Mars May Have a Hundreds-of-Degrees-Hotter Southern Interior

Mars’ dramatic north-south split may extend far below its surface. A study reported by Smithsonian Magazine suggests that the planet’s southern interior is hundreds of degrees hotter than its northern half and may be partially molten. The finding comes from a model built using variations in Mars’ gravitational field rather than seismic measurements alone. Researchers examined 16 years of data from three orbiters—the Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Small changes in the spacecrafts’ velocities helped reveal fluctuations in the planet’s gravity, which can provide clues about its internal structure. According to the study, published August 26 in Nature, the southern hemisphere’s interior is approximately 400 to 750 degrees Fahrenheit warmer than the northern hemisphere. The modeled temperature boundary also closely follows the visible division between Mars’ low, flat northern plains and its heavily cratered, mountainous south. Most previous models treated Mars as broadly uniform because much of what scientists know about its interior comes from NASA’s InSight lander, which operated near the equator from late 2018 through late 2022. The new approach offers a way to examine differences across the planet that a single seismic station could not capture. The proposed thermal imbalance could help account for several other observations. Magnetic anomalies have been detected in iron-bearing minerals in the south, potentially preserving evidence of an ancient magnetic field. Seismic waves recorded by InSight also travel faster through the southern region, something the researchers say could be related to higher temperatures underground. The difference may also bear on Mars’ ancient hydrology. Scientists increasingly think the planet once had substantial water, and the north-south division could preserve clues about how basins formed and whether they held water—an important consideration in studies of Mars’ past habitability. What created the heat difference remains unsettled. Possibilities discussed in the report include a giant impact that allowed the north to lose heat, hot material rising from the core into the southern mantle, or thick southern geological structures slowing heat loss. Mars’ southern crust is reported to be about 15.5 miles thicker on average than the northern crust. Researchers not involved in the study noted that questions remain, including why Mars’ largest volcanic province lies near the equator rather than directly above the proposed hotter region. The thermal model adds another piece to the ongoing effort to reconstruct Mars’ interior and geological past.