What created these mysterious hollows on the surface of Mercury?

Until NASA's spacecraft MESSENGER took up orbit around Mercury last year, the surface of the solar system's innermost planet was a great mystery. Now we've gotten a good look — and we have no idea what we're looking at.

These features spotted by MESSENGER have been dubbed "hollows." Though they don't actually glow bluish green like that, they are still plenty strange. While at first glance they look like craters, their shapes couldn't actually be formed by meteorite impacts, partially because most of them are actually inside actual impact craters.

Astronomers haven't spotted anything like this on the Moon or anywhere else in the solar system, and so whatever is creating these strange hollows must be unique to Mercury. In a new paper exploring the hollows, a team of NASA scientists offer this description of the hollows:

High-resolution images of Mercury's surface from orbit reveal that many bright deposits within impact craters exhibit fresh-appearing, irregular, shallow, rimless depressions. The depressions, or hollows, range from tens of meters to a few kilometers across, and many have high-reflectance interiors and halos. The host rocks, which are associated with crater central peaks, peak rings, floors, and walls, are interpreted to have been excavated from depth by the crater-forming process.

The best explanation for these hollows is that these depressions actually formed through the forced removal of the material that was once inside them. During the violent impact that formed the region's Raditladi basin, surface material was actually sublimated — turned from a solid straight to a gas — by the extreme conditions. That suggests there are far more volatile substances inside Mercury's interior than our current theories about its formation would suggest.

Original paper via NASA.