The next breath you take could have come from beyond Pluto. Researchers studying the composition of ancient gases trapped in deep wells in New Mexico have found convincing evidence that some of our planet's atmosphere originated in the far reaches of the solar system. The discovery could change long-standing thinking about how Earth's atmosphere evolved.
Since the 1950s, scientists have thought that as Earth congealed from the primordial cloud of dust and gases that also formed the sun and other planets, it trapped some of those gases within its mantle. Then, over hundreds of millions of years, volcanic eruptions returned the gases to Earth's surface, where gravity kept them from drifting off into space. The mixing of these gases--along with the oxygen and other molecules added by life--created the atmosphere we have today.
A reasonable idea, at least until researchers began collecting samples from the New Mexico wells several years ago. Their original purpose was to study the pristine bubbles of volcanic gases trapped underground for billions of years. These bubbles had never made it to the surface before and therefore remained uncontaminated by the modern atmosphere. In particular, the researchers were studying the isotope ratios--or the comparative abundances of the different forms of a particular element--of the gaseous elements krypton and xenon.
The krypton and xenon isotope ratios in the gas samples provided a surprising result: They did not match the ratios found in the primordial cloud that spawned the solar system. Those ratios persist today in the solar wind and have been measured by satellites. Instead, they matched the ratios contained within certain types of meteorites that formed 4.5 billion years ago at the beginning of the solar system, which have been measured from meteorites that have landed on Earth. The conclusion, the team reports today in Science, is that part of Earth's atmosphere must have arrived after the planet had fully formed--possibly carried by comets that hit the planet and whose ice would have evaporated upon impact, leaving water vapor and traces of krypton, xenon, and other elements in the atmosphere.
"This was an unexpected find," says isotope geochemist and co-author Christopher Ballentine of the University of Manchester in the United Kingdom. "The first gases captured by the Earth and trapped in its interior cannot [have contributed] to the gases now in the atmosphere," he says. "This means the atmosphere arrived far later than expected." To confirm that comets contributed to the atmosphere, scientists will need to analyze the krypton and xenon isotope ratios in the cometary samples recently captured by NASA's Stardust mission, says geochemist and lead author Greg Holland, also at Manchester.
The data the researchers have collected on the krypton and xenon isotope ratios are "superb," says physicist Bob Pepin of the University of Minnesota, Twin Cities. "They're of a quality not seen before." But Pepin notes that when early Earth was struck by the cataclysmic impact that resulted in the formation of the moon, primordial gases in the mantle could have been completely released to the atmosphere, leaving behind no evidence that they had ever existed there. So until new work can model the exact process involved, Pepin says, "the jury will probably stay out."