During NASA MESSENGER four-year orbital mission, the spacecraft X-Ray Spectrometer XRS instrument mapped out the chemical composition of Mercury and discovered striking regions of chemical diversity. These maps of magnesium/silicon (left) and aluminium/silicon (right) use red colors to indicate high values and blue colors for low values. In the maps shown here, the Caloris basin can be identified as a region with low Mg/Si and high Ca/Si on the upper left of each map. An extensive region with high Mg/Si is also clearly visible in the maps but is not correlated with any visible impact basin. Instrument: X-Ray Spectrometer (XRS) and Mercury Dual Imaging System (MDIS) Left Image: Map of Mg/Si Right Image: Map of Al/Si http://photojournal.jpl.nasa.gov/catalog/PIA19417