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. & 2014 Elsevier Ltd. All rights reserved. 1. Characterizing the composition of Phobos: science and exploration objectives There are several goals to acquiring detailed compositional measurements of Phobos or Deimos: solving the question of the moons' origins, understanding geochemical processes on these two moons of Mars, understanding the early geologic history of the Mars environment, and preparing for future human exploration. In situ measurements address aspects of the first and last goals, and can make contributions to the second. However only detailed measurements of returned samples in a geologic context are fully sufficient to address all four goals. 1.1. Solving the problem of the moons' origins There are two main classes of hypotheses to explain the origin of the Martian moons, capture or formation in situ (Table 1). Both are closely tied to compositional interpretations of their optical and spectral properties (e.g., Fraeman et al., 2012, 2014; Pieters et al., 2014). Capture of outer solar system objects composed of primitive, carbonaceous material (Hartmann, 1990; Burns, 1992) would explain Phobos' and Deimos' low densities, low albedos, and spectral similarity to D-type bodies in the outer asteroid belt and the Trojan asteroid population, by virtue of inherent properties of the constituent materials. In contrast, capture of inner solar system objects of presumed ordinary chondritic composition requires a high degree of space weathering to alter spectral properties of mafic silicate material to resemble the spectral properties of Phobos and Deimos. Space weathering occurs on the Moon and near-Earth asteroids through deposition of submicroscopic iron-rich rims on regolith grains from impactgenerated vapor and solar wind sputtering (Pieters et al., 2000; Hapke, 2001), darkening and reddening the spectra of silicate minerals and masking their mineral absorptions. If Deimos and Phobos are captured, then they are either of the same original composition or have experienced extensive space weathering to produce near-identical surfaces. If the latter, the two bodies may be unrelated and may sample the diversity of materials delivered to the inner solar system during the early history of the terrestrial planets. Dynamical models have been explored that allow the capture of Phobos and Deimos only through very specific conditions and by invoking aerodynamic drag by an early Mars protoatmosphere (Hunten, 1979; Sasaki, 1990). In this context, the orbits of Phobos and Deimos are difficult to explain because they are both nearcircular and near-equatorial with synchronous rotational periods,