Orbiter tracking data expose hot region in mantle beneath southern Mars

Satellites orbiting the Moon and Mars have given scientists a wealth of information about the surfaces of those worlds, from traces of water on the Moon to potentially habitable surface environments on Mars. Using the same tools to understand what lies beneath the surface has been much harder.

A technique called tidal tomography, developed by Brown University researchers, is now allowing scientists to use existing satellites to extract key information from deep inside planetary interiors.

In a study published in the journal Nature, a team led by Alexander Byrne of the University of Arizona used the technique to show that Mars has a massive heat anomaly in the mantle beneath its southern hemisphere. The study suggests the hot region is 200 to 400 degrees Celsius warmer than the rest of the planet, a finding that sheds new light on how Mars has evolved over the past 4 billion years.

Two Brown co-authors of the study have played central roles in developing and applying tidal tomography. Harriet Lau, an associate professor in the Department of Earth, Environmental and Planetary Sciences at Brown, previously used the technique to reveal new details of Earth's interior. Working with Nick Wagner, a postdoctoral researcher at Brown, she has adapted it for Mars, and the two hope it will soon reveal new details about the Moon as well.

How the technique works

Wagner explained that as Mars travels around the Sun, the Sun's gravity gently stretches and squeezes the planet. Because the orbit of Mars is not a perfect circle, the strength of that pull changes over a Martian year, and so does the amount by which Mars deforms. The deformation is called a tidal bulge.

The tidal bulge causes mass inside the planet to shift slightly, producing tiny changes in its gravitational pull. An orbiter passing over a region of slightly stronger gravity speeds up a little, and one passing over slightly weaker gravity slows down. The measured speed changes of orbiters can therefore be used to estimate how much Mars is deforming.

The study used about 16 years of data from three separate spacecraft. Once the response of the planet to these gravity changes is known, Wagner said, the interior state that causes it can be worked out mathematically. A hotter or less rigid interior deforms more easily, while a colder or more rigid interior resists deformation. The team found that the southern hemisphere of Mars would need to be about 20 percent less rigid than the northern part, which they infer is due to a difference in temperature.

Why planetary interiors matter

Wagner said learning what made Earth different from the other planets helps explain many things, including how life arose, and that understanding the structure and formation of other planets helps in understanding Earth, an approach known as comparative planetology. The reasoning also runs the other way: working out the interior of Mars can help explain why it looks so different from Earth. Mars once had water on its surface, and one large factor in why water flowed then but not now is the total heat engine of the planet, on which the present difference in internal heat may shed light.

What it says about the history of Mars

Wagner said there is a chicken-or-egg problem over how the temperature difference arose. Mars may have formed with the asymmetry in place, which then governed its geodynamic evolution, or something may have happened early in its history to cause it. One hypothesis holds that the northern hemisphere was struck by a large impactor early on, and the question is whether that impact could have left a mark lasting to the present. Alternatively, the southern hemisphere may have thicker crust than the north, insulating the mantle there and keeping it warmer. Both are possible, he said, and separating them is the next big step.

Comparison with Earth

Earth has comparable features. Using a very similar method, but with data from GPS stations instead of satellites, Lau helped constrain the buoyancy of two large regions beneath the Pacific Ocean and Africa called the Large Low Shear Velocity Provinces. Those have a compositional component as well as a temperature difference. Wagner said he thinks the Martian anomaly may also have a compositional component, but a follow-up study will be needed.

Next steps

For Mars, Wagner said, the two theories for the origin of the difference need to be disentangled, along with what they mean for the formation and evolution of the planet. The method is relatively new and is proving powerful for measuring internal variations in planets, and the team is working on applying it to the Moon.

CONTACT: https://www.nature.com/articles/s41586-026-10893-x