The origin of Phobos (L~ 27 km) and Deimos (L ~ 15 km) remains an enigma. There are three prevailing hypotheses: H1) they are captured small bodies from the outer main belt or beyond; H2) they are reaccreted Mars impact ejecta; H3) they are remnants of Mars’ formation. There are dynamical difficulties with each one of these hypotheses, one common to all being the relatively short dynamical lifetime of Phobos inside Mars’ synchronous orbit. It is also not clear if Phobos and Deimos share the same origin.
Aside from these bodies being the moons of Mars, the following properties of Phobos and Deimos make them remarkable small bodies at their current heliocentric location:
1) Phobos and Deimos are relatively large small bodies in Near-Earth space: if counted among near-Earth objects (NEOs), Phobos would rank #3 in size, and Deimos #5. Near-Earth asteroids (NEAs) 433 Eros (L ~ 34 km) and 1036 Ganymed (D ~ 33 km) rank #1 and #2, respectively, while NEA 3552 Don Quixote (D ~ 18 km) would rank #4. Thus, there is only one NEA that is intermediate in size between Phobos and Deimos: Don Quixote.
2) Deimos has a D-type spectrum, and most of Phobos as well, i.e., they are both very dark and very red. While the “Redder Unit” on Phobos might be an exogenous veneer of D-type-like material imported from Deimos, the D-type spectrum characterizing the majority of the surface of Phobos and all of the surface of Deimos, is remarkable. D-type asteroids are found mostly in the outer main belt and beyond, and among Jupiter Trojans. They are rare among NEAs: only ~1.5 % of NEAs are D-type. Given this scarcity, it is noteworthy that NEA 3552 Don Quixote stands out again: it is a D-type.
3) While it is not known if D-type asteroids are generally rich in volatiles (e.g., water and organics), the visible and near-IR spectrum of Phobos and Deimos, and indeed that of all D-type asteroids, is best matched by the Tagish Lake meteorite, an unusual carbonaceous chondrite particularly rich in water and organics.
One interpretation of the above observations, consistent with hypothesis H1, is that Phobos, Deimos, and 3552 Don Quixote might all be members of a rare type of body in the inner solar system: large, volatile-rich, D-type objects that originated in outer main belt or beyond. This possibility is particularly intriguing in light of the fact that 3552 Don Quixote has, since 2014, been revealed to be an active comet nucleus. While Don Quixote’s highly inclined (30o off the ecliptic) and Jupiter-and-Mars-crossing orbit suggested early-on that it might be an extinct comet nucleus, it is now known to be outgassing CO2 and other volatiles.
Given these considerations, might Phobos and Deimos have a connection to comets? We will discuss the potential implications of this hypothesis, in particular with regard to what Phobos and Deimos might offer as resources for human exploration, as well as the difficulties that the cometary hypothesis presents.