Wired Science (December 10, 2009)
"Isotopic analyses of the gases krypton and xenon suggest that much of Earth's atmosphere came from outer space, not inner space.
"Krypton and xenon appear in Earth's atmosphere — and in the universe as a whole — only in trace amounts. Detailed analyses of the gases provide clues about where those atmospheric components originated, says Greg Holland, an isotope geochemist at the University of Manchester in England. Those analyses, reported in the Dec. 11 Science, suggest that those gases, as well as many others now cloaking our planet, arrived via comets or were swept up from nearby gas clouds during the late stages of Earth's formation.
"Some scientists have proposed that the gases in Earth's atmosphere originated within the planet, says Holland. According to those arguments, the atmosphere either seeped out of the Earth as the planet gradually cooled or were expelled from the crust when large numbers of asteroids pummeled the planet and melted its surface around 3.9 billion years ago. But new isotopic evidence gathered by Holland and his colleagues suggests that those scenarios probably aren't right...."
First, a little background. An isotope is "one of two or more atoms with the same atomic number but with different numbers of neutrons" (Princeton's WordNet)
Elements - including noble gasses like krypton and xenon - from different planets have different ratios of isotopes. Which makes it possible to identify rocks found in Antarctica as having started out on Mars. 1
And it looks like quite a bit of the krypton and xenon in Earth's atmosphere didn't come from this planet.
One of the less-unlikely explanations for where the 'other' gasses came from is that comets crashed into our planet, mostly quite a long time ago.
- "Change, American Culture, Trilobites, Humanity's History, and the Big Picture"
(last updated December 9, 2009)
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New Scientist (August 28, 2009)
- "New Study Boosts Idea of Past Life on Mars"
University of Wisconsin-Madison (March 13, 1997)