Oxygen is obviously one of the gases we care most about here on Earth, and so it is not surprising that astronomers have been trying for over fifty years to determine its abundance in the cosmos. Oxygen not only lets us breathe, it is a key chemical constituent in space, combining with many other elements like carbon to form molecules like CO (carbon monoxide) that help to control the temperatures of prestellar, molecular clouds through a variety of thermodynamic processes. What is surprising is that astronomer still cannot find much of the oxygen in space. If the amount of oxygen we see on Earth (relative to other elements) is representative of its cosmic abundance, and scientists think that it is, then huge amounts of oxygen are hiding somewhere in space.
SAO astronomer Gary Melnick is part of a team of eight astronomers that used the Spitzer Space Telescope to search for one particularly promising reservoir for "unseen" oxygen: solid grains of carbon dioxide (CO_2) ice. The scientists observed eight stars buried deep within cold gas and dust in the interstellar medium in the constellation of Taurus. They discovered the absorption signature of CO_2 ice in all of these sources, and by careful modeling of the strength of that signature they determined that about one-quarter of all of the oxygen in space is neither in the form of atomic nor molecular gas, but is bound up in CO_2 ice. They also note that an additional 30% or so of the oxygen is found in interstellar dust particles (silicates for example). So, according to these astronomers, more than half of cosmic oxygen is trapped in solids of one type or another, with another 10% as carbon monoxide (CO) gas. That still leaves about 35% of the expected oxygen unaccounted for, but the scientists suggest ways in which future research can prospect for this most beneficial of gases.