Gaseous burps and excretions from Earth’s mantle are commonly believed to be the origin of our planet’s atmosphere, but how this escaping gas got into the mantle in the first place has long kept scientists guessing. One theory contends that gasless matter, which was accumulating to form the early Earth, gravitationally attracted solar nebula gases. Another theory argues that Earth’s gas actually comes from vapor-laden comets, meteorites, and dust that accumulated to form our planet. Now researchers led by Greg Holland, a geochemist at the University of Manchester, in England, report evidence for the latter (Science 2009, 326, 1522). The team dug deep into the mantle at a desolate spot in New Mexico and found that isotopes of the noble gas krypton correlated with comet and meteorite signatures—not those of solar nebula gases. Although the new work resolves where Earth’s gas initially came from, it also creates a new conundrum. The krypton and xenon isotope signatures in the mantle do not quite match those of the atmosphere, suggesting that mantle burps are at best only partially responsible for gas around our planet. Where our atmosphere came from is now an open question, Holland says.