Faint Young Sun

Here’s a climate puzzle — one that goes back to Earth’s infancy some 4.5 billion to 2.5 billion years ago. The sun was much dimmer back then. Far less solar radiation reached the planet. Earth should have been a frozen wasteland. But all geologic signs point to a young planet awash in liquid water, with the first life-forms emerging. Scientists call this conundrum the “faint young sun paradox.”

Carl Sagan and George Mullen identified the paradox in 1972. By then, researchers had determined that a newborn star’s brightness gradually increases over time as hydrogen atoms in the star’s core fuse into helium. Working backward, researchers estimated that the sun generated 20 to 30 percent less energy during the first half of Earth’s history than it does now.

Evidence of the paradox comes from clues in the rock record that indicate the presence of flowing water as far back as the Archean eon, 3.8 billion to 2.5 billion years ago. Geologists have found ancient pillow lavas — knobby volcanic rocks that form only when lava erupts under water — and ripple marks etched by waves on sedimentary rocks. No such rocks are known from the earlier Hadean eon (SN: 5/19/12, p. 22) 4.5 billion to 3.8 billion years ago, but the chemistry of Hadean-aged zircon minerals recycled into younger rocks suggests that liquid water must have been present by at least 4.2 billion years ago.

By this time, much of the heat from Earth’s formation would have dissipated, so it couldn’t account for the warm temperatures. The only explanation is that some unknown factor helped warm the planet. The dilemma seems impossible to resolve because data on fundamental climate factors are missing for this primordial period, says planetary scientist Robin Wordsworth of the University of Chicago. “The Earth has such an active system that the evidence gets erased quickly.”

There has been no dearth of theories, however. Over the last 40 years, climate scientists have offered a range of explanations — everything from high concentrations of insulating greenhouse gases in the atmosphere to changes in Earth’s proximity to the sun. Some ideas are more plausible than others, but even the most probable hypotheses present roadblocks for scientists.

Still, as researchers continue to mine the ground for more geologic clues and refine their simulations of early Earth’s climate, they inch closer to answers. “I’m rather confident that we can have a much clearer picture of what can solve the faint young sun problem in the next few years,” says Georg Feulner, a paleoclimate scientist at Germany’s Potsdam Institute for Climate Impact Research.

That solution, and scientists’ efforts to reach it, may shed light not just on early Earth but on the potential habitability of distant extrasolar planets.

A super greenhouse

Climate scientists acknowledge that the faint young sun paradox probably doesn’t have one simple solution. The Hadean and Archean stretch over 2 billion years. Multiple factors probably worked together to make Earth mild over that time, Feulner says.

Given the Hadean’s sparse geologic record, climate scientists tend to focus on ways to explain the paradox during the better understood Archean. The work is complicated by the fact that researchers don’t really know what global temperatures were like back then, only that they were warm enough for oceans of water to exist. “That’s the most frustrating thing,” says James Kasting, a planetary scientist at Penn State University in University Park.

With those limitations, the goal is to develop a scenario in which Earth’s average global temperature was at least above water’s freezing point — or else one with temperatures closer to today’s. Many climate scientists agree that some sort of enhanced greenhouse effect probably contributed to heating the planet. Greenhouse gases such as carbon dioxide allow sunlight to pass through the atmosphere and then trap heat bouncing back from the planet’s surface. If concentrations were high enough, greenhouse gases could have kept Earth temperate. The thorny part is figuring out which greenhouse gases were interacting.

Back in 1972, Sagan and Mullen thought ammonia was key. An abundance of ammonia seemed feasible based on now-outdated thinking about the chemical makeup of early Earth’s atmosphere and the gas’s supposed role in forming molecular precursors to life as lightning struck it. Although ammonia could have had a strong warming effect, other researchers in the 1970s and ’80s recognized a fundamental flaw in that hypothesis: The sun’s ultraviolet radiation would have broken down ammonia, reducing levels below those needed to insulate Earth’s atmosphere within a decade. For the ammonia idea to work, volcanoes would have had to continuously burp up hefty supplies of the gas, which is unlikely.

What’s more likely, some scientists argue now, is that early Earth had a large, stable supply of CO₂ through a balance between volcanic eruptions and weathering. (Weathering removes the gas from the atmosphere through chemical reactions, and the carbon eventually gets stored in seafloor sediments.) But that’s a lot of CO₂. Kasting and others have calculated that Earth would have needed as much as 1,000 times preindustrial levels of atmospheric CO₂ to compensate for low solar output during the early Archean. By the end of the Archean, as solar radiation grew, only up to 300 times preindustrial CO₂ would have been needed.

But as with the ammonia hypothesis, there are hitches. Geochemical clues indicate CO₂ levels probably weren’t high enough during the Archean. Most recently, geologist Nathan Sheldon of the University of Michigan in Ann Arbor and colleagues analyzed fossilized soil from Minnesota dating to 2.69 billion years ago. By comparing the chemical makeup of the ancient soil with that of the bedrock below, the researchers estimated how much weathering had occurred in the soil and thereby inferred how much CO₂ must have been in the atmosphere at the time. The most likely answer is about 40 times preindustrial CO₂ levels, Sheldon and colleagues reported in 2011 in Precambrian Research. “That’s not enough to overcome the faint young sun on its own,” Sheldon says.

Sheldon and Kasting agree that CO₂ probably had a little help from methane. Methane is a potent greenhouse gas — about 20 times as efficient as CO₂ in trapping heat. In an atmosphere with little oxygen to break down methane — as was the case during the Archean — it lasts thousands of years. So a little goes a long way.

Kasting calculates that the atmosphere needed 1,000 parts per million by volume of methane — combined with as much as 100 times preindustrial levels of CO₂ — to achieve temperatures during the middle Archean that would be equivalent to today’s, he reported in 2008 in Astrobiology. High levels of CO₂ would have been necessary to prevent the methane from forming a thick haze that would have blocked sunlight and chilled the planet.

Today’s methane concentration is in the parts per billion range because of interactions with oxygen. It’s hard to verify whether more methane was present 2.7 billion years ago. “We have OK ways to get at CO₂,” Sheldon says, “but there’s no clear proxy for how much methane was around.” However, the microbes that generate most of today’s methane had probably evolved by this time. In the absence of oxygen, if these organisms churned out as much methane as they do today, Kasting says, that would have led to 1,000 times today’s methane levels — enough to help offset the dimmer sun. There is one indication that a lot of methane was present: glaciations at the end of the Archean. The rise of oxygen at the end of the eon would have removed significant amounts of methane from the atmosphere, a plausible explanation for why temperatures plummeted.

Greenhouse gas helpers

Some scientists think there must be more to the solution than just CO₂ and methane. In the last few years, these researchers have been looking at other gases that could have contributed to warming. That’s where nitrogen and hydrogen come in.

Nitrogen is the most abundant gas in today’s atmosphere, accounting for more than three-fourths of the gas in air. Although it’s not a greenhouse gas, at higher levels than today’s, nitrogen would have enhanced the effects of greenhouse gases, says Colin Goldblatt, an earth scientist at the University of Victoria in Canada. With extra nitrogen, liquid water could have existed with lower levels of greenhouse gases, he says. That could help overcome some of the hurdles set by the geochemical evidence.

Adding more nitrogen to the atmosphere would have enhanced the greenhouse effect by causing more collisions between gas molecules. Ramming into a greenhouse gas molecule changes the way it wobbles, which controls how it absorbs radiation. With enough bumping, Goldblatt says, a molecule of greenhouse gas will start trapping radiation over a greater range of wavelengths. The general result: greenhouse gases that can suck up more heat overall.

“What nitrogen will do is give you more bang for your buck out of any greenhouse gases you’ve got,” Goldblatt says. At about 2.5 billion years ago, with an amount of CO₂ estimated from the geologic record along with some extra methane, a doubling of nitrogen relative to today would be enough to resolve the faint young sun paradox, Goldblatt and colleagues suggested in 2009 in Nature Geoscience. (But if the atmosphere built up too much nitrogen, he notes, the gas would have scattered incoming sunlight, ultimately cooling the planet.)

Hydrogen might have helped as well. If nitrogen and hydrogen levels were both higher in the past, collisions between the two would have created a lot of hydrogen-nitrogen molecules that would have stuck together for a little while, Wordsworth says. The way these molecules wobble makes them effective greenhouse gases, even though hydrogen and nitrogen on their own are not. And the molecules would have absorbed wavelengths of radiation not usually taken up by CO₂ or methane, Wordsworth and Raymond Pierrehumbert, also of the University of Chicago, reported in January in Science. Depending on the exact quantities of each gas, hydrogen-nitrogen pairings might have contributed several degrees Celsius of warming.

As with CO₂ and methane, however, it’s hard to assess how much nitrogen and hydrogen were actually in the atmosphere in the Archean. Goldblatt says the supply of nitrogen now stored in Earth’s crust and mantle is big enough that it’s possible the gas could have been twice as abundant in the past atmosphere as it is today. Hydrogen, meanwhile, might have been more of a player in the Hadean, before the arrival of methane-making microbes, Kasting says. Once these organisms evolved sometime near the onset of the Archean, they would have gobbled up most of the atmosphere’s hydrogen to make methane.

To really assess all of the various explanations, some sort of paleo­barometer is needed, Goldblatt says. If scientists had a record of the Archean’s total atmospheric pressure, they could figure out whether adding vast amounts of gas to the models — which would increase pressure — is a viable way to solve the faint young sun paradox.

Last year, researchers came up with a way to estimate pressure: fossil raindrops. Sanjoy Som, now at the NASA Ames Research Center in Moffett Field, Calif., and colleagues studied imprints left by raindrops on volcanic ash 2.7 billion years ago to estimate atmospheric pressure. The two are related through a long chain of inferences.

The shape of raindrop splatter depends in part on how fast the rain fell through the air. How fast rain falls, in turn, depends on air density, which exerts drag on the drops.

After analyzing how raindrops leave imprints in lab experiments, Som’s team concluded in 2012 in Nature that atmospheric density in the late Archean was probably no more than double today’s.

Since atmospheric density is proportional to air pressure, the assessment, if correct, puts an upper limit on how much gas climate scientists can add to models of the later Archean’s atmosphere. That might not be much of a barrier to hypotheses that rely on increasing concentrations of CO₂ or methane, which are currently at very small concentrations, Wordsworth says. But it’s more problematic for explanations that assume nitrogen was more abundant in the past.

With just one snapshot of what Archean atmospheric density might have been like, he says, “it’s still too early to definitively say how the atmospheric density changed that long ago.”

Climate in 3-D

Potential solutions to the faint young sun paradox have been investigated almost exclusively with one-dimensional climate simulations. The next step is to look at more complex, three-dimensional simulations to really understand what early Earth’s climate was like.

A one-dimensional simulation slices the atmosphere horizontally to see how sunlight travels down through its layers and how radiation reflected from Earth bounces back up. They’re useful in assessing the strength of a greenhouse effect, Wordsworth says. But “you’re getting a very, very idealized picture of what the climate would actually be like.”

Three-dimensional simulations add in how heat travels across Earth’s surface. They also take into account other factors, such as wind and clouds. “It gets pretty messy to do this problem right in 3-D,” Kasting says.

In December, Feulner and colleagues published the results of the first comprehensive three-dimensional simulation of Archean climate in Geophysical Research Letters. Feulner says researchers have been wary of 3-D simulations because so many factors are unknown. Some of the unknowns don’t really affect the results too much, but, Feulner says, two factors that aren’t accounted for in one-dimensional simulations make a big difference: Earth’s rotation and the presence of sea ice.

Because the moon was closer to Earth in the Archean, Earth probably rotated faster, with days that were perhaps only 15 hours long. Faster rotation changes how the air and oceans transport heat from the tropics to the poles. Sea ice influences warming because it reflects more sunlight back into space than land or liquid water, cooling the planet.

By accounting for these factors, the simulation’s results indicate early Earth might have needed seven times as much CO₂ to stay warm than simpler simulations have predicted, which is difficult to reconcile with current geochemical evidence. That’s just one result from one study, though. As others begin working with 3-D simulations, Feulner hopes researchers will get a better sense of how these studies change scientists’ understanding of the faint young sun paradox.

Expanding habitability

By thinking up new ways Earth could have stayed warm during the sun’s dim days, climate scientists have also inadvertently expanded the definition of what makes a planet habitable.

The search for life-sustaining planets outside the solar system is guided by the existence of a theoretical habitable zone, the area around a star where liquid water could exist. “You might be able to extend the outer edge of the habitable zone farther,” Goldblatt says, if some planets have vast quantities of nitrogen or hydrogen in their atmospheres, making greenhouse effects stronger than expected. Even free-floating planets that don’t appear to orbit any star could possess liquid water if their chemistries were just right, Wordsworth adds.

Right now, this is all conjecture. Planet hunters will need to do some detailed calculations to see whether the solutions to the faint young sun paradox also help solve the mystery of whether life exists somewhere else in the universe.