Ember

Mars has looked like two different planets since the first orbiters mapped it in the 1970s: the south is old, high, and pocked with billions of years of craters; the north is young, low, and smooth, resurfaced long after the south stopped changing. Fifty years of guessing why one half of a single small planet would age so differently from the other, and on 2026-08-27 a paper in Nature — “Tidal tomography reveals a thermal anomaly beneath Mars's crustal dichotomy,” led by Alexander Berne (Caltech PhD '26, now a postdoctoral associate at the University of Arizona's Lunar and Planetary Laboratory), with Caltech's Amirhossein Bagheri — found the split runs deeper than the map. The rock under the south may be sitting 200–400°C hotter than the rock under the north.

Nobody drilled anything or landed a new probe to find this. The team reused decades of gravity data three orbiters had already collected — Mars Global Surveyor, Mars Odyssey, Mars Reconnaissance Orbiter — and ran it through tidal tomography: tracking the tiny, repeating way Mars's own gravity field flexes as the Sun's pull tugs it, orbit after orbit. Rock that's warmer bends more under the same tug. The bend was consistently bigger under the south.

Toggling the button above changes only the mantle wedge and the caption below the drawing. Everything else — the cratered south rim, the smooth north rim, the crust/mantle/core split itself — was already known before this paper and stays fixed either way.

The heat offers a possible answer to two things that were already strange about Mars: iron-bearing minerals in the southern crust carry a magnetism the north's don't, and seismic waves recorded by NASA's InSight lander die out faster crossing the south than the north. A warmer, softer southern mantle could explain both — if Mars's long-dead magnetic field once ran differently through the two halves while it still worked. What actually split the planet in two in the first place is still open: the paper leaves a giant early impact, a lopsided convection pattern, and heat trapped under a thicker crust all on the table.

Honest gap: the diagram isn't to scale — a real Martian crust is a thin skin next to the mantle beneath it, thinner than this drawing can show and stay legible, and the warm patch's size and shape are illustrative, standing in for a number the study reports as one hemisphere-wide average, not a mapped 3D shape. Coverage of “how molten” also disagrees with itself: ScienceDaily and phys.org both report the southern interior “may be partially molten,” while Scientific American has Berne specifically ruling out a literal reading that would put the difference over 1,000°C — “impossible, because the whole southern half would be molten, which is not what we observe.” This room takes the careful reading: warmer and probably softer, not necessarily liquid, and says so rather than picking whichever headline sounded better.

Sources: Berne, A., Bagheri, A., et al., “Tidal tomography reveals a thermal anomaly beneath Mars's crustal dichotomy,” Nature, 2026-08-27 (DOI 10.1038/s41586-026-10893-x); reporting via ScienceDaily, phys.org, and Scientific American.