THE GEOPHYSICAL EVOLUTION OF MERCURY

After two flybys of Mercury last year by the MESSENGER spacecraft, images of areas seen for the first time at close range together with observations of the planet's internal magnetic field, neutral and ionized exosphere, surface spectra, and topography are yielding fresh insight into Mercury's tectonic and magmatic history and the formation and interior evolution of the planet. MESSENGER confirmed the prediction, made from Mariner 10 images of less than half the surface, that tectonic features on Mercury are primarily contractional. The strain accommodated by lobate scarps, the surface expressions of huge thrust faults, constrains the global thermal history since the end of heavy impact bombardment, particularly cooling of the planet's core. Core cooling is the likely power source for Mercury's magnetic dynamo, the probable origin of the planet's internal magnetic field on the basis of the field's dominantly dipolar geometry and the lack to date of detected crustal magnetic anomalies. On the basis of scarp distribution and embayment relations, scarp formation began before the emplacement of many smooth plains units and continued until after the youngest expanse of smooth plains material had formed. The ubiquity of smooth plains, many of which are volcanic, indicates that compressive lithospheric stress levels were not sufficient to prevent widespread early volcanism. Impact basins provided major foci for volcanism and deformation. Basin formation amplified magma production at depth by the removal of overburden pressure and the emplacement of impact energy as heat, and also changed the lithospheric stress state by removing pre-existing stress within the basin interior and modifying stress within a damage zone that extended to several basin radii. The latter processes favored magma ascent and plains formation and affected the style and timing of basin deformation. MESSENGER images have documented examples of volcanic centers surrounded by material interpreted to be pyroclastic deposits. The implied magmatic contents of candidate volatiles indicate that Mercury's accretion included the incorporation of volatile-rich planetesimals or embryos formed farther from the Sun and that some of those volatiles were preserved during whatever processes imparted to Mercury its anomalously high ratio of metal to silicate.