Mercury is the innermost planet in our Solar System, and until recently it was mostly considered to be unchanging. Scientists having sent a couple of probes towards it conclided that there wasn’t much changing on its surface. That is, until the MESSENGER mission started sending back images of the surface that showed some rather odd changes to Mercury’s surface:
These pock-marks, or hallows were found on Mercury’s surface and were as big as a mile across. The weren’t impact craters as they were not deep enough. While rockey planet surfaces are not uncommon in our Solar System (our own Moon and many of the outer planet’s moons).
The likely culprit would seem to be minerals brought to the surface by actual impacts – the dramaticly changing temperature of Mercury’s surface means some elements and chemicals might be ‘erupting’ in a chemical, rather than a geological volcano of sorts.
In any case, it sheds some interesting light on little Mercury. You can read more as the story develops on NASA’s webpage