[ExI] "Stardust blurs line between asteroids and comets"

Amara Graps amara at amara.com
Fri Jan 25 19:27:26 UTC 2008


Damien Broderick thespike at satx.rr.com :

>http://www.cosmosmagazine.com/node/1825
>Friday, 25 January 2008
>Cosmos Online
>SYDNEY: A new analysis of comet grains collected by NASA's Stardust
>probe, has surprised experts by revealing that they don't contain
>samples of the most ancient material in the Solar System.

>Rather than primordial dust, the samples contain material that has
>been more commonly in asteroids from the inner Solar System.

>The find reported today in the U.S. journal Science by cosmochemists
>led by Hope Ishii of the Lawrence Livermore National Laboratory in
>California, blurs the distinction between asteroids and comets.

We discussed this paper this morning at our SwRI science meeting, but a
couple of dynamical people, whose opinion I would like, were not there,
so maybe I will say more on this in the future. There isn't a surprise
that sometimes asteroids look like comets and vice versa, that has been
part of the field of small bodies and dust for decades. Also there are
some intriguing dynamics with the discovery a few years ago of comets in
the asteroid belt ("Main Belt comets") that has relevance to these
Stardust measurements too. What is surprising is that, in comet Wild 2,
there has, so far, been no evidence of materials made in cold
environments side-by-side with materials made in hot environments. The
comet is all 'hot', that is: primarily of materials made in the inner
solar system. So then what is the radial distance for mixing in the
solar nebula? The X-wind theory gives some parameters, which I thought
were for the region of the asteroid belt, only, but now I must check.

Comet Wild 2 is a Jupiter family comet, traveling in from the scattered
disk part of the Kuiper belt. Do the dynamics of early solar system
formation say it is possible for a body made in the inner solar system
to be flung to the Kuiper belt scattered disk? It seems unlikely, but
this is a question I would like to discuss more with dynamical experts
here.

The original paper is in Science (by subscription, though).

-------------

Science 25 January 2008:
Vol. 319. no. 5862, p. 401
DOI: 10.1126/science.319.5862.401a

The main paper is here:
http://www.sciencemag.org/cgi/content/full/sci;319/5862/447

Comparison of Comet 81P/Wild 2 Dust with Interplanetary Dust from Comets
Hope A. Ishii, John P. Bradley, Zu Rong Dai, Miaofang Chi, Anton T.
Kearsley, Mark J. Burchell, Nigel D. Browning, and Frank Molster (25
January 2008) Science 319 (5862), 447. [DOI: 10.1126/science.1150683]

Abstract
http://www.sciencemag.org/cgi/content/abstract/sci;319/5862/447

The Stardust mission returned the first sample of a known outer solar
system body, comet 81P/Wild 2, to Earth. The sample was expected to
resemble chondritic porous interplanetary dust particles because many,
and possibly all, such particles are derived from comets. Here, we
report that the most abundant and most recognizable silicate materials
in chondritic porous interplanetary dust particles appear to be absent
from the returned sample, indicating that indigenous outer nebula
material is probably rare in 81P/Wild 2. Instead, the sample resembles
chondritic meteorites from the asteroid belt, composed mostly of inner
solar nebula materials. This surprising finding emphasizes the
petrogenetic continuum between comets and asteroids and elevates the
astrophysical importance of stratospheric chondritic porous
interplanetary dust particles as a precious source of the most
cosmically primitive astromaterials.

Here is the lead-in article (printed below)

http://www.sciencemag.org/cgi/content/summary/319/5862/401a

News of the Week
GEOCHEMISTRY:
Where Has All the Stardust Gone?
Richard A. Kerr

Surprise has followed surprise for cosmochemists analyzing the dust
sample that the Stardust spacecraft returned from comet Wild 2 in
January 2006. First, they found tiny flecks of once-molten
minerals--material very different from the raw, primordial dust they
expected to see. Such unaltered, so-called presolar material was the
prime ingredient of the rocky planets and was thought to abound in icy
comets. But on page 447, researchers report that they have failed to
find a single speck of it.

"For those of us who study presolar materials, it's turned out to be a
bit of a bust," says cosmochemist Larry R. Nittler of the Carnegie
Institution of Washington's Department of Terrestrial Magnetism in
Washington, D.C. "Wild 2 seems more related to asteroids than comets,"
because all asteroids were altered from the solar system's primitive
starting materials. Still, "the mission's been a huge success," says
John Bradley of Lawrence Livermore National Laboratory (LLNL) in
California, a co-author of the Science paper. "It's changing the way we
think about comets."

Before Stardust's return, cosmochemists thought of comets as vaults
where the primitive ingredients of the planetary recipe had been locked
up. Their best look at the likely ingredients list came from the study
of certain meteoritic particles collected in Earth's stratosphere by
retired spy planes. Because of their exotic isotopic composition, these
particular interplanetary dust particles (IDPs) looked as though they
might be comet dust. Presumably, such primitive dust fell into the cold,
outer reaches of the nebula that gave rise to the planets and combined
with nebular ices to form comets, in which the dust has been preserved
ever since.

Figure 1 An unfortunate match. Globs of mineral-riddled glass (left)
from a comet sample were created during sample collection, as replicated
in the lab (right).

One of the unaltered components of cometlike IDPs was so-called GEMS
(glass with embedded metal and sulfides). And early analyses of
particles captured near Wild 2 by Stardust tantalizingly revealed
GEMS-like particles. But cosmochemist Hope Ishii of LLNL and her
colleagues report in this issue that the GEMS-like particles in Stardust
samples were actually forged as Wild 2 dust particles plowed into the
wispy glass of the Stardust sample collector at a blistering 22,000
kilometers per hour. The researchers made some themselves by shooting
mineral particles into collector material at Stardust velocities.
Stardust principal investigator Donald Brownlee of the University of
Washington, Seattle, does allow that any true GEMS--which tend to be
submicrometer in size--might have been lost on impact with the Stardust
sample collector.

Ishii's group also found only one microscopic "whisker" of the mineral
enstatite. Such threadlike crystals are common in primitive, cometlike
IDPs, but the lone Stardust find has the wrong orientation to have come
from a comet. And what little organic matter could be found in the
Stardust sample has a much lower deuterium-hydrogen ratio than the
organic matter of cometlike IDPs.

All in all, "it's looking as if Wild 2 is more like an asteroid than a
primitive comet," says Ishii. Brownlee agrees. Rather than preserving
the original ingredients of planets, comets--or at least Wild 2--seem to
be loaded with materials first altered by the great heat near the young
sun, he says. Then those altered materials must have been carried
outward to the outer reaches of the nebula, where comets incorporated
them. "I would say a large fraction of the [outermost] nebular materials
were probably transported there" from much nearer the sun, Brownlee
says, "which is pretty amazing." Now, no one is at all sure where the
solar system's lingering primitive materials might reside.

-------------

-- 

Amara Graps, PhD      www.amara.com
Research Scientist, Southwest Research Institute (SwRI), Boulder, Colorado



More information about the extropy-chat mailing list