A new study suggests that roughly 22% of the element xenon found in Earth's atmosphere may have come from comets. The finding -- shedding light on a decades-long mystery about the source for some of this gas on Earth -- could be important for understanding comets' contribution of other materials, such as water, to our planet, as well. Xenon is the heaviest stable noble gas. It has nine different isotopes (essentially "weights"), which scientists can trace through the cosmos and use to determine its origins. Yet, models of xenon's origin on Earth require an additional unknown source which has been unidentified for decades. Between May 14 and 31, 2016, an important clue about a xenon source was uncovered in data collected by the Rosetta spacecraft, as it carried out a series of low-altitude orbits around comet 67P/Churyumov-Gerasimenko. Upon analyzing the spectrometry data, Bernard Marty et al. found that the xenon leaking from 67P appears to have been trapped within the cometary ice since before the solar system formed. What's more, the isotopic signature of this cometary xenon closely mirrors the signature of the xenon on Earth derived from a previously unknown source. The authors discuss several other possibilities for how the mysterious isotopic signature of xenon came to be on Earth, but ultimately rule these out. They propose that a substantial portion of atmospheric xenon on Earth - roughly 22% - was delivered by comets.