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标 题: [NS]Saturn's moon Enceladus surprisingly comet-like
发信站: BBS 未名空间站 (Sun Mar 30 16:24:40 2008)
Saturn's moon Enceladus surprisingly comet-like
* 21:24 26 March 2008
* NewScientist.com news service
* Stephen Battersby
Saturn's curious moon Enceladus appears to have the same chemical makeup as
a comet, according to the latest results from the Cassini probe. That's a
big surprise, as Enceladus should have formed in very different conditions
from those of comets.
On 12 March, Cassini flew through the huge plume of steam and other gases
that spews from fissures at the moon's south pole. A glitch prevented the
spacecraft's dust analyser from studying the makeup of the plume, but
another instrument, called the Ion and Neutral Mass Spectrometer (INMS), did
sample its chemistry.
As well as water vapour, the INMS detected carbon dioxide, methane and a
range of more complex organic chemicals such as propane.
"The organics are clearly there in abundance beyond what we expected," says
INMS lead scientist Hunter Waite of the Southwest Research Institute in San
Antonio, Texas, US. "And the composition is very like the composition of a
"This is very exciting," says Cassini scientist Julie Castillo of NASA's Jet
Propulsion Laboratory in Pasadena, California, US. "It indicates that
Enceladus and comets were made of the same initial materials, and/or
affected by similar internal processes," she told New Scientist.
That is rather puzzling because comets are thought to have formed far from
the Sun, out in the region of Uranus and Neptune, says INMS co-investigator
Roger Yelle of the University of Arizona in Tucson, US. Enceladus, on the
other hand, is thought to have grown within the "Saturnian subnebula" – the
cloud of gas that coalesced into Saturn and its major moons.
"The temperature and pressure should have been very different, so you should
get different gases," Yelle told New Scientist.
Enceladus is almost certainly not a captured giant comet, but cometary stuff
might have been incorporated into the moon. Ices from the outer solar
system might have infiltrated the Saturnian subnebula, suggests William
McKinnon of Washington University in St Louis, Missouri, US.
Or comets might have hit Enceladus during a period of upheaval in the solar
system around 4 billion years ago called the late heavy bombardment.
During the flyby, Cassini's infrared camera mapped the heat emissions of the
south pole more clearly than before, showing that a great quantity of heat
is coming out along the four fractures called "tiger stripes". Temperatures
along these stripes are higher than their surroundings by up to 90 °Celsius.
"The closer we look, the higher the temperatures," says John Spencer of the
Southwest Research Institute in Boulder, Colorado, US, who works on the
infrared detector. "It is entirely possible that there is liquid water below
the surface of these fractures."
Altogether, the results are quietly encouraging for the possibility of life
on this cold moon.
"We see on Enceladus the three basic requirements for the origin of life,"
says Larry Esposito of the University of Colorado in Boulder, principal
investigator of another Cassini instrument, the Ultraviolet Imaging
Spectrograph (UVIS). "There is water – although it may not be liquid –
plus organics and heat."
Cassini will revisit Saturn's comet impersonator in August and October, when
it might fly even closer to the moon.
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