Scientists using a novel technique called tidal tomography have detected a massive thermal anomaly deep beneath Mars' surface: the mantle under the southern hemisphere is up to 750°F hotter than that of the north, according to a study published Wednesday in Nature. This discovery adds a crucial new clue to the decades-old puzzle of why the red planet's northern half is dominated by low-lying plains, while the south is covered in highlands miles higher in average elevation.
Researchers led by planetary scientist Nick Wagner of Brown University analyzed 16 years of tracking data from three NASA orbiters — Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter — to measure minuscule gravitational wobbles of the planet as it orbits the Sun. This allowed them to infer temperature variations deep inside Mars, a technique previously used on Earth and the Moon but applied to Mars for the first time.
The results revealed a sharp and consistent asymmetry: the mantle beneath the southern highlands is 400 to 750°F hotter than the northern mantle. This temperature difference mirrors the surface dichotomy, suggesting that the roots of the Martian divide lie deep beneath the crust.
"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," Wagner told 404 Media. The team suggests the anomaly could be caused by ongoing convection in the mantle — akin to slow boiling — or it may represent a long-lived insulating layer beneath the southern highlands that has persisted for billions of years.
The origin of the Martian dichotomy has been hotly debated for decades. One leading theory holds that a giant ancient impact blasted away the northern crust; another argues that internal processes like mantle convection or early plate tectonics sculpted the two-faced planet. The new findings do not rule out the impact hypothesis, but they provide strong evidence that internal dynamics play a major role.
Unraveling the full complexity of Mars — from its atmosphere down to its core — is key to understanding its past and present habitability and whether it ever hosted life. As Wagner noted, "The Mars interior is still really unknown." Tidal tomography could become a powerful tool for future missions seeking to characterize the interiors of other planetary bodies without the need for landers or seismometers.
The study was published in the journal Nature and relied on data from the aforementioned NASA orbiters, highlighting the value of long-duration robotic missions even after their primary objectives are complete.
Analysis
Why This Matters
- Unlocks a key chapter in Mars' geologic history: Understanding why Mars is split into two dramatically different hemispheres is fundamental to piecing together the planet's formation and evolution. The thermal anomaly provides a new constraint on models of planetary differentiation and mantle dynamics.
- Implications for past habitability: A hot, dynamic interior could drive volcanic activity, outgassing, and hydrothermal systems — key ingredients for sustaining liquid water and potentially life. This discovery may help narrow down where to search for biosignatures.
- Methodological milestone: Tidal tomography on Mars opens the door to probing the interiors of other rocky bodies without expensive and risky landings. This could revolutionize the study of moons, asteroids, and even exoplanets in the future.
Background
The Martian dichotomy was first revealed by the Mariner 9 mission in the 1970s. The northern hemisphere is flat and low, covered in smooth plains that some scientists believe were once the floor of a vast ocean. The southern hemisphere is rugged, heavily cratered, and stands up to 8 km higher in elevation. For 50 years, two primary hypotheses have battled for acceptance: the "impact hypothesis" — a single giant impact or multiple large impacts — and the "endogenic hypothesis" — internal processes such as mantle convection, plumes, or early plate tectonics. This new study adds weight to the endogenic side, though many scientists remain cautious.
Key Perspectives
[Lead author Nick Wagner (Brown University)]: The team sees the finding as a strong piece of evidence for an internally driven asymmetry. Wagner emphasized that the result was "sharper and more confident" than expected, reinforcing the idea that Mars' surface dichotomy reflects a deep-seated mantle structure. He acknowledges the impact hypothesis remains possible, but the thermal data suggests internal processes are at least a major contributor.
[Proponents of the impact hypothesis]: Some planetary scientists argue that a giant impact could still have set off the asymmetry, triggering long-lived mantle convection patterns that persist today. The new data does not rule out this scenario; indeed, the heat anomaly may be a secondary effect of an ancient collision that thickened the southern crust and insulated the mantle.
[Skeptics and independent experts]: While praising the novel technique, several researchers caution that tidal tomography on Mars is still in its infancy. Inferring temperature from gravitational wobbles requires many assumptions about mantle composition and structure. Cross-validation with seismic data (from the InSight lander) would strengthen the conclusions. Others note that the three-orbiters dataset spans 16 years, which is a long but still limited window for measuring subtle wobbles.
What to Watch
- Future tidal tomography studies on Mars: Are the results reproducible with newer orbiters (e.g., MAVEN, Tianwen-1)? A second independent analysis could confirm the anomaly.
- Seismic data from InSight: Although InSight's mission has ended, continued analysis of its data for mantle structure could either support or challenge the findings.
- Upcoming missions: NASA's Mars Sample Return and ESA's Rosalind Franklin rover could target the dichotomy boundary for direct rock sampling, potentially revealing the cause of the asymmetry.
- Application to other bodies: Watch for proposals to use tidal tomography on Venus, the Moon, or the icy moons of Jupiter and Saturn.