Though the sun was so much dimmer billions of years ago that the young Earth should have been literally freezing, the planet remained largely covered with liquid water. That was thanks to a substantially darker surface and a dearth of light-scattering clouds, a new study suggests.

“All other things being equal, Earth should have been frozen over for the first half of its existence,” says James F. Kasting, a geoscientist at Pennsylvania State University in University Park who was not involved in the research. “But it wasn’t.”

Previously scientists have explained the presence of liquid water at that low-light time, during the Archean eon of geologic history, by suggesting that Earth’s atmosphere held large amounts of planet-warming greenhouse gases such as carbon dioxide and methane. But new analyses show that greenhouse gases weren’t dramatically higher then compared with today, a team of earth scientists reports in the April 1 Nature. The researchers now propose that early Earth stayed above freezing because the planet was darker then and therefore absorbed more of the sun’s energy — the same phenomenon that renders dark vinyl car seats scorching hot while light-colored seats stay relatively cool.

Early in the sun’s lifetime, the portion of solar core where the light- and heat-generating fusion reactions take place was much smaller than it is today. So, for an extended period, the sun could have been up to 30 percent dimmer than it is now, says Minik Rosing, a geologist at the University of Copenhagen’s Nordic Center for Earth Evolution. Although Earth’s surface temperature should have been well below freezing, geological signs of liquid water in that era abound — a puzzler that scientists have dubbed the “faint young sun paradox.”

Some studies have suggested that carbon dioxide concentrations in Earth’s early atmosphere were more than 100 times current levels. But the new analyses of ancient rocks known as banded iron formations reveal proportions of iron-bearing minerals that could appear only if carbon dioxide levels were no more than three times modern values — a concentration too low to keep the planet from freezing beneath a faint young sun. Methane probably didn’t help make up the difference, Rosing adds, because at high concentrations methane reacts chemically to form a light-scattering haze that would have cooled Earth’s surface rather than warming it.

What probably kept Earth above freezing during the dim-sun era was its darker surface, Rosing and his colleagues contend. The continents were much smaller then, so the planet’s oceans — which are typically much darker than landmasses — could absorb more heat. About 3.8 billion years ago, continents covered less than 5 percent of Earth’s surface, a proportion that gradually rose to reach today’s value of 30 percent around 1.5 billion years ago.

Second, the researchers suggest, light-scattering clouds covered much less of Earth’s surface long ago — another net gain for surface warmth. Because early Earth lacked plants and other complex life, the biologically produced particles and chemicals that water droplets coalesce around weren’t available. In the few clouds that did form, droplets were larger and scattered light less efficiently, allowing more warming radiation to reach ground level.

In their paper, the researchers present a numerical simulation that shows how these two rather straightforward phenomena could have kept Earth’s average temperature above freezing.  

“A lot of the reviewers of our paper were kicking themselves and asking, ‘Why didn’t we think of this first?’” Rosing notes.

Despite the new findings, the faint young sun problem may not be fully solved, Kasting notes. For one thing, the new analyses don’t consider the effect of high-latitude ice masses on the planet’s albedo. “We clearly need additional constraints to understand why the Archean Earth remained habitable,” he comments in Nature.