The challenging detection, by ESA's Rosetta mission, of several isotopes of the noble gas xenon at Comet 67P/Churyumov-Gerasimenko has established the first quantitative link between comets and the atmosphere of Earth. The blend of xenon found at the comet closely resembles U-xenon, the primordial mixture that scientists believe was brought to Earth during the early stages of Solar System formation.

These measurements suggest that comets contributed about one fifth the amount of xenon in Earth's ancient atmosphere. Xenon - a colourless, odourless gas which makes up less than one billionth of the volume of Earth's atmosphere - might hold the key to answer a long-standing question about comets: did they contribute to the delivery of material to our planet when the Solar System was taking shape, some 4.6 billion years ago? And if so, by how much?

The noble gas xenon is formed in a variety of stellar processes, from the late phases of low- and intermediate-mass stars to supernova explosions and even neutron star mergers. Each of these phenomena gives rise to different isotopes of the element. As a noble gas, xenon does not interact with other chemical species, and is therefore an important tracer of the material from which the Sun and planets originated, which in turns derives from earlier generations of stars.

"Xenon is the heaviest stable noble gas and perhaps the most important because of its many isotopes that originate in different stellar processes: each one provides an additional piece of information about our cosmic origins," says Bernard Marty from CRPG-CNRS and Universite de Lorraine, France. Bernard is the lead author of a paper reporting Rosetta's discovery of xenon at Comet 67P/C-G, which is published in Science.

It is because of this special 'fingerprint' that scientists have been using xenon to investigate the composition of the early Solar System, which provides important clues to constrain its formation. Over the past decades, they sampled the relative abundances of its various isotopes at different locations: in the atmosphere of Earth and Mars, in meteorites deriving from asteroids, at Jupiter, and in the solar wind - the flow of charged particles streaming from the Sun.

The blend of xenon present in the atmosphere of our planet contains a higher abundance of heavier isotopes with respect to the lighter ones; however, this is a result of lighter elements escaping more easily from Earth's gravitational pull and being lost to space in greater amounts. By correcting the atmospheric composition of xenon for this runaway effect, scientists in the 1970s calculated the composition of the primordial mixture of this noble gas, known as U-xenon, that was once present on Earth.

This U-xenon contained a similar mix of light isotopes to that of asteroids and the solar wind, but included significantly smaller amounts of the heavier isotopes.

"For these reasons, we have long suspected that xenon in the early atmosphere of Earth could have a different origin from the average blend of this noble gas found in the Solar System," says Bernard.

One of the explanations is that Solar System xenon derives directly from the protosolar cloud, a mass of gas and dust that gave rise to the Sun and planets, while the xenon found in the Earth's atmosphere was delivered at a later stage by comets, which in turn might have formed from a different mix of material.

With ESA's Rosetta mission visiting Comet 67P/Churyumov-Gerasimenko, an icy fossil of the early Solar System, scientists could finally gather the long-sought data to test this hypothesis.

"Searching for xenon at the comet was one of the most crucial and challenging measurements we performed with Rosetta," says Kathrin Altwegg from the University of Bern, Switzerland, principal investigator of ROSINA, the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis, which was used for this study.

Xenon is very diffuse in the comet's thin atmosphere, so the navigation team had to fly Rosetta very close - 5 km to 8 km from the surface of the nucleus - for a period of three weeks so that ROSINA could obtain a significant detection of all the relevant isotopes.

Flying so close to the comet was extremely challenging because of the large amount of dust that was lifting off the surface at the time, which could confuse the star trackers that were used to orient the spacecraft.

Eventually, the Rosetta team decided to perform this operation in the second half of May 2016. This period was chosen as a compromise so that enough time would have passed after the comet's perihelion, in August 2015, for the dust activity to be less intense, but not too much for the atmosphere to be excessively thin and the presence of xenon hard to detect.

As a result of the observations, ROSINA identified seven isotopes of xenon, as well as several isotopes of another noble gas, krypton; these brought to three the inventory of noble gases found at Rosetta's comet, following the discovery of argon from measurements performed in late 2014.

"These measurements required a long stretch of dedicated time solely for ROSINA, and it would have been very disappointing if we hadn't detected xenon at Comet 67P/C-G, so I'm really glad that we succeeded in detecting so many isotopes," adds Kathrin.

Further analysis of the data revealed that the blend of xenon at Comet 67P/C-G, which contains larger amounts of light isotopes than heavy ones, is quite different from the average mixture found in the Solar System. A comparison with the on-board calibration sample confirmed that the xenon detected at the comet is also different from the current mix in the Earth's atmosphere.

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Caltech: Chemical Engineer Explains Oxygen Mystery on Comets
Pasadena CA (SPX) May 09, 2017
A Caltech chemical engineer who normally develops new ways to fabricate microprocessors in computers has figured out how to explain a nagging mystery in space - why comets expel oxygen gas, the same gas we humans breathe. The discovery that comets produce oxygen gas - also referred to as molecular oxygen or O2 - was announced in 2015 by researchers studying the comet 67P/Churyumov-Gerasime ... read more

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