[meteorite-list] Frozen Comet Had a Watery Past, UA Scientists Find (Stardust)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 6 Apr 2011 09:53:02 -0700 (PDT)
Message-ID: <201104061653.p36Gr2qT020138_at_zagami.jpl.nasa.gov>

http://uanews.org/node/39041

Frozen Comet Had a Watery Past, UA Scientists Find
By Daniel Stolte
University of Arizona
April 5, 2011

The discovery of minerals requiring liquid water for their
formation challenges the paradigm of comets as "dirty snowballs"
frozen in time.

For the first time, scientists have found convincing evidence for the
presence of liquid water in a comet, shattering the current paradigm
that comets never get warm enough to melt the ice that makes up the bulk
of their material.

"Current thinking suggests that it is impossible to form liquid water
inside of a comet," said Dante Lauretta,
an associate professor of cosmochemistry and planet formation at the
UA's Lunar and Planetary Laboratory. Lauretta is the principal
investigator of the UA team involved in analysis of samples returned by
NASA's Stardust mission.

UA graduate student Eve Berger, who led the study, and her colleagues
from Johnson Space Center and the Naval Research Laboratory made the
discovery analyzing dust grains brought back to Earth from comet Wild-2
as part of the Stardust mission. Launched in 1999, the Stardust
spacecraft scooped up tiny particles released from the comet's surface
in 2004 and brought them back to Earth in a capsule that landed in Utah
two years later.

"In our samples, we found minerals that formed in the presence of liquid
water," Berger said. "At some point in its history, the comet must have
harbored pockets of water."

The discovery is to be published in an upcoming online edition of the
journal Geochimica et Cosmochimica Acta.

<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V66-52FDSX8-6&_user=9555371&_coverDate=03%2F22%2F2011&_alid=1706890677&_rdoc=1&_fmt=high&_orig=search&_origin=search&_zone=rslt_list_item&_cdi=5806&_sort=r&_st=13&_docanchor=&view=c&_ct=1&_acct=C000055186&_version=1&_urlVersion=0&_userid=9555371&md5=0801055df7bbf1b7d7b07b4e9f9d1ad7&searchtype=a>

Comets are frequently called dirty snowballs because they consist of
mostly water ice, peppered with rocky debris and frozen gases. Unlike
asteroids, extraterrestrial chunks made up of rock and minerals, comets
sport a tail - jets of gas and vapor that the high-energy particle
stream coming from the sun flushes out of their frozen bodies.

"When the ice melted on Wild-2, the resulting warm water dissolved
minerals that were present at the time and precipitated the iron and
copper sulfide minerals we observed in our study," Lauretta said. "The
sulfide minerals formed between 50 and 200 degrees Celsius (122 and 392
degrees Fahrenheit), much warmer than the sub-zero temperatures
predicted for the interior of a comet."

Discovered in 1978 by Swiss astronomer Paul Wild, Wild-2 (pronounced
"Vilt") had traveled the outer reaches of the solar system for most of
its 4.5 billion year history, until a close encounter with Jupiter's
field of gravity sent the 3.4 mile-wide comet onto a new, highly
elliptical orbit bringing it closer to the sun and the inner planets.

Scientists believe that like many other comets, Wild-2 originated in the
Kuiper belt, a region extending from beyond Neptune's orbit into deep
space, containing icy debris left over from the formation of the solar
system. Wild-2 is thought to have spent most of its time in the Kuiper
belt, transiting on unstable orbits within the planetary system before
Jupiter's gravity hurled it into the neighborhood of the sun.

The discovery of the low-temperature sulfide minerals is important for
our understanding of how comets formed - which in turn tells us about
the origin of the solar system.

In addition to providing evidence of liquid water, the discovered
ingredients put an upper limit to the temperatures Wild-2 encountered
during its origin and history.

"The mineral we found - cubanite - is very rare in sample collections
from space," Berger said. "It comes in two forms - the one we found only
exists below 210 degrees Celsius (99 degrees Fahrenheit). This is
exciting because it tells us those grains have not seen temperatures
higher than that. "

Cubanite is a copper iron sulfide, which is also found in ore deposits
on Earth exposed to heated groundwater and in a particular type of
meteorite.

"Wherever the cubanite formed, it stayed cool," she added. "If this
mineral formed on the comet, it has implications for heat sources on
comets in general."

According to Berger, two ways to generate heat sources on comets are
minor collisions with other objects and radioactive decay of elements
present in the comet's mixture.

Heat generated at the site of minor impacts might generate pockets of
water in which the sulfides could form very quickly, within about a year
(as opposed to millions of years). This could happen at any point in the
comet's history. Radioactive decay on the other hand, would point to a
very early formation of the minerals since the radioactive nuclides
would decay over time and cause the heat source to flicker out.

The presence of the cubanite and the other sulfide minerals helps
scientists better understand cometary heat sources. The interior of the
comet must have been warm enough to melt ice yet cool enough - below 210
degrees Celsius - to form cubanite.

"Such detailed thermal constraints will allow for detailed analysis of
the role temperature played during the history of comet Wild 2,"
Lauretta said.

Each sample Berger's team analyzed consisted of a microscopic speck of
cometary dust about the size of a bacterial cell. The group then studied
the chemical composition by electron microscopy and X-ray analysis,
during which the chemical elements revealed their presence by giving off
a characteristic beam. Turning the sample in different orientations gave
the scientists clues about its crystallographic structure.

According to Lauretta, the findings show that comets experienced
processes such as heating and chemical reactions in liquid water that
changed the minerals they inherited from the time when the solar system
was still a protoplanetary disk, a swirling mix of hot gases and dust,
before it cooled down enough for planets to form.

The results demonstrate the increasingly apparent connections between
comets and asteroids.

"What we found makes us look at comets in a different way," Lauretta
said. "We think they should be viewed as individual entities with their
own unique geologic history."

"This study shows the high science value of sample return missions,"
Lauretta said. "These grains would never been detected by remote sensing
or by flying a spacecraft past the comet to make observations without
collecting a sample."

Lauretta believes so strongly in the value of sample return missions
that he has spent the past seven years developing the OSIRIS-REx
Asteroid Sample Return mission
<http://gsfctechnology.gsfc.nasa.gov/ORIRIS.htm>, which is currently a
finalist in the NASA New Frontiers mission competition. Selections are
expected in early June.
Received on Wed 06 Apr 2011 12:53:02 PM PDT