Mars Is Hundreds of Degrees Hotter on One Side, Study Finds

Half of Mars is quietly running hundreds of degrees hotter than the other half, according to a study published this week in the journal Nature — and nobody is entirely sure why.

Researchers led by Alexander Berne, a Caltech-trained planetary scientist now at the University of Arizona, found that the interior of Mars's southern hemisphere sits roughly 200 to 400 degrees Celsius warmer than the north, and is partially molten. The team didn't drill into the planet or land a new probe to find this. They read it in the wobble.

Mars has looked lopsided since the first orbiters got a good look at it: towering highlands and deep craters in the south, low flat plains in the north. Planetary scientists call that split the "crustal dichotomy," and for decades the assumption was that it was mostly a surface story — a scar from an ancient impact or some early volcanic episode, sitting on top of an interior that behaved more or less like a sphere. Berne's team just found that assumption doesn't hold. The asymmetry goes all the way down.

"Scientists usually assume that the interiors of planetary bodies are generally spherically symmetric, but this is not necessarily true. As we get more gravity data, we can determine the three-dimensional intricacies of a planet's interior structure."

Alexander Berne, lead author, in a statement from Caltech

How do you take Mars's temperature without landing on it?

The method is called tidal tomography, and Berne developed it during his graduate work at Caltech. Mars orbits the sun on a slightly elliptical path, tilted on its axis, which means the sun's gravitational pull on the planet flexes and releases in a regular seasonal rhythm — a tide, essentially, running through solid rock instead of ocean water. How a planet's interior responds to that flexing depends on what it's made of and how hot it is.

To measure the response, the team didn't need a new mission at all. They used decades of tracking data from three spacecraft already in the Mars archive — Mars Global Surveyor, Mars Odyssey, and the Mars Reconnaissance Orbiter — measuring tiny variations in each craft's velocity as Mars's gravity tugged on it. Small shifts in a spacecraft's speed reveal small shifts in the mass distribution beneath it. String enough of those measurements together over enough years, and a three-dimensional map of the interior starts to emerge, layer by layer, without ever touching the ground.

Does this explain anything else that's been puzzling about Mars?

It might explain two things researchers had already noticed but couldn't fully connect. Iron minerals in Mars's southern crust carry a stronger magnetic signature than those in the north — a hotter, more active southern mantle could account for the difference, since heat and past magnetic field strength are closely linked in planetary interiors. Separately, NASA's InSight lander, which spent four years listening to marsquakes before its mission ended, had already clocked something odd: seismic waves lose energy faster as they pass through the south than the north. A warmer south is exactly the kind of thing that would slow those waves down.

Amirhossein Bagheri, a Caltech postdoctoral scholar and co-author who previously worked on the InSight team, said the temperature split matters beyond geology. 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.

So what actually caused it?

Nobody knows yet, and the paper is candid about that. Berne's team lays out three competing explanations, none ruled out. One: a colossal early impact gouged out the northern lowlands and, in the process, shed heat that left the north cooler ever since. Two: the southern mantle underwent its own slow-motion convection, churning warmer material up from deeper inside the planet over billions of years. Three: the thick southern crust itself acts like insulation, trapping radioactive heat generated deep inside Mars the way a blanket traps body heat, while the thinner northern crust lets that same heat escape more freely.

Untangling which of those three actually happened will take more than gravity data. It's the kind of question a future orbiter or a longer-lived seismic network could chip away at — Berne's group has already framed the finding as a "blueprint for designing future missions," a rare case of old spacecraft data pointing directly at what the next mission ought to go looking for.

The study, first reported by Phys.org, drew on twelve researchers spread across ten institutions in the United States, Taiwan, Italy and the Netherlands, funded by NASA — a reminder that answering a question about a dead planet's insides doesn't require landing on it. Sometimes it just requires watching how it wobbles for long enough.

NASA has already picked a contractor for a future Mars orbiter mission, one of several efforts that could eventually carry instruments sensitive enough to test Berne's three hypotheses directly against each other.