#rosettawatch: IF THE comet lander Philae were alive, it might be able to breathe. The Rosetta spacecraft has discovered oxygen in 67P Churyumov-Gerasimenko’s atmosphere – and the team thinks it may date back to the birth of the solar system, when comets and planets first formed.

Molecular oxygen – the kind we breathe – has never been seen on a comet before. Oxygen is a volatile chemical that shouldn’t stick around for long in space, and the team couldn’t be sure that it wasn’t coming from the spacecraft itself. But after seven months of observing, they were confident that the oxygen had been buried in the comet since its birth about 4.5 billion years ago (Nature, DOI: 10.1038/nature15707).

Oxygen levels stayed high from September 2014 to March 2015, even while the comet’s upper few centimetres hissed away into space. That rules out chemical reactions between ice and sunlight, which would only have happened in the comet’s outermost skin, and hence would have declined as this layer disintegrated.


The oxygen levels also varied in step with water levels as Rosetta flew around the comet, suggesting that ice and oxygen in 67P’s atmosphere are coming from the same places in its nucleus. That could help solve a mystery about when oxygen first appeared in the solar system.

“This goes to the core of the question of whether this primitive solar system material was made in the interstellar medium, or whether it was made in the solar nebula later,” says Klaus Pontoppidan of the Space Telescope Science Institute in Maryland.

Astronomers have long searched for oxygen in the kinds of interstellar clouds where we think solar systems are born, but without much luck. So how did 67P come to have oxygen?

Andre Bieler of the University of Michigan and his team have two theories. In one scenario, interstellar clouds contain tricky-to-see oxygen gas, which got caught up as the cloud collapsed into a disc, then flash-froze and stuck to tiny grains of ice. But the more likely explanation, they think, is that the oxygen was chemically made later – inside ice grains in the disc, where it stayed trapped for eons.

Either theory would require that when grains combined into pebbles, boulders and eventually the comet, they never got too hot or pressurised. “The whole accretion of these icy grains has to be pretty gentle,” Bieler says.

Recent models of 67P’s formation agree with the idea that the comet’s birth was relatively calm. The fusion of its two lobes seems to have been a peaceful affair, not a dramatic collision.

That’s not the only comet chemistry in the news this week. In another study, researchers observing comet Lovejoy, which veered closest to the sun in January, spotted alcohol and simple sugars in its atmosphere – another first for comets (Science Advances, DOI: 10.1126/sciadv.1500863). The Rosetta team says 67P hosts similar stuff.

If other comets have oxygen, this glimpse into the early solar system’s chemistry could help us piece together a record of conditions on early Earth, too.

“It’s linked to how well we know the composition of the planets early on, especially with regards to habitability,” Pontoppidan says. “It changes how we may understand the conditions for life.”

(Image: ESA)

This article appeared in print under the headline “Rosetta’s comet is first with oxygen”