[meteorite-list] Bizarre Boulders Litter Enceladus' Surface

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Jul 19 14:44:39 2005
Message-ID: <200507191843.j6JIhob23552_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn7692--bizarre-boulders-litter-saturn-moons-icy-surface.html
 
Bizarre boulders litter Saturn moon's icy surface
Stuart Clark
New Scientist
19 July 2005
 
The Cassini spacecraft has coasted to its closest encounter yet -
skimming just 175 kilometres above Saturn's icy moon Enceladus. But
astronomers are at a loss to explain its observations.

On 14 July, Cassini swooped in for an unprecedented close-up view of the
wrinkled moon. Its Imaging Science Subsystem (ISS) camera has since
returned pictures of a boulder-strewn landscape that is currently beyond
explanation. The "boulders" appear to range between 10 and 20 metres in
diameter in the highest-resolution images, which can resolve features
just 4 m across.

"That's a surface texture I have never seen anywhere else in the solar
system," says David Rothery, a planetary geologist at the Open
University in Milton Keynes, UK.

Cracks crisscross Enceladus's surface - possibly as a result of the moon
being repeatedly squeezed and stretched by the gravity of Saturn and
other moons nearby. But Rothery points out the boulders avoid - rather
than fill - the cracks. This might indicate that the fracturing took
place after the boulders had already formed.

Alien landscape

John Spencer, a Cassini team member at the Southwest Research Institute
in Boulder, Colorado, US, agrees that the images are puzzling. "You
would expect to see small craters or a smooth, snow-covered landscape at
this resolution," he told New Scientist. "This is just strange. In fact,
I have a really hard time understanding what I'm seeing."

NASA scientists have been locked in discussions since 15 July and are
expected to pass judgment on what they think this peculiar surface might
be later on Tuesday.

But Elizabeth Turtle, a Cassini imaging team member at the University of
Arizona in Tucson, US, warns there will be no quick answers. "Trying to
figure out what is going on is going to take a lot longer than a weekend
of swapped emails," she says.

Heat source

These images - like those from previous flybys - reveal a surface clawed
with fractures and swollen with ridges. It could point to a substantial
heat source within the moon, driving the internal convection of ice. And
this raises the possibility that Enceladus could possess a sub-surface
ocean similar to that on Jupiter's moon Europa.

That could be a problem, according to Spencer. Superficially, the two
worlds bear a passing resemblance, but Enceladus is six times smaller
than Europa. "Enceladus seems too small to have enough internal heat to
create a sub-surface ocean," he says. "But, since we don't understand
the surface, we might not understand the interior either," he says.
Turtle, however, is sceptical of the ocean hypothesis and says "we see
no evidence of liquid flows on the surface".

Key information in this debate may come from Cassini's Dual Technique
Magnetometer. It was fluctuations in Europa's magnetic field that
finally convinced scientists that it harboured a subsurface ocean.
Perhaps the same will be true of Enceladus. At present, the data is
being analysed by scientists at Imperial College in London, UK.

Regardless of the outcome, NASA has already decided that Enceladus is
worth an even closer look. They have scheduled another grazing flyby of
the moon in 2008, when Cassini will skim even closer than ever - to
within 100 km of the boulder-strewn surface.
Received on Tue 19 Jul 2005 02:43:49 PM PDT