Mercury (planet)

"The planet Mercury is dotted with holes that appear to be unlike any other landform yet seen in the solar system. High-resolution photographs from NASA's MESSENGER spacecraft revealed the shallow, rimless, irregularly shaped depressions—similar to the holes in Swiss cheese—in impact craters all over Mercury."

"The features are widespread both in latitude and longitude. Dubbed hollows, the odd landforms can be tens of meters to a few kilometers wide, whereas the impact craters that contain them are tens of kilometers wide or bigger. The hollows are often seen in clusters on the walls, floors, and peaks of the craters. Many hollows have smooth, flat bottoms and feature highly reflective material...While Mercury had previously been thought of as a geologically dead planet, with few changes to it the hollows are much smaller than known volcanic pits, and the holes appear in places on Mercury that aren't likely to have experienced volcanic activity."

"The Martian depressions form as carbon-dioxide ice sublimates—turns directly from a solid to a gas—during seasonal temperature changes, hinting that some type of sublimation may be happening on Mercury. But on Mercury it's happening in solid rock, not in ice, so it's sort of a unique expression of geological processes that happen elsewhere, but maybe not as vigorously."

"...that the hollows could form when volatile materials such as sulfur on the surface are exposed to the harsh solar wind—actually a stream of charged particles from the sun. Since the tiny planet has no atmosphere, these particles can hit the surface directly, vaporizing volatile minerals. Or the close sun's intense heat could "boil" the minerals away."

"...using MESSENGER's x-ray spectrometer [it is] found that Mercury's surface has much more sulfur than that of any other rocky planet in the solar system."

"The old thinking was, Oh, Mercury, it's an old burned-out cinder and not so interesting. Now, here's this jaw-dropping thing that nobody ever predicted."