The Origins and Geological Histories of Deimos and Phobos: Hypotheses and Open Questions

Kenneth R. Ramsley & James W. Head

Space Science Reviews, Volume 217, Article number: 86 (2021)


“Quantitative measurements and photographic imaging of the moons of Mars set limits on several key geological conclusions-such as the orbits of the moons. Other examples include their estimated bulk densities where Phobos has a density of ∼1.876 g/cm3 and Deimos has a lower and less certain density of ∼1.48 g/cm3. Imaging data reveals a low albedo of both moons – Phobos: ∼0.07; Deimos: ∼0.08. Also, Phobos manifests distinct redder and bluer units, whereas the surface of Deimos is globally comparable to the Phobos redder units with localized streaks of brighter material. Further, the libration of Phobos suggests interior density distributions potentially ranging from a generally homogeneous interior to a slightly heterogeneous composition, whereas Deimos lacks sufficient evidence to estimate its interior density distribution. Both moons are mantled in a global equivalent of dozens to hundreds of meters of regolith deposits, yet they differ in their surface characteristics. Other than scattered boulders and an underabundance of fresh craters, the surface of Deimos is generally smooth with most observable impact craters filled with regolith. Conversely, thousands of boulders are observed on Phobos, and the moon is heavily modified by larger craters that are crosscut by grooves and mantled with regolith deposits and smaller generally fresh and unfilled craters. Yet models that focus on many key questions are inadequately supported by definitive evidence, or lack definitive evidence altogether – and these questions must be considered open. For example, how were Deimos and Phobos produced? Where are their geological histories similar, and where do they diverge? Why are the global densities of Deimos and Phobos different from each other, and why are these densities less than typical rocky bodies? What are their present-day geochemical compositions, density distributions, and deeper interior structures? What are the natures of the Phobos red and blue surface units? When did the Stickney Crater (D∼9 km) impact take place on Phobos and how did the Stickney impact alter the surface and interior of Phobos? How and when were the Phobos grooves produced? Do large impact craters on Mars emplace observable deposits of martian ejecta on the martian moons? Finally, how might we collect definitive evidence to address these open questions?”