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A two-faced Mars has been hiding under its skin for as long as the planet has had a southern half and a northern half. The surface has always advertised the asymmetry: cratered, elevated highlands to the south and smooth, low-lying plains to the north, with a roughly 5 kilometer elevation drop at a boundary that traces most of a great circle around the planet. A study published in Nature on August 27, 2026 now reports that the split goes deeper than anyone had measured, all the way into the mantle. Alexander Berne, formerly at Caltech and now a postdoctoral associate at the University of Arizona, and twelve collaborators used nearly two decades of radio tracking data from three NASA Mars orbiters to infer that the mantle beneath the southern highlands is between 200 and 400 degrees Celsius warmer than the mantle beneath the northern lowlands, and that the contrast is preserved to the present day.

The crustal dichotomy has been one of the most stubborn puzzles in Mars science since the first global topographic maps arrived from the Mars Orbiter Laser Altimeter in the late 1990s. The northern lowlands sit about 5 kilometers lower than the southern highlands on average, and the southern crust is roughly 25 kilometers thicker. Magnetic stripes recorded in the southern crust by Mars Global Surveyor’s magnetometer imply that a once-active dynamo imprinted its field there, while the northern lowlands carry almost no such signature. Seismic waves from marsquakes detected by NASA’s InSight lander between 2018 and 2022 lose energy much more quickly when they travel through the mantle under the southern Terra Cimmeria region than when they pass through Cerberus Fossae in the north. Each of those observations had an individual explanation; what was missing was a single piece of physics that could pull them together.

Tidal tomography is a way to read a planet’s interior by watching how its shape responds to changing gravitational stresses from a partner body. Mars’s orbit is eccentric and its rotation axis is tilted, so the Sun’s tidal pull on the planet varies over the 687-day seasonal cycle. The deformation of the planet under that varying pull depends on the rigidity of the mantle at depth, and a stiffer or softer region flexes by a different amount, leaving a small but measurable fingerprint on the spacecraft flying overhead. Berne and his team analyzed X-band Doppler tracking collected by NASA’s Deep Space Network from Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter over sixteen years and extracted the time-varying component of the Martian gravity field, as described in the Caltech press release. The degree-3 spherical harmonic coefficients of that field, which should be vanishingly small for a spherically symmetric planet, deviated from zero by up to 300 percent, with high statistical significance. A Bayesian Markov-chain Monte Carlo inversion using the semi-analytical code LOV3D translated those gravity deviations into a map of effective shear modulus in the mantle, which in turn implies the temperature contrast.

The map lines up almost perfectly with the surface topography. The team recovered a peak-to-peak variation in mantle shear modulus of 81 percent with a 60 percent uncertainty, with stiffer mantle beneath the northern lowlands, centered near Vastitas Borealis at 45 degrees north, 138 degrees west, and softer mantle beneath the southern highlands, centered near the Hellas basin at 45 degrees south, 42 degrees east. A colder mantle is stiffer, so the inversion translates the stiffness pattern into a present-day temperature anomaly of 200 to 400 degrees Celsius, biased hot to the south. The team also constrained compositional differences: up to 5 percent iron enrichment in the southern mantle relative to the north, a modest but non-negligible contribution that helps the gravity field match without breaking the observed center-of-figure to center-of-mass offset, per Space.com’s coverage. The paper notes that even this 5 percent compositional contribution requires the temperature anomaly; pure composition cannot reproduce the rigidity contrast without violating independent constraints on Mars’s interior structure.

The InSight lander had already left a strong clue, as ScienceDaily summarized. Quality factors, a measure of how much energy a seismic wave loses as it travels through a given region, came in around 500 for events in Terra Cimmeria in the south and between 800 and 2000 for events in Cerberus Fossae in the north. A few hundred kelvin of extra heat in the southern mantle can drive that contrast, and the new tidal tomography result sits right at the temperature the seismic attenuation had implied. Magnetic anomalies in the southern highlands crust also make more sense if the mantle beneath them was unusually hot for part of Mars’s early history: a deep heat source under the south could have driven stronger convection in the underlying outer core, sustaining a localized dynamo that imprinted the crust as it cooled through the Curie temperature before the global field died out around 4 billion years ago. The InSight quality factors are still well above what fully molten rock would produce, so the southern mantle is warm rather than liquid, and three competing explanations remain in play. A giant impact, the so-called Borealis basin formation often invoked to explain the lowlands, would have left the southern mantle iron-enriched relative to the north, consistent with what the team infers, but a single impact is not expected to keep a hemisphere several hundred kelvin hotter than the other for four billion years without help. Spontaneous degree-1 convection would naturally produce a thermal asymmetry but predicts the opposite compositional signature. Thermal insulation by the thick southern crust can sustain the observed temperatures in geodynamic simulations and matches the seismic data, but does not by itself explain why the southern crust is thick in the first place. Berne and his co-authors favor a hybrid story: a giant impact removed or depleted the northern mantle early, leaving the south iron-enriched, and the resulting thicker southern crust then insulated the mantle beneath it long enough for the present-day thermal contrast to survive.

The next probes of this structure will not be landers but gravity missions. Today’s Mars orbiters can resolve degree-3 variations in the time-varying field only because the team pooled two decades of data. A dedicated gravity mission, the Mars-equivalent of GRACE on Earth or GRAIL at the Moon, could resolve similar or finer heterogeneities in a small fraction of the time, and the same technique generalizes to other asymmetric worlds. The authors specifically call out Ganymede, Mercury, Io, and Enceladus as targets where tidal tomography could reveal deep interior structure that no lander has yet touched. The deeper message is that the Martian dichotomy is not just a story about an ancient surface; it is a story about an interior that has remembered its first billion years and is still telling the tale, one radio photon at a time, to anyone with sixteen years of patience to listen.