Oxygen is a very reactive element and finding molecular oxygen (O2) in space is quite rare. So it was surprising to see it being released by Rosetta’s comet in 2015. Several ideas were put forward to explain it and chemical engineers at Caltech think they have now figured it out.
In a paper published in Nature Communications, the team suggests the oxygen is produced from collisions. Water is released by the light from the Sun and then ionized by ultraviolet rays. It is then pushed back onto the surface of the comet where it interacts with the oxygen-rich minerals, liberating molecular oxygen.
The team, led by Professor Konstantinos P. Giapis, doesn’t usually work on astronomical problems. Their specialization is colliding high-speed ions with superconductors to create better computer chips. But high-speed ions are very common in space so they were intrigued to see if they could use their expertise elsewhere.
"I started to take an interest in space and was looking for places where ions would be accelerated against surfaces," Giapis said in a statement. "After looking at measurements made on Rosetta's comet, in particular regarding the energies of the water molecules hitting the comet, it all clicked. What I've been studying for years is happening right here on this comet."
The team tested this idea by producing a surface similar in composition to Comet 67P/Churyumov-Gerasimenko, which Rosetta studied for several years until last summer. The surface contained oxygen bound in molecules like sand and rust, which the researchers shot ionized water at. The surface then released molecular oxygen.
"This original chemistry mechanism is based on the seldom-considered class of Eley-Rideal reactions, which occur when fast-moving molecules, water in this case, collide with surfaces and extract atoms residing there, forming new molecules. All necessary conditions for such reactions exist on comet 67P," Giapis continued.
Co-author Yunxi Yao added: "We have shown experimentally that it is possible to form molecular oxygen dynamically on the surface of materials similar to those found on the comet."
An alternative idea suggested that the molecular oxygen was present (and trapped) in the ice of the comet. As the comet melts, the oxygen is released. A big question mark in this hypothesis though is why oxygen didn’t react with any other element for billions of years?
In space, oxygen is mostly seen as a single atom and in water molecules and carbon dioxide. So far, molecular oxygen has been detected only twice in deep space observations of star-forming nebula, so a mechanism that can form oxygen seems more likely than trapped bubbles beneath the ice.
Visual representation of ionized water molecules liberating molecular oxygen from a surface. Caltech