THE DISTRIBUTION OF MAGNESIUM ON MERCURY'S SURFACE AS MEASURED BY THE MESSENGER X-RAY SPECTROMETER

We have used >6000 individual measurements acquired over two years by MESSENGER’s X-Ray Spectrometer (XRS) to construct a global map of the Mg/Si ratio across Mercury’s surface. In the northern hemisphere, spatial coverage and resolution are sufficient to compare our map with geologic features. The global Mg/Si ratio varies from 0.15 to 0.90, a range we attribute mainly to variations in Mg since MESSENGER gamma-ray measurements indicate little variation in Si. We confirm earlier results that showed the northern volcanic plains and plains interior to the Caloris basin have low and relatively uniform Mg/Si values. The older surrounding intercrater plains and heavily cratered terrain (IcP-HCT), however, display large variability in Mg/Si. The highest Mg/Si ratios (>0.7) are for low-reflectance ejecta deposits that surround the Rachmaninoff impact basin, and in a large (>5×10⁶ km²), relatively homogeneous region that is centered at ~30°N, 290°E. The lowest Al, and highest Ca and S abundances derived from XRS data are also found in this region. A map of crustal thickness, consistent with Mercury’s gravity field and topography, indicates that the large Mg-rich region we observe coincides with an area of relatively thin crust. The shared boundaries between high and low Mg, and between thick and thin crust are particularly sharp along the northern and eastern edges of this area; the eastern edge is also near a major north–south-oriented fold-and-thrust belt and a region with a high density of hollows. There is some evidence that Mg content and surface age are correlated. The large Mg-rich region partly overlaps with some of Mercury’s oldest terrain, as inferred from impact crater densities. Younger parts of the IcP-HCT (e.g., around Amaral crater) have significantly lower Mg/Si than the planetary average. Mercury’s surface is likely to have been extensively resurfaced by volcanism early in the planet’s history, so Mg/Si variations within regions of IcP-HCT may be indicative of temporal variations in the depth or degree of partial melting and/or chemical heterogeneity in Mercury’s mantle.