New research into the interior of Mars further emphasizes that the Red Planet is a world of two very different halves.
Deep inside the southern hemisphere of Mars, the temperature is up to 750 degrees Fahrenheit (400 degrees Celsius) warmer than beneath the northern hemisphere, deepening the mystery of the strange dichotomy between the Red Planet's flattened north and craggy southern highlands.
"Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true," said Alexander Berne, formerly of Caltech and now at the University of Arizona, in a statement. Berne is the lead author of a new research paper revealing the discovery of the strange southern hotspot.
Berne and his team went back and looked at archival data from three NASA spacecraft – Mars Global Surveyor, Mars Odyssey and Mars Reconnaissance Orbiter – to look for subtle changes in their orbital velocity that might reflect variations in Mars's gravitational field. Using a technique developed by Berne called 'tidal tomography' that also accounts for the sun's varying gravitational influence on Mars as the Red Planet progresses around its elliptical orbit, he and his team were able to infer a model of the interior structure and temperature of Mars.
How the gravitational field that the various spacecraft experienced changed over time differs substantially from what would be expected were Mars's interior perfectly spherically symmetrical. To explain this difference, the mantle beneath Mars's southern hemisphere must be between 390 and 750 degrees Fahrenheit (200 and 400 degrees Celsius) warmer than the mantle in the north, and indeed it is so warm that it could still be partially molten. This could have repercussions for both Mars's habitability and how long Mars remained volcanically active.
Perhaps we shouldn't be surprised that Mars's interior is different in the south compared to the north, because we already know that its surface is asymmetrical. The north of Mars is characterized by lowland plains, while the planet's crust in the south is on average 15.5 miles (25 kilometers) thicker than in the north and is covered by cratered highlands. The northern lowlands may have once been home to a large ocean.
"The dichotomy that we see between north and south is important to understand because it gives information about processes that may have influenced the hydrology of Mars, including the formation of basins that may have held water," said Caltech's Amirhossein Bagheri, who is the second author of the research.
A hotter southern mantle could also explain some other puzzling discoveries on Mars. NASA's InSight lander, which ended its mission in December 2022 and which had a seismometer on board, found that seismic waves dissipate more quickly in the south, which can be explained by the hotter temperature there. Additionally, magnetic anomalies found in iron-bearing minerals in the south now make more sense.
Today, Mars lacks a global magnetic field, but over four billion years ago it had one. If upwelling from a warmer southern mantle heated the crust in excess of the 'Curie temperature', which is the temperature at which materials lose their inherent magnetic properties and any residual magnetism in them is induced, it could have resulted in creating the mysterious magnetic remnants in the iron-bearing minerals.
"Understanding the interior structure of planetary bodies helps us unravel the processes that shaped their formation and evolution," said Berne, although the origin of Mars's north–south divide is still up in the air. It's not even clear whether the surface differences have the same causes as the interior differences.
The leading contender is that it's all the result of an ancient giant impact over four billion years ago and that the northern lowlands are therefore actually an enormous impact basin. By gouging out a huge hole in the north, the impact would have facilitated the release of a lot of internal heat, allowing the northern mantle to cool faster than the south.
Alternatively, the thicker crust in the south could have proven to be an efficient lid, preventing the leaking of much heat from the mantle. This can actually be tested – magma upwelling would have stalled as it encountered the thick, dense crust, creating melt intrusions beneath the surface that could be inferred in higher-resolution gravity data.
These are questions for future missions, but the tidal tomography technique could be employed for other worlds too, from Mercury to the large moons of Jupiter.
"As we get more gravity data, we can determine the three-dimensional intricacies of a planet's interior structure," said Berne. "These inferences in turn give us a blueprint for designing future missions and scientific exploration of these worlds."
The discovery of Mars's thermal anomaly was reported on Aug. 27 in Nature.
