Compositional Terranes on Mercury: Information from Fast Neutrons

We report measurements of the flux of fast neutrons at Mercury from 20 degrees S to the north pole. On the basis of neutron transport simulations and remotely sensed elemental compositions, cosmic-ray-induced fast neutrons are shown to provide a measure of average atomic mass, , a result consistent with earlier studies of the Moon and Vesta. The dynamic range of fast neutron flux at Mercury is 3%, which is smaller than the fast-neutron dynamic ranges of 30% and 6% at the Moon and Vesta, respectively. Fast-neutron data delineate compositional terranes on Mercury that are complementary to those identified with Xray, gamma-ray, and slow-neutron data. Fast neutron measurements confirm the presence of a region with high , relative to the mean for the planet, that coincides with the previously identified high Mg region and reveal the existence of at least two additional compositional terranes: a low- region within the northern smooth plains and a high- region near the equator centered near 90 degrees E longitude. Comparison of the fast-neutron map with elemental composition maps show that variations predicted from the combined element maps are not consistent with the measured variations in fast-neutron flux. This lack of consistency could be due to incomplete coverage for some elements or uncertainties in the interpretations of compositional and neutron data. Currently available data and analyses do not provide sufficient constraints to resolve these differences. (C) 2016 The Authors. Published by Elsevier Inc.