NASA’s Messenger spacecraft entered orbit around Mercury one year ago this week, and the spacecraft has been hard at work. It has captured nearly 100,000 images, mapped Mercury’s gravity field, and taken sensitive altimetry measurements that are shedding light on the planet’s surface features like never before. This week, scientists on the Messenger mission published another round of new findings about the innermost planet, which turns out to be an altogether weirder world than we'd thought.

 

The planet’s crust is thicker in low latitudes and thinner at the poles, a distribution that suggests the planet could have a liquid outer core. Its core is also large relative to the planet, comprising 85 percent of the planet’s radius, much more than Earth. The findings suggest that a layer of liquid iron sulfide lies beneath Mercury’s crust, which would make the planet much different from the other terrestrial planets.

Mercury’s elevation changes are also much smaller than those of Mars or the moon, the new research found. The most prominent topographic feature is an uplift inside a major volcanic plain in the planet’s northern latitudes. Somehow this area lifted up after the plains formed.