Mercury's a bit weird, say NASA scientists, with a different surface composition and magnetic field to other planets.
Data from the Messenger spacecraft, six months into its Mercury orbit,calls into question current theories for the planet's formation.
The surface material looks like what you'd expect if Mercury formed from building blocks that were less oxidized than those which made up other planets - perhaps because of a different proportion of ice, says NASA.
Measurements by the probe's X-ray and gamma-ray spectrometers also reveal much more sulfur and potassium than predicted. Both elements vaporize at relatively low temperatures, so the new data rules out several popular scenarios in which Mercury experienced extremely high temperatures early on.
"Theorists need to go back to the drawing board on Mercury's formation," says Larry Nittler of the Carnegie Institution for Science.
"Most previous ideas about Mercury's chemistry are inconsistent with what we have actually measured on the planet's surface."
Messenger's also discovered a huge expanse of volcanic plains surrounding the north polar region of Mercury and covering more than six percent of the planet's total surface. The deposits appear typical of flood lavas.
The team's also discovered vents, up to 15 miles in length, that appear to be the source of some of the lava.
Another surprise was a new type of landform, possibly created by a previosuly unknown geological process. These small, bright craters are found in clusters and have been designated 'hollows'. They have a fresh appearance and haven't accumulated small impact craters, indicating that they're relatively young.
"Analysis of the images and estimates of the rate at which the hollows may be growing lead to the conclusion that they are actively forming today," says planetary scientist David Blewett of the Johns Hopkins University Applied Physics Laboratory.
"The old conventional wisdom was that 'Mercury is just like the moon.' But from its vantage point in orbit, Messenger is showing us that Mercury is radically different from the moon in just about every way we can measure."
The other big surprise was that Mercury's magnetic field is much weaker than those of Earth, Jupiter, Saturn, Uranus, and Neptune. It lacks a Van Allen radiation belt - a band of charged particles that interact with the magnetic field and surround the planets - and its magnetic equator is also well to the north of the planet's geographic equator.
"Our results tell us is that Mercury's weak magnetosphere provides very little protection of the planet from the solar wind," says the University of Michigan's Thomas Zurbuchen. "Extreme space weather must be a continuing activity at the surface of the planet closest to the Sun."