How did the early Earth not freeze? The Earth’s sun has been…

How did the early Earth not freeze? The Earth’s sun has been…

How did the early Earth not freeze?

The Earth’s sun has been fusing hydrogen into helium for about 4.56 billion years. This timespan is less than ½ of the sun’s likely lifetime, but there’s another interesting detail about main-sequence stars like sol; they heat up with time.

During the first billion years of Earth’s history, the planet received about 25% less energy from the sun than it receives today. These changes in the sun don’t happen rapidly; that rate translates to an increase of less than 1% every 100 million years, but it leaves us with a major scientific quandary for the early Earth; how did it not freeze?

Today, even with a warm sun, there are still ice caps at the poles. It’s likely that at some points in Earth’s history glaciers have covered large portions of the planet. But, the first billion years of Earth’s history should have had even less input from the sun. If it can freeze partially today, how did the planet not completely freeze early in its history?

If you’ve followed this site, you might know that the natural answer for how to keep the planet warmer is greenhouse gases like CO2. If the early Earth’s atmosphere had substantial quantities of carbon dioxide and methane, maybe hundreds of times what is present today, that could overcome the lower input from the sun and keep the planet warm.

Problem is, people have looked for evidence of these high values of greenhouse gases and they haven’t found them.

One way to fix this disconnect might be the rest of the atmosphere. The main inert gases in the Earth’s atmosphere are nitrogen and argon; if there was a lot more nitrogen in the early atmosphere, that gas could make the greenhouse effect of CO2 stronger.

That’s where the rocks you’re looking at come in. These rocks come from the Dresser formation, part of a series of rocks known as the Pilbara craton in Western Australia; some of the oldest continental rocks on Earth. These rocks are 3.49 billion years old and were formed in a shallow sea.

One mineral they contain is quartz. Certain grains of quartz in these rocks have small inclusions of gas. When they were growing, small cavities formed inside the crystals and those crystals were able to take in samples of the atmosphere which they have stored for three and a half billion years.

Argon is actually produced by radioactive decay in the Earth, so we know roughly how much argon should have been in the Earth’s early atmosphere. To measure the composition of the early atmosphere, researchers from CNRS in France broke these inclusions apart and measured the ratios of nitrogen to argon in the inclusions. If the Earth had a thick nitrogen atmosphere that was lost, there should be very large ratios of nitrogen to argon in the samples.

Instead, the samples contain nitrogen to argon ratios similar to today’s atmosphere. In fact, the nitrogen abundances might even be a bit lower, suggesting that the atmosphere was still growing in the early Earth and nitrogen was still being supplied.

Combined with other data, this result suggests that the early atmosphere might have been about 2x as dense as the current atmosphere made up of about 50-75% CO2 and 25-50% nitrogen as the major gases. This atmosphere may or may not have been enough to overcome the faint young sun, and it’s uncertain whether the abundances of CO2 required are consistent with other geologic records (3.5 billion year old rock records can be really hard to interpret). If the CO2 on its own wasn’t enough, then a small portion of methane or perhaps another gas could have been required to keep the Earth from freezing.

-JBB

Image credit: Wikimedia Commons

http://pilbara.mq.edu.au/wiki/Image:Wrinkle_mats.jpg

Original paper:

http://www.sciencemag.org/content/342/6154/101.short

Press report:

http://phys.org/news/2013-10-climate-puzzle-life-earth.html