Strange cliff patterns on Mercury baffle scientists
The solar system's smallest planet and the one closest to the sun is offering a baffling mystery. Just what are all those giant cliffs and ridges on its surface?
Where did they come from? The ridges were detailed by NASA's messenger spacecraft.
It was the first spacecraft to orbit mercury and spent the last four years doing so. It ended its mission in April by crashing into Mercury's surface.
There's been speculation the ridges were created when Mercury cooled long ago.
But for that to be the case they would have to be uniformly scattered around the planet. We now know they aren't but are grouped in some intriguing clusters.
The next spacecraft to visit Mercury won't launch until 2017, so it may be a while before we have some answers.