The origin of the xenon in Earth’s atmosphere has been a mystery for decades. Now, using data from the Rosetta spacecraft’s tight orbits around a comet, researchers have determined that 22 per cent came from comets. This strengthens suspected connections between these celestial bodies and Earth’s evolution.
The xenon gas in Earth’s atmosphere contains more heavy isotopes than xenon in the solar wind or meteoroids, and for decades researchers couldn’t figure out where this heavy component came from. The idea that it could have been brought here by comets was often suggested, but evidence was limited.
In 2014, the Rosetta spacecraft orbited the comet 67P/Churyumov-Gerasimenko mere kilometres from the surface, allowing it to sample the gas coming off the comet’s ice patches. Bernard Marty at the University of Lorraine in France and his colleagues found that those gases closely matched the composition of Earth’s heavy xenon.
“The Earth’s atmospheric xenon is a mixture between meteoritic and cometary, and we know the composition of each now,” says Marty. “So we mix them, we make up a cocktail, until we find the taste of the atmosphere.”
The cocktail that best matched our atmosphere was about 22 per cent cometary xenon, with the rest of the xenon coming from meteors. “It’s a nice, elegant explanation for xenon in the atmosphere, something which has eluded geochemists to date,” says Colin Jackson at the Smithsonian Institution in Washington DC.
However, this does assume that all comets are similar to 67P. “This is based on measurements on this one comet, and the study of materials in the solar system always emphasises how diverse chemistry is throughout the solar system,” says Jackson
Should the type of xenon on 67P prove to be representative of a large group of comets, it also has larger implications for Earth’s evolution.
In addition to matching the heavy components of xenon in Earth’s atmosphere, the comet samples also had a surprising amount of a particular type of the gas, xenon-129.
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.
Looking at Earth’s mantle gives us a hint as to when our xenon delivery must have occurred, and it means comets would have had a difficult job to reach us. The mantle doesn’t contain the cometary signature of xenon, so the xenon in the modern atmosphere must have been delivered after the mantle stopped incorporating gases from the atmosphere. At that time, 4.5 to 3.5 billion years ago, comets would have had to navigate a treacherous solar system to get here.
Astronomers think Jupiter, Saturn, Neptune and Uranus formed a kind of barrier between the inner and outer parts of the solar system. Their gravitational fields would have attracted small bodies, like these xenon-laden comets, lowering their chances of making it to Earth.
“But comets are from the outer solar system, and now we know they came to Earth,” says Marty. He says if the orbits of the giant planets changed at some point 100 million years or more after the solar system began to form, which some theories have suggested that they did, they could have allowed some comets through toward Earth.
Those comets could have brought with them not just xenon, but also the volatile elements crucial for life, like hydrogen and nitrogen. The inner solar system was probably too hot for these elements to survive the cloud of dust and gas from which Earth was formed, so it’s long been postulated that at least some portion was delivered after the planet’s formation by comets.
This new evidence that our planet was visited by comets relatively early in its formation could strengthen the connection between comets and Earth’s hospitality to life. “Comets could have potentially carried a lot of organic molecules out to Earth,” says Marty. “It doesn’t mean that comets brought life, but they could have brought the bricks of life.”
Journal reference: Science, DOI: 10.1126/science.aal3496