Origin of Phobos and Deimos Awaiting Direct Exploration

Two major hypotheses have been proposed for the origin of the Martian moons Phobos and Deimos: the in situ formation theory, supported by the fact that they have circular orbits nearly parallel to the Martian equator, and the asteroid capture theory, supported by the similarity of their reflectance spectra to those of carbonaceous asteroids. Regarding the in situ formation theory, recent theoretical studies have focused on the huge impact scenario, which proposes that debris ejected into orbits during the formation of a giant impact basin on Mars accumulated to form the Martian moons. On the other hand, gas drag from a Martian gas envelope composed of gravitationally attracted solar nebula gas has been proposed as a mechanism for trapping the approaching asteroidal objects in areocentric orbits. In particular, an object entering a temporarily captured orbit in the Martian gravitational sphere would easily evolve into a fully captured moon with a near-equatorial orbit under realistic gas densities. The upcoming Phobos sample return mission is expected to elucidate the origin of both moons, with implications for material transport in the early Solar System and the early evolution of Mars. ▪ The origin of Mars’ small moons, Phobos and Deimos, has long been an open question. ▪ The leading hypotheses are asteroid capture, inferred from their appearance like primitive asteroids, and giant impact, implied by the regularity of their orbits. ▪ The origin of Phobos will be precisely determined by a sample return mission to be conducted in the late 2020s to early 2030s. ▪ Determining the origin of the Martian moons will provide clues to clarifying how the parent planet Mars formed and came to have a habitable surface environment.Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 52 is May 2024. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.