Reconsidering Our Ideas of Mercury

Friday, February 15, 2013
Room 204 (Hynes Convention Center)
David T. Blewett , Johns Hopkins University, Laurel, MD
Mercury has long been viewed as a planetary end member, but NASA's MESSENGER mission is finding that Mercury is even stranger than we thought. MESSENGER became only the second probe to visit Mercury as it flew past three times in 2008-09, and it made history in March 2011 as the first spacecraft to orbit the innermost planet. The instruments in the payload were chosen to characterize Mercury's surface morphology, topography and composition, identify species in the highly rarefied atmosphere, map the gravitational and magnetic fields, and measure the mix of charged particles in and around the planet’s small magnetosphere.

Mapping campaigns by the multispectral and monochrome cameras have confirmed that voluminous flood lavas sculpted and covered the surface. Explosive volcanism has also been documented. Tectonic modification includes both compressive faulting related to cooling and contraction of the interior and extension within large craters and basins. Discovery of  “hollows” suggests that modification of the surface through loss of volatiles, driven by sublimation or perhaps by the harsh "space weathering" environment, may continue to the present. Mapping of areas of permanent shadow near the poles shows a close correspondence with locations of high backscatter in Earth-based radar images, strengthening the case that deposits of water ice lurk in these cold traps. Gamma- and X-ray spectra reveal that Mercury's surface differs in composition from those of other terrestrial planets. Surprisingly high abundances of sulfur and potassium have been found, supporting the view that Mercury formed from highly reduced but not volatile-depleted precursor material. Magnetometer data indicate that Mercury's magnetic equator is offset from the geographic equator. Models of the interior structure from gravity and topography suggest that a giant iron core occupies more than 80% of the planetary radius, leaving only an outer 400-km-thick shell of silicate mantle and crust. A solid iron sulfide layer may overlie the top of the fluid core.

The emerging picture is one of a planet that is unusual in essentially all its characteristics. Mercury differs radically from the Moon, a body with which it has often been compared. The harvest of knowledge provided by MESSENGER is forcing ideas about planetary formation to be reconsidered. If we want to understand planet formation and evolution in general (whether in our Solar System or beyond), we must be able to explain how Mercury arrived at its present state.