[meteorite-list] Meteorites Harder to Trace Than Family Trees

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:09 2004
Message-ID: <200102121746.JAA22293_at_zagami.jpl.nasa.gov>

http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2001/02/12/MN192684.DTL

Meteorites Harder to Trace Than Family Trees

Chemicals, radiation help determine celestial parentage of former asteroids

Keay Davidson
San Francisco Chronicle
February 12, 2001

In a modern-day version of the legendary quest for the source of the Nile,

space scientists seek the source of the heavens' gift to Earth: meteorites.

These fragments of rock and metal are chips off multibillion-year-old flying
mountains -- asteroids -- that hurtle through the night sky and occasionally
crash to Earth.

Like birds and bugs, meteorites and asteroids aren't homogeneous: rather,
they come in different varieties. For decades, space scientists have debated
which types of meteorites come from which types of asteroids.

"This, of course, is the Holy Grail of asteroid science -- to establish
parent body asteroid types for each of the major classes of meteorites,"
says Donald K. Yeomans, a noted space scientist at NASA's Jet Propulsion
Laboratory in Pasadena.

Resolving this debate is a key goal of NASA's Near Earth Asteroid Rendezvous
mission to the asteroid Eros. Today, if all goes as planned, the one-ton,
9-foot-long robotic spaceship will end its five-year, highly successful
journey by crashing into the asteroid surface.

One of NEAR's prime goals has been to use X-ray and gamma-ray spectrometers
to analyze the surface of Eros. The scientists hope to determine how well
Eros' chemical composition matches that of the commonest type of meteorites,
technically dubbed "ordinary chondrites."

Not until the early 19th century did scientists accept the idea that rocks
fall from the heavens. Poetry and fiction would never be the same again.

In the film "American Beauty," for example, one of the few cherished
memories of the embittered character played by Kevin Spacey is of a summer
night in childhood, when he watched "shooting stars" crisscross the sky.

Most of the objects seen by Spacey's character -- or by anyone else who
glances starward for a few minutes -- are grain-size fragments of giant ice
balls called comets. These grains quickly burn up from friction during their
high-speed flight into Earth's atmosphere.

By contrast, meteorites are rock-iron fragments of asteroids and are hefty
enough to survive their incendiary trip through the atmosphere.

Thousands of meteorites rest inside museums and laboratories. There,
scientists try to unravel their celestial secrets by sprinkling them with
acidic chemicals and blasting them with radiation.

Meteorites are among the oldest objects in the solar system. They offer
clues to its origin from a primordial cloud of dust and gas more than 4
billion years ago. A small percentage of meteorites come not from asteroids
but from the moon and Mars. They were presumably knocked off the lunar or
Martian surfaces by asteroid impacts.

Genealogists know the frustrations of tracing family trees: Does one's uncle
stem from the family line that goes back to Queen Victoria or to Jack the
Ripper? Tracing the origins of meteorite types is as difficult.

Traditionally, scientists have tried to trace particular types of meteorites
to particular types of asteroids by comparing their spectral "signatures" --
the unique spectrum of colors emitted by an object according to its chemical
composition.

Sometimes the asteroidal and meteoritic signatures jibe closely. For
example, certain types of meteorites are basaltic, like volcanic lava. This
implies they come from a world that underwent volcanic activity at some time
in the past.

"The best match is probably the Eucrite meteorites with the asteroid Vesta,
" says Mark Robinson of the NEAR scientific team. Eucrites are basalt-like
rocks, and their spectral signature closely resembles that of Vesta when it
is observed through ground-based telescopes. Conclusion: Vesta was
apparently volcanic early in its history.

Otherwise, despite decades of research, to date "we have not convincingly
traced most meteorites back to specific classes of asteroids," Robinson
says.

The solar system is believed to have formed from materials similar to those
in a rare, strange type of meteorite called a carbonaceous chondrite. These
are rich in water and organic molecules, and break down quickly after
landing on Earth. When heated, carbonaceous chondrites form a new
concentration of materials similar to those in ordinary chondrites.

Some 80 percent of meteorites are ordinary chondrites. Presumably they're
relics of a later stage in solar system evolution, when the early Sun began
generating heat.

Ordinary chondrites are generally believed to have come from "S-type" (so
called because they're rich in silicates) asteroids, of which Eros is one.
However, the case is not ironclad: The spectral signatures of ordinary
chondrites and S-type asteroids do not precisely match.

Why? One theory is that the surface of S-type asteroids changes over time
due to "space weathering" -- the steady bombardment by extremely small
meteorites, cosmic rays and the solar wind.

To determine whether Eros is a parent of ordinary chondrites, the NEAR robot
has analyzed its surface with both X-ray and gamma-ray-detecting
instruments. The X-ray instrument can identify minerals as deep as about
100- millionth of a meter beneath the surface, while the gamma-ray device
sees deeper -- about four inches. Observations will continue to be made
during the probe's descent to the asteroidal surface today, thereby
providing higher- and higher-resolution data.

A major puzzle: Eros' surface appears to contain a lower percentage of
sulfur than do ordinary chondrites.

Perhaps space weathering is to blame. Sulfur is, as every high school
chemistry enthusiast knows, a volatile element, and space weathering might
readily drive it off the asteroidal surface, says Jacob Trombka, team leader
of NEAR's XGRS experiment at Goddard Space Flight Center in Greenbelt, Md.

Says a leading authority on asteroids, Richard Binzel of MIT: "There has
been a long debate, going back 20 years at least, as to whether this vague
resemblance between S asteroids -- the most common asteroids we see --
indicates that they are related to the most common meteorites," that is
ordinary chondrites.

In trying to explain the difference between S-type asteroids and ordinary
chondrites, scientists have attributed it to "space weathering."

Unfortunately, Binzel adds, the term space weathering is rather vague, too
vague to resolve the issue definitively. "Not being able to define what
space weathering is -- well, that drives scientists crazy, trying to pin an
answer on a process that they don't understand."
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What We've Learned

The NEAR Shoemaker spacecraft used its on-board sensors to collect
information on Eros' mass, structure, geology, composition, gravity and
magnetic field. Scientists hope to use the data to learn more about the
formation of the Earth and other planets. Here's what scientists have
learned about Eros and other asteroids:Two kinds of asteroids:

Before reaching Eros, NEAR passed by the asteroid Mathilde. Both asteroids
are thought to have broken away from the main asteroid belt between Mars and
Jupiter.C-type asteroids like Mathilde are rich in carbon compounds, prevail
in the outer part of the asteroid belt and appear dark gray in color. (x)
S-type asteroids like Eros are rich in silicates, prevail in the inner part
of the asteroid belt and have colors consistent with a rocky composition.(x)
The image of Mathilde has been brightened for viewing purposes; the image of
Eros shows approximately how the naked eye would see it. They are shown in
relative size to one another.. Some notable features of ErosComposite
black-and-white image of Eros' opposite hemispheres: CRATERS: Result of
impacts with asteroid fragments; more recent craters have sharper rims

SIZE: 21 miles long, 8 miles wide

SADDLE REGION: Lack of crater activity indicates it was formed late in Eros'
history

GROOVES AND STREAKS: Marks formed as loose surface rock drains into cracks,
exposing lighter-colored subsurface material

GRAVITY: A 200-pound person would weigh only about two ounces on
Eros.Source: Johns Hopkins University Applied Physics LaboratoryTodd
Trumbull / The Chronicle

E-mail Keay Davidson at kdavidson_at_sfchronicle.com.
Received on Mon 12 Feb 2001 12:46:51 PM PST