Mercury Has a Liquid Core, and Other New Surprises From the Innermost Planet

New observations from the Messenger spacecraft suggest a deep reservoir of high-density material exists within Mercury. Measurements show that Mercury must have sustained intense geophysical activity for most of its history, scientists say. Future observations will help answer some questions about how this material formed. NASA/JHUAPL/CIW-DTM/GSFC/MIT/Brown University. Rendering by James Dickson and Jim Head

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.

There are also some weird features relating to Mercury’s largest impact basin, Caloris, actually one of the largest craters in the solar system. The floor of the 1,500-kilometer (932-mile)-wide crater is now higher than the rim, scientists found. This implies that Mercury underwent some major topographic changes after it formed — perhaps through tectonic forces. This is still up for debate, though, and an extended one-year mission could help settle it.

The two Mercury papers will be published online today in Science Express. Check out the photo gallery for some more interesting finds from the Messenger mission’s first year.