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Posted by baalke on August 15, 2007, 12:50 pm
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http://www.news.uiuc.edu/news/07/0814enceladus.html
Frigid enceladus: an unlikely harbor for life
University of Illinois at Urbana-Champaign
Contact:
James E. Kloeppel, Physical Sciences Editor
1-217-244-1073 kloeppel@uiuc.edu
Released 8/14/07
CHAMPAIGN, Ill. - A new model of Saturn's icy moon Enceladus may
quell
hopes of finding life there. Developed by researchers at the
University of
Illinois, the model explains the most salient observations on
Enceladus
without requiring the presence of liquid water.
Orbiting Saturn since June 30, 2004, the Cassini spacecraft has
revealed a
south polar region of Enceladus with an elaborate arrangement of
fractures
and ridges, intense heat radiation and geyser-like plumes consisting
of ice
crystals and gases such as methane, nitrogen and carbon dioxide. The
plumes
erupt from vents located in large fractures called "tiger stripes"
that
cut across the south pole.
The plumes monitored by Cassini had a rate of discharge similar to
Old
Faithful geyser in Yellowstone National Park. Dubbed "Cold Faithful,"
the first model that was proposed to explain the plumes suggested the
plumes
tap into shallow pockets of liquid water in a water-ice shell.
Last year, U. of I. geology professor and planetary scientist Susan
Kieffer
and colleagues proposed an alternate model, which they called "Frigid
Faithful." In this model, the plumes originate in the dissociation of
certain stiff compounds of ice, called clathrates, which may cover
Enceladus
to a depth of tens of kilometers. The researchers published their
model in
the Dec. 15, 2006, issue of the journal Science.
"Frigid Faithful gives a straightforward account of the measured
composition, including the gases left unaccounted by Cold Faithful,"
said
Kieffer, who holds a Charles R. Walgreen Jr. Chair at Illinois and is
also a
professor in the University's Center for Advanced Study, one of the
highest forms of campus recognition.
"Perhaps more important, the plumes of Frigid Faithful could remain
active
far below the freezing point of water, under the frigid conditions
that
might be surmised inside a tiny, icy moon," Kieffer said.
Now, Kieffer, mechanical science and engineering professor Gustavo
Gioia,
geology research associate Pinaki Chakraborty and geology professor
and
department head Stephen Marshak have expanded the model to account for
both
the tectonic features and the heat transport in the southern
hemisphere.
They describe the model in a paper accepted for publication in the
Proceedings of the National Academy of Sciences and posted on the
journal's Web site.
By examining the deformation of a clathrate-rich shell containing a
mildly
warm heat source buried under the south pole, the researchers show it
is
possible for a frigid, stiff Enceladus without a shifting interior
(such as
plate tectonics on Earth) to develop fractures and ridges, and convey
heat
at the observed rate.
"As the heat source warmed at depth, it expanded and stretched the
clathrate-rich shell above, giving rise to tensile stresses in the
south
polar cap," said Gioia, lead author of the paper. "As a result, the
shell cracked, forming the four 130 kilometer-long fractures known as
tiger
stripes."
The researchers estimate the heat source could have been only 40
degrees
warmer than the surrounding shell. "In this model, the tiger stripes
are
analogous to the cracks that form in the glazing of a porcelain vessel
when
the vessel is filled with hot tea," Gioia said.
The researchers also show that, northwards of the south polar cap (in
which
the stresses were tensile), the stresses turned first from tensile to
compressive - forming the ring of ridges that circles the tiger
stripes
- and then back to tensile - forming the set of "starfish" fractures
that radiates northward from the ring of ridges. Thus the model
explains the
formation of the entire arrangement of fractures and ridges observed
by
Cassini on the southern hemisphere of Enceladus.
The Illinois researchers estimate the tiger stripes cut through the
shell of
Enceladus to a depth of about 35 kilometers. After the tiger stripes
formed,
the clathrates exposed on the cracked surfaces of the tiger stripes
were
decompressed. Upon decompression, the exposed clathrates absorbed heat
from
the source at depth and dissociated explosively, exposing more
clathrates to
decompression, in a process that continues today.
The gaseous products of clathrate dissociation rush up the tiger
stripes,
transporting heat to the surface where they may occasionally leak in
the
form of plumes. The transport of heat by fast-moving gases is called
"heat
advection." The cracked shell of Frigid Faithful acts as a gigantic
"advection machine," which efficiently conveys heat from the source
to
the surface.
In contrast to "heat conduction", where the transport of heat (in a
bar
of steel, for example) can only occur from points at higher
temperature
towards points at lower temperature, heat advection takes place at a
nearly
uniform temperature.
The implication is that Frigid Faithful's shell remains close to the
surface temperature to a depth of about 35 kilometers, Gioia said.
According
to the Cassini measurements, the surface temperature might be as many
as 150
degrees below the freezing point of water.
"This is indeed a frigid Enceladus," Gioia said. "It appears that
high
heat fluxes, geyser-like activity and complex tectonic features can
occur
even if moons do not have hot, liquid or shifting interiors."
While the Enceladus envisioned by the Illinois researchers is unlikely
to
possess liquid water and therefore unlikely to harbor life, it is
compatible
with the available evidence and is the only model that has been shown
to
explain the origin of the arrangement of fractures and ridges
documented by
Cassini."
The National Science Foundation funded the work.
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Posted by John Schutkeker on August 15, 2007, 4:50 pm
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baalke@earthlink.net wrote in news:1187196641.387665.322930
@b79g2000hse.googlegroups.com:
> http://www.news.uiuc.edu/news/07/0814enceladus.html
>
> Frigid enceladus: an unlikely harbor for life
>
> They describe the model in a paper accepted for publication in the
> Proceedings of the National Academy of Sciences and posted on the
> journal's Web site.
>
> The researchers also show that, northwards of the south polar cap (in
> which the stresses were tensile), the stresses turned first from
> tensile to compressive - forming the ring of ridges that circles the
> tiger stripes - and then back to tensile - forming the set of
> "starfish" fractures that radiates northward from the ring of ridges.
> Thus the model explains the formation of the entire arrangement of
> fractures and ridges observed by Cassini on the southern hemisphere of
> Enceladus.
Jeez, this looks like good work. !:)
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Posted by Dr J R Stockton on August 15, 2007, 5:26 pm
Please log in for more thread options In sci.astro message <1187196641.387665.322930@b79g2000hse.googlegroups.
com>, Wed, 15 Aug 2007 09:50:41, baalke@earthlink.net posted:
>CHAMPAIGN, Ill. - A new model of Saturn's icy moon Enceladus may
>quell
>hopes of finding life there. Developed by researchers at the
>University of
>Illinois, the model explains the most salient observations on
>Enceladus
>without requiring the presence of liquid water.
Such wrapping is horrible to read. I have written freely-available
software to fix it, packing all paragraphs to (default) 72 chars/line.
Paste it in, click, copy it out.
--
(c) John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v6.05 IE 6
news:comp.lang.javascript FAQ <URL:http://www.jibbering.com/faq/index.html>.
<URL:http://www.merlyn.demon.co.uk/js-index.htm> jscr maths, dates, sources.
<URL:http://www.merlyn.demon.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links.
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