Mars' Southern Mantle May Be Hundreds of Degrees Hotter Than Its North

The mantle beneath the southern highlands of Mars still preserves a thermal anomaly of 200 to 400 °C. The study, published Aug. 26 in Nature, inferred this internal temperature from radio tracking of three NASA orbiters by measuring how the planet's gravity field changes as Mars flexes.

Mars is stretched and squeezed as it moves along its eccentric orbit around the Sun. The team, led by Alexander Berne, used tracking data from Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter to measure the resulting changes in the gravity field. The degree-3 components of that time-varying field, one set of the large-scale patterns the signal is broken into, differ by up to 300% from predictions for a planet whose interior is the same in every direction.

Those deviations can be explained if the effective shear modulus of the mantle, a measure of how stiffly it resists being deformed, varies by more than 20% across a roughly north-south pattern. That pattern closely aligns with the surface expression of the Martian crustal dichotomy, the divide between the low northern plains and the elevated southern highlands. The temperature range follows from that correlation.

The variation "could reflect regional mantle convection or insulation by the thick southern highlands crust of Mars that has persisted over several billion years," the authors write.

This approach, known as tidal tomography, uses a planet's response to tidal forcing to map its internal structure.