Many cosmic dust particles have escaped the aqueous and thermal processing, the gravitational compaction, and the impact shocks that often overprint the record, in most larger samples, of how Solar System materials formed. The least-altered types of cosmic dust can, therefore, act as probes into the conditions of the solar protoplanetary disk when the first solids formed. Analyses of these "primitive" particles indicate that the protoplanetary disk was well mixed, that it contained submicron grains formed in a diversity of environments, that these grains were aerodynamically transported prior to aggregation, which was likely aided by organic grain coatings, and that some minerals that condensed directly from the disk are not found in other materials. These protoplanetary aggregates are not represented in any type of meteorite or terrestrial rock. They can only be studied from cosmic dust.

Targeted MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) X-Ray Spectrometer measurements of Mercury's largest identified pyroclastic deposit are combined with neutron and reflectance spectroscopy data to constrain the composition of volatiles involved in the eruption that emplaced the pyroclastic material. The deposit, northeast of the Rachmaninoff basin, is depleted in S (relative to Ca and Si) and C, compared with the rest of Mercury's surface. Spectral reflectance measurements of the deposit indicate relatively high overall reflectance and an oxygen-metal charge transfer (OMCT) absorption band at ultraviolet wavelengths. These results are consistent with oxidation of graphite and sulfides during magma ascent, via reaction with oxides in the magma or assimilated country rock, and the formation of S-and C-bearing volatile species. Consumption of graphite during oxidation could account for the elevated reflectance of the pyroclastic material, and the strength of the OMCT band is consistent with similar to 0.03-0.1 wt% FeO in the deposit.

Mercury's global surface is markedly darker than predicted from its measured elemental composition. The darkening agent, which has not been previously identified, is most concentrated within Mercury's lowest-reflectance spectral unit, the low-reflectance material(1). This low-reflectance material is generally found in large impact craters and their ejecta(2,3), which suggests a mid-to-lower crustal origin. Here we present neutron spectroscopy measurements of Mercury's surface from the MESSENGER spacecraft that reveal increases in thermal-neutron count rates that correlate spatially with deposits of low-reflectance material. The only element consistent with both the neutron measurements and visible to near-infrared spectra(4) of low-reflectance material is carbon, at an abundance that is 1-3 wt% greater than surrounding, higher-reflectance material. We infer that carbon is the primary darkening agent on Mercury and that the low-reflectance material samples carbon-bearing deposits within the planet's crust. Our findings are consistent with the formation of a graphite flotation crust from an early magma ocean(5), and we propose that the heavily disrupted remnants of this ancient layer persist beneath the present upper crust. Under this scenario, Mercury's globally low reflectance results from mixing of the ancient graphite-rich crust with overlying volcanic materials via impact processes or assimilation of carbon into rising magmas during secondary crustal formation.

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.