Mars isn’t just lopsided on the surface. New research shows the asymmetry runs deep into the planet’s interior.

The southern half sits hundreds of degrees hotter than the north and may still hold pockets of molten rock today. The study was led by Caltech alumnus Alexander Berne, who is now a postdoctoral associate at the University of Arizona.

Using gravitational data, the researchers found Mars‘s southern interior runs roughly 200 to 400 degrees Celsius warmer than its northern counterpart.  Such a temperature gap is large enough to add new context to when, and where, the planet might once have supported conditions favorable to life. The technique behind this discovery started as Berne’s graduate work at Caltech, where he built a model that translates tiny variations in gravitational data into a picture of what’s happening deep inside a planetary body.

Once the model existed, the obvious next step was pointing it at Mars. The team pulled together decades of tracking data from three separate missions: Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter. Each spacecraft’s velocity shifts, ever so slightly, in response to the gravitational field it’s flying through, and those shifts can be measured with remarkable precision. Because Mars has a slightly elliptical orbit and a tilted rotation axis, the Sun’s gravitational pull on the planet actually varies with the seasons. Tracking how Mars’s own gravitational signature responds to that seasonal tug, a method called tidal tomography, lets researchers reconstruct a three-dimensional model of what’s happening far below the surface.

“Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true,” Berne explained. “As we get more gravity data, we can determine the three-dimensional intricacies of a planet’s interior structure.” “These inferences in turn give us a blueprint for designing future missions and scientific exploration of these worlds.” “Understanding the interior structure of planetary bodies helps us unravel the processes that shaped their formation and evolution.”

Mars has always worn its asymmetry on its sleeve. The southern hemisphere bristles with towering mountains and deep, ancient craters, while the north flattens out into low plains, a divide geologists have puzzled over for decades. What nobody expected was for that same north-south split to show up in the planet’s internal temperature.

Yet that’s exactly what turned up. The southern interior runs hundreds of degrees hotter than the north. This asymmetry is buried far beneath the surface features that first drew scientists’ attention to Mars’s split aspect. The thermal anomaly doesn’t just sit there in isolation. It offers a possible explanation for a handful of other Martian mysteries that have puzzled researchers for years.

Take the strange magnetic patterns locked into iron minerals across the southern crust. A hot patch in the southern mantle could point to a stronger ancient magnetic field there, one strong enough to leave behind the north-south magnetic differences scientists have already mapped.