Bright 'hollows' on Mercury are unique to solar system

Mercury is covered with pits that are unlike anything else in the solar system, new observations from NASA's Messenger spacecraft show. They may have been formed by processes still active today, and change our view of the small rocky planet's history.

"The fact that these things are there at all is a big surprise," says David Blewett of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland.

Messenger flew past Mercury three times before settling into orbit on 18 March. Those flybys ? in 2008 and 2009 ? revealed that some craters were filled with unidentified bright stuff.

Now, after more than a Mercury year (88 Earth days) in orbit, Messenger's high-resolution camera has revealed that the mysterious bright patches are clusters of shallow, rimless, irregularly-shaped pits.

Each such "hollow" is between tens of metres to a few kilometres across and looks fresh and young ? by planetary standards.

Spongy matrix

Other instruments on Messenger have found that Mercury's rocks contain a lot more volatile elements than expected. Blewett thinks the hollows formed when these elements ? which are vaporised easily ? were liberated from the surface, leaving behind a spongy, fragile matrix of soil that then collapsed.

Micrometeoroids and charged particles from the solar wind might have vaporised volatiles in the rocks, triggering the collapses.

Volatiles might also have been concentrated in pockets by volcanic eruptions that occurred during Mercury's long, frigid nights. In that case, volatile volcanic gases from the eruptions could freeze solid and get buried under lava flows. Later, a meteoroid impact could expose this material to sunlight, causing it to evaporate and the surrounding rock to crumble.

Smack unlikely

In either case, a lot of volatile stuff seems to be hiding beneath the surface of Mercury. That presents problems for theories that attempt to explain the planet's high density ? an enormous two-thirds of its mass is made up of its metal core.

One theory argued that early on in the planet's life, the sun might have vaporised part of its rocky exterior. Another suggested that a large protoplanet smacked into it during its formation phase, blasting away its outer layers. In both cases, the heating involved would have evaporated volatile elements, allowing them to escape into space.

The fact that Messenger has found more volatiles than expected is stumping theorists trying to model Mercury's past. "The people who think about planetary-formation sorts of things are running back to their drawing boards," Blewett says.

Journal reference: Science, DOI: 10.1126/science.1211997

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