For decades, one of Mars' greatest mysteries has been evident on its surface its northern hemisphere is low-lying and largely flat, while its southern hemisphere is high, rugged, and heavily cratered. Scientists have sought an explanation for this division in crust and surface, but a recent study suggests that the roots of this difference run much deeper, to the planet's mantle itself.
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False-color topographic view of heavily cratered terrain on Mars from the Mars Orbiter Laser Altimeter instrument on board NASA's Mars Global Surveyor spacecraft. Red areas are high elevations and blue regions are low areas. The instrument collected altimetry data about the height of surface features.
The study, published in the journal Nature on August 26, 2026, showed that the Martian mantle beneath the southern highlands currently retains a thermal region that is approximately 200 to 400 degrees Celsius hotter than its northern counterpart. The findings suggest that this temperature difference may have persisted for billions of years, potentially making parts of the southern mantle weaker and more susceptible to partial melting.
How did scientists read what was beneath the surface?
The team reached this conclusion without drilling or directly measuring the temperature of the Martian interior, but rather through precise tracking of the movement of three NASA spacecraft: the Mars Global Surveyor, the Mars Odyssey, and the Mars Reconnaissance Orbiter, benefiting from nearly two decades of spacecraft tracking data.
The researchers relied on a technique called "tidal tomography," a method that takes advantage of small changes in a planet's gravitational field caused by the sun's gravity.
Mars orbits the Sun in a slightly inclined, non-circular path, and its axis of rotation is also tilted, resulting in periodic variations in tidal forces. This response leaves a signature in its gravitational field that can be used to infer the properties of the materials within its interior.
Gravity data reveals a massive thermal anomaly in Mars' southern mantle
The research was led by Alexander Byrne, a postdoctoral researcher at the University of Arizona and a graduate of the California Institute of Technology, who during his graduate studies developed a model to infer the internal structure of planets from gravitational changes.
Byrne says that scientists often assume that the interior of planets is roughly symmetrical with their center, but more accurate gravity data allows for the revelation of more complex three-dimensional details.
Southern heat reveals the history of Mars
The hottest region of the mantle corresponds to the known geological divide between the two Martian hemispheres. The southern crust is thicker and higher, while low-lying plains cover most of the north. The model suggests that the difference in mantle stiffness between the two hemispheres can be largely explained by a temperature difference of between 200 and 400 degrees Celsius, with a possible minor difference in composition as well.
This discovery may help explain a number of phenomena that have puzzled scientists. Iron-rich rocks in southern Mars bear traces of the planet's ancient magnetic field, and data from the InSight lander showed a difference in the way seismic waves lose energy as they cross the southern regions.
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