Thursday, January 31, 2013


Many theories have been explored over the years as to how the Earth stayed warm during the first two billion years of its development, as even though it was warm enough to support life, it was not from heat provided by the Sun. Earth at that time only received 70 percent of the solar radiation that it receives today and its average temperature was up to 25 °C colder. There is geological evidence that Earth had liquid water at this time, despite the average surface temperature being around -10 °C. Robin Wordsworth and Raymond Pierrehumbert, geologists at the University of Chicago, have suggested in a paper published in the journal Science that it was collisions between hydrogen and nitrogen molecules in the atmosphere that kept early Earth warm.

Previous work had focused on the hypothesis that it was methane released by organisms consuming hydrogen that was acting as a greenhouse gas, trapping the small amount of heat coming from the Sun. In this new study however, Wordsworth and Pierrehumbert suggest that collisions between hydrogen and nitrogen molecules create "dimer" molecules that wobble when hit by infrared light from the Sun. This wobbling would allow for heat capture for Earth’s atmosphere. Evidence for this theory would be indications that there was more hydrogen in Earth’s atmosphere in the past than there is today.

The two researchers reference new work by others which suggests that that is the case: there are some calculations that show that early Earth’s atmosphere may have contained as much as 30 percent hydrogen. According to Wordsworth and Pierrehumbert, if the early Earth had as much as 10 percent more hydrogen in its atmosphere than it does today and nitrogen was present at double or triple today's concentrations, Earth's average surface temperature would have been 10 to 15°C higher.

This research has implications for other planetary bodies, as they too may be experiencing similar warming effects. If these worlds have a lot of hydrogen within their atmosphere, they may be worth further observing as a potentially habitable world if they are within a habitable zone.

Saturn’s largest moon, Titan, has liquid on its surface despite being so far from the Sun. Its atmosphere has high concentrations of hydrogen and nitrogen; these gases are under so much pressure that their molecules constantly collide. These collisions cause a chemical reaction that traps the energy of the Sun.

This new model does not explain fossilized raindrop imprints on Earth that date back to 2.7 billion years ago. The size of these imprints suggests that the raindrops fell quickly to Earth through a thin atmosphere similar to that of the present day, rather than an atmosphere thick with greenhouse gases. Hydrogen is a light gas, so the raindrops would have passed through it more quickly than through an atmosphere rich with CO2 or methane. The concentrations of hydrogen and nitrogen needed by this new model would have slowed the raindrops too much to make them consistent with the imprints.

Wordsworth does admit that there is little geological evidence that hydrogen and nitrogen levels were as high as suggested by this new model, but believes there are other factors that could have created such an atmosphere. It is possible that Earth’s volcanoes of 2 billion years ago emitted more hydrogen than today’s volcanoes. The atmosphere at that time would have been able to hold more hydrogen as oxygen levels were lower; hydrogen would have been less likely to combine with this oxygen to form water. Microbes that consume hydrogen may have been rarer than they are today as there were fewer nutrients.

Though the model itself is good, according to Chris McKay of the NASA Ames Research Center in Moffett Field, California, there will need to be strong evidence of nitrogen and hydrogen levels being that high.

The image shows Earth vs Titan, courtesy Victoria Jaggard of National Geographic News
Hydrogen-Nitrogen Greenhouse Warming in Earth's Early Atmosphere, Science 4 January 2013: Vol. 339 no. 6115 pp. 64-67 DOI: 10.1126/science.1225759

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