Mercury doesn’t care about your expectations.
For decades, astronomers assumed the solar system’s innermost planet was geologically dead—a cratered husk baked into submission by the Sun’s relentless radiation. Then NASA’s MESSENGER spacecraft, which orbited Mercury from 2011 to 2015, started sending back images that made planetary scientists do a collective double-take. Smooth plains. Strange pit craters. Volcanic vents that looked suspiciously like they’d been active relatively recently in geological terms. Which, granted, could mean a billion years ago, but still.
When a Planet That Should Be Dead Decides to Get Volcanic Anyway
Here’s the thing: Mercury shouldn’t have volcanoes. It’s tiny—barely larger than Earth’s Moon—and conventional wisdom says small planets cool down fast, their internal heat engines sputtering out like dying campfires. Mars lost most of its volcanic mojo billions of years ago. The Moon’s last volcanic hiccup happened around 1 billion years ago, maybe less. Mercury should have been cold and dormant since the Hadean eon.
Turns out the universe doesn’t read our textbooks.
MESSENGER found volcanic plains covering roughly 6% of Mercury’s surface, mostly in the northern hemisphere. These aren’t towering stratovolcanoes like Mount Fuji or explosive calderas like Yellowstone. They’re flood basalts—vast sheets of lava that oozed out from fissures and spread across hundreds of thousands of square kilometers. The Caloris Basin, a 1,550-kilometer-wide impact scar, is surrounded by volcanic deposits that flowed out after the asteroid collision that created it around 3.9 billion years ago.
The Chemistry Doesn’t Make Sense But Here We Are
Wait—maybe the weirdness goes deeper than just “unexpected volcanoes.” Mercury’s volcanic rocks have a chemical composition that planetary geologists find genuinely baffling. Earth’s volcanoes spew basalt rich in iron and magnesium. Mercury’s lavas? They’re loaded with sulfur and have unusually low iron content compared to what you’d expect from a planet with such a massive iron core. Some volcanic vents detected by MESSENGER’s spectrometers showed sulfur concentrations reaching 4% by weight—way higher than terrestrial basalts.
This suggests Mercury’s mantle chemistry is fundamentaly different from Earth’s, possibly because the planet formed closer to the Sun where temperatures were high enough to vaporize volatile elements that would normally get incorporated into planetary building blocks.
The volcanic pits are particularly strange. MESSENGER identified dozens of irregular depressions, some up to 30 kilometers across, with flat floors and no obvious impact origin. They look like collapse features—what happens when underground magma chambers empty out and the roof caves in. On Earth, we call these calderas, and they’re unmistakable volcanic signatures.
Maybe Mercury Was Volcanically Active More Recently Than Anyone Wants to Admit
The age estimates for Mercury’s volcanic plains cluster around 3.5 to 4 billion years old, based on crater counting techniques. But some regions show surprisingly few impact craters, which could mean—could mean—they’re younger. How much younger? That’s the billion-dollar question BepiColombo, the European Space Agency and Japan’s joint mission that arrived at Mercury in 2021, is trying to answer with higher-resolution imaging and more sophisticated spectrometers.
If Mercury had volcanic activity within the last billion years, that rewrites our understanding of how small planetary bodies retain internal heat. Tidal heating from the Sun? Radioactive decay lasting longer than models predict? Core crystallization releasing latent heat? Nobody knows for sure, but the implications ripple outward to every rocky world we study.
The Volcano That Isn’t Really a Mountain at All
Mercury’s volcanoes don’t build towering peaks. They don’t have the dramatic profiles of Earth’s stratovolcanoes or Mars’s Olympus Mons. The lava was too fluid, the eruptions too effusive. Instead, you get something more like Iceland’s fissure eruptions or the Columbia River Basalts that flooded eastern Washington with lava flows up to 400 meters thick around 15 million years ago.
One particularly intriguing feature is the so-called “ghost craters”—ancient impact basins that were completely flooded by lava, leaving only faint circular outlines visible in elevation data. It’s like geological crime scene investigation, trying to reconstruct what happened billions of years ago based on subtle topographic clues.
What This Means for Planets Nobody Thought Could Have Volcanoes
Mercury’s unexpected volcanism forces us to reconsider other small worlds. The Moon might have had volcanic activity more recently than the 1-billion-year-old samples Apollo astronauts brought back. Some lunar volcanic deposits might be only 50 million years old—practically yesterday in geological time. Asteroids like Vesta show signs of ancient volcanism. Even icy moons in the outer solar system have cryovolcanoes spewing water and ammonia instead of molten rock.
The solar system is volcanically weirder than we gave it credit for. Mercury just had the audacity to prove it first, one sulfur-rich lava flow at a time.
