Fossilized imprints of raindrops that were sealed into stone 2.7 billion years ago indicate that Earth's early atmosphere could have been packed with greenhouse gases, according to new research that addresses a long-standing paradox of the planet's early history.
About 2 billion years ago, the young sun was far less bright, emitting less than 85% of the light and heat it puts out today. With such weak sunlight, Earth should have remained frozen. But ancient water-damaged rocks and algae-like fossils show clear evidence that there was indeed liquid water in the distant past.
How did Earth manage to stay so warm? Some scientists theorize that the atmosphere must have been thicker, or perhaps more densely packed with greenhouse gases that are very good at trapping heat from sunlight.
To get at the answer, a team of scientists from the University of Washington in Seattle adopted an unusual method employed in 1851 by a 19th-century geologist named Charles Lyell.
Lyell suggested that the thickness of the ancient atmosphere could be pinned down by measuring the size of fossil raindrops. When a raindrop splatters on something soft, it makes a little crater — and the faster that droplet is falling, the bigger that crater will be. In a thin atmosphere, that droplet will plummet to the ground, while a thicker atmosphere would slow it down, buffering its fall and resulting in a smaller dent.
With this in mind, the modern-day researchers examined 2.7-billion-year-old sedimentary rock from South Africa. Tiny pits in the rock made by falling raindrops were quickly preserved by a thin layer of fine ash that acted rather like a protective varnish. That was followed by a second layer of coarser ash that was later worn away, leaving the raindrop imprints intact.
"What's so incredible, in a sense, is that they were preserved for so long," said study leader Sanjoy Som, now an astrobiologist at NASA Ames Research Center in Mountain View, Calif.
Som and his colleagues poured latex over the rocks, creating casts of 955 raindrops that they studied back in the lab.
But before they could draw any conclusions from the fossils, the researchers needed to see how raindrops behaved in action. They sprinkled water onto ash gathered from two sites — fresh ash from the 2010 Eyjafjallajökull eruption in Iceland, and much older ash from the Pleistocene era from Pahala in Hawaii — to see how the size and speed of raindrops affected the size of the dents they made.
Using these data as a guide, the researchers calculated that to make the fossilized craters, the raindrops were probably falling through an atmosphere similar to ours, or perhaps thinner — one that was 50% to 105% of its current thickness. They reported their results in Thursday's edition of the journal Nature.
The findings appear to rule out the possibility that young Earth was being warmed by a super-thick atmosphere. It's possible that the atmosphere back then contained a higher proportion of potent greenhouse gases like methane or carbonyl sulfide than it does today, said William Cassata, a geochronologist at UC Berkeley who was not involved in the study.
There is one caveat: The atmospheric estimates rely on the assumption that ancient and modern raindrops were about the same size, Cassata said.
"There's no reason to think raindrops in the past were any larger," he said. "However, if the raindrops were anomalously large, then they can accommodate pressures twice what we observe today."
Som sees use for this approach elsewhere in the solar system. If fossil raindrops are ever discovered on Mars, this technique could be useful for learning more about the Red Planet's past atmosphere.
And, Som said, for scientists tired of looking for life on other planets by comparing them to Earth, understanding the markedly different atmosphere of ancient times would give researchers an entirely new point of comparison.
"It was essentially a completely different world," he said.