Earth's life kindled under a cold sun?

WHY are we here? As questions go, it's a big 'un, beloved of philosophers and theologists in a navel-gazing, hand-wringing sort of way. Scientists often find themselves raising an objection before the others even start: we probably shouldn't be here to ask the question in the first place.

The existence of life on Earth seems to have been the product of many lucky turns of events. Take the sun's early history. According to everything we know about how stars like it develop, it should have been born feebly dim, only gradually warming to its present level. Earth, born with the sun 4.5 billion years ago, should have spent its first two billion years or so as a frozen ball of ice, devoid of life.

Yet in rocks laid down during this time we find sediments clearly deposited in aquatic environments, and ample fossil evidence of bacteria that indicate our planet was already a clement, inhabited world, perhaps within a billion years or so from the off. This mismatch, known as the faint young sun paradox, has many potential solutions. None quite has the ring of truth. But as suggestions accumulate and are discarded, one conclusion seems ever harder to ignore: we are even luckier to be here than we thought.

The faint young sun paradox has its origins in the 1960s, when astrophysicists ran the first crude computer simulations of how changes in chemical composition affect the luminosity and heat output of stars such as our sun. The results were clear: the greater abundance of hydrogen in the early sun's core would have given it a higher internal pressure, expanding the star's nuclear heart and lowering its temperature. As a result, the sun's output in its early years was 25 to 30 per cent lower than it is today. That translates into an average surface temperature of the early Earth some 20 degrees cooler - about 10 degrees below water's freezing point.

Yet records of liquid water on Earth go back almost as far as the planet itself. Deposits of the mineral zircon in rocks from Jack Hills in Western Australia have been dated to 4.4 billion years ago, and contain oxygen isotopes that point to their having formed in a watery environment. In the same region there are fossil stromatolites, layered structures formed in shallow water by microbial communities, thought to date to 3.5 billion years ago.

"This clearly tells us that simple models for planetary habitability are wrong," says David Minton, a planetary scientist at Purdue University in West Lafayette, Indiana. "There was life on Earth when it should have been a frozen wasteland." Minton was one of a few dozen astrophysicists and geophysicists who met in Baltimore, Maryland, last year to discuss ways out of this bind. "It turned out that there were almost as many potential solutions as there were participants," he says.

An early proposal is still the most popular: that some greenhouse gas allowed the early Earth's atmosphere to trap more of the weak sun's rays. The suggestion was first made in 1972 in Science by astronomers Carl Sagan and George Mullen. But as they discovered, finding the right gas is tricky.

Correct cocktails

Carbon dioxide seems unlikely to be the sole culprit. CO2 enters soil either in raindrops or through direct diffusion, and drives chemical weathering that is reflected in the mineral composition of rocks known as palaeosols. Studies of ancient palaeosols do suggest atmospheric CO2 levels were higher back in the Archean era, which ran from 3.8 billion years ago to 2.5 billion years ago. But to keep the oceans at a surely liquid temperature of 5 degrees above freezing, they would need to be some 300 times the current amount - 10 times more than even the most generous palaeosol estimates.

James Kasting, a palaeoclimatologist at Penn State University in Philadelphia, still thinks a CO2-based greenhouse effect is the solution, pointing to other evidence of its role in mediating Earth's temperature (see "Carbon control"). "I pay attention to those estimates even if I don't completely agree with some them," he says. All that is needed is to find the correct cocktail of other gases that was mixed in with the CO2.

Back in 1972, Sagan and Mullen suggested ammonia and methane. But ammonia is highly susceptible to ultraviolet light and, with no protective ozone layer around the early Earth, would have been destroyed easily even by the faint young sun's rays. Methane is a powerful greenhouse gas but above a certain concentration forms an organic haze that absorbs sunlight, radiating it back into space. Too much methane cools a planet's surface instead of warming it - an effect astronomers have seen on Saturn's moon Titan.

Titan suggests other ways of making Earth's early atmosphere more of a comforting blanket. Robin Wordsworth and Raymond Pierrehumbert at the University of Chicago have recently investigated whether high levels of nitrogen and hydrogen, such as are found on Titan, can have a warming effect. While the answer is yes, there is no evidence that Earth's atmosphere was ever dense enough to hold the required quantities.

"It turns out that all the gases are more problematic than you hope," says Georg Feulner of the Potsdam Institute for Climate Impact Research in Germany. He believes one reason the paradox has yet to be resolved is that the computer models generally used to study ancient climates are too crude to provide meaningful results.

Relentless activity

The models are crude because they typically ignore factors such as Earth's rotation, which has slowed over the years owing to the effect of the moon's gravity. This slowing would have altered the pattern of heat transport from the equator to the poles, perhaps changing the extent of ice cover and so the amount of energy that was reflected straight back up into space rather than being absorbed by Earth.

This quantity, the albedo, is a general problem. "We know nothing at all about the albedo of the early Earth," says Kasting. Oceans tend to absorb more heat than land does, so the albedo will be affected by factors such as the arrangement of the continents. Thanks to Earth's relentless tectonic activity, this would have been very different in the distant past. Minik Rosing and his colleagues at the University of Copenhagen, Denmark, have even controversially argued that considerably reduced continental cover, plus chemical differences in cloud cover, would have reduced the albedo enough to explain the faint young sun paradox without the need to invoke higher levels of greenhouse gases at all (Nature, vol 464, p 744).

All of these factors - atmospheric composition, rotation, albedo, the effect of clouds - could be the key to solving the paradox. Or they could be red herrings. We simply do not know. Feulner's own latest attempt at a more sophisticated climate model suggests that previous studies have underestimated the cooling effects of faster rotation and ice cover, making the faint young sun paradox even more of a problem (Geophysical Research Letters, vol 39, p L23710).

A few years down the line, he hopes to bring together the teams working on simulations of the early Earth's climate to compare results. That way they can see which effects are a result of the theoretical assumptions that go into making individual models. Any warming effects that pop up in all models, regardless of the assumptions, stand a greater chance of being the key to the problem.

Meanwhile some geophysicists continue to cast a suspicious eye at the sun. Is it possible that astrophysicists have not got the workings of our star nailed down? "Every 10 years or so, someone proposes that the sun must have been more massive in the past," says Kasting. The excess would have to have been substantial - about 2.5 per cent, or 8250 Earth masses - to have made the sun shine brightly enough. Although the sun is constantly flinging particles into space, creating the solar wind, it currently takes 150 million years to lose the mass equivalent to a single Earth. That means the solar wind must have been stronger in the past - a great deal stronger. "That's a sustained mass loss which is at least 10 times larger than anything we infer through the observation of other stars," says Minton.

The book is not closed on all astronomical suggestions. Minton's own involves a game of planetary billiards, and is inspired by the work of Jacques Laskar of the Paris Observatory in France. In 2009, Laskar made headlines with a series of computer simulations that showed that the orbits of the solarsystem's inner planets are not necessarily stable over billions of years. In one particularly alarming scenario, the gravity of the outer solar system's giant, Jupiter, might destabilise Mercury's orbit, flinging it outwards and potentially causing collisions between it, Venus, Earth and Mars in about 3.5 billion years' time.

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