NASA's MESSENGER spacecraft has observed strange hollows on the surface of Mercury with reflective walls and peculiar depressions that range in size from 60 feet to over a mile across and 60 to 120 feet deep. What's even more peculiar is that NASA scientists are stumped as to how these hollows got there.

"These hollows were a major surprise," says David Blewett, science team member from the Johns Hopkins University Applied Physics Laboratory.

"We've been thinking of Mercury as a relic - a place that's really not changing much anymore, except by impact cratering. But the hollows appear to be younger than the craters in which they are found, and that means Mercury's surface is still evolving in a surprising way."

What makes the hollows unique is their location. The Mars Orbiter spotted similar depressions at Mars' South Pole. These depressions were found in the carbon dioxide ice in Mars. But in Mercury, MESSENGER found them in rock.

"There's essentially no atmosphere on Mercury," explains Blewett. "And with no atmosphere, wind doesn't blow and rain doesn't fall. So the hollows weren't carved by wind or water. Other forces must be at work."

Blewett believes that the hollows are a product of the extreme weather on Mercury. The hollows are thought to be made of materials from the depths of the planet that becomes unstable after facing the surface's fierce heat.

"Certain minerals, for example those that contain sulfur and other volatiles, would be easily vaporized by the onslaught of heat, solar wind, and micrometeoroids that Mercury experiences on a daily basis," he says.

"Perhaps sulfur is vaporizing, leaving just the other minerals, and therefore weakening the rock and making it spongier. Then the rock would crumble and erode more readily, forming these depressions."

MESSENGER has revealed that Mercury isn't as boring a planet as many thought. New data has unearthed even more mysteries like how Mercury was really formed. MESSENGER has shown that Mercury is rich in sulfur which contradicts how scientists thought Mercury formed. The prevailing theory is that Mercury was formed early in the history of the solar system. However a hot phase of the early Sun would have driven off a volatile element like sulfur. Scientists are now re-thinking the history of this small planet and even the whole solar system.

"It's the anchor at one end of the Solar System", says Blewett.

"Learning how Mercury formed will have major implications for the rest of the planets. And MESSENGER is showing that, up to now, we've been completely wrong about this little world in so many ways!"