EVIDENCE FOR A BASALT-FREE SURFACE ON MERCURY AND IMPLICATIONS FOR INTERNAL HEAT

Microwave and mid-infrared observations reveal that Mercury's surface contains less FeO + TiO2 and at least as much feldspar as the lunar hi ghlands. The results are compatible with the high albedo (brightness) of Mercury's surface at visible wavelengths in suggesting a rock and s oil composition that is devoid of basalt, the primary differentiate of terrestrial mantles. The occurrence of a basalt-free, highly differen tiated crust is in accord with recent models of the planet's thermal e volution and suggests that Mercury has retained a hot interior as a re sult of a combination of inefficient mantle convection and minimal vol canic heat loss.