[meteorite-list] Strange Comet Unlike Anything Known (Stardust)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jun 17 19:35:06 2004
Message-ID: <200406172334.QAA22184_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/stardust_results_040617.html

Strange Comet Unlike Anything Known
By Robert Roy Britt
space.com
17 June 2004

A detailed analysis of the comet Wild 2 (pronounced "Vilt 2") has left
astronomers astounded at an object that has no known peers in the solar
system.

The comet, examined in a close flyby in January by NASA's Stardust
spacecraft, has towering protrusions and steep-walled craters that seem
to defy gravity. More than a dozen jets of material shoot out from its
insides. Dust swirls around the comet in unexpectedly dense pockets.

Among the bizarre features are two depressions with flat floors and
nearly vertical walls that resemble giant footprints. They aren't
structured like typical impact craters. The features have been named
Left Foot and Right Foot in a new map of the comet, which is roughly 3
miles (5 kilometers) wide.

Only two other comets have been seen up close,
but both appeared fairly smooth and were nowhere near so heavily
cratered. Nor do the pockmarked surfaces common to asteroids and moons
bear much stylistic resemblance to the shapes seen on Wild 2.

"So far, as far as we know Wild 2 is a unique object," said Donald
Brownlee, an astronomy professor at the University of Washington and
Stardust's principal investigator.

Brownlee told SPACE.com that Wild 2 could represent a unique class of
comet. He and his colleagues had expected it to be relatively
featureless with a dusty, charcoal-like coating. Instead they found a
place riddled with apparently ancient impact craters. Broad mesas and
steep canyons stand out clearly.

It is more likely, Brownlee added, that Wild 2 will turn out to be a
garden variety comet once more of them are studied up close.

Brownlee's group and other research teams present four analyses of Wild
2 in the June 18 issue of the journal Science, building on preliminary
results released in March.

Almost no gravity

Scientists don't know exactly what comets are made of. But they're
thought to represent the composition of the outer solar system in its
primordial state. They're loaded with frozen water and other ices, plus
organic materials and silicates, or rock. Many theorists believe comets
delivered the water and other pre-biotic ingredients that led to life on
Earth.

Stardust flew to within 147 miles (237 kilometers) of Wild 2 on Jan. 2.
The observations -- and dust samples that will be returned to Earth in
2006 for lab study -- should improve understanding of the solar system's
formation.

Craters on Wild 2, presumably caused by run-ins with smaller objects,
are strangely free of the powder, rocks and other debris commonly seen
in impact craters on other bodies. Brownlee thinks this is because the
comet is a bit like hard, frozen dirt that takes a hit but is brittle,
so material flies out.

And because the comet is so small, the material does not fall back.

"There's almost no gravity at the surface," Brownlee said. "If you were
standing on [the surface], you could jump into orbit."

Yet Wild 2 is not a fractured pile of rubble that would all fly apart
when hit, as some astronomers expected. Brownlee: "We're sure this is a
rigid material because it can support cliffs and spires."

Spires shooting up from inside craters "looks like Monument Valley in
Arizona," Brownlee said at a press conference today. They likely formed
as material around them eroded, indicated that the comet has lost about
328 feet (100 meters) of its surface since birth.

What sort of material can crumble under impact, leave sheer walls and
allow its parent body to remain intact?

Previous research in Europe may provide the answer. Scientists made
artificial comets of finally powdered organic materials and ice in a
vacuum. "They always ended up with a rigid, crusty material," Brownlee
said. "If you were standing on it you might go right through it."

He suggested the consistency of the comet is something like freeze-dried
astronaut ice cream.

Comets 'blow up'

That all assumes the "footprints" and other depressions on Wild 2 are in
fact impact craters. Other forces could be at work.

"Comets do blow up unexpectedly," Brownlee pointed out, adding that
built-up internal pressure and "steam explosions" might be responsible
for some of the surface features.

In another baffling surprise, Brownlee said, dozens of photos
show no small craters on Wild 2, only the large craters that are
presumably billions of years old. Perhaps small craters erode away, he said.

Brownlee is also intrigued by the utter lack of similarities between
Wild 2 and Phoebe, a fairly small moon of Saturn recently imaged up
close by the Cassini spacecraft. Phoebe is thought to be a captured
object, having originated -- like Wild 2 -- beyond Neptune. But Phoebe's
gently sloping craters, which are riddled with boulders, resemble those
seen on asteroids. And Phoebe has many small craters embedded in larger,
older craters.

"It's fascinating that they're so different," Brownlee said in a
telephone interview.

The two objects may have started out with the same stuff, he speculates,
but then underwent different histories. Perhaps like larger objects --
planets and other moons -- Phoebe was once heated enough to melt its
insides, which produces a different chemical and mineral structure.

Phoebe is considerably larger than Wild 2 -- about 137 miles (220
kilometers) wide -- so self-gravity could have something to do with the
differences. And Phoebe has likely never traveled inside the orbit of
Saturn, so it probably has not been hit with solar radiation to the
extent now experienced by Wild 2.

Swarms of particles

Comet Wild 2 probably gathered itself together 4.5 billion years ago,
just after the Sun was born, in a region beyond Neptune known as the
Kuiper Belt.

In 1974 it had a close encounter with Jupiter and was thrown onto a new
orbit that brings it closer to the Sun. A comet loses material when it
approaches the Sun, as solar radiation causes ice from its surface to
"sublimate" into space, carring dust and larger particles with it. The
process creates a cloud of material that reflects sunlight and creates
the familiar head of a comet (scientists call it a coma) and sometimes a
tail.

Among the new findings: Wild 2 has lost about 3 feet (1 meter) of its
surface since 1974.

Stardust flew right through some of that coma material, avoiding
boulder-sized objects and grabbing more than 1,000 tiny bits in a
specially designed catcher's mitt.

"These things were like a thunderbolt," said Anthony Tuzzolino, a senior
scientist at the University of Chicago's Enrico Fermi Institute. "I
didn't anticipate running into this kind of show."

Stardust encountered two intense swarms of dust with relative voids in
between, suggesting that ejections from Wild 2 come in intense spurts.
Comet Halley, studied in a 1986 flyby, exhibited a much smoother
distribution of particles.

Large chunks of material are probably ejected from the comet and then
fragment into swarms of particles, said Thanasis Economou, another Fermi
Institute researcher involved in the study.

More to learn

Comets also lose material when jets of stuff apparently shoot out from
inside or, possibly, from surface pockets of especially volatile
material. A separate analysis of Stardust data found 20 jets, on both
the sunlit side of the comet and, surprisingly, the dark side.

The jets likely emanate from inside the comet, researchers concluded,
and they contain water, carbon dioxide and rock particles.

What remains of Wild 2, along with the bits captured by Stardust, is
considered pristine. Because this is the comet's first trip to the inner
solar system, its composition has been mostly unaltered for the billions
of years it spent in the deep freeze. Studying the samples in a lab
should provide clues to the state of the solar system at its birth.

But learning whether Wild 2 is common or unique will require visits to
other comets, Brownlee said. Two flashy missions are in the works:

The European Rosetta craft, recently launched, will
land on a comet in 2014. NASA's Deep Impact will slam a probe
into comet Tempel 1 on Independence Day 2005.

Claudia Alexander, a program scientist for Rosetta from NASA's Jet
Propulsion Laboratory, has modeled comets for years. She did not expect
the number of jets or their ability to lift the large chunks thought to
then break up and create the particle swarms. And she's surprised that
the comet is apparently not a loosely cemented rubble pile.

"I would have told you it wasn't going to be like that," Alexander said.
"We are astonished and intrigued."
Received on Thu 17 Jun 2004 07:34:53 PM PDT


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