Chlorine on the Surface of Mercury: MESSENGER Gamma-Ray Measurements and Implications for the Planet s Formation and Evolution

Abstract

Orbital measurements obtained by the MESSENGER Gamma-Ray Spectrometer have been analyzed to determine the surface abundance of chlorine in Mercury s northern hemisphere. The derived Cl/Si mass ratio is 0.0057 ± 0.001, which for an assumed Si abundance of 24.6 wt% corresponds to 0.14 ± 0.03 wt% Cl. The abundance of Cl is a factor of 2.9 ± 1.3 higher in the north polar region (>80° N) than at latitudes 0°-60° N, a latitudinal variation similar to that observed for Na. Our reported Cl abundances are consistent with measured bulk concentrations of neutron-absorbing elements on Mercury, particularly those observed at high northern latitudes. The Cl/K ratio on Mercury is chondritic, indicating a limited impact history akin to that of Mars, which accreted rapidly. Hypotheses for the origin of Mercury s high metal-to-silicate ratio must be able to reproduce Mercury s observed elemental abundances, including Cl. Chlorine is also an important magmatic volatile, and its elevated abundance in the northern polar region of Mercury indicates that it could have played a role in the production, ascent, and eruption of flood volcanic material in this region. We have identified several candidate primary mineralogical hosts for Cl on Mercury including the halide minerals lawrencite (FeCl2), sylvite (KCl), and halite (NaCl), as well as Cl-bearing alkali sulfides. Amphiboles, micas, apatite, and aqueously-deposited halides, in contrast, may be ruled out as mineralogical hosts of Cl on Mercury.