Scientists have found that Mars is not the geologically uniform ball of rock it might appear from orbit. New analysis of nearly two decades of spacecraft tracking data shows that the mantle beneath the planet’s southern highlands runs between 200°C and 400°C hotter than the mantle beneath its northern lowlands, a temperature gap wide enough to leave the southern interior noticeably softer than the north. According to the study published in Nature, titled ‘Tidal tomography reveals a thermal anomaly beneath Mars’s crustal dichotomy’, the results come from a technique called tidal tomography, which reads the planet’s internal structure from the way it flexes under the Sun’s gravity. The result gives researchers their first direct look at how deep the Martian crustal dichotomy, the long-known split between the smooth northern plains and the rugged southern highlands, actually reaches, and hints that pockets of the mantle beneath the south could still be warm enough to hold trapped magma today.
Why is Mars’ southern interior 200-400°C hotter than its northern half
Mars has long been known for its crustal dichotomy: low, smooth plains cover the north, while the south sits higher, rougher and far more heavily cratered. Researchers have debated for decades whether this asymmetry was carved out by a colossal ancient impact or produced by uneven convection within the mantle, and whether any trace of that formative event survives today. This hemispheric split in surface geology is accompanied by roughly a 25-kilometre difference in average crustal thickness between the two hemispheres, alongside separate differences in crustal density, magnetism and how strongly the crust dampens seismic waves.Until now, it has been difficult to say whether these asymmetries were only skin-deep or extended into the planet’s mantle. The research team notes that resolving this question matters because the processes that created the dichotomy would have shaped the ancient hydrology of Mars and influenced how long the surface stayed suitable for water, and potentially life, in its earliest history.
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How scientists studied Mars’ interior
To probe the deep interior, the team turned to a technique called tidal tomography, which infers a planet’s internal structure from how it deforms in response to periodic tidal forcing. Mars is pulled and stretched slightly by the Sun’s gravity as it moves along its eccentric, tilted orbit, and the way the planet flexes in response depends on the stiffness of the rock inside it. The researchers reprocessed sixteen years of Earth-based Doppler tracking data gathered from three NASA orbiters, Mars Global Surveyor, Mars Odyssey and the Mars Reconnaissance Orbiter, to detect tiny seasonal wobbles in the planet’s gravity field.The critical signal came from so-called degree-3 gravity coefficients, higher-order ripples in the gravity field that should be almost non-existent if Mars behaved like a uniform sphere. Instead, after carefully subtracting the gravitational influence of the Martian atmosphere, the team found that one of these coefficients differed from the spherically symmetric prediction by roughly 300%. As the authors describe it, this scale of deviation implies that lateral variations inside the mantle are coupling with the everyday tidal forcing to produce a much larger, and clearly detectable, gravitational signature.
Why Mars’ southern mantle is much hotter
Turning the gravity measurements into a picture of the interior required a Bayesian statistical inversion, run using the LOV3D modelling code, to work out what pattern of internal stiffness could explain the observed signal. The results pointed to a greater-than-20% variation in the mantle’s effective shear modulus, arranged in a broadly north–south pattern that lines up closely with the boundary of the surface dichotomy: stiffer mantle rock beneath the northern lowlands near Vastitas Borealis, and softer, weaker rock beneath the southern highlands near the Hellas basin.Because temperature has a far stronger effect on mantle stiffness at the slow, year-long tidal timescale than it does at faster seismic frequencies, the team was able to translate this stiffness contrast into a thermal one. The paper states that the observed shear modulus variation is best explained by a hemispheric temperature anomaly of approximately 200–400°C in the present-day Martian mantle, with the data also allowing for a modest amount of additional iron enrichment, up to around 5%, in the southern mantle.
Possibility of hidden magma beneath Mars
The authors weigh up explanations for why the southern mantle might have stayed warmer for billions of years. One possibility is that the thick crust of the southern highlands has acted as an insulating blanket, trapping heat and radiogenic warmth beneath it since the earliest history of the planet. Another is that a pattern of mantle upwelling, potentially set in motion after the giant impact thought to have created the northern lowlands, has kept the region beneath the highlands warmer than its surroundings.The team is careful to note that its results do not require the presence of melt, but they do leave the door open to it: thermal models cited in the study suggest that if the southern mantle really is 200–400°C hotter, some magma production there is plausible, even though recent volcanic activity on Mars is concentrated far to the north, at Cerberus Fossae. The researchers propose that any melt generated in the south may simply be trapped by the thick overlying crust rather than ever reaching the surface, and they suggest that future missions carrying dedicated gravity experiments, or techniques such as electromagnetic sounding, could help determine whether this trapped heat corresponds to isolated pockets of magma or a more continuous molten layer deep within the planet.
