Although Mars is our neighboring planet, it still poses a number of mysteries. This includes details of its internal structure and past history, but also the striking differences between the rugged highland terrain in the south of the planet and the lowlands of the north, several kilometers below.
The origin of these hemisphere differences is controversial: “Some studies suggest that a massive impact changed the original mantle of Mars,” explain Alexander Berne of the University of Arizona in Tucson and his colleagues. This impact could then have thickened the crust in the southern hemisphere or thinned it in the north. However, it is also being discussed whether there may be a stronger upwelling beneath the crust of Mars’ southern hemisphere that deposits rising mantle material on the underside of the crust.
What seasonal tidal effects reveal about the Martian mantle
And something else is unclear: Does this asymmetry of the halves of Mars continue down into the Martian mantle? To clarify this, Berne and his team carried out a special form of gravity measurement: They analyzed how the gravity field of the Martian hemispheres changes due to the changing tidal forces of the sun. Because of Mars’ elliptical orbit around the sun, its attraction varies slightly depending on the season.
If a planet is spherically symmetric, these tidal effects only cause certain, clearly defined deformations in the planet’s gravity field. However, if there are deep deviations in the composition, temperature or mass of a planetary half, this also produces higher-order changes in the gravity field – in the overtones of the complex measured values, so to speak. Using data from the three Mars orbiter probes Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter, Berne and his colleagues searched for such third-degree effects.
To do this, the researchers evaluated 16 years of radio communication between these probes and the Earth. Subtle changes in the spacecraft’s altitude and position produce signals in these radio data that reflect the response of Mars’ gravity field to the Sun’s seasonally varying tidal forces.
Topographic map of Mars and above it, semi-transparent, the shear modulus of the Martian mantle determined via seasonal tidal effects. Accordingly, the mantle rock in the north of the planet is stronger. — © Berne et al./Nature, CC by 4.0
Mars’ mantle is 200 to 400 Gad hotter in the south
In fact, Berne and his team found what they were looking for: “The third-degree components in Mars’ gravity field differ by up to 300 percent from the predictions for a spherically symmetric planet,” they report. “This suggests a heterogeneous internal structure of the Martian mantle.” The shear modulus of the mantle rock, which indicates elasticity, differs by more than 20 percent between the two hemispheres.
Specifically, this means: The Martian mantle in the southern half of Mars is softer and warmer than in the north. The temperature difference between the two halves of the mantle must be between 200 and 400 degrees, as Berne and his team determined using a geophysical model. Mars is therefore not only quite asymmetrical in terms of the landscapes on its surface – its dichotomy extends into the depths of the mantle rock.
The southern half of Mars’ mantle is hotter and softer than the northern half. Perhaps molten magma will even rise to the Martian crust. — © Berne et al./Nature, CC by 4.0
Three candidates for “jacket heating”
But what is the cause? “There are theoretically three possible candidates for this: a huge impact, spontaneous upwellings in the Martian mantle that continue to this day, and increased heating of the southern mantle due to radioactive decay in the rock and a thicker, more insulating crust,” explain the researchers. However, not all explanations fit the geological features of Mars.
The impact of a huge asteroid in the early days of Mars could explain why the northern hemisphere has a significantly thinner crust. But the heat generated by the impact should have long since disappeared by now. If stronger mantle convection in Mars’ southern hemisphere continues to this day, the hot magma would have to enrich the Martian crust there with iron. This could potentially explain why the crust of the southern highlands is more magnetized than that of the north, according to Berne and his team.
The third scenario would also be conceivable, according to which the thicker crust of the Martian highlands acted like an insulating blanket and thereby kept the Martian mantle warmer for longer. According to the researchers, a combination of this insulating effect with stronger currents in the southern mantle of Mars would also be possible.
The answer remains open – for now
It is still unclear which of these scenarios applies. For now, the Red Planet still needs an answer in this regard. “But these data provide us with valuable information to plan future Mars missions and the exploration of these phenomena,” says Berne. “And understanding the internal structure of Mars will, in turn, help us decipher the processes that shaped its formation and evolution.”
Source: Alexander Berne (University of Arizona, Tucson) et al., Nature, 2026; doi: 10.1038/s41586-026-10893-x