Comets from outer space may have created Earth's atmosphere – not volcanoes spewing out gases from deep within the planet.

The origin of the gases in Earth's atmosphere has long been a puzzle. One of the main theories is that the gases bubbled up out of the mantle via volcanoes.

Greg Holland of the University of Manchester, UK, and colleagues have arrived at a different theory after collecting samples of the noble gas krypton from several hundred metres beneath New Mexico.

They found that the mantle's chemical fingerprint was rich in "heavy" isotopes of krypton such as krypton-86 and krypton-84, and poorer in "lighter" forms such as krypton-82. This is a composition that closely resembles meteorites –- support for the ideas that gas-rich meteorites colliding in the early solar system formed our planet.

"The results confirm one of the basic ideas of planetary formation theory, that most of the Earth formed by collisions of smaller objects like carbonaceous chondrites," says Scott Kenyon at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
Light atmosphere

But where did the atmosphere come from –- it is rich in lighter isotopes so the mantle cannot be the source, says Holland. Earth's atmosphere cannot have gained a greater proportion of lighter isotopes since it formed. Because light isotopes of krypton escape into space more quickly than heavy isotopes, the atmosphere can only get "heavier"

If not the mantle then what? Chris Ballentine, a co-author and colleague of Holland's suggests that comets could be the answer. At the outer edges of the solar system, in the Kuiper Belt, are millions of icy bodies that formed when the solar system was born. These comets have noble gas signatures that resemble that of our modern atmosphere.

A shift in Jupiter's orbit around 4.5 billion years ago may have jarred the Kuiper Belt, flinging icy comets at the Earth. "Ancient Earth was strewn with huge volcanoes spewing out gas, but our research shows that the real source of Earth's first atmosphere was actually outer space," says Ballentine, a co-author of the paper and colleague of Holland's.

Source: Science, DOI: 10.1126/science.1179518

Read More..