geotimesheader
Highlights
Meteorites
Derek W.
Sears
Last year began with optimism at the launch of
NASA’s Stardust spacecraft to collect interplanetary dust and dust
from comet Wild 2. A month later, as if to underscore that missions are
not the only way to recover extraterrestrial material, the 14th martian
meteorite was found in the Libyan Sahara, not far from the site of a similar
find of a year before. These two fragments of a weathered, 2-kilogram basaltic
rock have been dubbed Dar al Gani 489. It is likely that these fragments
are pieces of the same Mars rock that disintegrated in the atmosphere or
in the desert.
The deserts of North Africa and Australia, the
prairies of North America, and the ice fields of Antarctica yielded their
usual large crop of meteorites in 1999, including many from the small and
rare groups.
At the same time, meteorite researchers still
enjoyed returns from serendipitous falls. The first such fall was a shower
of 54 pieces of an H6 chondrite that fell on June 13, 1998, near the small
New Mexico town of Portales. The 105 kilograms of meteorites were found
over the familiar elliptical strewn field, 8 kilometers long.
The class is fairly common, but what makes the
Portales Valley meteorites unique is the their abundance of centimeter-sized
veins of metal and the presence in these veins of a Widmanstätten
structure — a well-known structure normally found in iron meteorites and
indicating very slow cooling. Dave Kring and his colleagues at the University
of Arizona published their description of Portales Valley in
Meteoritics
and Planetary Science (v. 31, p. 663-669). They conclude that the meteorite
originated from the metal-veined floor of a 20-kilometer impact crater
on its parent body.
A second serendipitous event occurred when an
H chondrite fell on the small Texas town of Monahans (M.E. Zolensky et
al., Science, v. 285, p. 1377-1379). Researchers were excited because
H chondrites are normally bone dry, but this one contains halite and sylverite,
minerals produced by the evaporation of brine. Furthermore, these minerals
contain inclusions of trapped water. Unlike the abundant water found in
carbonaceous meteorites, this water is unreacted primordial water, dating
from the meteorite’s origins. In principle it should be possible to obtain
pressures and temperatures of water entrapment, and chemical and isotopic
compositions for the fluid. The Monahans fall could offer a unique window
into the parent body and its formation.
Oxygen isotopes
Many researchers think water played a part in
determining the curious patterns of oxygen isotopes in meteorites. For
three decades, these oxygen isotope fingerprints have been defined and
refined and used to establish genetic links between the meteorite classes.
However, there has been little understanding of what caused these patterns.
In 1999, Ed Young and his colleagues in the United Kingdom showed that
fluids flowing down temperature gradients could explain most of the oxygen
isotope patterns observed in meteorites and their inclusions, assuming
that they started with the 16O fractionation trend for ordinary
chondrites (Science, v. 286, p. 1331-1335).
Another United Kingdom group, led by Grenville
Turner of Manchester University, showed that the ordinary chondrite trend
exists across a chondrule that had clearly been a chemically open system
during its formation. Chondrules are glass beads almost unique to chondrites
(hence their name), formed by flash heating of dust. The new data could
indicate that the ordinary trend in oxygen isotope abundances was produced
during chondrule formation. Data from NASA’s upcoming Genesis project,
which should launch a spacecraft in 2001 that will orbit at the first Lagrangian
point to capture solar particles, will help define the solar oxygen isotopic
composition (R.C. Weins et al., Meteoritics and Planetary Science,
v. 34, p. 99-107).
Oxygen isotope analysis also played an important
part in defining another major new direction for meteorite research: the
realization that presolar grains might be present in primitive meteorites.
Normally these grains are found to be carbon-rich phases, but in recent
years oxygen-bearing phases have been found. To date these have been Al2O3,
but last year Larry R. Nittler and Conel Alexander, using an automated
ion microprobe at the Carnegie Institution in Washington, found presolar
TiO2 grains in ordinary chondrites.
Perplexing chondrites
The enstatite chondrites form a relatively small
group and have perplexed meteorite researchers because they are much more
reduced than the other classes. Until recently, their study was hampered
by a lack of unaltered meteorites, but the Antarctic meteorite collection
has remedied that. Yubin Guan and colleagues found evidence for 26Al
in the refractory inclusions of enstatite chondrites, showing that this
isotope was present in all the major chondrite classes as they formed.
The importance of 26Al, of course, is that it is a short-lived
isotope of a relatively abundant element. It can thus help determine time
scales and heat sources of the early solar system.
The enstatite chondrites form a relatively small
group and have perplexed meteorite researchers because they are much more
reduced than the other classes. Until recently, their study was hampered
by a lack of unaltered meteorites, but the Antarctic meteorite collection
has remedied that. Yubin Guan and colleagues found evidence for 26Al
in the refractory inclusions of enstatite chondrites, showing that this
isotope was present in all the major chondrite classes as they formed.
The importance of 26Al, of course, is that it is a short-lived
isotope of a relatively abundant element. It can thus help determine time
scales and heat sources of the early solar system. |
Eros, as seen by the NEAR Shoemaker camera during the spacecraft's
orbit of the asteroid earlier this year. For a complete view of the asteroid,
digital images returned by the spacecract are draped over a computer model
of the asteriod's shape. NASA. |
The debate over where these meteorites originate
continues, and last year some asteroid astronomers claimed they could associate
every major asteroid type with a class of meteorites, albeit a rare class.
By this reckoning, the H chondrites come from asteroid 6 Hebe. The spectral
match is not exact, but requires the addition of considerable amounts of
metal to the asteroid’s surface. For this reason, the Portales Valley meteorite
mentioned above is important.
Other asteroid astronomers argue that the space
environment alters the surfaces of asteroids and that most of the S asteroids
are ordinary chondrites in disguise. Fuel was added to this idea by papers
presented at the International Conference on Asteroids, Comets and Meteors,
held July 26-30 at Cornell University. Michael Hicks of NASA’s Jet Propulsion
Laboratory, and colleagues argued that the smaller asteroids most closely
matched the spectra of ordinary chondrites. They believe that small asteroids
were more recently produced by fragmentation and therefore have fresher
surfaces. This idea does not explain why S asteroids have much lower densities
than ordinary chondrites.
The question of the meteorite-asteroid connection
will only be resolved by spacecraft missions to multiple asteroids along
with studies of the samples returned.
At the end of last year, meteorite researchers
applauded the successful completion of the Deep Space 1 mission,
with all it promises for future research of the planetary system. And we
looked forward to the adventures of NEAR (now NEAR Shoemaker)
which took orbit around the asteroid 433 Eros on Feb. 14, 2000.
Never before have meteorite researchers been so
close to resolving many questions fundamental to their science. Never before
has their presence been felt in mission design nor have they felt the impact
of missions. We have left the era when meteorite research was a fringe
activity for those fascinated with tales of rocks from the sky. It is now
emerging as a central part of small-body planetary research.
Sears is a professor at the
University of Arkansas and studies primitive meteorites and the possibility
that asteroids and comets are meteorite parent bodies. He is editor of
Meteoritics
and Planetary Science and director of the Arkansas-Oklahoma Center
for Space and Planetary Science.
