MESSENGER: IMPLICATIONS FOR MERCURY FORMATION HYPOTHESES

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has characterized the surface abundances of major elements and the ratios K/Th and K/U in surface rocks. One of the goals of the MESSENGER mission has been to test, by measurement of surface composition, hypotheses advanced to account for Mercury’s anomalously large core mass fraction (~65%) compared to those for Earth and Venus (~30%). One set of hypotheses requires primary chemical or mechanical fractionation of metal and silicates in the high-temperature inner annulus of the solar system's nebular disk where Mercury accreted. Another hypothesis calls for the catastrophic loss of the earliest crust and a substantial fraction of the mantle in a high-velocity impact by a planetary-embryo-sized body of unknown composition. MESSENGER's Gamma-Ray Spectrometer and X-Ray Spectrometer, respectively, indicate terrestrial K/Th abundance ratios and high S/Si ratios. Mercury is thus richer in volatiles than expected for some models, and may have a higher bulk S/Si ratio than Earth. Models for crust and mantle removal do not necessarily result in volatile depletion in what remains of the target body; the proposed catastrophic impact model for Mercury is less fractionating of volatiles than the Moon’s likely origin via accretion within a volatile-depleted disk of ejected material. Enrichment of S in Mercury (relative to Earth), however, points to initial accretion chemistry. Sulfur enrichment may have resulted from highly reduced accretion conditions, consistent with disk mid-plane enrichment in C-rich interplanetary dust. In a reduced, differentiating body, incorporation of Si in the core would decrease core incorporation of S. Observed correlations of Mg/Si and Ca/Si with S/Si may be due to the stability of Mg and Ca sulfides in a reducing mantle. MESSENGER’s orbital geochemical, geological, geophysical, plasma science, and other results constitute a diverse set of observations demanding explanation.