The smallest planet in the solar system keeps serving up big surprises.

A team led by geophysicist Maria Zuber of the Massachusetts Institute of Technology reported in the journal Science that the team found the planet's surface to be unusually flat when compared with the terrain of the moon or Mars.

Messenger mission scans also showed that in the 960-mile-wide Caloris impact basin, the northern part of its floor is higher than the south, with parts standing higher than its rim.

This lopsidedness may have resulted from tectonic forces -- and is one of a growing number of clues that Mercury may have been geologically active more recently than previously thought.

Other MIT researchers found that the core is even larger than they thought -- encompassing 83 percent of Mercury's radius. (Earth's core is a little more than half of its radius.)

Perhaps most oddly of all, there must be a solid layer of iron sulfide lying between Mercury's liquid outer core and its thin mantle. It all helps scientists to better understand what could make potentially habitable planets beyond our solar system.