"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 kilometres 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 Institute Center for Astrophysics in Cambridge, Massachusetts." - Steven Perez from Bookmarklet