MESSENGER Science Team’s maps reveal previously unrecognized geochemical terranes, large regions that have compositions distinct from their surroundings, and the presence of these large terranes has important implications for the history of the planet.
These are the first global geochemical maps of Mercury, and the first maps of global extent for any planetary body acquired via the technique of X-ray fluorescence, by which X-rays emitted from the Sun’s atmosphere allow the planet’s surface composition to be examined.
The global magnesium and aluminum maps were paired with less spatially complete maps of sulfur/silicon, calcium/silicon, and iron/silicon, as well as other MESSENGER datasets, to study the geochemical characteristics of Mercury’s surface and to investigate the evolution of the planet’s thin silicate shell.
The most obvious of Mercury’s geochemical terranes is a large feature, spanning more than 5 million square kilometers. This terrane exhibits the highest observed magnesium/silicon, sulfur/silicon, and calcium/silicon ratios, as well as some of the lowest aluminum/silicon ratios on the planet’s surface, writes Shoshana Weider, a planetary geologist and Visiting Scientist at the Carnegie Institution of Washington.
Weider and colleagues suggest that this high-magnesium region could be the site of an ancient impact basin. By this interpretation, the distinctive chemical signature of the region reflects a substantial contribution from mantle material that was exposed during a large impact event.
From these maps scientists may infer the distribution of thermal-neutron-absorbing elements across the planet, including iron, chlorine, and sodium, wrote lead author Patrick Peplowski. This information has been combined with other MESSENGER geochemical measurements, including the new XRS measurements, to identify and map four distinct geochemical terranes on Mercury.
According to Peplowski, the results indicate that the smooth plains interior to the Caloris basin, Mercury’s largest well-preserved impact basin, have an elemental composition that is distinct from other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury’s mantle. Mercury’s high-magnesium region, first recognized from the XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements.
The study is published in Earth and Planetary Science Letters.