[meteorite-list] First Detection of Molecular Oxygen At A Comet (Rosetta)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sun, 1 Nov 2015 20:55:27 -0800 (PST)
Message-ID: <201511020455.tA24tRK5002519_at_zagami.jpl.nasa.gov>

http://sci.esa.int/rosetta/56727-first-detection-of-molecular-oxygen-at-a-comet/

First detection of molecular oxygen at a comet
European Space Agency
28 October 2015

ESA's Rosetta spacecraft has made the first in situ detection of oxygen
molecules outgassing from a comet, a surprising observation that suggests
they were incorporated into the comet during its formation.

[Images]
Rosetta's detection of molecular oxygen at comet 67P/C-G. Credit: Spacecraft:
ESA/ATG medialab; comet: ESA/Rosetta/NavCam - CC BY-SA IGO 3.0; Data:
A. Bieler et al. (2015)

Rosetta has been studying Comet 67P/Churyumov-Gerasimenko for over a year
and has detected an abundance of different gases pouring from its nucleus.
Water vapour, carbon monoxide and carbon dioxide are the most prolific,
with a rich array of other nitrogen-, sulphur- and carbon-bearing species,
and even 'noble gases' also recorded.

Oxygen is the third most abundant element in the Universe, but the simplest
molecular version of the gas, O2, has proven surprisingly hard to track
down, even in star-forming clouds, because it is highly reactive and readily
breaks apart to bind with other atoms and molecules.

For example, oxygen atoms can combine with hydrogen atoms on cold dust
grains to form water, or a free oxygen split from O2 by ultraviolet radiation
can recombine with an O2 molecule to form ozone (O3).

Despite its detection on the icy moons of Jupiter and Saturn, O2 had been
missing in the inventory of volatile species associated with comets until
now.

"We weren't really expecting to detect O2 at the comet - and in such
high abundance - because it is so chemically reactive, so it was quite
a surprise," says Kathrin Altwegg of the University of Bern, and principal
investigator of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
instrument, ROSINA.

"It's also unanticipated because there aren't very many examples of the
detection of interstellar O2. And thus, even though it must have been
incorporated into the comet during its formation, this is not so easily
explained by current Solar System formation models."

The team analysed more than 3000 samples collected around the comet between
September 2014 and March 2015 to identify the O2. They determined an abundance
of 1-10% relative to H2O, with an average value of 3.80 +/- 0.85%, an
order of magnitude higher than predicted by models describing the chemistry
in molecular clouds.
The O2/H2O ratio at comet 67P/C-G. Credit: A. Bieler et al. [2015]

The amount of molecular oxygen detected showed a strong relationship to
the amount of water measured at any given time, suggesting that their
origin on the nucleus and release mechanism are linked. By contrast, the
amount of O2 seen was poorly correlated with carbon monoxide and molecular
nitrogen, even though they have a similar volatility to O2. In addition,
no ozone was detected.

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 O2/H2O ratio remained constant
over time, and it also did not change with Rosetta's longitude or latitude
over the comet.

In more detail, the O2/H2O ratio was seen to decrease for high H2O 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 O2 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 O2 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 O2 seen today.

More recent generation of O2 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 O2 then we would have expected
to see a decrease in the O2/H2O 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.

"The instantaneous generation of O2 also seems unlikely, as that should
lead to variable O2 ratios under different illumination conditions. Instead,
it seems more likely that primordial O2 was somehow incorporated into
the comet's ices during its formation, and is being released with the
water vapour today."

In one scenario, gaseous O2 would first be incorporated into water ice
in the early protosolar nebula stage of our Solar System. Chemical models
of protoplanetary discs predict that high abundances of gaseous O2 could
be available in the comet forming zone, but rapid cooling from temperatures
above -173C to less than -243C would be required to form water ice
with O2 trapped on dust grains. The grains would then have to be incorporated
into the comet without being chemically altered.

"Other possibilities include the Solar System being formed in an unusually
warm part of a dense molecular cloud, at temperatures of 10-20C above
the -263C or so typically expected for such clouds," says Ewine van
Dishoeck of Leiden Observatory in the Netherlands, co-author of the paper.

"This is still consistent with estimates for the comet formation conditions
in the outer solar nebula, and also with previous findings at Rosetta's
comet regarding the low abundance of N2."

Alternatively, radiolysis of icy dust grains could have taken place prior
to the comet's accretion into a larger body. In this case, the O2 would
remain trapped in the voids of the water ice on the grains while the hydrogen
diffused out, preventing the reformation of O2 to water, and resulting
in an increased and stable level of O2 in the solid ice.

Incorporation of such icy grains into the nucleus could explain the observed
strong correlation with H2O observed at the comet today.

"Regardless of how it was made, the O2 was also somehow protected during
the accretion stage of the comet: this must have happened gently to avoid
the O2 being destroyed by further chemical reactions," adds Kathrin.

"This is an intriguing result for studies both within and beyond the comet
community, with possible implications for our models of Solar System evolution,"
says Matt Taylor, ESA's Rosetta project scientist.
Notes for Editors

"Abundant molecular oxygen in the coma of comet 67P/Churyumov-Gerasimenko,"
by A. Bieler et al. is published in the 29 October 2015 issue of the journal
Nature.
For further information, please contact:

Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer at esa.int

Kathrin Altwegg
Principal investigator for ROSINA
University of Bern, Switzerland
Email: kathrin.altwegg at space.unibe.ch

Andre Bieler
University of Michigan
Email: abieler at umich.edu

Ewine van Dishoeck
Leiden Observatory, University of Leiden, the Netherlands
Email: ewine at strw.leidenuniv.nl

Matt Taylor
ESA Rosetta Project Scientist
Email: matt.taylor at esa.int
Received on Sun 01 Nov 2015 11:55:27 PM PST