A new study suggests that the mantle beneath southern Mars could be 200° to 400° Celsius warmer than the mantle beneath much of the north.
The researchers analyzed about 16 years of tracking data from NASA’s Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter.
Schematic cross-section showing the inferred thermal anomaly beneath Mars’ southern highlands.
But if southern Mars is hotter, why is some of the planet’s youngest volcanism found farther north around Cerberus Fossae?
The researchers suggest the southern highlands’ thick crust may trap rising magma underground, while compressional stresses there make eruptions harder.
Artistic rendering of Mars interior. Credit: NASA/Theophilus Britt Griswold
Contrary to Earth, present-day Mars has little to no volcanic or tectonic activity. Furthermore, almost half of the planet’s surface is more than 3.5 billion years old, which can only mean that there has been no extensive crustal recycling — a phenomenon usually powered by tectonics such as those on Earth in which surface material is recycled into the mantle — since then.
For all intents and purposes, the Red Planet is geologically deceased. Or so we were led to believe. It seems Mars may still have planetary geological processes still active.
A new study suggests that the mantle beneath southern Mars could be 200° to 400° Celsius warmer than the mantle beneath much of the north. The anomaly broadly follows the planet’s famous north-south divide, with rugged, heavily cratered highlands in the south and low plains in the north.
This means Mars is not simply a uniformly cooling ball of rock. Its interior may preserve the consequences of violent events and large-scale geological processes that began billions of years ago—and perhaps still influence the planet today.
“Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true,” lead author Alexander Berne, a planetary scientist at the University of Arizona, said in a statement.
Taking Mars’s Temperature
Berne and his colleagues could not measure heat inside Mars directly, but did the next best thing. They watched Mars deform.
As Mars moves around the Sun on its eccentric, tilted orbit, changing solar tides gently deform the planet. That movement shifts mass inside Mars, slightly changing its gravitational pull. Orbiting spacecraft respond by speeding up or slowing down by tiny amounts.
The researchers analyzed about 16 years of tracking data from NASA’s Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Some seasonal gravity changes differed by as much as 300% from what scientists would expect if Mars were internally uniform.
The technique, called tidal tomography, essentially turns a planet’s tides into a probe of its interior. A hotter mantle bends more readily than a colder, stiffer one. Researchers recently used a similar approach to uncover a roughly 100-to-200-kelvin thermal asymmetry inside the Moon, suggesting the method could become a powerful way to probe the geology of distant worlds remotely without placing seismometers on them.
The new findings make sense in earlier context. A recent analysis of marsquakes recorded during the InSight mission found that seismic waves traveling through the southern highlands lost energy more rapidly than those beneath the northern lowlands, something researchers argued could reflect a hotter southern mantle.
An Ancient Scar?
Schematic cross-section showing the inferred thermal anomaly beneath Mars’ southern highlands. Credit: Nature
What produced such an enormous temperature difference is less clear.
“There’s a bit of a chicken-or-egg problem now as to how primordial this temperature difference came to be,” study co-author Nick Wagner told Futurity.
One possibility begins with catastrophe. Scientists have long proposed that an immense early impact helped create Mars’s northern lowlands. But impact heat alone probably could not maintain a hemispheric temperature difference for billions of years, the researchers note.
Instead, the impact may have started a chain reaction. It could have altered the mantle and crust, leaving the southern hemisphere with thicker crust that subsequently acted like insulation. Large-scale mantle upwelling could also have helped maintain the heat.
But if southern Mars is hotter, why is some of the planet’s youngest volcanism found farther north around Cerberus Fossae? Previous work has even identified evidence for an active mantle plume beneath nearby Elysium Planitia.
The researchers suggest the southern highlands’ thick crust may trap rising magma underground, while compressional stresses there make eruptions harder.
The team therefore considers several possibilities—including a giant impact, large-scale mantle convection, and long-term insulation beneath the thick southern crust—rather than pinning the anomaly on a single ancient event.
Mars may look quiet from above. Deep below, its ancient history may still be playing out.
The study was published in the journal Nature.
