Mars Has Been Hiding a Massive Hot Spot Under Its Southern Half

Keypoints:

• • Gravity measurements reveal Mars's southern interior runs 200 to 400 degrees Celsius hotter than its north
  • Part of the southern mantle may still be partially molten today
  • The research was led by Caltech alumnus Alexander Berne, now at the University of Arizona
  • Findings were published August 27 in the journal Nature
  • The temperature gap may explain Mars's mysterious southern magnetism and faster-fading seismic waves

Mars has been quietly running two different internal temperatures for billions of years, and nobody had the tools to notice until now. New gravity measurements show the planet's southern interior sits roughly 200 to 400 degrees Celsius hotter than its northern half, hot enough that part of the southern mantle may still be partially molten today.

The discovery came from a new modeling approach, not a new spacecraft. Alexander Berne, who led the research as a Caltech graduate student and now works as a postdoctoral associate at the University of Arizona, built a method that uses subtle shifts in a planet's gravity field over time to infer what's happening deep inside it. 

Applying that model to years of accumulated Mars data revealed an asymmetry nobody had confirmed before, since researchers had generally assumed planetary interiors stayed roughly symmetrical without strong evidence to check that assumption against.

The finding helps explain a handful of Martian mysteries that had been sitting unconnected for years. NASA's InSight lander, before its mission ended in 2022, had already noticed that seismic waves faded faster when they traveled through the southern hemisphere, a quirk that makes far more sense if that region genuinely runs hotter. 

Strange magnetic signatures locked into iron-rich minerals across the south point the same direction, since a hotter mantle pushing crust above a certain temperature threshold, called the Curie point, would erase and later reset the magnetic imprint left by Mars's ancient global magnetic field.

The temperature gap likely traces back over four billion years, to an ancient, giant impact that many scientists believe carved out Mars's northern lowlands in the first place. That kind of impact would have released enormous heat into the mantle beneath it, causing the northern interior to cool off faster than the south ever since. 

Beyond explaining old puzzles, the discovery also reshapes how researchers think about Mars's ancient hydrology, since a hotter, more geologically active south may have played a direct role in forming the basins that once may have held liquid water.