Mercury’s dark surface could be the remnant of an astonishing graphite crust formed early in the planet’s history. Graphite is a naturally occurring crystalline form of pure carbon, gray to black in color, which notably makes up the core of our pencils.
To understand its origin, we must go back to a time when Mercury was still extremely hot. Much of its rock was then molten, forming a vast magma ocean. The carbon present in this mixture could follow several paths: remain in the magma, enter the metallic core, or form graphite.
Artist’s impression of the MESSENGER probe orbiting Mercury.
Credit: NASA
Image Wikimedia
Researchers reproduced in the laboratory the conditions that prevailed in the depths of young Mercury. They subjected different materials to high pressures and temperatures that could exceed 2,000 °C. They then measured how the carbon was distributed between the molten rock and the metal.
The result depends strongly on the amount of oxygen available for chemical reactions. Mercury formed in a very oxygen-poor environment. Under these particular conditions, carbon enters the metal less easily. A larger amount can therefore remain in the magma ocean and then transform into graphite as it cools.
Graphite, however, is much less dense than the surrounding rocks and metal. It can therefore rise toward the surface of the magma. As cooling progressed, this separation would have produced an initial graphite-rich crust. The researchers calculate that the initial layer that best matches the observations would have been approximately 40 to 120 meters thick.
Schematic representation of the distribution of carbon in Mercury, from its formation to the present day.
This ancient surface is no longer intact today. For billions of years, meteorite impacts and volcanic activity have churned up the materials. Nevertheless, the MESSENGER probe detected particularly dark, carbon-rich terrains. These observations had already led researchers to propose the existence of an ancient floating graphite crust.
The study also provides information about Mercury’s gigantic core, which accounts for approximately 70% of the planet’s mass. The calculations indicate that carbon could only be present there in relatively small quantities. It would therefore be difficult for carbon alone to explain why the core is less dense than pure iron. Silicon and sulfur probably play a more important role.
These results can be compared with new measurements of Mercury. The BepiColombo mission is set to study its surface, composition, and internal structure using several instruments. This data will notably make it possible to better determine the amount of carbon present and test the scenario of this ancient graphite crust.