New data suggests that Mercury has undergone much more dynamic processes than previously believed and that its core is unlike any of the other rocky planets in our solar system. NASA’s MESSENGER spacecraft, which has been in orbit around the solar system’s smallest and innermost planet for just over a year, has beamed back plenty of surprises for scientists here on Earth. “I thought the surface of Mercury would turn out to be complex and the interior simple,” said planetary scientist Maria Zuber of MIT, who is a member of the MESSENGER team and co-author of two new papers on the planet that appear March 21 in Science. “Instead, our data has been such a surprise that we kept thinking we were interpreting it wrong.” Mercury’s tiny size and heavily cratered surface suggested that the planet cooled into an inert lump soon after its formation 4.5 billion years ago. The two new papers show that the planet had active geologic and tectonic processes occurring until at least the planet’s middle age, around 2 billion years ago. Here, Wired takes a look at some of the weirdest new findings of what is turning out to be a strange little world. Above: Surface Heights MESSENGER has measured the height of surface features over much of Mercury. This image shows off altimetry data from ancient volcanic plains in the planet’s northern high latitudes. These smooth plains were created at the earliest stages of the planet’s history and have been subsequently deformed. The large white feature in the lower left is an enormous rise in the plains that was created through active geologic processes. Image: NASA/JHUAPL/CIW-DTM/GSFC/MIT/Brown University. Rendering by James Dickson and Jim Head

Basin Rising This view shows the topography of many craters around Mercury’s North Pole. The largest is Caloris Basin, a giant impact crater more than 900 miles across that likely formed during the heavy bombardment period of our solar system’s history, around 4 billion years ago. If Mercury were a dead planet, with no active geology, the crater is expected to have remained mostly unchanged since then. By measuring the height of surface features on Mercury, MESSENGER was able to show that some process caused the floor of Caloris Basin to expand upward, so that it now rises higher than the crater’s rim. “We’ve never seen anything like it on the planet,” said planetary scientist Maria Zuber. The findings suggest that an active interior on Mercury pushed the floor up, and that this process occurred later in the planet’s history than previously thought possible. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Active Geology This detailed map of Caloris Basin show that its floor has long been shaped by active tectonic processes. Complex folds and ridges deform the surface while parallel sunken ditches known as graben are present nearly everywhere. Image: Byrne et al. (2012). A tectonic survey of the Caloris basin, Mercury. 43rd Lunar and Planetary Science Conference, The Woodlands, Texas, abstract #1722.

Odd Tilt Many craters on Mercury show evidence of lively geologic processes. The northward tilt of Atget crater’s floor suggests that the ground was still active after its formation. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Strange Core Gravitational measurements from MESSENGER suggest that Mercury’s dense iron core is enormous, taking up about 85 percent of its interior (Earth’s core, in contrast, occupies half its interior). Researchers have also discovered that the area above the iron core is much denser than rock, though not as dense as iron. The best interpretation for this finding is that a layer of iron sulfide 124 miles thick exists around the central core. This means that the crust and mantle of Mercury are only 186 miles thick “It’s an extremely unusual structure,” said planetary scientist Maria Zuber. “If Mercury was an orange, the core plus the iron sulfide would be the orange flesh, while the crust and mantle would be the peel.” Image: A comparison of the various interiors of Earth and Mercury. Case Western Reserve University.