Comets hurtling through space 3.5–4.5 billion years ago may have deposited heavy isotopes of xenon into Earth’s atmosphere. This clue to the isotopes’ mysterious origins suggests that comets visited Earth in its earliest days, that the moon and atmosphere are younger than we originally thought, and that comets might have delivered the first organic material to our fledgeling planet.
While there are only trace amounts of xenon in the atmosphere, the noble gas doesn’t chemically react with other elements, meaning it’s a relatively stable marker whose origin is easy to trace.
But the presence of a surprisingly high percentage of heavy xenon isotopes in Earth’s atmosphere has hampered our understanding of our planet’s origins. Scientists previously couldn’t find any plausible sources for these heavier isotopes—solar wind and meteors, for example, only contain lighter isotopes.
The new data comes from gas collected during the Rosetta mission, which flew a spacecraft near the surface of 67P/Churyumov-Gerasimenko, a member of the family of comets controlled by Jupiter’s gravity. The samples contained a mixture of three noble gases, krypton, argon, and xenon. This comet’s xenon contained the same perplexing heavy isotopes, suggesting that Earth may have received a fraction of its xenon from comets. The model that best fits the atmosphere’s xenon makeup estimates that about 22% of it came from comets.
On Earth, we presumed the presence of this isotope was primarily a result of decaying iodine. We know the rate at which iodine decays, so we use the quantity of xenon-129 to gauge the time at which planetary events took place. But if 22 per cent of the xenon in Earth’s atmosphere was transported by comets, models based on iodine decay aren’t accurate. They overestimate the age of Earth’s atmosphere and the moon.
Since this study only examines data from one comet, missions to other comets will need to verify the results. But this cometary xenon further links Earth’s birth with the rest of the solar system. Results from other comets will provide further details of Earth’s formative years and perhaps even an explanation for how the first organic molecules arrived on Earth.