The Dark Spots of Mercury

Expanded-color image of Mercury’s 52-km Degas crater, showing an abundance of dark material (NASA)

Today brings interesting color themed news from outer space. The Messenger space craft (which was destroyed when it was deliberately crashed into Mercury in spring of 2015) spotted numerous mysterious dark spots on Mercury. Indeed the Messenger spacecraft probably now is a dark spot on Mercury. Apparently the small dense planet has a dark layer close beneath the surface. Asteroid impacts, volcanoes, space probe collisions, and other events which disturb the surface of the planet reveal this extremely dark black/gray layer.

Scientists have been analyzing the data from Messenger and it now seems that this black layer which is the color and texture of pencil lead is actually composed of…graphite, the same material as pencil lead! Apparently when Mercury formed (which featured strange geological processes unseen anywhere else in the solar system) a planet-sized ocean of lava covered the entire world. As Mercury cooled the heavier elements of this lava field crystallized and sank leaving the buoyant pure carbon at the top. This dark layer has been subsequently covered with ejecta, dust, and fragments, but any disruption shows the crystallized carbon is still there.

Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

There is a lot more data from Messenger left to analyze. I wonder what other surprises the closest planet to the sun still holds.

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