Mercury's mysterious 'darkness' revealed

March 7, 2016
This oblique image of Basho shows the distinctive dark halo that encircles the crater. The halo is composed of so-called Low Reflectance Material (LRM), which was excavated from depth when the crater was formed. Basho is also renowned for its bright ray craters, which render the crater easily visible even from very far away. Credit: Courtesy NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Scientists have long been puzzled about what makes Mercury's surface so dark. The innermost planet reflects much less sunlight than the Moon, a body on which surface darkness is controlled by the abundance of iron-rich minerals. These are known to be rare at Mercury's surface, so what is the "darkening agent" there?

About a year ago, scientists proposed that Mercury's darkness was due to that gradually accumulated from the impact of comets that traveled into the inner Solar System. Now scientists, led by Patrick Peplowski of the Johns Hopkins University Applied Physics Laboratory, have used data from the MESSENGER mission to confirm that a high abundance of carbon is present at Mercury's surface. However, they also have 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 results are published in the March 7, 2016, Advanced Online Publication of Nature Geoscience.

Co-author and Deputy Principal Investigator of the MESSENGER mission, Carnegie's Larry Nittler, explained: "The previous proposal of comets delivering carbon to Mercury was based on modelling and simulation. Although we had prior suggestions that carbon may be the darkening agent, we had no direct evidence. We 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, we 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."

This enhanced color image highlights Mercury’s low-reflectance material, which appears blue in this image, and its association with impact-excavated material. The young rayed crater Degas appears near the center, and to the far left is the crater Akutagawa, whose extensive low-reflectance (dark blue) material was studied by MESSENGER and found to be carbon rich. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

MESSENGER obtained its statistically robust data via many orbits on which the spacecraft passed lower than 60 miles (100 km) above the surface of the planet during its last year of operation. The data used to identify carbon included measurements taken just days before MESSENGER impacted Mercury in April 2015. Repeated Neutron Spectrometer measurements showed higher amounts of low-energy neutrons, a signature consistent with the presence of elevated carbon, coming from the surface when the spacecraft passed over concentrations of the darkest material. Estimating the amount of carbon present required combining the neutron measurements with other MESSENGER datasets, including X-ray measurements and reflectance spectra. Together, the 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. 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 laboratory 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 we see today. This result is a testament to the phenomenal success of the MESSENGER mission and adds to a long list of ways the innermost planet differs from its planetary neighbors and provides additional clues to the origin and early evolution of the inner Solar System," concluded Nittler.

Explore further: New Mercury surface composition maps illuminate the planet's history

More information: Nature Geoscience, DOI: 10.1038/ngeo2669

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4 comments

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FredJose
1.6 / 5 (7) Mar 08, 2016
" provides additional clues to the origin and early evolution of the inner Solar System,"

For the naturalist, this is as far as it goes. There'll be forever a searching for clues and never coming to the truth of the matter - just how and when did the planet come into existence. There's only so much one can infer from the existing physical evidence and then you get stuck. In order to illicit the truth about the origin of the planet, one requires an actual eye-witness account or some recorded documentary on it. With our current limitations, this is not going to happen since no one can go back into the past to bring back the history of the origin of this planet.
FredJose
1 / 5 (6) Mar 08, 2016
Mercury still presents the naturalist with some really big challenges.
1. Just how did this tiny blob manage to retain a magnetic field?
2. Just how did it manage to hang on to its gaseous sulfurous elements when those supposedly should have been blown away by the solar wind pressure?
3. How can the naturalist explain the remaining magnetic field in terms of a liquid core when the rotating core model fails miserably to explain how a magnetic field can be generated and sustained?
4. Why does the core of Mercury appear to be so dense?
5. How to explain the big offset of the magnetic center?
6. How to explain what appears to be geological activity on a planet this small and supposedly this old? Makes one think of Pluto and the challenge that that rock represents for the current planetary formation model.
torbjorn_b_g_larsson
4.4 / 5 (7) Mar 08, 2016
"naturalist".

Philosophical bullshit of an anti-science troll.

There are real facts, read the article (before commenting inanities).
Captain Stumpy
5 / 5 (4) Mar 08, 2016
blah blah blah
1. Just how did this ...
[insert crap here]
6. How to explain...
@fred
you forgot #7
7. - why do you come to a science site to push a delusional fanatical belief that has been proven false?

also, to answer your above:
START by reading what is written in the article, THEN, as noted at the end, open the further information, including http://dx.doi.org...ngeo2669

of you still don't have answers, contact the actual author

but this also requires you to accept evidence that is validated and refuse to accept things which can't be proven, like your faith

so.. meh

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