Tidal tomography reveals a thermal anomaly beneath Mars's crustal dichotomy

Mars undergoes seasonal tidal forcing as a result of its eccentric orbit and the tilt of its rotation axis relative to the Sun¹. Response to this forcing produces temporal variations in the Martian gravity field and is sensitive to the internal structure of the planet^2-4. Using tracking data from the Mars Global Surveyor (MGS), Mars Odyssey (ODY) and Mars Reconnaissance Orbiter (MRO) spacecraft^5-7, we demonstrate that degree-3 components of the time-variable gravity field of Mars differ by up to 300% from predictions for a spherically symmetric planet^8-10. These deviations can be explained if the effective shear modulus of the mantle varies by >20% over a roughly north-south pattern that closely aligns with the surface expression of the Martian crustal dichotomy¹¹. On the basis of this correlation, we infer preservation of a 200-400 °C thermal anomaly in the present-day mantle below the southern highlands of Mars. This temperature variation could reflect regional mantle convection^12,13 or insulation by the thick southern highlands crust of Mars that has persisted over several billion years^14,15.