The Composition of Geological-Scale Features Resolved by MESSENGER’s X-Ray Spectrometer

Abstract:

During MESSENGER’s four years in orbit, its X-Ray Spectrometer (XRS) measured Mercury’s surface abundances of Mg, Al, Si, S, Ca, Ti, and Fe. Analysis of the distribution of these key rock-forming elements revealed the presence of several distinct geochemical terranes with boundaries that do not always correlate with mapped geological units or other datasets. Later measurements, made during MESSENGER’s low-altitude campaign near the end of the mission, have unveiled smaller-scale chemical heterogeneities within the geochemical terranes. Updated global Mg/Si and Al/Si maps as well as a new Al/Mg map are now sufficiently high in resolution to correlate some compositional heterogeneities with geological features identified in other datasets. For example, the interior smooth plains of Caloris basin were already known to feature among the lowest Mg/Si and highest Al/Si ratios on the planet, but the low-altitude data have allowed us to resolve the higher Mg/Si and lower Al/Si of the low-reflectance material (LRM) excavated by craters within the basin. Similarly, Rachmaninoff basin now shows an interesting substructure in which its bright interior smooth plains are consistent with lower Mg/Si than the rest of the basin floor. An area of high Mg/Si within the basin, however, does not match the distribution of LRM along the basin edges. The preponderance of additional chemical trends that correlate with craters, fresh deposits, and low-reflectance material demonstrates that Mercury’s crust is both laterally and vertically heterogeneous.