Scientists involved in the European Space Agency's (ESA) Rosetta mission have found high levels of oxygen streaming off of a comet that transited past the sun back in August.

In a study featured in the journal Nature, experts at the agency said the gas was detected by the Rosetta spacecraft while conducting an analysis of the 67P/Churyumov-Gerasimenko comet.

The new discovery has led researchers to rethink what elements could have been present in the solar system in earlier times.

"It was a big surprise to actually detect the O2 [oxygen]," Andre Bieler, a researcher from the University of Michigan and one of the co-leaders of the study, said.

What makes the recent discovery perplexing to the researchers is that it was initially believed that such molecular oxygen would likely have been annihilated during the formation of the solar system.

This type of gas is considered to be highly reactive to hydrogen, which was known to be in abundance in space while the sun and planets were being created.

According to present-day models of the solar system, molecular oxygen should no longer have existed by the time the 67P comet, approximately 4.6 billion years ago in the past.

While the findings point to a need for revising current solar system models, the researchers, who mostly have backgrounds in cometary science, were limited in providing insights as to how to modify these models.

Possible Molecular Oxygen on Halley's and Other Comets

Aside from the 67P comet, scientists are also trying to look for molecular oxygen on Halley's Comet based on data collected by the Giotto spacecraft in 1986.

Despite identifying faint traces of oxygen from the comet, these were not enough to confirm that molecular oxygen is indeed present in other comets.

The ESA's Rosetta spacecraft has spent over a year on the trail of the 67P comet as it transited around the sun.

The exploration vehicle was able to detect several elements in the gas cloud around the comet's rocky nucleus known as a coma. Some of the elements Rosetta identified included traces of carbon monoxide, carbon dioxide and water, which researchers also observed in other comets.

The discovery of molecular oxygen on the 67P comet, however, was not anticipated. The mass spectrometer on the Rosetta, called ROSINA-DFMS, was able to detect the gas for several months. The researchers had to make sure that the detection of the oxygen was indeed true and not a result of a glitch in the spacecraft's instrument.

The research team found that the oxygen was considerably denser when the Rosetta approached the comet but then became less dense when the vehicle moved farther away.

The gas also appeared to "follow" the 67P comet, and its quantities remained constant even when the comet shed some of its external layers to the sun.

With the presence of the molecular oxygen on the 67P comet confirmed, astronomers are now trying to figure out how the gas came to be on the transiting space rock to begin with.

Theories behind the Presence of Molecular Oxygen

Two of the leading theories the scientists are debating on involve the possibility that the oxygen was a result of the freezing or dissolving of icy grains or the breaking down of water ice by a bombardment of radioactive particles.

The first theory suggests that the molecular oxygen were likely formed because of the dissolution of grains of ice, which scientists believe could have constructed the 67P comet itself.

Other experts, however, contest this theory because of the rarity of oxygen in the solar system. There have only been two other instances the gas was detected beyond the solar system, thought researchers could not determine the exact location.

Chemistry also suggests that the gas should have been converted into water ice in space instead of remaining as molecular oxygen.

The second theory features the possibility that the 67P's water ice disintegrated after radioactive particles hit the regolith, or the dust found on the surface of the comet.

Bieler explained that the atoms of oxygen and hydrogen that form water could be broken down into gaseous molecular oxygen, which would subsequently be combined with voids formed from the ice.

Some researchers believe the molecular oxygen on the 67P comet must have already existed even before the solar system came about around 4.5 billion years ago. It is likely that particles of high energy could have bombarded the dark nebulae, known as the sun's birthplace, and broke down the water found in the nebula into hydrogen and oxygen.

This second theory is supported by data from the 67P suggesting that much of the comet's internal material predates the formation of the solar system. The composition of the materials also resembles dark nebulae.

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