Researchers using data from NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission have created global-scale maps of Mercury’s surface chemistry that reveal the presence of distinct geochemical terranes. The results appear in two separate papers, one in the journal Icarus and the other in the journal Earth and Planetary Science Letters.
Map of magnesium/silicon abundance ratio across Mercury’s surface – red indicates high values, blue low. Image credit: NASA / JHUAPL / Carnegie Institution of Washington.
The MESSENGER mission, now nearing the end of its 4th and final year of orbital operations at Mercury, is well into a low-altitude campaign that is returning images and measurements of the planet’s surface and interior that are unprecedented in their resolution.
Since MESSENGER was inserted into orbit around the planet in 2011, data from the spacecraft’s instruments have provided information on the concentrations of potassium, thorium, uranium, sodium, chlorine, and silicon, as well as ratios relative to silicon of magnesium, aluminum, sulfur, calcium, and iron.
Until now, however, geochemical maps for some of these elements and ratios have been limited to one hemisphere and have had poor spatial resolution.
The first study, led by Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory in Laurel, MD, presents the first maps of the absorption of low-energy neutrons across Mercury’s surface.
The data used in this study were obtained with the anti-coincidence shield of the MESSENGER’s Gamma-Ray Spectrometer (GRS), which is sensitive to neutron emissions from the surface of Mercury.
“From these maps we may infer the distribution of thermal-neutron-absorbing elements across the planet, including iron, chlorine, and sodium,” Peplowski said.
“This information has been combined with other MESSENGER geochemical measurements, including the new X-Ray Spectrometer (XRS) measurements, to identify and map four distinct geochemical terranes on Mercury.”
The results indicate that the smooth plains interior to the Mercury’s largest well-preserved impact basin, Caloris, 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 the planet’s mantle.
Map of thermal neutron absorption across Mercury’s surface – red indicates high values, blue low. Image credit: NASA / JHUAPL / Carnegie Institution of Washington.
The second study, headed by Shoshana Weider of the Carnegie Institution of Washington, used a novel methodology to produce global maps of the magnesium/silicon and aluminum/silicon abundance ratios across Mercury’s surface from data acquired by MESSENGER’s XRS.
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,” Weider said.
The scientists 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.
_____
Patrick N. Peplowski et al. Geochemical terranes of Mercury’s northern hemisphere as revealed by MESSENGER neutron measurements. Icarus, published online February 13, 2015; doi: 10.1016/j.icarus.2015.02.002
Shoshana Z. Weider et al. 2015. Evidence for geochemical terranes on Mercury: Global mapping of major elements with MESSENGER’s X-Ray Spectrometer. Earth and Planetary Science Letters, vol. 416, pp. 109–120; doi: 10.1016/j.epsl.2015.01.023
