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Hydrogen and the faint-young-sun paradox

by CIFAR Mar 15 / 13

In the earliest days of our Solar System, the sun was much fainter than it is today and the average temperature on Earth would have been correspondingly cooler if the composition of the atmosphere were the same as today.

young-sun
Planet Earth and the setting sun. Photo: Shutterstock

Yet, scientists have long shown that this period had evidence of water, and that the young Earth was not an icy ball, despite the faint Sun. Carbon dioxide at the time may have been insufficient to have provided enough greenhouse gas effect to keep the Earth from freezing over. So what was keeping the Earth warm? This so-called ‘faint-young-sun paradox’ has puzzled researchers until now.

A paper published in Science by Senior Fellow Ray Pierrehumbert (University of Chicago) and his colleague, Robin Wordsworth, provides new clues. The researchers suggest that a gas that is less prevalent today, hydrogen, could have provided the warming effect.

“Normally, we don’t think about hydrogen as a greenhouse gas; it is methane and carbon dioxide that work today to keep our oceans from freezing over,” says Pierrehumbert. But drawing from his previous work on Titan, Saturn’s largest moon, and the planet Jupiter, Dr. Pierrehumbert points to hydrogen as being potentially one of Earth’s early greenhouse gases.

“Both Titan and Jupiter can keep hydrogen trapped because their coldness, and in Jupiter’s case its gravity, won’t let molecules escape from the atmosphere quickly. We know that hydrogen is one of the main contributors to their temperature, so we began to think that it could also have been possible in Earth’s history.”

Using sophisticated mathematical models, the researchers developed a new theory to estimate the warming that would be caused by the amount of hydrogen believed to be present in the Earth’s early atmosphere.

These findings can help researchers understand what planetary conditions are suitable for life and may help scientists to reconsider which planets to study for evidence of life beyond Earth.