Thermal Anomaly Discovered Below Mars's South Pole

Researchers have discovered a thermal asymmetry deep beneath the surface throughout Mars's southern hemisphere.

Based on gravitational measurements that give clues to the Red Planet's interior structure, Mars's interior southern hemisphere is around 200 to 400 degrees Celsius warmer than the northern half of the planet and partially molten. The surprising finding adds additional context to Martian history and the periods of time in which it may have hosted conditions favorable for life.

The research was led by Caltech alumnus Alexander Berne (PhD '26), who is now a postdoctoral associate at the University of Arizona. The findings are reported in a paper appearing in the journal Nature on August 27.

During his graduate studies at Caltech, Berne developed a model that uses variations in gravitational data to infer the structure of a planetary body's interior. Berne and his collaborators then aimed to apply his model to understanding Mars's interior. Using data collected over decades from three different Mars missions—Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter—the team measured tiny variations in these spacecrafts' velocities and used them to reconstruct the gravitational field around Mars. The gravitational forces exerted by the Sun on Mars vary over seasonal timescales as a result of Mars's slightly elliptical orbit and its tilted rotation axis. A technique called tidal tomography measures how those gravitational signatures vary over time and results in a model of the planet's interior.

"Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true," Berne says. "As we get more gravity data, we can determine the three-dimensional intricacies of a planet's interior structure. These inferences in turn give us a blueprint for designing future missions and scientific exploration of these worlds. Understanding the interior structure of planetary bodies helps us unravel the processes that shaped their formation and evolution."

On the surface, Mars is a geologically asymmetrical planet: Its southern hemisphere contains towering mountains and deep craters, whereas the northern hemisphere is composed of low flat lands. In the new study, the team was surprised to discover that Mars's interior is also thermally asymmetric—the southern hemisphere is hundreds of degrees hotter than the north.

The new observation also happens to suggest explanations for other phenomena observed on Mars, such as magnetic anomalies found in iron minerals in the south. A thermal anomaly in the southern mantle could mean that a magnetic field existed strong enough to cause magnetic differences between the north and south. Additionally, NASA's InSight mission had previously discovered that seismic waves dissipate more quickly in the south, which could be explained if the region were hotter.

"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," says Amirhossein Bagheri, a postdoctoral scholar at Caltech and co-author on the paper. Bagheri is also a former member of the InSight team.

It is still unclear what created the thermal anomaly, and there are several hypotheses for its origins, including a giant impact releasing heat from the north, past spontaneous convection in the Martian southern mantle, and thick geological features trapping excess heat from escaping.

The paper is titled "Tidal Tomography Reveals a Thermal Anomaly Beneath Mars's Crustal Dichotomy." In addition to Berne and Bagheri, co-authors are Nicholas Wagner and Harriet Lau of Brown University, Isamu Matsuyama and Angela Marusiak of the University of Arizona, Sander Goossens of NASA Goddard Space Flight Center, Karwai Cheng of the Institute of Astronomy and Astrophysics at Academia Sinica in Taiwan, Antonio Genova of the University of Rome in Italy, Marc Rovira-Navarro of the Delft University of Technology in the Netherlands, Chuan Qin of UCLA, Douglas Hemingway of the University of Texas at Austin, Shijie Zhong of the University of Colorado Boulder, and Francis Nimmo of UC Santa Cruz. Funding was provided by NASA.