On Earth there are meteorites that arrived naturally from the Moon and also ones from Mars that arrived the same way. Both are fragments that were ejected into space at escape velocity in the aftermath of big meteorite impacts.
A new paper published in Planetary and Space Science by Kenneth Ramsley and James Head (both Brown University) examines this “far-flung rocks” scenario as it apples to Mars debris landing on Phobos, the larger and closer of the two Martian moons.
Bottom line? Every part of Phobos should have bits of debris from the Martian surface embedded within it. Furthermore, because the orbit of Phobos is slowly decaying, bringing the moon closer to Mars, in the last billion years, the amount of Mars debris hitting Phobos has increased about 10 times.
The scientists ran computer simulations for the trajectories of rocks and debris thrown off Mars in large meteorite impacts, and they also examined what happens to Phobos ejecta. They note, “Primary ejecta from Mars typically impacts Phobos at velocities of about 2 to 3 kilometers per second” or 1.2 to 2 miles per second. But because Phobos has a low escape velocity – 4 to 10 meters per second, or 9 to 22 miles per hour – almost all the secondary ejecta from Phobos is kicked off 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,” the scientists write. Because of orbital dynamics, they explain that debris ejected from the leading side of Phobos will eventually land on the trailing side. This tends to distribute the debris approximately evenly over the moon.
Tiny fragments (less than a third of a millimeter) orbiting Mars typically fall into the Martian atmosphere or escape into the solar system due to pressure from sunlight. Larger fragments gradually build up in the soft regolith covering Phobos, reaching a bulk concentration of roughly 250 parts per million. Because of the slow increase in the captured ejecta as Phobos moved inward over its history, the uppermost half meter (20 inches) in the regolith will be richer in Mars bits.
“Our prediction of about 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,” they note. “At depth, Mars ejecta fragments are likely to be found in bulk concentrations that are 10 to 60 times lower than at the surface of Phobos.”
What about Phobos meteorites landing on Mars? “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.”
Finally, a caution: Natural impacts on Phobos “may produce a space debris hazard that imperils Mars-orbiting spacecraft.” Thus attempts to study the interior of Phobos or to collect samples by regolith-impacting probes should be discouraged.