"It's Molecular, my dear Watson"

As the third most abundant element in the Universe, you would think finding two of these atoms joined together wouldn’t exactly be Astronomy’s version of a detective mystery. However, up until now definitive evidence of molecular oxygen, a necessary molecule for life, has eluded scientists. This month the solar system’s best private investigators will publish their results into exactly where it’s been hiding.

Earth’s own atmosphere has been pulling the wool over scientist’s eyes for years. Elements in space are detected using wavelength spectra, and specific bright spots seen on these spectra are clues to the identity of each element. The Earth’s atmosphere is composed of about 21% oxygen. Unfortunately this was found to be absorbing any evidence of those wavelengths specific to oxygen. Only recent advances in technology have allowed us to measure spectra from space, without interference.  

In theory oxygen should only be 100 000 times less abundant than hydrogen. However, the Herschel Space Observatory scientists have been repeatedly baffled to find, experimentally, it is 10 000 000 times less abundant. However, as Sherlock Holmes once said “It is a capital mistake to theorize before one has all the data.” So where is all the oxygen?

The new evidence suggests the culprit is water ice. Oxygen atoms are disguising themselves as water ice by clinging to dust grains in freezing cold parts of the universe. However, the Orion Nebula, has caused their mask to fall. The Orion Nebula, is the nearest massive star forming region to Earth. Here newborn stars radiate heat, which is able to evaporate the ice, allowing molecular oxygen to form and leaving it exposed to discovery.

"Theory suggests we should find lots of oxygen atoms locked in molecular oxygen (O₂), but previous searches kept falling short of such a large amount," comments Paul Goldsmith from the Jet Propulsion Laboratory in the US. Goldsmith is the NASA Herschel Project Scientist "With these new data, we finally have a strong hint at where cosmic oxygen might be hiding," he adds.

The mystery of the missing molecule may have been solved for now, but the Herschel observations of extraordinary sources project has many more riddles to help unravel yet.

Ref: P. Goldsmith, R. Liseau, et al., "Herschel measurements of molecular oxygen in Orion", Astrophysical Journal, in press.