Published online by Cambridge University Press: 05 June 2012
The exploration of other planets increasingly involves combining the detailed chemical analyses of samples (laboratory or in situ) with chemical mapping by orbiting spacecraft to provide geologic context. In this chapter, we illustrate this approach to exploration by reviewing what has been learned about the Moon and Mars.
Lunar surface materials (Apollo and Luna returned samples and lunar meteorites) are classified into three geochemical end members – anorthosite, mare basalt, and KREEP. These components are clearly associated with the various geochemically mapped terrains of different age on the lunar surface. The composition of the lunar interior is inferred from the geochemical characteristics of basalts that formed by mantle melting, and geochemistry provides constraints on the Moon's impact origin and differentiation via a magma ocean.
Martian meteorites and Mars rover analyses suggest that it is a basalt-covered world, a conclusion supported by orbital measurements. Basalts of different ages appear to have distinct compositions. Since its original differentiation, the Martian mantle has remained geochemically isolated, although it is periodically melted to produce basalts. The core has an appreciable amount of sulfide, as inferred from trace elements in basalts. Water, once important in producing clays and sulfates, has now retreated into the subsurface.
Why the Moon and Mars?
The Moon and Mars are the only large bodies for which we have both samples for laboratory analysis and considerable chemical data from orbiting and landed spacecraft. Both bodies have experienced melting and differentiation.