Mars Hidden South Is Hundreds of Degrees Hotter—And It May Hold Clues to the Red Planet’s Watery Past

Mars may look cold, dry, and lifeless today, but deep beneath its surface, the Red Planet appears to be hiding a surprising secret. Scientists have discovered that the interior of Mars’ southern hemisphere is about 200 to 400 degrees Celsius hotter than its northern half.

The finding, reported in the journal Nature, suggests that Mars’ interior is not as uniform as scientists once thought. The southern region may also be partially molten, potentially explaining several unusual features observed on the planet’s surface and beneath it.

The research was led by Alexander Berne, a Caltech alumnus who is now a postdoctoral associate at the University of Arizona. His team used decades of spacecraft observations to study tiny variations in Mars’ gravity and reveal what may be happening deep inside the planet.

Reading Mars Through Its Gravity

Scientists cannot directly drill deep into Mars to examine its interior. Instead, they rely on indirect clues, including seismic waves, magnetic measurements and gravity.

Gravity is particularly useful because it is influenced by the distribution and movement of material inside a planet. Differences in density and internal structure can produce extremely small changes in a spacecraft’s motion.

During his graduate studies at Caltech, Berne developed a model capable of using these gravitational variations to investigate the interiors of planetary bodies. His team applied this technique to Mars using data collected over several decades.

The researchers combined observations from three NASA missions: Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter.

These spacecraft experienced tiny changes in their velocities as they orbited Mars. By carefully studying those changes, scientists were able to reconstruct details of Mars’ gravitational field.

The team then used a technique known as tidal tomography.

Mars does not orbit the Sun in a perfect circle, and its rotational axis is tilted. As a result, the Sun’s gravitational influence on Mars changes slightly over seasonal timescales. These changing gravitational effects contain information about how mass is distributed inside the planet.

Tidal tomography essentially allows researchers to use these variations as a kind of internal scan of Mars.

“Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true,” Berne explained.

The new research demonstrates why that assumption can be misleading. Mars’ interior appears to have significant differences between its northern and southern hemispheres.

A Hotter Southern Hemisphere

Mars already has a dramatic difference between its northern and southern surfaces.

The southern hemisphere is dominated by ancient, heavily cratered terrain and enormous highlands, while much of the northern hemisphere consists of relatively smooth, low-lying plains.

This major difference is known as the Martian hemispheric dichotomy.

Scientists have debated for decades how this striking division formed.

The new research adds another surprising layer to the mystery: the difference is not limited to the surface.

The interior beneath Mars’ southern hemisphere appears to be significantly hotter than the interior beneath the north.

According to the study, the southern interior is approximately 200–400°C warmer. The researchers also found evidence suggesting that parts of the southern interior could be partially molten.

This discovery could help explain several other observations scientists have made on Mars.

Explaining Mars’ Magnetic Mysteries

One important clue comes from magnetic anomalies in the southern hemisphere.

Mars does not have a strong global magnetic field today like Earth. However, parts of the Martian crust—particularly in the ancient southern highlands—contain minerals that preserve evidence of an ancient magnetic field.

The newly identified thermal difference could be connected to this magnetic history.

A hotter southern mantle may have influenced conditions when Mars possessed a stronger global magnetic field. If the planet’s interior behaved differently between its northern and southern regions, this could have contributed to the magnetic differences preserved in ancient rocks.

The finding may therefore provide scientists with another piece of the puzzle surrounding the disappearance of Mars’ ancient magnetic dynamo.

Why Mars’ Seismic Waves Matter

The discovery also connects with observations made by NASA’s InSight mission.

InSight placed a highly sensitive seismometer on Mars to detect marsquakes and study how seismic waves travel through the planet.

Scientists had previously noticed that seismic waves dissipate more rapidly in the southern regions of Mars.

The new thermal model provides a possible explanation.

Hotter and partially molten material can interact with seismic waves differently from colder, solid rock. If the southern interior is substantially warmer, seismic energy could be absorbed or scattered more efficiently as waves travel through it.

In other words, two seemingly separate observations—Mars’ unusual gravity field and the behavior of its seismic waves—may be pointing toward the same hidden feature: a significantly warmer southern interior.

What Does This Mean for Ancient Water?

Perhaps the most exciting implication concerns Mars’ ancient climate and water.

Billions of years ago, Mars appears to have been very different from the cold desert world we see today. Evidence from river valleys, ancient lakebeds and minerals formed in water suggests that liquid water once existed on the Martian surface.

Understanding why Mars changed from a potentially wetter world into the dry planet of today requires scientists to understand what happened inside the planet as well.

The north-south difference in Mars’ interior could have influenced the planet’s ancient hydrology—the movement and storage of water.

According to Amirhossein Bagheri, a postdoctoral scholar at Caltech and co-author of the study, the hemispheric difference could provide information about processes that affected Mars’ hydrology, including the formation of basins that may once have contained water.

This means the heat hidden deep beneath Mars could ultimately help scientists understand where ancient water accumulated and how the planet’s surface evolved.

What Made Mars’ South So Hot?

Although the discovery is significant, scientists do not yet know exactly what created the thermal asymmetry.

The researchers are considering several possible explanations.

One possibility is that a giant impact affected Mars early in its history. A massive collision could have released enormous amounts of heat and changed the planet’s internal structure.

Another possibility involves mantle convection. Deep inside Mars, hot material can potentially rise while cooler material sinks. Such movement may have occurred differently in the southern hemisphere and could have redistributed heat.

A third idea is that unusually thick geological structures in the southern interior may have acted like an insulating layer. Instead of allowing heat to escape efficiently, these structures could have trapped excess heat beneath the surface for billions of years.

More observations will be needed to determine which explanation—or combination of explanations—is correct.

A New Way to Explore Planetary Interiors

The importance of the research goes beyond Mars.

Berne’s work demonstrates how gravity measurements can reveal three-dimensional details hidden deep inside planetary bodies. As spacecraft collect increasingly precise data, scientists may be able to build more detailed maps of the interiors of Mars and other worlds.

These maps could help researchers understand how planets form, how their interiors evolve and why their surfaces become so different over time.

For Mars, the discovery of a dramatically hotter southern interior adds a new dimension to one of planetary science’s biggest mysteries.

The Red Planet’s surface may tell the story of ancient impacts, volcanoes, rivers and climate change—but its hidden interior may contain another chapter.

And that chapter could help explain how Mars became the cold, dry world we see today, while also revealing whether its ancient environment once provided conditions suitable for life.

Mars may appear frozen and inactive from the outside, but deep beneath its surface, the planet could still be carrying the thermal fingerprints of its dramatic past.

Reference: Berne, A., Wagner, N., Matsuyama, I. et al. Tidal tomography reveals a thermal anomaly beneath Mars’s crustal dichotomy. Nature 656, 848–853 (2026). https://doi.org/10.1038/s41586-026-10893-x