Geology of Planet Mercury
Data from the January, 2008 flyby of Mercury by the Messenger Spacecraft has revealed new information about the magnetic field of the planet and how volcanoes have played an important role in shaping its surface.
One question that had scientists debating for decades was the origin of Mercury's vast plains. Some believed that they were a blanket if impact ejecta but others felt that they were a resurfacing of the planet by lavas. However, a lack of volcanic vents foiled the later idea. Those vents may have been found by Messenger.
Accurate altitude measurements revealed that the craters on Mercury are much shallower than craters on Earth's moon. Messenger data also revealed that Mercury's core is at least 60% of its mass and that its magnetic field drives interactions amoung the planet's interior, surface, exosphere and magnetosphere.
Perhaps the most interesting observation was related to the planet's change in volume. Large lobate scarps with huge cliffs are evidence that the planet has become smaller in volume over time. The reduction in volume is thought to be a result of a drop in Mercury's internal temperature.