Mars Hides a Hotter Hemisphere Under Its Crust
Sixteen years of spacecraft tracking reveal that Mars flexes unevenly under the Sun. The wobble points to a deep southern mantle hundreds of degrees warmer than the north.
The Discovery
Mars has two faces: smooth northern lowlands and an older, cratered southern highland crust. Now its annual flex under the Sun has exposed a matching divide far below the surface.
A peer-reviewed Nature study analysed 16 years of radio tracking from Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Tiny seasonal changes in the planet's gravity field show that the mantle does not deform as a uniform sphere.
What Makes It Strange
The Sun raises a slow solid-body tide on Mars every 687 Earth days. If the planet's interior were laterally uniform, that forcing would produce a predictable gravity pattern. Instead, the researchers found higher-order signals that remained after modelling the atmosphere and seasonal carbon-dioxide movement at the poles.
- The recovered mantle rigidity varies on a broad hemispheric scale.
- The anomaly is concentrated beneath the southern highlands rather than the crust alone.
- Thermal models translate the contrast into a southern mantle roughly 200–400°C warmer than its northern counterpart.
- The team used tracking data from three orbiters rather than a new lander or seismic network.
Mars keeps its oldest divide alive below the landscape.
The Implications
A warmer, weaker southern mantle fits models in which the thick highland crust insulated the interior and preserved radiogenic heat. The study also finds the scale of the asymmetry difficult to explain through iron composition alone.
That does not settle how the crustal dichotomy began. A giant impact, long-lived mantle convection or a hybrid of both could have created the original imbalance. The gravity result shows that whatever happened left a deep structure that survives today.
What Happens Next
The authors describe the method as tidal tomography: using a planet's response to periodic forcing to map variations inside it. Applying the same technique to longer tracking records and other worlds could reveal structures that static gravity maps blur together.
Mars is not geologically symmetrical beneath its scars. Once each orbit, the Sun makes the hidden difference move.