There is also a geochemically intermediate plains composition, found mostly in intercrater plains, which has the highest modeled enstatite content (> 25%) and > 45% plagioclase, and the high-Mg region appears to have the highest forsterite (> 25%) and diopside (> 20%) but lowest plagioclase (< 40%) content. On this basis, it has been suggested that the mineralogy of Borealis Planitia is 10–15% forsterite, > 15% diopside, > 50% plagioclase, enstatite-free and quartz normative, whereas the plains within the Caloris basin have the highest-calculated plagioclase fraction, > 60%, with low proportions of forsterite, diopside and enstatite.

The standard approach is to calculate the normative mineralogy of the bedrock from the elemental abundances in regolith, even though the abundances of some elements are averaged across > 100 km wide areas (BepiColombo's X-ray spectrometer will provide higher-resolution element maps, sensitive to more elements).

Until BepiColombo arrives with its thermal infrared imaging capability and sensitivity to some diagnostic mineralogical markers, our understanding of Mercury's mineralogy remains model-dependent. The simplest explanation of this is that it is a region of ancient plains produced by an exceptionally high degree of mantle partial melting, but it lacks expression as either a morphologically distinct unit or as a color anomaly. However, the range of elemental composition seen across all smooth plains does not diverge widely from what is seen in older units, with the notable exception of a high-Mg region of intercrater plains extending across about 80 degrees of longitude to the south of Borealis Planitia, where the Mg/Si ratio is about twice the global average. Borealis Planitia can be divided into a northern low-Mg member and a southern intermediate-Mg member. Neither instrument produced worthwhile data outside the northern hemisphere.īroadly speaking, variations in major element abundances, such as Mg/Si v Al/Si, correlate with visible smooth plains boundaries. By mapping changes in the flux of thermal and epithermal neutrons, the neutron spectrometer was able to detect H (assumed to be in water–ice) in north polar craters and to infer the presence of C in one low reflectance material target during a very low altitude pass. MESSENGER's gamma-ray spectrometer was able to estimate abundances of K, Th and U because of natural radioactivity, and of Na and Cl thanks to gamma-emission triggered by cosmic-ray bombardment. Data were collected across the whole planet, but MESSENGER's orbital eccentricity means that spatial resolution becomes very poor south of the equator. Credit: NASA/Johns Hopkins University/Carnegie Institute of Washington.Įlements detected on Mercury by X-ray spectroscopy, usually reported as abundances relative to an assumed absolute abundance of Si in the region of 24.6 wt%, are Si, Mg, Al, Ca, Ti, Fe, Mn, Cr and S. Extracted from a rotating global view from the MESSENGER image gallery. A partial annulus of dark blue “low reflectance material” is exposed on the floor of Rachmaninoff crater in the upper left, and there are several other examples of “low reflectance material” in the visible part of the southern hemisphere. The similarly-colored region in the far north is part of Borealis Planitia. The circular region of orange plains in the upper right is the smooth plains of the Caloris basin.

Enhanced MESSENGER color view of Mercury centered at about 115° E.

Moon atlas with images at 24km