Evidence for more dust than ice in comets (Forwarded)

ESA News
http://www.esa.int

12 October 2005

Evidence for more dust than ice in comets

Observations of Comet 9P/Tempel 1 made by ESA's Rosetta spacecraft after
the Deep Impact collision suggest that comets are 'icy dirtballs',
rather than 'dirty snowballs' as previously believed.

Comets spend most of their lifetime in a low-temperature environment far
from the Sun. Their relatively unchanged composition carries important
information about the origin of the Solar System.

On 4 July this year, the NASA Deep Impact mission sent an 'impactor'
probe to hit the surface of Comet 9P/Tempel 1 to investigate the
interior of a cometary nucleus.

The 370 kg copper impactor hit Comet Tempel 1 with a relative velocity
of 10.2 kilometres per second. The collision was expected to generate a
crater with a predicted diameter of about 100-125 metres and eject
cometary material. It vaporised 4500 tonnes of water, but surprisingly
released even more dust.

Tempel 1's icy nucleus, roughly the size of central Paris, is dynamic
and volatile. Possibly the impact would also trigger an outburst of dust
and gas, and produce a new active area on the comet's surface.

Just before impact, the Hubble Space Telescope spotted a new jet of dust
streaming from the icy comet. No one knows for sure what causes these
outbursts.

Rosetta, with its set of very sensitive instruments for cometary
investigations, used its capabilities to observe Tempel 1 before, during
and after the impact.

At a distance of about 80 million kilometres from the comet, Rosetta was
in the most privileged position to observe the event.

European scientists using Rosetta's OSIRIS imaging system observed the
comet's nucleus before and after the impact. OSIRIS comprises a
narrow-angle camera (NAC) and a wide-angle camera (WAC). Both cameras
imaged the extended dust coma from the impact in different filters.

OSIRIS measured the water vapour content and the cross-section of the
dust created by the impact. The scientists could then work out the
corresponding dust/ice mass ratio, which is larger than one, suggesting
that comets are composed more of dust held together by ice, rather than
made of ice comtaminated with dust. Hence, they are now 'icy dirtballs'
rather than 'dirty snowballs' as previously believed.

The scientists did not find evidence of enhanced outburst activity of
Comet 9P/Tempel 1 in the days after the impact, suggesting that, in
general, impacts of meteoroids are not the cause of cometary outbursts.
Scientists also hope to make a 3D reconstruction of the dust cloud
around the comet by combining the OSIRIS images with those taken from
ground observatories.

For more information:

Michael Küppers, lead author of the results
Max-Planck Institut für Sonnensystemforschung
Katlenburg-Lindau, Germany
E-mail: kueppers @ mps.mpg.de

Gerhard Schwehm, ESA Rosetta Project Scientist
E-mail: gerhard.schwehm @ esa.int

The paper by Küppers and colleagues entitled 'A large dust/ice ratio in
the nucleus of comet 9P/Tempel 1' has been featured in the 12 October
press release for Nature.

More about...

* ESA's comet chaser: Rosetta
http://www.esa.int/SPECIALS/Rosetta/index.html
* Rosetta factsheet
http://www.esa.int/esaSC/SEMJUZS1VED_index_0.html
* XMM-Newton factsheet
http://www.esa.int/esaSC/SEM14YS1VED_index_0.html
* Hubble factsheet
http://www.esa.int/esaSC/SEMB5E1A6BD_index_0.html

Related articles

* Life of a comet
http://www.esa.int/SPECIALS/Rosetta/SEM3NV0PGQD_0.html
* Tempel 1: Biography of a comet
http://www.esa.int/SPECIALS/Rosetta/SEMIUG0DU8E_0.html
* Rosetta monitors Deep Impact
http://www.esa.int/esaCP/SEMCOZ1DU8E_index_0.html
* XMM-Newton to observe Deep Impact
http://www.esa.int/esaCP/SEMSZC2DU8E_index_0.html
* Hubble sees outburst from Deep Impact comet
http://www.esa.int/esaCP/SEMSBN5DIAE_index_0.html
* ESA observes Deep Impact from Earth
http://www.esa.int/SPECIALS/Rosetta/SEMNRO5DIAE_0.html
* Dust and gas from Comet 9P/Tempel 1 seen by ESA OGS
http://www.esa.int/esaCP/SEMQR06DIAE_index_0.html
* Tempel 1 is weak X-ray source, XMM-Newton confirms
http://www.esa.int/esaCP/SEMXA46DIAE_index_0.html

Related links

* NASA Deep Impact
http://www.nasa.gov/mission_pages/de...ain/index.html
* ESO Deep Impact news
http://www.eso.org/outreach/press-re.../pr-15-05.html

IMAGE CAPTION:

[Image 1:
http://www.esa.int/esaCP/SEMUSK5Y3EE_index_1.html]
This image shows the initial ejecta that resulted when NASA's Deep
Impact probe collided with Comet 9P/Tempel 1 at 07:52 CEST, 4 July 2005.
It was taken by the spacecraft's high-resolution camera 13 seconds after
impact. The image has been digitally processed to better show the
comet's nucleus.

Credits: NASA/JPL-Caltech/UMD

[Image 2:
http://www.esa.int/esaCP/SEMUSK5Y3EE...html#subhead1]
The Hubble Space Telescope was 120 million kilometres away from the
comet when this image was taken by the high-resolution Advanced Camera
for Surveys (ACS). The telescope's views complement close-up images
being taken by cameras aboard Deep Impact, which is speeding toward the
comet.

This image, snapped at 14:15 (UT/GMT) on 14 June, reveals the 'jet' (the
bright fan-shaped area). The jet extends about 2200 kilometres, which is
roughly the distance from Copenhagen to Athens. It is pointing in the
direction of the Sun.

Credits: NASA, ESA, P. Feldman (Johns Hopkins University), and H. Weaver
(Applied Physics Lab)