Saturn‘s moon Titan is unique in our solar system, being the only natural satellite to boast a significant atmosphere, somewhat like Earth’s.
Also like Earth, Titan has bodies of liquid on its surface that support processes akin to our water cycle—the huge moon has clouds, spring rains, and fog—and even shows signs of a lake effect similar to the one seen over North America’s Great Lakes.
The key difference, of course, is that the liquid on chilly Titan is methane, a carbon-based chemical that, on our world, is the prime component in natural gas.
Still, Titan’s hydrocarbon haze is exciting to scientists who are hoping to get a glimpse of how life might have been sparked on Earth: Lab experiments, for example, suggest that the moon’s atmosphere may be flush with the building blocks of life, such as amino acids and DNA bases.
And now, for the first time, scientists say they have proof that early Earth had a very Titan-like atmosphere … at least periodically.
On today’s Earth, dry air contains roughly 78% nitrogen and 21% oxygen, with trace amounts of other gases. But ancient rocks show that, before about 2.5 billion years ago, atmospheric oxygen was a scarce commodity on our planet.
The widely held theory has been that before the so-called Great Oxygenation Event, Earth’s atmosphere was dominated by Titan-esque organics—but no one had yet found evidence for such a chemical makeup in the planet’s rocky history.
In a study published online yesterday in Nature Geoscience, Aubrey Zerkle of Newcastle University in the U.K. and colleagues report their analyses of rocks from South Africa that date to between 2.65 and 2.5 billion years ago.
Core samples from these rocks contain microbial mats, which show that some of the tiny creatures in shallow seas were producing oxygen long before the Great Oxygenation of our atmosphere.
The rocks also contain carbon and sulfur isotopes—chemicals that would have reacted with oxygen. The levels of the different kinds of isotopes present indicate that sometimes oxygen production was happening when the atmosphere was thick with methane—but other times the atmosphere must have been haze-free.
The clarity of the early atmosphere seems to flip flop roughly every few million years, Zerkle and co. report, hinting at a push and pull between microbes that generated oxygen and those that belched methane.
At last, though, *something* happened about 2.5 billion years ago to trigger the planet’s permanent oxygen high. (What that “something” was is still a mystery, although theories abound.)
Of course, this pattern has so far been seen only in the South African rocks, so more research on samples from around the world will be needed to truly tell whether Earth was once a Titan—atmospherically speaking.
Victoria Jaggard is a senior editor for National Geographic News, specializing in all things space. You can follow Victoria on Twitter @vmjaggard99.