Vigorous volcanic eruptions cool planets down. But larger worlds, like Earth and Venus, take longer to release their internal heat, allowing them to stay volcanically active for longer. On average, Earth has about 20 known volcanoes erupting on any given day. We can’t see through Venus’s thick clouds, but thanks to a vast amount of circumstantial evidence, scientists think it’s almost certainly still erupting.
Smaller worlds seem to lose their fuel faster. In the four centuries since humans first pointed their telescopes at Mars, no eruption has been seen, leading to the assumption that it’s volcanically dead.
But the estimates of when, exactly, Mars went dark have shifted in recent years. One 2017 study found that lava flows in the cauldron atop Arsia Mons, another gigantic volcano, could have appeared 50 million years ago or perhaps even more recently, long after the Tyrannosaurus rex died out on Earth.
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And then there’s Cerberus Fossae.
In 2019, several seismic events, or “marsquakes,” were tracked back to Cerberus Fossae (“rifts of Cerberus,” named after the terrifying three-headed hound that guards the entrance to Hades in Greek mythology). Covered in crevasses hundreds of miles long, this place became the first active fault zone found on Mars. And it’s a seriously strange part of the planet.
The best way to estimate the age of a planet’s surface is to analyze its rocks, using the radioactive decay of its elements as a geologic clock. But for the time being, the only Martian rocks we have on Earth arrived in the form of meteors, and any meteor could have come from anywhere on Mars’s surface. So researchers have to rely on counting craters.
Crudely put: If a planet like Mars doesn’t have any major erosional processes, then older surfaces should show more impact craters from asteroid hits. Crater counting can’t give scientists a surface’s absolute age, but it gives a relative age.
Cerberus Fossae is notably lacking in craters. “It’s definitely amongst the youngest surfaces on the planet,” Gregg says.
Early orbital reconnaissance of Cerberus Fossae suggested that not too long ago, in geologic terms, some sort of fluid washed over the area. For a while, scientists couldn’t tell if it had been water or lava. But after analyzing images taken by higher-resolution cameras in the past two decades, Gregg says, “we were able to determine that both of those things came out of Cerberus at different times.”
One of these apparent outpourings of lava, the Athabasca Valles flood lava, erupted less than 20 million years ago, says Hamilton. That torrent of lava, erupting in just a few weeks, would have covered an area larger than the United Kingdom.
A paper that Hamilton co-authored, currently undergoing revisions for publication in the journal Icarus, looked at a smaller volcanic deposit at Cerberus Fossae. This deposit may have resulted from an explosive eruption that blanketed the area in ash. Careful crater counting suggests it could be as old as 1 million years, or as youthful as 53,000 years—yesterday, in planetary terms.