The value of Phobos sample return

Phobos occupies a unique position physically, scientifically, and programmatically on the road to exploration of the solar system. It is a low-gravity object moderately inside the gravity well of Mars. Scientifically, it is both an enigma and an opportunity: an enigma because the origins of both it and Deimos are uncertain, and provide insights into formation of the terrestrial planets; and an opportunity because Phobos may be a waypoint or staging point for future human exploration of the Mars system. Phobos is a low albedo, spectrally bland body with a red-sloped continuum. It appears similar to D-type objects more commonly found in the outer asteroid belt and Jovian space (Rivkin et al., 2002), but occurs in an orbit that is difficult to explain by capture (Burns, 1992). It might have a primitive composition like that inferred for outer solar system objects or it could be related to Mars and, for example, be composed of Martian basin ejecta. Regardless, Phobos has acted as a witness plate to Martian debris over the age of the solar system. The moons may possibly be a source of in situ resources that could support future human exploration in circum-Mars space or on the Martian surface. in situ compositional analyses can address many questions relevant to preparation for future human exploration. Sample return resolves those questions while also enabling detailed analyses in terrestrial laboratories to address higher order questions, many of which have not yet been asked.