Mars hides a hot southern interior under its crust
A Nature paper from 27 August uses tidal tomography on decades of Mars gravity data to show the southern mantle is 200 to 400 degrees Celsius warmer than the north and partly molten. The red planet is not just lopsided on the surface; it is lopsided inside.
On the surface Mars is a famously lopsided world. Its southern hemisphere rises in cratered highlands while the north sinks into low, smooth plains, a split so visible it is called the crustal dichotomy. A paper published in Nature on 27 August argues that the asymmetry goes all the way down: the interior of the southern hemisphere is roughly 200 to 400 degrees Celsius hotter than the northern half, and parts of it are close to molten.
The work uses a method called tidal tomography. Planetary scientists usually assume that the inside of a planet is roughly symmetric, a sphere layered like an onion. That assumption is convenient, but it is also an assumption. Alexander Berne, a Caltech alumnus now at the University of Arizona, and his colleagues instead squeezed more information out of spacecraft gravity measurements. Over decades, three missions, Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter, have flown through a gravity field that the Sun tugs on differently as Mars moves along its elliptical orbit and tilts on its axis. By tracking tiny variations in the spacecraft’s velocities, the team reconstructed how the gravity signal changes through the seasonal cycle, and inverted those variations into a three-dimensional picture of what the interior must look like to produce them.
The picture that came back was not symmetric. The southern mantle runs far hotter than the northern one. That is a surprising result for a small, long-cooled planet, where a cold, dead interior is the quiet expectation. A partly molten southern mantle also helps explain two puzzles that have sat around Mars for years. The first is the pattern of magnetic anomalies frozen into ancient southern rocks, which looks as though an early magnetic field was far stronger or more active in the south. The second comes from the InSight lander, which found that seismic waves die out faster through the southern crust than the north, a symptom that fits a hotter, less rigid interior.
What is not settled is why the south is warm. The authors lay out several possibilities without declaring a winner. A giant early impact could have stripped heat from one side of the planet, leaving the other relatively insulated. Slow, spontaneous convection in the southern mantle could have kept it stirred. Or a thick, heavily cratered southern crust could simply trap heat that the thinner northern crust lets escape. Each explanation points to a different Mars, and choosing among them means better interior models and, eventually, new missions.
The deeper significance is methodological. For years the interior of a planet beyond Earth has been inferred as a smooth average, one number for density, one for temperature, one for state. Tidal tomography shows that, with enough gravity data, the average stops being good enough. The three-dimensional interior of a planet turns out to carry the fingerprint of how it formed and how it has cooled. Mars, it turns out, has been keeping a thermal secret in its southern half, and the secret is large enough to rewrite the story of which side of the planet was habitable, and when.