Mars’s interior was long modelled as broadly symmetrical — but a Nature study published on 27 August found evidence that the mantle beneath its southern highlands remains 200 to 400°C hotter than the north and may be partially molten, an enormous thermal divide whose origin remains unexplained.

Mars looks divided before anyone tries to see beneath its surface. The north is dominated by broad lowlands. The south rises into older, heavily cratered highlands underlain by substantially thicker crust. Now a gravity-based analysis suggests that this division continues deep into the mantle.

The estimate is enormous. Mantle beneath the southern highlands may be 200 to 400 degrees Celsius warmer than comparable material in the north. The warmer region may also permit some rock to remain partially molten.

Neither result came from a thermometer or a sample. They emerged from tiny changes in the velocities of three spacecraft measured over 16 years, followed by an inversion linking Mars’s changing gravity to the mechanical properties of its interior.

That distinction is central. The spacecraft supplied a very precise gravitational signal. Temperature and melt are the physical interpretation that best reconciles that signal with models of Martian rock. The evidence is strong enough to challenge the familiar picture of Mars as a series of uniform spherical shells, but not detailed enough to explain what created the divide.

The August 27 date needs one small note

The study is titled “Tidal tomography reveals a thermal anomaly beneath Mars’s crustal dichotomy”. Nature’s online record lists 26 August 2026 as its publication date. Caltech’s release, also dated August 26, described the paper as appearing in Nature on August 27. That accounts for the date used in much of the following day’s coverage.

This article is also a deeper companion to Space Daily’s earlier account of the same thermal anomaly. The first report set out the core calculation and the three broad origin hypotheses. The closer question here is how a change in an orbiter’s radio signal becomes a map of heat inside another planet, and where the interpretation remains open.

The research was led by Alexander Berne, a Caltech alumnus now at the University of Arizona, with colleagues from several institutions. It introduces an application of “tidal tomography,” a technique that uses the way a planet deforms under an external gravitational pull to constrain variations hidden inside it.

Tomography is an apt word, but it should not evoke the resolution of a medical scan. The result is a broad three-dimensional outline of hemispheric structure. It cannot pick out a narrow magma chamber or provide a temperature at a particular coordinate.

Three orbiters became probes of the mantle

The dataset combined tracking of Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Ground stations in NASA’s Deep Space Network sent X-band radio signals to the spacecraft and received signals back.

If an orbiter accelerates towards Earth, the received radio frequency shifts slightly upward. If it moves away, the frequency shifts downward. This Doppler measurement allows navigators to reconstruct changes in velocity with extraordinary precision.

Engineers normally use that information to know where a spacecraft is and keep it on course. Planetary geodesists can reverse the logic. Once they account for solar radiation pressure, manoeuvres and other known forces, the spacecraft’s motion reveals small variations in the gravity field it is flying through.

The orbiters therefore acted as test masses. Each repeatedly fell around a planet whose mass is not distributed perfectly evenly. By combining many years of tracking, the researchers could search for gravitational changes far too small to recognise in one pass.

The final record covered roughly 16 years. Long duration mattered because the signal of interest varied over Mars’s seasons, while atmospheric mass, polar carbon dioxide and unmodelled forces on the spacecraft could produce competing changes.

The Sun raises a tide in solid Mars

Mars takes about 687 Earth days to travel around the Sun, and its orbit is more elliptical than Earth’s. The strength and geometry of the Sun’s pull therefore change appreciably over a Martian year.

That pull deforms solid Mars by a small amount. It is the planetary equivalent of a tide, although there is no visible wave washing across the ground. The size and spatial pattern of the response depend on how stiff the crust and mantle are and on how mass is distributed inside the planet.

Researchers describe broad gravity patterns using spherical harmonics. A perfectly layered planet responding to the Sun would be dominated by a degree-two tide, the large-scale pattern associated with stretching along one axis and compression along another.

The team found seasonal behaviour in degree-three gravity coefficients that a spherically symmetric model could not reproduce. One coefficient differed by about 300 per cent from the modelled value after the estimated atmospheric contribution was included.

This did not mean gravity in one hemisphere was three times stronger. It meant that a small, specific time-variable component of the gravity field was much larger than the symmetric model predicted. The most successful interior models coupled the ordinary solar tide into this degree-three response through a major north–south change in mantle properties.

Gravity measured stiffness before temperature

The direct interior quantity recovered by the inversion was not temperature. It was a contrast in effective shear modulus, a measure of how strongly material resists being distorted.

The southern mantle appeared more than 20 per cent less rigid than the northern mantle at the long period of the solar tide. Warmer rock generally deforms more readily, so the researchers compared the inferred mechanical contrast with laboratory-based models of olivine-rich mantle material.

That comparison produced the 200 to 400°C thermal excess. It is a difference between two broad regions, not an absolute temperature. It does not mean the southern mantle is only 400°C, nor that every part of the hemisphere shares one value.

Composition can imitate part of the mechanical and density signal. The models allowed the southern mantle to contain as much as roughly 5 per cent more iron. By combining rigidity with Mars’s offset between its centre of mass and centre of figure, the study favoured temperature as the main contributor while leaving room for a compositional difference.

As in every inversion, the answer depends on the assumed physics. The team accounted for this by testing interior structures and enlarging the formal uncertainties on the relevant gravity coefficients by a factor of 15, particularly to cover unmodelled non-gravitational forces on the orbiters. The anomaly persisted through that conservative treatment.

“Partially molten” is plausible, not a global magma ocean

At the upper end of the inferred temperature range, some southern mantle material could cross the melting threshold. Caltech’s summary of the research describes the region as partially molten.

The phrase can easily be pictured too dramatically. Partial melting usually means a mostly solid matrix containing a modest fraction of melt between mineral grains. It is not an underground ocean extending across half of Mars.

The paper treats melt with appropriate caution. A warm anomaly could support local melting below the rigid lithosphere. Buoyant melt might rise and then stall inside the unusually thick southern crust, forming intrusions rather than erupting at the surface.

There is also a reason not to claim widespread melt. Seismic waves travelling through hot or molten rock tend to lose energy. InSight observations indicate greater attenuation for some southern events, consistent with a warmer region, but not the extreme attenuation expected if a large fraction of the southern mantle were broadly molten.

The current evidence therefore supports a warm, mechanically softer mantle that may contain partial melt in some places. It does not locate that melt, measure its fraction or demonstrate active volcanism throughout the southern highlands.

The surface dichotomy is now an interior problem

Mars’s hemispheric dichotomy is not subtle. Much of the northern hemisphere consists of relatively smooth plains several kilometres below the southern terrain. Southern crust is commonly estimated to be roughly 25 to 30 kilometres thicker.

The ancient south also preserves stronger remanent magnetism in its rocks. Mars no longer has a global magnetic field like Earth’s, but portions of old crust record magnetisation acquired while an early dynamo operated.

A deep thermal boundary aligned with the surface division offers a way to connect these observations. Warmer mantle could have generated melt that built and altered the southern crust. Differences in mantle and crustal cooling could also have influenced where strong magnetisation was acquired or preserved.

Alignment is not proof of a single cause. Thick crust might be the result of warm mantle, but it could also be the reason the mantle stayed warm by limiting heat loss. Both could descend from an earlier event that changed the entire hemisphere.

The dichotomy also shaped Martian water. Low northern basins are among the places proposed to have held ancient seas, while highland valleys delivered sediment and water downhill. A thermal history that affected crustal elevation and basin formation therefore belongs in any reconstruction of when and where Mars could sustain habitable environments.

The magnetic connection has been building for some time. Space Daily reported in 2025 on simulations linking hemispheric mantle differences to Mars’s uneven magnetic history. The new thermal result gives that class of model an observational constraint, although it does not by itself reconstruct the ancient dynamo.

Three origin stories remain in play

The first explanation begins with mantle convection. Even without Earth-style plate tectonics, hot material can rise and cooler material can sink inside a planet. A long-lived, planet-scale upwelling beneath the south could preserve a degree-one thermal pattern, with one hemisphere warmer than the other.

The difficulty is longevity. Mars formed more than 4.5 billion years ago. Any convection model must explain how such a large contrast survived rather than being mixed away. Mars’s stagnant outer shell and comparatively sluggish mantle may make that preservation possible, but the model must still reproduce the observed crust and volcanism.

The second explanation is insulation. The southern highlands have much thicker crust, which can slow the escape of interior heat. If that crust is also enriched in radioactive elements that produce heat as they decay, the blanket effect could be stronger.

This mechanism reverses the usual causal story. The mantle need not have built the thick crust; the existing crust could have protected a warm mantle. Higher-resolution gravity observations might reveal frozen or partly molten intrusions expected where rising magma encountered that lid.

The third explanation begins with a colossal impact. An object striking early Mars may have excavated the northern lowlands, removed insulating crust and allowed the northern mantle to release heat more efficiently. The south would then appear anomalously warm partly because the north cooled faster.

Space Daily covered giant-impact models for the Martian dichotomy in 2008. Those simulations showed that an oblique collision by a body hundreds or thousands of kilometres across could create a northern basin without globally melting the planet. The new gravity result makes that old possibility more testable, but does not select it over convection or insulation.

InSight supplies an independent, incomplete check

Gravity is not the only evidence that Mars’s mantle varies from place to place. NASA’s InSight lander recorded 1,319 marsquakes before its mission ended in 2022, allowing researchers to examine how seismic waves travelled through the planet.

A 2025 study found lower seismic quality factors for events associated with the southern highlands than for quakes near Cerberus Fossae in the north. Lower quality factor means waves lost energy more rapidly. Higher temperature and some melt can cause that behaviour.

The agreement is encouraging because orbital gravity and seismic attenuation are independent measurements. It remains incomplete because Mars had only one seismic station. Event locations, paths and material properties are less tightly constrained than they would be with a network of landers.

The same limits appear elsewhere in Mars interior science. Space Daily’s discussion of possible liquid water in the Martian mid-crust showed how a seismic velocity pattern can fit water-saturated fractured rock while also depending strongly on assumptions about minerals and pore shape.

These are not failures of the method. They are the nature of remote geophysics. Scientists measure waves, gravity and deformation at accessible locations, then ask which hidden structures could have produced them. Independent datasets narrow the possibilities.

Symmetrical models were a starting point, not a mistake

It is tempting to frame the result as overturning a naive belief that Mars must be perfectly symmetrical. Planetary scientists have long known that its crust and surface are uneven. Spherical models persisted because sparse data make radial layers the most stable starting point.

A one-dimensional model can estimate average crustal thickness, mantle density and core size without pretending that every longitude is identical. The new study shows that the available gravity record may now be rich enough to recover a large lateral contrast that those baseline models deliberately omit.

That is a methodological advance as much as a Martian discovery. Tidal tomography could be applied to other planets and moons if tracking data contain a measurable changing tide. Mercury and the large moons of Jupiter are obvious candidates, although each presents different orbital and observational complications.

For Mars, the next decisive evidence would come from better spatial coverage. Multiple seismometers could compare northern and southern paths directly. Electromagnetic measurements could test for conductive melt. Denser and longer gravity tracking could determine whether the degree-three response survives improved atmospheric and spacecraft-force models.

A large anomaly with an unfinished explanation

The strongest result is not that half of Mars is a sea of magma. It is that a seasonal gravitational response recorded far above the surface is difficult to explain unless the mantle beneath the southern highlands is much softer than the mantle beneath the north.

Mineral physics translates that softness into a thermal contrast of roughly 200 to 400°C. The warmer end permits partial melting, while seismic observations offer a separate hint of greater attenuation in the south.

What remains unsettled is the direction of history. Warm mantle may have helped build the southern highlands. Thick southern crust may have kept the mantle warm. A giant impact may have stripped the north and accelerated its cooling. Long-lived convection may have divided the interior before the modern landscape took shape.

Mars has preserved the outcome but blurred the sequence. The new study reveals that the planet’s most obvious surface boundary is rooted far deeper than the topography. Explaining why will require measurements capable of separating heat, composition, melt and ancient structure rather than folding them into one broad gravitational signature.