[meteorite-list] Rosetta Reveals Comet's Water-Ice Cycle

[Ron Baalke]

http://sci.esa.int/rosetta/56513-rosetta-reveals-comet-s-water-ice-cycle/

Rosetta reveals comet's water-ice cycle
European Space Agency
23 September 2015

ESA's Rosetta spacecraft has provided evidence for a daily water-ice cycle 
on and near the surface of comets.

Comets are celestial bodies comprising a mixture of dust and ices, which 
they periodically shed as they swing towards their closest point to the 
Sun along their highly eccentric orbits.

As sunlight heats the frozen nucleus of a comet, the ice in it - mainly 
water but also other 'volatiles' such as carbon monoxide and carbon dioxide 
- turns directly into a gas.

This gas flows away from the comet, carrying dust particles along. Together, 
gas and dust build up the bright halo and tails that are characteristic 
of comets.

[Image]
Hapi region on comet 67P/C-G.
Credit: ESA/Rosetta/VIRTIS/INAF-IAPS/OBS DE PARIS-LESIA/DLR; M.C. De Sanctis 
et al. (2015)

Rosetta arrived at Comet 67P/Churyumov-Gerasimenko in August 2014 and 
has been studying it up close for over a year. On 13 August 2015, the 
comet reached the closest point to the Sun along its 6.5-year orbit, and 
is now moving back towards the outer Solar System.

A key feature that Rosetta's scientists are investigating is the way in 
which activity on the comet and the associated outgassing are driven, 
by monitoring the increasing activity on and around the comet since Rosetta's 
arrival.

Scientists using Rosetta's Visible, InfraRed and Thermal Imaging Spectrometer, 
VIRTIS, have identified a region on the comet's surface where water ice 
appears and disappears in sync with its rotation period. Their findings 
are published today in the journal Nature.

"We found a mechanism that replenishes the surface of the comet with fresh 
ice at every rotation: this keeps the comet 'alive'," says Maria Cristina 
De Sanctis from INAF-IAPS in Rome, Italy, lead author of the study.

The team studied a set of data taken in September 2014, concentrating 
on a one square km region on the comet's neck. At the time, the comet 
was about 500 million km from the Sun and the neck was one of the most 
active areas.

As the comet rotates, taking just over 12 hours to complete a full revolution, 
the various regions undergo different illumination.

"We saw the tell-tale signature of water ice in the spectra of the study 
region but only when certain portions were cast in shadow," says Maria 
Cristina.

"Conversely, when the Sun was shining on these regions, the ice was gone. 
This indicates a cyclical behaviour of water ice during each comet rotation."

The data suggest that water ice on and a few centimetres below the surface 
'sublimates' when illuminated by sunlight, turning it into gas that then 
flows away from the comet. Then, as the comet rotates and the same region 
falls into darkness, the surface rapidly cools again.

However, the underlying layers remain warm owing to the sunlight they 
received in the previous hours, and, as a result, subsurface water ice 
keeps sublimating and finding its way to the surface through the comet's 
porous interior.

But as soon as this 'underground' water vapour reaches the cold surface, 
it freezes again, blanketing that patch of comet surface with a thin layer 
of fresh ice.

Eventually, as the Sun rises again over this part of the surface on the 
next comet day, the molecules in the newly formed ice layer are the first 
to sublimate and flow away from the comet, restarting the cycle.

"We suspected such a water ice cycle might be at play at comets, on the 
basis of theoretical models and previous observations of other comets 
but now, thanks to Rosetta's extensive monitoring at 67P/Churyumov-Gerasimenko, 
we finally have observational proof," says Fabrizio Capaccioni, VIRTIS 
principal investigator at INAF-IAPS in Rome, Italy.

>From these data, it is possible to estimate the relative abundance of 
water ice with respect to other material. Down to a few cm deep over the 
region of the portion of the comet nucleus that was surveyed, water ice 
accounts for 10-15% of the material and appears to be well-mixed with 
the other constituents.

The scientists also calculated how much water vapour was being emitted 
by the patch that they analysed with VIRTIS, and showed that this accounted 
for about 3% of the total amount of water vapour coming out from the whole 
comet at the same time, as measured by Rosetta's MIRO microwave sensor.

"It is possible that many patches across the surface were undergoing the 
same diurnal cycle, thus providing additional contributions to the overall 
outgassing of the comet," adds Dr Capaccioni.

The scientists are now busy analysing VIRTIS data collected in the following 
months, as the comet's activity increased around the closest approach 
to the Sun.

"These initial results give us a glimpse of what is happening underneath 
the surface, in the comet's interior," concludes Matt Taylor, ESA Rosetta 
Project Scientist.

"Rosetta is capable of tracking changes on the comet over short as well 
as longer time scales, and we are looking forward to combining all of 
this information to understand the evolution of this and other comets."
Notes for Editors

"The diurnal cycle of water ice on comet 67P/Churyumov-Gerasimenko," 
by Maria Cristina De Sanctis et al. is published in the 24 September 2015 
issue of Nature.

The results are based on images and spectra taken at visible and infrared 
wavelengths of light on 12-14 September 2014 with VIRTIS.

These results will be presented next week at the European Planetary Science 
Congress, taking place from 27 September to 2 October 2015 in Nantes, 
France.

About Rosetta

Rosetta is an ESA mission with contributions from its Member States and 
NASA. Rosetta's Philae lander is contributed by a consortium led by DLR, 
MPS, CNES and ASI.

For further information, please contact:

Maria Cristina De Sanctis
INAF-IAPS, Rome, Italy
Email: mariacristina.desanctis at iaps.inaf.it

Fabrizio Capaccioni
VIRTIS principal investigator
INAF-IAPS, Rome, Italy
Email: fabrizio.capaccioni at iaps.inaf.it

Matt Taylor
ESA Rosetta Project Scientist
Email: matt.taylor at esa.int

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