Earth's atmosphere may have come from meteorites

15 December 2009 by Tamera Jones

The gases that formed the Earth's atmosphere may have come from outer space and not from gases spewed from ancient volcanoes, say scientists in a report published in Science.

Meteor shower

A shower of fiery meteorites falling about 4000 million years ago onto the hot, glowing rocks of Earth

Scientists have long thought that the gases that make up our atmosphere came from the mantle inside the Earth and were released when huge volcanoes erupted.

The theory is that our solar system formed when a huge cloud of dust and gas collapsed under its own weight. The Sun was the first to form and then the planets followed. It makes sense then to suppose that gases inside the Earth must be similar to gases found in the Sun.

"But no-one has proved this and we haven't had the technology to test this out until now," says Dr Greg Holland from the University of Manchester and lead author of the report.

An alternative view is that our atmospheric gases may have come from comets, meteorites and other dust from outer space.

We thought that mantle gases would come from the Sun, but we found that the noble gases in the mantle are like the gases you'd find in meteorites.

- Dr Greg Holland, University of Manchester

A team of scientists led by Professor Chris Ballentine from the University of Manchester collected ancient gases from a commercial carbon dioxide (CO2) mine in the Bravo Dome gas field in New Mexico in the United States. The gases come from 600 to 700 metres down and have been trapped for several million years.

These gases are made up of 99.9 per cent CO2; the rest is made up of traces of noble gases, like helium, neon or krypton.

Noble gases are chemically inert which means scientists can use them to figure out the physical processes that happened inside the Earth millions of years ago.

The researchers found that the chemical fingerprint of krypton showed more of the heavy isotopes (different types of atoms of the same chemical element) krypton-86 and krypton-84 than the light isotope of krypton-82.

This ratio is essentially the same as scientists find in gas-containing meteorites from the asteroid belt.

"We thought that mantle gases would come from the Sun, but we found that the noble gases in the mantle are like the gases you'd find in meteorites," says Holland.

"This was a big surprise. It's difficult to make models of the Earth's atmosphere without a solar composition. Gas has to come out of the Earth when volcanoes erupt. But our results suggest the atmosphere couldn't have come from inside of the Earth. So it must have come from outside, from meteorites," adds Holland.

The findings suggest that textbook images of ancient Earth with huge volcanoes spewing gas into the atmosphere may have to be re-drawn.

Meteorite Kr in Earth's Mantle Suggests a Late Accretionary Source for the Atmosphere
Greg Holland, Martin Cassidy, Chris J Ballentine
Science 11 December 2009: Vol. 326. no. 5959, pp. 1522 - 1525
DOI: 10.1126/science.1179518