Exclusive oxygen finally spotted in space

Digital Sky Survey image of the region around the Orion Nebula.

LONDON: After decades of searching, oxygen molecules have finally been found in space by an international team of scientists.

The discovery revealed that oxygen is so hard to find because it is locked up inside ice in interstellar clouds. The finding, to be published in The Astrophysical Journal, helps to explain why until now, levels of oxygen detected in space were much lower than predicted.

"There was something wrong with the theory and that's always something that gets scientists going," said lead author Paul Goldsmith from NASA's Jet Propulsion Laboratory in Maryland. "We were determined to keep trying to resolve this mystery."

Hide and seek

The inset is an artist's impression of a collection of oxygen molecules, where two connected balls represent two oxygen atoms bound together as an oxygen molecule. The image of the Orion nebula was taken in infrared light by NASA's Spitzer Space Telescope.

Despite oxygen being the third most abundant element in the universe, scientists have previously failed to locate molecular oxygen, or O₂, in space. This is the form of oxygen consisting of two oxygen atoms, the same kind we need to breathe to survive.

Atomic oxygen, which consists of a single oxygen atom and is highly reactive, has been spotted, but even atomic oxygen seems to be much more elusive than expected.

Now, Goldsmith and his colleagues report that they have finally found molecular oxygen in the Orion Nebular. "We thought that Orion would be a good choice because it has a lot of warm dust and gas in it, which according to our models should contain molecular oxygen," said Goldsmith.

To search for oxygen, the team used the most powerful telescope in space - Hershel. The telescope is equipped with a piece of kit called a HIFI spectrometer, which can search for oxygen at several different frequencies. It is much more sensitive than satellites used in the past because of this on-board technology, and because it has a bigger telescope.

Water lock

Using the sensitive beams of the Hershel telescope, the team detected signals that are indicative of there being O₂ in Orion.

According to Goldsmith, oxygen is incredibly elusive in space because interstellar clouds are so cold that the oxygen atoms end up locked into water ice. Oxygen atoms bump into dust grains and stick to them, then hydrogen atoms interact with the oxygen to form water.

The result is so-called 'icy grain mantels' - water ice stuck to dust particles. "This is where all the oxygen is locked up," Goldsmith said. Once a star is formed and it starts heating up these clouds, the ice comes off the grains and some of the water will get converted to molecular oxygen.

Giant step forward

In the past, any possible identification of molecular oxygen in space was never confirmed, but the new study detected oxygen at three different frequencies. This reduces the chance of error - that the results come from some other molecule that happens to give off energy at that frequency. "The fact that we saw three different frequencies means we are relatively confident the signal did come from molecular oxygen," Goldsmith said.

"The authors have made a giant step forward to solving the mystery of why there is so little O₂ in dense molecular clouds where stars form," said Sara Seager from the Massachusetts Institute of Technology. "The Herschel spacecraft has done a great job with a robust measurement of molecular oxygen, still finding that there is much less O₂ than expected from chemistry theory."

"I think the most important aspect of this detection is to guide us in the future in targeting just the right regions: warm enough to contain O₂ gas but not too warm or UV-irradiated to destroy the molecule," added Daniel Apai, from the University of Arizona. "With more Herschel observations it will be possible to map the O₂ content of the region and better understand the processes in which it is involved."

Next, Goldsmith's team plans to investigate why it is they found O₂ in this location but not elsewhere. "We can't really believe that one spot in Orion is totally unique, but it might be that it's a little bit warmer. The new stars in Orion are massive, putting out lots of light and heat," Goldsmith said. "But we have a hint at one other place and we're hoping we can confirm that."