[meteorite-list] A Comet's Tale (Stardust)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Feb 13 20:05:22 2006
Message-ID: <200602140103.k1E13eE18347_at_zagami.jpl.nasa.gov>


A comet's tale
Mark Peplow
13 February 2006

Scientists are just beginning to examine the pieces of a comet brought
back to Earth by NASA's Stardust mission. Mark Peplow tagged along to
one lab to watch researchers examine their prize catch.

It's not every day you get to play with a comet's tail. And for Phil
Bland, an expert on meteorites and space dust at Imperial College
London, UK, it's like having all his birthdays come at once. "I keep
having flashbacks to being a nine-year-old, dreaming about space," he grins.

"This is comet dust," he exclaims, brandishing an anonymous-looking
cardboard box. "How cool is that?"

The box has just arrived from NASA's Johnson Space Center in Houston,
Texas, but its contents have rather more distant origins. When the
Stardust probe swooped low over the comet Wild 2 on 2 January 2004, it
scooped up samples from the stream of dust that forms the comet's tail.

Embedded safely in a lightweight foam called aerogel, thousands of the
tiny grains were parachuted back to Earth on 15 January after travelling
more than a billion kilometres from the comet (see 'Comet dust delivered
to Earth <http://www.nature.com/news/2006/060116/full/060116-1.html>' ).

Stardust's scientists are wont to remind us, in hushed tones, that these
grains are the first geological samples returned from space for 20
years. "It's also the first time we've ever got material from a comet,"
says Bland. Stardust skimmed just 240 kilometres above the
5-kilometre-wide comet nucleus, picking up dust that had blown off the
comet's surface just minutes before.

Because comets are thought to be frosty leftovers from the Solar
System's formation, that dust has a lot of history that could help to
reveal where the Earth's oceans came from, and more besides.

Bland is one of the first scientists in the United Kingdom to get his
hands on some of these grains, each just a few micrometres, or
millionths of a metre, across. With so much precious dust to study, more
than 150 teams around the world are making a preliminary analysis to
find out what the probe brought back, and single out the most
interesting samples for further research.

What's in the box?

Jittery with excitement, Bland opens up the box to reveal the objects of
his affection, buried in bubble wrap. His team has received two types of
randomly selected sample dust grains, prepared for them by staff at NASA
to suit their analysis techniques. Both of these are the
'non-destructive' type: for the moment scientists are only allowed to
look at the samples without altering them, saving them for further
analyses down the line.

Bland's first sample is a credit-card-sized slab of plastic, which
appears to be dotted with 50 poppy seeds arranged in a rectangular grid.
Each of these grains, no bigger than the full stop at the end of this
sentence, is a fraction of a dust particle.

The second sample is made of epoxy resin and shaped like a small bullet.
A single grain is embedded in the very tip, which has been sliced and
polished to give a clean face. This is destined for the Natural History
Museum's scanning electron microscope (SEM).

Electron scan

SEMs can resolve tiny structural details by watching how a stream of
electrons bounce off a surface. But it can also reveal the presence of
different chemical elements from the characteristic X-rays given off as
the electrons collide with different atoms. So it should give Bland a
first glimpse of the minerals inside the grain.

We take the sample to the SEM laboratory, followed by a gaggle of
Bland's PhD students and resident SEM expert, Anton Kearsley. I expected
clean-room conditions, with white paper suits and airlocks to prevent
the samples becoming contaminated. How wrong I was. For these
experiments, air exposure doesn't significantly affect the mineral
chemistry, says Bland, which means that it's a lot easier to work with
the samples.

That's just as well, as Bland is getting impatient. "Come on, let's have
a look at it," he says, bouncing on his seat like a restless schoolboy.

"Three billion miles to get that?"

Kearsley cradles the epoxy bullet on a small mound of aluminium foil and
places it delicately inside the machine, which is the size of a small
refrigerator. He then manoeuvres the sample into precise position using
twin joysticks, while an internal camera relays magnified images of the
operation to a monitor. On screen, the bullet looks enormous, like a
missile rolling into place in a Bond villain's underground silo.

With ten of us crowded into the small room, and the lights dimmed to
avoid reflections on the screens, palpable tension steals over the group
as we crane forward to see the SEM images as they rise out of a static fog.

And there, behold, a series of smooth, interconnected surfaces that look
like a section from a geodesic dome: a perfect crystal. I'm impressed,
until Kearsley explains that this is merely the tip of the epoxy bullet.
The grain itself is there, he says, pointing at half a dozen bright
pixels lost in the middle of the screen. "Three billion miles to get
that?" mutters someone at the back of the room.

"Whatever it is, it's weird"

After fiddling around to improve the picture slightly, Kearsley starts a
more intensive scan of the grain that will reveal its chemical make-up.
As the analysis comes through, there are cries of surprise. "Whatever it
is, it's weird," says Bland.

The team agonize over the decision to make another scan to get more
accurate results, but Kearsley is worried about "frying" the sample.
"These grains have had a long journey and rather a lot of money spent on
them," he cautions.

They can see that just a few minutes exposure to high-energy electrons
has changed the structure of some of the epoxy surrounding the grain, so
they finally decide that the initial results are so astonishing that
they should contact Mike Zolensky at Johnson Space Center immediately to
tell him about the find, and wait for further instructions. Zolensky is
in charge of the preliminary analysis of the samples, and is collating
all the information from these first tests.

Poor fit

The team discusses the first results over lunch, before the second set
of samples in the credit-card grid are tackled in the afternoon by
Bland's colleague Matt Genge. He wants to look at them using
transmission electron microscopy (TEM), which delivers little chemical
information but can show up a lot more of the grain's internal structure.

But after an hour of work, Genge admits defeat. The samples in the grid
are a poor fit for the piece of brass designed to hold them inside the
microscope, and the grid itself is so delicate that Genge fears damaging
it irreparably. He plans to hunt down a different holder, delaying the
experiment until the following week.

Meanwhile, Bland and Kearsley spend almost two hours putting an e-mail
together to Zolensky. On several occasions they are about to send the
images and data, but suddenly notice another feature that sets them off
on a different train of thought, delaying the e-mail.

Wild and wacky

By the time the e-mail is finally sent, the day is almost done. Bland
will travel to Liverpool University, UK, the following week to use
another type of electron microscope that should reveal more about the
structure of crystals in the samples, helping to pin down exactly what
they are made of, and what conditions they formed in.

"But the first day was as exciting as I'd anticipated," says Bland. "And
to actually find something wacky straight off was way more than I expected."

Exactly what Bland found, however, isn't yet public information. NASA is
keen for all the results, from the many teams around the world, to be
gathered together before they are announced. "If it comes out in dribs
and drabs it'll be total chaos," explains Zolensky. He and his team will
put together all the results, which they expect to present at the Lunar
and Planetary Science Conference in Houston, Texas, in March.

"We're just happy we've done our bit," says Bland.

But Bland is still optimistic that their unusual find could get special
attention. "I'll be jumping up and down at the back, shouting, 'That's
my grain!'" he says.
Received on Mon 13 Feb 2006 08:03:39 PM PST

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