According to a team of planetary scientists led by Dr. Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory, planet Mercury’s mysterious ‘darkness’ results from mixing of the ancient graphite-rich crust with overlying volcanic materials.
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER’s primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet’s surface. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
“Mercury’s global surface is markedly darker than predicted from its measured elemental composition. The darkening agent is most concentrated within Mercury’s lowest-reflectance spectral unit, the low-reflectance material,” Dr. Peplowski and his colleagues said.
“This low-reflectance material is generally found in large impact craters and their ejecta, which suggests a mid-to-lower crustal origin.”
In 2015 Dr. Megan Bruck Syal of Lawrence Livermore National Laboratory and co-autors proposed that Mercury’s darkness was due to carbon that gradually accumulated from the impact of comets.
Now Dr. Peplowski’s team has used data from NASA’s MESSENGER mission to confirm that a high abundance of carbon is present at the surface of the planet.
However, the team has also found that, rather than being delivered by comets, the carbon most likely originated deep below the surface, in the form of a now-disrupted and buried ancient graphite-rich crust, some of which was later brought to the surface by impact processes after most of Mercury’s current crust had formed.
“The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation,” said team member Dr. Larry Nittler, from the Carnegie Institution of Washington.
“Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence.”
The scientists used MESSENGER’s Neutron Spectrometer to spatially resolve the distribution of carbon and found that it is correlated with the darkest material on Mercury, and this material most likely originated deep in the crust.
Moreover, they used both neutrons and X-rays to confirm that the dark material is not enriched in iron, in contrast to the Moon where iron-rich minerals darken the surface.
“The MESSENGER data indicate that Mercury’s surface rocks are made up of as much as a few weight percent graphitic carbon, much higher than on other planets,” the researchers said.
“Graphite has the best fit to the reflectance spectra, at visible wavelengths, and the likely conditions that produced the material.”
When Mercury was very young, much of the planet was likely so hot that there was a global ‘ocean’ of molten magma.
From lab experiments and modeling, scientists have suggested that as this magma ocean cooled, most minerals that solidified would sink.
A notable exception is graphite, which would have been buoyant and floated to form the original crust of Mercury.
“The finding of abundant carbon on the surface suggests that we may be seeing remnants of Mercury’s original ancient crust mixed into the volcanic rocks and impact ejecta that form the surface we see today,” said Dr. Nittler, who is a co-author on a study published this week in the journal Nature Geoscience.
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Patrick N. Peplowski et al. Remote sensing evidence for an ancient carbon-bearing crust on Mercury. Nature Geoscience, published online March 7, 2016; doi: 10.1038/ngeo2669
