The enigmatic noble gas xenon has been presenting scientists with puzzling riddles for decades, including how it got to Earth, and then its apparent disappearance since arriving.
But geochemists may have finally figured out where at least some of the xenon in Earth's atmosphere might have originated - and it's not from here.
New results from the Rosetta spacecraft reveal that almost a quarter of the xenon found in our atmosphere may have come from comets.
In addition to solving the long-held mystery about the origins of the rare gas, the new findings could help scientists understand how comets have potentially delivered other materials, such as water, to our planet.
"The xenon isotopic composition matches that of a primordial atmospheric component," says Bernard Marty, lead author and geochemist at the University of Lorraine, France. "The present-day Earth atmosphere contains 22 percent cometary xenon."
Much like helium and argon, xenon is a noble gas that is odourless, colourless, and is mostly unreactive, though some of its compounds can be highly explosive. In Earth's atmosphere, xenon is a trace gas that only occurs at around one part in 20 million.
Even though it's rare, you likely come across xenon on a regular basis in the form of car headlights, digital film projectors and plasma screen televisions.
Xenon is also the heaviest stable noble gas at about four times heavier than air. It has nine stable isotopes, which act as 'weights' that can be used to tell the story of how our Solar System formed.
But the weird thing about xenon is that it doesn't act the way it's supposed to when compared with its more common noble relatives.
For a start, over 90 percent of the xenon that's expected to be in Earth's atmosphere is mysteriously missing. There have been several arguments over where this missing xenon could be hiding, including deep in Earth's core.
And the riddle doesn't stop there. Another mystery scientists have been trying to solve for decades is where a portion of the xenon found in our atmosphere came from in the first place.
The problem is, current models suggest that at least some of the xenon found on Earth had to come from an unknown source, which hadn't been identified - until now.
Enter the Rosetta spacecraft, which first set out to investigate the comet 67P/Churyumov-Gerasimenko in 2004.
As part of its mission, Rosetta circled the comet to find out about the isotopic signature of the xenon trapped in the ice, uncovering possible clues about where Earth's atmospheric xenon had come from.
After analysing the spectrometry data collected by the spacecraft, Marty and his team discovered that the xenon in the comet's ice had been there before the birth of the Solar System.
Additionally, the weird isotopic signature of the comet's xenon matched the xenon in Earth's atmosphere which had been derived from an unknown source.
"Cometary noble gases are concentrated in ice and a presolar origin for xenon would imply that cometary ice is also presolar," writes the team.
The team explored other possible explanations for how this exotic type of xenon came to be on Earth. They investigated whether the unique xenon found in 67P/Churyumov-Gerasimenko could be due to other processes, such as the comet's ice melting or interactions with the solar wind.
But the results revealed that the isotopic signature of the comet's xenon differed from the typical signature found in Solar System xenon, indicating that it was primordial in origin.
Ruling out these other explanations, Marty is confident that some of Earth's xenon was brought here by comets.
"The peculiar xenon isotopic composition of 67P/Churyumov-Gerasimenko provides a fingerprint of the contribution of comets to the terrestrial atmosphere," the team writes in their paper.
With one answer for the origin of our planet's xenon uncovered, it leads to broader questions of what else comets may have brought down to Earth in our planet's early days.
The research has been published in Science.