Download CHEMICAL IMPACT Buckyballs Teach Some History

CHEMICAL IMPACT
Buckyballs Teach Some History
bout 250 million years ago, 90% of life on earth was
destroyed in some sort of cataclysmic event. This
event, which ended the Permian period and began the
Triassic (the P-T boundary), is the most devastating mass
extinction in the earth’s history—far surpassing the catastrophe 65 million years ago that wiped out the dinosaurs
(the K-T boundary). In 1979 geologist Walter Alvarez and
his Nobel Prize–winning physicist father Luis Alvarez suggested that unusually high concentrations of iridium in
rocks laid down at the K-T boundary meant that an asteroid had hit the earth, causing tremendous devastation. In
the last 20 years much evidence has accumulated to support this hypothesis, including identification of the location of the probable crater caused by the impact in the
ocean near Mexico.
Were the P-T boundary extinctions also caused by an
extraterrestrial object or by some event on earth, such as a
massive volcano explosion? Recent discoveries by geochemists Luann Becker of the University of Washington and
A
Robert J. Poreda of the University of Rochester seem to
strongly support the impact theory. Examining sediment
from China and Japan, the team found fullerenes encapsulating argon and helium gas atoms whose isotopic composition indicates that they are extraterrestrial in origin. For
example, the ratio of 32He to 42He found in the fullerenes is
100 times greater than the ratio for helium found in the
earth’s atmosphere. Likewise, the isotopic composition of
the fullerene-trapped argon atoms is quite different from that
found on earth.
Fullerenes include spherical C60 carbon molecules
(“buckyballs”) whose cavities can trap other atoms such as
helium and argon. (See the accompanying figure.) The scientists postulate that the fullerenes originated in stars or
collapsing gas clouds where the noble gas atoms were
trapped as the fullerenes formed. These fullerenes were
then somehow incorporated into the object that eventually
hit the earth. Based on the isotopic compositions, the
geochemists estimate that the impacting body must have
been 10 kilometers in diameter, which is comparable in
size to the asteroid that is assumed to have killed the
dinosaurs.
One factor that had previously cast doubt on an asteroid collision as the cause of the P-T catastrophe was
the lack of iridium found in sediments from this period.
However, Becker and other scientists argue that this absence probably means the impacting object may have been
a comet rather than an asteroid. It is also possible that
such a blow could have intensified the volcanism already
under way on earth at that time, delivering a “one-two
punch” that almost obliterated life on earth, according to
Becker.
It is ironic that “buckyballs,” which made big news
when they were recently synthesized for the first time in the
laboratory, actually have been around for millions of years
and have some very interesting history to teach us.
Figure from Chemical and Engineering News, Feb. 26, 2001, p. 9.
Reprinted by permission of Joseph Wilmhoff.
Isotope ratios of the noble gas atoms inside celestial buckyballs
indicate that these ancient carbon cages formed in a stellar
environment, not on earth.