RADIOACTIVE ELEMENTS MEASURED ON MERCURY BY MESSENGER: IMPLICATIONS FOR THE PLANET'S FORMATION AND EVOLUTION

Measurements of the surface composition of Mercury offer a special window into the epoch of planet formation in the inner solar system. Mercury likely preserves a more complete record of early crustal formation than do Venus, Earth, or Mars, each of which experienced extensive and prolonged resurfacing and near-surface alteration since earliest crustal formation. The MErcury Surface, Space ENvironment, GEochemisty, and Ranging (MESSENGER) spacecraft, inserted into orbit about Mercury on 18 March 2011, carries a suite of instruments designed for elemental and mineralogical remote sensing. We report measured surface abundances of radioactive elements on Mercury and their implications for hypotheses regarding the planet’s formation and thermal evolution. The average surface abundances of radioactive elements over the region of Mercury sampled by the Gamma-Ray Spectrometer are 1150 ± 220 ppm K, 220 ± 60 ppb Th, and 90 ± 20 ppb U. Ratios of the moderately volatile incompatible element K to the refractory incompatible elements Th and U provide insights into the volatile inventory of planetary bodies. The measured K/Th ratio for Mercury (5200 ± 1800) is comparable to values for the other terrestrial planets. By contrast, the lunar K/Th value (360) is an order of magnitude lower, indicative of the depletion of lunar volatiles relative to Earth. Mercury’s K/Th ratio, combined with the high abundance of the volatile element sulfur measured by MESSENGER’s X-Ray Spectrometer, indicates that the planet has a volatile inventory similar to those of the other terrestrial planets. Hypotheses proposed to explain the unusually high ratio of metal to silicate on Mercury also carry predictions for the ratios of volatile to refractory elements that can be tested against the K, Th and U abundances measured by MESSENGER. The abundance of the moderately volatile element K, relative to Th and U, is inconsistent with physical models for the formation of Mercury requiring extreme heating of the planet or its precursors and supports formation from relatively volatile-enriched material comparable to known chondritic meteorites. Abundances of K, Th, and U indicate that heat production declined substantially in the past 4 Gy, consistent with widespread volcanism near the end of late heavy bombardment and only limited volcanic activity since.