Gravity Data Reveals Mars Is Hundreds of Degrees Hotter in the South

Researchers have found a major temperature imbalance deep inside Mars, with the planet's southern interior appearing significantly hotter than its northern half and possibly partially molten. The discovery, drawn from decades of gravity measurements, offers a new window into the Red Planet's geologic history, including periods when Mars may have had environments capable of supporting life.

Mars has long been known to look very different across its two hemispheres. The southern surface is dominated by tall mountains and heavily cratered terrain, while the northern hemisphere consists largely of broad, low-lying plains. The new results show that this contrast may extend far below the surface, with the interior beneath the southern hemisphere measuring about 200 to 400 degrees Celsius warmer than the north.

The study was led by Caltech alumnus Alexander Berne, now a postdoctoral associate at the University of Arizona, and published August 27 in Nature. While at Caltech, Berne developed a model that uses subtle variations in gravity to estimate the internal structure of a planetary body, then applied it to Mars. The team analyzed observations from three missions, Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter, tracking extremely small changes in the spacecrafts' velocities to reconstruct the planet's gravitational field.

The researchers used a method known as tidal tomography, studying how gravitational signatures change over the course of the Martian year as the Sun's pull varies along the planet's slightly elliptical orbit and tilted axis. "Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true," Berne said. "As we get more gravity data, we can determine the three-dimensional intricacies of a planet's interior structure."

The hidden heat could help explain several other puzzling features of Mars. Scientists have detected unusual magnetic signatures in iron-rich minerals in the southern hemisphere, and a hotter southern mantle could indicate that Mars once had a magnetic field strong enough to produce differences in magnetism between the two hemispheres. NASA's InSight mission also previously found that seismic waves lose energy more quickly in the south, behavior that higher temperatures in the region could help account for.

The finding also carries implications for the planet's watery past. "The dichotomy that we see between north and south is important to understand because it gives information about processes that may have influenced the hydrology of Mars, including the formation of basins that may have held water," said Amirhossein Bagheri, a postdoctoral scholar at Caltech, co-author on the paper, and a former member of the InSight team. Understanding the temperature difference could provide new clues about how Mars evolved during periods when liquid water may have existed on the surface.

Researchers do not yet know what caused the thermal anomaly, and several possibilities remain under consideration. One hypothesis is that a giant impact released heat from the north. Another suggests that spontaneous convection may once have occurred within the southern Martian mantle. A third possibility is that thick geological structures in the south trapped heat and prevented it from escaping efficiently. Berne noted that such inferences give scientists a blueprint for designing future missions and scientific exploration of these worlds. The research was funded by NASA.