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文章阅读:[BBC]Sodium issue clouds Enceladus
[同主题阅读] [版面: 航空航天] [作者:ZZGR] , 2007年12月17日05:42:33
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发信人: ZZGR (闲逛), 信区: Aviation
标  题: [BBC]Sodium issue clouds Enceladus
发信站: BBS 未名空间站 (Mon Dec 17 05:42:33 2007)

By Molly Bentley
Science reporter, San Francisco

An ocean is not the source of the jets emanating from Saturn's moon
Enceladus, a new study concludes.

The research questions the moon's promise as a target in the search for life
beyond Earth and has stirred controversy among scientists who dispute its
conclusions.

A chemical analysis of Enceladus, led by University of Colorado planetary
scientist Nick Schneider, failed to detect sodium, an element scientists say
should be in a body of water that has had billions of years of contact with
rock.

"If you have a long-lived ocean, it's going to have salt in it," said Dr
Schneider, at the American Geophysical Union (AGU) meeting in San Francisco
this week, "but that ocean, if it exists on Enceladus, isn't leaking out
into space."

Spectral analysis with the Keck Telescope found no sodium in the plumes or
in the vapour that's in orbit around the moon.

The source of the plumes is "very, very pure water," Dr Schneider concluded,
and proposed clean ice, melt water or clathrates - a crystal of water,
carbon dioxide and ammonia - as alternative sources.

Heated debate

The fountains on Enceladus tantalise scientists by suggesting an ocean
beneath the moon's icy crust. An ancient sea is the best bet for where life
might evolve off Earth, scientists say.

At stake is whether Saturn's moon could support alien life and is a worthy
target for a US space agency (Nasa) exploratory mission to detect it. Such a
mission to Enceladus is one of four currently under review for further
development.

Dr Schneider didn't rule out the possibility of an ocean on Enceladus, only
that it is the source of the spraying water.

Critics of the study accept his observations, but disagree with his
conclusion; and it has led to some robust exchanges here at the AGU meeting
this past week.

"There is tremendous dispute about his interpretation of the results," said
Carolyn Porco, the Imaging Team leader on the Cassini spacecraft.

He may not have detected sodium, she said, but it did not follow that the
plumes were not connected to an ocean.

The absence of detectable sodium might mean only that; it's not detectible,
said Dr Porco. It could be in a solid form that eludes detection by this
method, she said.

Salty secrets

Dr Schneider used the Keck Telescope to look for a glow from sodium atoms,
the same colour found in sodium streetlights. He failed to detect it in the
plume or in the ring of particles that encircle Saturn at Enceladus' orbit.

Yet sodium is quite abundant in the Solar System, said Torrence Johnson,
Cassini Imaging Team member at Nasa's Jet Propulsion Laboratory.

"It's a very surprising result not to find sodium at all," he explained. "So
the question is: can you hide the sodium?"

One way to hide sodium is to put it in a salt crystal that becomes the
nucleus of a water particle. If a sodium atom were tied up in a solid form,
the Keck Telescope would not detect it. It only detects liberated sodium
atoms, he said.

"If you took salt from a salt-shaker and threw it into the air, the
telescope wouldn't see any sodium, even though half the salt is sodium,"
added Dr Johnson.

Dr Schneider said that the molecules would release sodium as they made their
way into the particles that encircle Saturn.

Near fly-by

Scientists have detected sodium around Jupiter's moons Io and Europa.
Volcanoes produce Io's sodium and their heat liberates it. Scientists
believe Jupiter's energetic radiation belt kicks the sodium out of minerals
that entrap it on Europa.

But Saturn is cooler in temperature and its radiation levels may also be too
weak to free sodium.

At any rate, said Dr Johnson, the sodium-free test results do not rule out
the existence of an ocean on the moon or an ocean as the source of the
plumes.

The Cassini spacecraft is scheduled to fly close to the jets in March of
next year. It will analyse the water further, but is not able to test for
sodium.

Nonetheless, the discovery of water spewing from cracks - dubbed "tiger
stripes" -on Enceladus in 2006, has promoted the moon into an elite club of
outer Solar System bodies that are considered high priority for future Nasa
missions.

Design competition

The possibility that the plumes tap directly into a lunar ocean is the
impetus behind a flagship mission that would explore Enceladus further.

A spacecraft that flew through the spray might be designed to sample the
water directly and run tests to detect the presence of alien microbes.

The mission is much more difficult - and expensive - if the data is not
obtainable at altitude and a lander has to be put on the icy surface to
reach it.

The Enceladus flagship mission is one of four - along with those to Europa,
Titan and Jupiter - competing for funding and currently under review by Nasa.

Dr Johnson says Dr Schneider's study might influence how inclined people are
to send a spacecraft to Enceladus and fly through the plumes.

"If Nick is right," he said, "all they'd see is pure water."

Nasa is scheduled to select which flagship missions will advance at the end
of December.

The concept that eventually emerges in the process will launch no earlier
than 2015 and is likely to include sizeable input from the Europeans.
--

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