecta fragments in the regolith of Phobos of ~250 ppm. Phobos has orbited at least 4000 km farther from Mars during all but the most recent ~500 Myr, and throughout the majority of the early geological history of Phobos, ejecta plumes from Mars would have expanded to a much larger volumes and would have diffused to substantially lower volumetric densities at the point of intersection with Phobos. This suggests that our prediction of ~250 ppm for the bulk concentration of Mars ejecta in the present day may only be found preferentially closer to the younger upper regolith of Phobos, and at depth, Mars ejecta fragments are likely to be found in bulk concentrations that are 10–60× less than at the surface of Phobos. We also calculate that ejecta fragments from the regolith of Phobos (Phobos meteorites) may be found on Mars in abundances comparable to lunar meteorites on Earth.