picture of the day
Credit: NASA/JPL-Caltech/University of Washington
This image shows a comet particle collected by the Stardust spacecraft.
particle is made up of the silicate mineral forsterite, also
known as peridot in its gem form.
It is surrounded by a thin rim of
melted aerogel, the substance used to collect the comet
dust samples. The particle is about 2 micrometers across.
Mar 16, 2006
“Stardust” Shatters Comet Theory
The first results from NASA's Stardust mission are in, leaving mission
scientists in a state of shock and awe. The tiny fragments of comet
dust brought back to Earth did not accrete in the cold of space, but
were formed under “astonishingly” high temperatures.
It seems that the gulf between the
impressive successes of modern technology and the depressing failure
of theory has grown by another giant leap.
NASA’s celebrated Stardust mission was a
technical triumph, achieved at a respectable cost. The mission
collected the first samples ever of the dust discharged by comets.
On January 2, 2004, the Stardust craft had entered the dusty clouds
Comet Wild 2 (pronounced
VILT 2), gathering samples of the minute particles as they struck
the “aerogel” in a 100-pound
capsule. The capsule returned to Earth and parachuted to touchdown
on a Utah desert January 15, 2006.
A surprise—the particles revealed abundances of minerals that can only
be formed at high temperatures.
Mineral inclusions ranged from anorthite, which is made up of
calcium, sodium, aluminum and silicate, to diopside, made of calcium
magnesium and silicate. Formation of such minerals requires
temperatures of thousands of degrees.
How could that be? For decades we have been
assured that comets accreted uneventfully from the leftovers of a
cold “nebular cloud” in the outermost regions of the solar system.
The theoretical assumption has been stated as fact repeatedly in
popular scientific media, and its proponents believed it. Indeed,
the implication of a fiery past was so unexpected that an early
sample of dust was thought to be contamination from the spacecraft.
materials formed by fire end up on the outermost reaches of the
solar system, where temperatures are the coldest?” asked
Associated Press writer Pam Easton.
"That's a big surprise. People thought comets would just be cold
stuff that formed out ... where things are very cold," said NASA
curator Michael Zolensky. "It was kind of a shock to not just find
one but several of these, which implies they are pretty common in
Researchers were forced to conclude that the
enigmatic particle material formed at a superheated region either
close to our Sun, or close to an alien star. “In the coldest part of
the solar system we’ve found samples that formed at extremely high
temperatures,” said Donald Brownlee, Stardust’s principal
investigator at the University of Washington in Seattle, during a
Monday press conference. “When these minerals formed they were
either red hot or white hot grains, and yet they were collected in a
comet, the Siberia of the Solar System.”
reports that the finding “perplexed Stardust researchers and added a
new wrinkle in astronomers’ understanding of how comets, and
possibly the Solar System, formed”. But did it really?
do not die easily. Our own impression is that comet researchers have
yet to revisit their “big picture” assumptions. A litany of
surprises has not deterred them, and they continue to discuss the
formation of comets “at the outermost regions of the solar system”.
The idea does not deserve such unyielding devotion. It was never
more than a guess, and it never successfully predicted any of the
milestone discoveries in cometology.
So the paradoxes and contradictions continue to accumulate. Michael
Zolensky, Stardust curator and a mission co-investigator at NASA’s
Johnson Space Center (JSC), said astronomers believed that a sort of
material “zoning” occurred during the Solar System’s formation. In
the eons-long collapse of the primordial “nebular cloud”, material
closer to the emerging “sun” formed under hotter conditions, while
farther away from the sun everything remained dark and cold. The
comet was supposed to be the case par excellence of a body accreted
in the outermost region and constituted primarily of water ice and
Speculations erupted. Could it be that something occurred in or very
near the Sun in its formative phase, flinging immense quantities of
material out to the periphery of the Sun’s domain (far, far beyond
the orbit of Pluto), to the “Oort cloud”, the legendary—but
never-witnessed—sea of comets?
Then the researchers reminded themselves that this would produce a
mixing and contradict the zoning that is evident in the asteroid
belt. “If this mixing is occurring, as suggested by these results,
then how do you preserve any kind of zoning in the solar system”,
Zolenksy asked. “It raises more mysteries.”
Perhaps the paradigm could be redeemed by finding the signature of
primordial water, whose existence is essential to the survival of official comet
A report by the journal Nature is
illuminating. A writer for the journal spent a day with
Phil Bland, a planetary scientist at Imperial College London, as he and
his team analyzed part of a grain. When he found large amounts of
calcium, Bland was excited. Could the calcium be present in the form
of calcium carbonate, a mineral that almost always forms in water?
He bet his colleague Matt Genge that this would indeed be the case.
lost the bet, owing Genge a dinner. According to the Nature report
NASA “scientists have not yet found any carbonates in their grains”.
Today, the study of comets has
Every key finding comes as a surprise, but no one seems to
realize that the surprises are not random— they are
predictable under a different perspective. The tragedy
is the way inertia can leave well-intentioned scientists with
their feet in the sand. The momentum of prior belief, working in
concert with pressing demands of funding, creates nearly endless
obstructions to open-minded exploration and discourse. Even a
brief vacation from an oppressive paradigm could do wonders.
Coming March 17: The Rilles are Electric
Coming March 18: Stardust Shatters Comet Theory (2)
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