Titan holds clue to faint young sun paradox

Earth should have been an ice cube in the very early days of our solar system, when the sun was cooler than today. Yet the primordial Earth was a watery sphere. This "faint young sun" paradox has puzzled researchers for decades, but now a look at the atmosphere of one of Saturn's moons suggests a new solution.

For the first two billion years of the Earth's existence, the sun was up to 25 per cent dimmer than it is today, making Earth's average temperature up to 25 ?C colder. But the geological evidence is that Earth had liquid water at this time, even though the average surface temperature should have been around -10 ?C.

Robin Wordsworth and Raymond Pierrehumbert, both of the University of Chicago, looked to Saturn's moon Titan to solve the paradox. Despite being far from the sun, Titan has liquids on its surface ? although no known liquid water. That is because its atmosphere has high concentrations of hydrogen and nitrogen, and the gases are under so much pressure that their molecules constantly collide, causing a chemical reaction that traps the sun's energy, much like greenhouse gases do.

Wordsworth and Pierrehumbert created a simulation to determine whether an atmosphere rich in hydrogen and nitrogen would cause the same effect on the ancient Earth. According to their model, if hydrogen made up 10 per cent of the Earth's atmosphere during its early years and nitrogen was present at double or triple today's concentrations, Earth's average surface temperature would have been 10 to 15 ?C higher.

Volcanic role

Wordsworth admits that there is little geological evidence that hydrogen and nitrogen levels were actually that high, but says several factors could have created such an atmosphere. Hydrogen is spewed into the atmosphere by volcanoes, and although the researchers' model assumes that the amount of hydrogen released in the primordial era was similar to today, ancient volcanoes could have emitted more hydrogen. The atmosphere would have been able to hold more of that hydrogen than today, too, because levels of oxygen were lower and so hydrogen would have been less likely to combine with it to form water. And microbes that consume hydrogen may have been rarer, their growth limited by a lack of nutrients.

The model is good, says Chris McKay of the NASA Ames Research Center in Moffett Field, California, but he adds that strong evidence will be needed to show nitrogen and hydrogen levels were that high. The bacteria that convert hydrogen to methane are just too efficient, he says, and would have quickly pulled most of the hydrogen out of the atmosphere.

Wordsworth and Pierrehumbert's study appears in Science, accompanied by a commentary by James Kasting of Pennsylvania State University in University Park. Kasting says that the model's findings also have implications for life on exoplanets: high concentrations of hydrogen in a planet's atmosphere might suggest it is able to harbour life, even if it is cold and far from its sun.

Raindrop enigma

But the new model cannot explain a set of fossilised raindrop imprints dating back to Earth's early days. The size of the imprints suggest the raindrops fell quickly through a thin atmosphere similar to our own, rather than slowly through an atmosphere dense with greenhouse gases.

Because hydrogen is light, raindrops would have passed through it more quickly than they would have through greenhouse gases such as CO₂ or methane, both of which have been proposed as solutions to the faint young sun paradox, Wordsworth says. However, the concentrations of hydrogen and nitrogen demanded by his model would have slowed the raindrops too much to be consistent with the imprints.

Journal reference: Science, doi.org/j47

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