Deep beneath Mars, the planet’s southern hemisphere appears to be roughly 200 to 400 degrees Celsius hotter than the north, with evidence suggesting that part of the southern interior is molten. The finding comes from an analysis of tiny changes in Mars’ gravitational field and points to a strong thermal difference between the two halves of the planet.
The evidence comes from gravitational measurements collected over decades by three Mars missions: Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter. Researchers examined tiny changes in the spacecrafts’ velocities and used them to reconstruct Mars’ gravitational field.
The analysis was led by Alexander Berne, a Caltech alumnus who is now a postdoctoral associate at the University of Arizona. During his graduate work at Caltech, Berne developed a model for using changes in gravity to infer the internal structure of planetary bodies.
The team applied that approach to Mars using a technique called tidal tomography. Mars’ orbit around the Sun is slightly elliptical, and its rotation axis is tilted. As a result, the Sun’s gravitational influence on Mars changes over seasonal timescales. By tracking how those gravitational signatures change, researchers can build a model of what lies inside the planet.
The approach also allows researchers to look beyond the assumption that a planet’s interior is evenly structured in all directions.
“Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true,” Berne says.
The southern interior is much hotter
Mars already has a pronounced difference between its northern and southern surfaces. The south contains high mountains and deep craters, while the north consists largely of low-lying flatlands.
The new analysis indicates that the difference extends below the surface. The southern interior is estimated to be about 200 to 400 degrees Celsius warmer than the northern half, and the southern region appears to be partially molten.
That thermal difference may also help account for other observations from Mars.
Iron minerals in the southern hemisphere contain magnetic anomalies. The researchers suggest that a hotter region in the southern mantle could have been associated with a magnetic field strong enough to produce differences in magnetism between the planet’s northern and southern regions.
The finding may also be consistent with observations from NASA’s InSight mission. Seismic waves were previously found to dissipate more quickly in the southern part of Mars, something that could be explained by higher temperatures there.
Several explanations remain possible
The researchers do not yet know what produced the thermal difference. They identify several possible explanations.
One possibility is that a giant impact released heat from the northern part of Mars. Another is that convection once occurred spontaneously in the southern mantle. A third possibility is that thick geological structures trapped heat and prevented it from escaping.
The difference between Mars’ northern and southern interiors may also provide information about processes that affected the planet’s hydrology, including the formation of basins that could have held water, according to Amirhossein Bagheri, a postdoctoral scholar at Caltech and co-author of the study who was formerly part of the InSight team.
The study was published in Nature.
