Scientists detect vast thermal anomaly deep inside Mars that may explain the planet's split geography

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Scientists have discovered a thermal “anomaly” deep underground on Mars that could help explain the red planet’s divided geography and shed light on whether it once hosted alien life, according to a study published Wednesday in Nature .

Using a novel approach involving three Martian orbiters, the team peered into the interior of the planet and detected that its mantle—the layer between its crust and the core—is 400 to 750°F hotter in the southern hemisphere compared with its northern half.

The temperature anomaly could help explain the longstanding mystery of the so-called “Martian dichotomy,” the term for the bizarre difference between the Martian north and south hemispheres. Whereas Mars’ northern half is dominated by lowlands that may have once hosted a giant ocean, the southern half is covered in highlands that are miles higher in average elevation.

For decades, scientists have puzzled over whether this dichotomy originates from an ancient impact, or if it might have emerged from early internal processes in the mantle, such as convection or plate tectonics. By tracking tiny changes on Mars during its orbit around the Sun, scientists detected much higher temperatures in the planet’s southern mantle; the crustal dichotomy is at least partly a reflection of a similarly divided interior.

The origin of the dichotomy “is still an ongoing question in Mars science, and a pretty fundamental one,” said Nick Wagner, a planetary scientist at Brown University who co-authored the study, in a call with 404 Media. “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.”

To get a look under Mars’ hood, Wagner and his colleagues used a novel approach called tidal tomography. The team analyzed 16 years of tracking data from three NASA orbiters—Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter—to measure the slight gravitational wobbles of Mars during its orbit around the Sun.

The results exposed a clear asymmetry in temperatures between the hemispheres that mirrors the crustal dichotomy. While it will take a lot of future research to narrow down the exact cause of this thermal anomaly, the team suggested that it might be due to present-day activity in the mantle, known as convection, or perhaps it acts as insulation under the highlands that has lasted for billions of years.

“I was kind of surprised how sharp the result was, and how confident it was,” said Wagner. “But maybe we shouldn't be surprised, because Mars seems to be dominated by these large scale variations from the north to south, so it makes sense that these variations also extend into the mantle.”

This hypothesis is not mutually exclusive to the idea that an ancient impact may have contributed to the crustal dichotomy, and it will take more research to tease out the causes of Mars’ strange structure.

“The Mars interior is still really unknown,” Wagner noted.

Tidal tomography has been used to glimpse the interiors of Earth and the Moon, but this is the first time the approach has been applied to Mars. The technique allows scientists to remotely probe a celestial body’s interior without having to land spacecraft on the surface, which could make it “a powerful tool for future missions seeking to characterize the interiors of planetary bodies,” according to the study.

Unraveling the full complexity of Mars—from its atmosphere down to its core—will yield insights into its past and present habitability, and whether it ever hosted life. Some four billion years ago, the Martian surface was home to rivers and lakes, and perhaps an enormous ocean, providing the right conditions for life as we know it on Earth. NASA’s Perseverance rover, which is exploring a dried-up lakebed, has discovered tantalizing compounds that could have been made by ancient microbes, but could also have a geological origin.

To determine if Mars has ever been home to aliens, scientists need to reconstruct its complete evolution, including its internal dynamics.

“Habitability is a complicated thing,” Wagner said. “There's a lot of different things that feed into it: heat, chemistry, water. Earth is special for a lot of reasons, habitability included, but I think what gets undersold, potentially, is the geodynamics of it. And what I mean by that is [Earth has] plate tectonics—a way for the surface of Earth to transfer material with its interior and regulate its carbon, regulate its nitrogen and its oxygen. As far as we know, Mars doesn't have plate tectonics, and it never did.”

“I'm wondering how this anomaly plays into the idea of Mars maybe having the ability to have plate tectonics at one point, or maybe not having the ability,” he concluded. “It's another piece in this puzzle to figure out what past Mars looked like, both habitability-wise, but also just for pure science.”