Comet on 18 October 2015 – NavCam

Over the six-month study period, Rosetta was inbound towards the Sun along its orbit, and orbiting as close as 10–30 km from the nucleus. Despite the decreasing distance to the Sun, the O 2 /H 2 O ratio remained constant over time, and it also did not change with Rosetta’s longitude or latitude over the comet.

In more detail, the O 2 /H 2 O ratio was seen to decrease for high H 2 O abundances, an observation that might be influenced by surface water ice produced in the observed daily sublimation–condensation process.

The team explored the possibilities to explain the presence and consistently high abundance of O 2 and its relationship to water, as well as the lack of ozone, by first considering photolysis and radiolysis of water ice over a range of timescales.

In photolysis, photons break bonds between molecules, whereas radiolysis involves more energetic photons or fast electrons and ions depositing energy into ice and ionising molecules – a process observed on icy moons in the outer Solar System, and in Saturn’s rings. Either process can, in principle, lead to the formation and liberation of molecular oxygen.

Radiolysis will have operated over the billions of years that the comet spent in the Kuiper Belt and led to the build-up of O 2 to a few metres depth. But these top layers must all have been removed in the time since the comet moved into its inner Solar System orbit, ruling this out as the source of the O 2 seen today.

More recent generation of O 2 via radiolysis and photolysis by solar wind particles and UV photons should only have occurred in the top few micrometres of the comet.

“But if this was the primary source of the O 2 then we would have expected to see a decrease in the O 2 /H 2 O ratio as this layer was removed during the six-month timespan of our observations,” says Andre Bieler of the University of Michigan and lead author of the paper describing the new results in the journal Nature this week.