For the first time ever, molecular oxygen has been found on a comet. Comet 67P/Churyumov–Gerasimenko, currently being orbited by the European Space Agency’s Rosetta spacecraft, is the subject of a study published in Nature. According to ESA researchers, the oxygen present in the comet’s surrounding gasses has likely been there since the formation of the comet. “It is the most surprising discovery we have made so far, because oxygen was not among the molecules expected in a cometary coma,” the University of Bern’s Kathrin Altwegg said at a news conference Tuesday held by Nature. Altwegg, one of the new study’s authors, is in charge of Rosetta’s ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis). This instrument is used to “sniff” out the atmospheric composition of the comet.
About a year ago, she and the rest of the team now report, it sniffed out some O2. The gas enveloping a comet, known as its coma is usually comprised of carbon monoxide, carbon dioxide and water. There are atoms of water in all three of those compounds, of course, but atmospheric atoms of oxygen, the stuff present in our own atmosphere were unexpected.”We all went a little bit into denial,” she said. The result was so unexpected that the team wanted to be absolutely certain that the oxygen wasn’t the result of some kind of instrumentation failure or contamination. But the oxygen, Altwegg explained, “follows the comet very well.” The team has observed that oxygen levels are higher close to the comet, and disappear as the instrument pulls away. Furthermore, her co-author André Bieler of the University of Michigan explained, the oxygen to hydrogen ratios have stayed level throughout their observations.This is important, because Comet 67P was getting ever closer to the sun during that time. As it approached its closest pass of the sun (called its perihelion) in August, the comet became more “active” with more and more of its ice sublimating into gas.
Since molecular oxygen is highly reactive, it was assumed that it would have combined with hydrogen then present to form water. “We had never thought that oxygen could ‘survive’ for billions of years without combining with other substances,” said Prof. Altwegg. Alas, the discovery of molecular oxygen has shaken the very foundation of our understanding of oxygen in comets. “Molecular oxygen was constant over a long period of time. When the comet is orbiting the Sun, it loses more material from the surface. So a fresh layer gets exposed over time. Since the ratio of water to oxygen is remaining constant [in different locations on the comet], it means that molecular oxygen must be present in the whole body [of the comet]. If it is present only on the top surface then there would be a decrease [in amount] over a period of time,” Prof. A. Bieler the first author of the paper from the University of Michigan, U.S. told during the briefing.
During the period of study August 2014 to March 2015 the authors estimate that several centimetres thick layer of material must have been lost from the surface areas of the comet. Since the ratio of water to oxygen has not changed in different locations on the comet or over time (nearly 4.6 billion years), there is a stable correlation between water and oxygen. Since oxygen is present in the whole body of the comet, the oxygen must be primordial and must be present even before or at the formation of the comet, Prof. Bieler said. In other words, the oxygen originated very early, before the formation of the Solar System. “Specifically, high-energy particles struck grains of ice in the cold and dense birthplaces of stars, the so-called dark nebulae, and split water into oxygen and hydrogen. The oxygen was then not further “processed” in the early solar system. The oxygen measurements show that at least a significant part of the comet’s material is older than our solar system and has a composition typical of dark nebulae, from which solar nebulae and later planetary systems originate,” notes a University of Bern release.
The molecular oxygen has been trapped in the water ice of the comet 67P. The ice has never been heated up to get reprocessed. “[You have] ice grains with oxygen embedded into it and you have them even today means that the ice was never sublimated, never went back into the gas phase,” Prof. Altwegg said. According to her, most of the Solar System info models predict heavy inflow of material from outside to the centre of the Solar System and then also a heavy outflow of material leading to a mixing of intrasolar system during formation. “But these [models] are not compatible with ice grains containing oxygen. They must have stayed out, never mixed, or never come close to the young Sun,” she said. “The preferred explanation of our observations is the incorporation of primordial oxygen into the cometary nucleus,” they note in the paper.
Radiolysis of icy grains before accretion is one of the possible mechanisms that the author think could have preserved the oxygen over a long period of time. “When produced by radiolysis in water ice, oxygen can remain trapped in voids, while hydrogen can diffuse out. This prevents the hydrogenation of oxygen, which is otherwise a dominant reaction for the destruction of molecular oxygen, and could lead to increased and stable levels of oxygen in the solid ice,” they write.
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