Scientists Discover Vast Underground 'Anomaly' on Mars

Researchers have detected a vast thermal anomaly deep beneath the Martian surface, finding that the planet's southern mantle is 400 to 750°F hotter than its northern counterpart, according to a study published Wednesday in Nature. The finding adds a new layer of evidence to one of planetary science's longest-running puzzles.

The team used a technique called tidal tomography — analyzing 16 years of tracking data from three NASA orbiters, Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter — to measure subtle gravitational wobbles as Mars orbits the Sun, per 404 Media. By parsing those tiny shifts, scientists were able to map temperature variation in the planet's mantle, the layer sitting between the crust and the core. The approach has previously been applied to Earth and the Moon, but this marks its first use on Mars.

The thermal asymmetry mirrors what's visible on Mars' surface — the so-called "Martian dichotomy," in which the northern hemisphere is dominated by low-lying plains that may once have held a vast ocean, while the southern hemisphere is blanketed by highlands sitting miles higher in average elevation. "The origin of the dichotomy is still an ongoing question in Mars science, and a pretty fundamental one," Nick Wagner, a planetary scientist at Brown University and co-author of the study, told 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."

Scientists have debated for decades whether the dichotomy originated from a massive ancient impact or from early internal processes such as mantle convection or plate tectonics. The study does not resolve that question, but it does establish that the crustal divide has a thermal counterpart extending deep into the interior. The team suggested the anomaly could stem from ongoing mantle convection, or from the highlands acting as long-term thermal insulation — possibilities the authors say are not mutually exclusive with an impact origin.

Wagner told 404 Media he was surprised by how sharp and confident the result turned out to be, though he noted it may follow logically given Mars' north-south dominance at the surface. "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," he said.

The findings carry implications beyond geology. Understanding Mars' internal dynamics is directly relevant to questions of past habitability: roughly four billion years ago, the Martian surface hosted rivers, lakes, and possibly an ocean. NASA's Perseverance rover, currently exploring a dried lakebed, has discovered compounds that could have been produced by ancient microbes, though a geological origin remains possible, per 404 Media. Wagner pointed to the absence of plate tectonics on Mars — a mechanism Earth uses to cycle carbon, nitrogen, and oxygen between its surface and interior — as a key factor in evaluating whether the planet could ever have sustained life.

The study describes tidal tomography as a potentially powerful tool for future planetary missions, since it allows interior probing without landing on a surface. Wagner said the anomaly raises further questions about whether Mars ever had the internal conditions for plate tectonics, calling it "another piece in this puzzle to figure out what past Mars looked like."