Mars's Interior Reveals a Striking Temperature Divide Between North and South

A Thermal Divide Beneath Mars's Surface

New research has uncovered a dramatic temperature difference deep within Mars, suggesting that the planet's southern hemisphere interior is hundreds of degrees Celsius warmer than its northern half. The findings, published in the journal Nature, offer a fresh perspective on the Red Planet's internal structure and its long history.

Using gravitational measurements from three Mars missions, scientists have determined that the interior beneath Mars's southern hemisphere is about 200 to 400 degrees Celsius warmer than the north and may be partially molten. The discovery adds context to the planet's evolution, including periods when it might have hosted conditions suitable for life.

The study was led by Caltech alumnus Alexander Berne (PhD '26), now a postdoctoral associate at the University of Arizona, and appears in the August 27 issue of Nature.

Reading Mars Through Gravity

Berne developed a model that uses subtle variations in gravitational data to infer the internal structure of a planetary body. He and his collaborators applied this approach to Mars, analyzing decades of observations from three spacecraft: Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter. By tracking extremely small changes in the spacecrafts' velocities, the team reconstructed the gravitational field around Mars.

The technique, known as tidal tomography, relies on the fact that the Sun's gravitational pull on Mars changes over the seasons because the planet travels along a slightly elliptical orbit and rotates on a tilted axis. By measuring how these gravitational signatures vary over time, researchers can build 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 said. "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."

A Deep North-South Divide

Mars has long been known to look strikingly different across its two hemispheres. The southern surface is characterized by towering mountains and deep craters, while the northern hemisphere consists of low-lying flatlands. This surface asymmetry, known as the Martian dichotomy, has puzzled scientists for decades.

The new findings indicate that this divide extends deep into the planet's interior. The temperature difference in the mantle — the layer between the crust and the core — is so significant that it may help explain the dichotomy, which could have originated from an ancient impact or from internal processes such as mantle convection.

The discovery also offers clues about other Martian phenomena. Scientists suggest that a thermal anomaly in the southern mantle could account for magnetic anomalies found in iron minerals in the south, potentially indicating that a strong magnetic field once existed there. Additionally, NASA's InSight mission previously found that seismic waves dissipate more quickly in the southern region, a pattern that could be explained if the area were hotter.

Unanswered Questions

While the findings provide new evidence, the ultimate cause of the thermal anomaly remains unclear. The study does not settle the origin of the Martian dichotomy, but it adds another line of evidence to the ongoing investigation.

Nick Wagner, a planetary scientist at Brown University and a co-author of the study, noted that the dichotomy remains "still an ongoing question in Mars science, and a pretty fundamental one." He added, "This study doesn't provide an answer to it, but adds another line of evidence to figure out what is actually going on underneath Mars."

Possible explanations for the temperature difference include a giant impact that released heat in the north, ongoing mantle convection, or thick geological features that trap heat beneath the southern highlands.

Broader Implications

The thermal asymmetry could also shed light on the planet's hydrological history. Berne pointed out that understanding the north-south dichotomy is important because it provides information about processes that may have influenced the formation of basins that could have held water, and thus the potential for life.

The research was funded by NASA and involved collaborators from multiple institutions, including Brown University, the University of Arizona, NASA Goddard Space Flight Center, Caltech, and others.

While the study primarily focuses on Mars's interior, it underscores a fundamental shift in how scientists approach planetary science. Berne's model demonstrates that planetary interiors are not necessarily uniform, opening new avenues for understanding other worlds, including those beyond our solar system.