alle@ihuxb.UUCP (Allen England) (03/06/84)
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[Reprinted from Newsweek, March 5, 1984]
A Death-Star Theory Is Born: Nemesis
The *really* bad news last week was from scientists reporting that
a "death star" periodically wipes out much of the life on earth. The
suspect, a.k.a. Nemesis, has yet to be found, but when it is, astronomers
don't expect it to look like a killer-just a mousy little orb one-tenth
the size of the sun. The problems start, though, when Nemesis's wide-
ranging orbit takes it through the Oort Cloud, a sort of holding pen
for some 200 billion comets at the outskirts of the solar system. What
occurs is the cosmic equivalent of a bar fight as the star's gravity
picks up billions of comets and sends them plunging out of the cloud.
Most miss the earth, but two dozen or so land direct hits. The impact
fills the atmosphere with dust and poisons, changes the planet's climate
and, in time, causes the extinction of myriad species of plants and
animals.
So, at least, goes a new theory that seeks to explain mass extinctions
of life on earth. The best known impact wiped out the dinosaurs and other
creatures 65 million years ago; other cataclysms occurred about 12 million,
38 million, 92 million and 125 million years ago. The regularity was
first noted by paleontologists J. John Sepkoski and David Raup of the
University of Chicago, and it sent astronomers scurrying to find a
cosmic event with the same frequency as the extinctions. The smoking
gun was the impact craters from comets and meteors that scar the
earth's surface: the ages of seven of the largest coincide with the
dates of mass extinctions. NASA scientists suggested that the
impacts, and the craters, occur when the solar system passes through
the debris-laden plane of the Milky Way galaxy. But "we are [doing
that] now," argues astrophysicist Richard Muller of the University
of California, Berkeley, and aren't getting a comet barrage. Muller,
colleague Marc Davis and Piet Hut of the Institute for Advanced Study
propose instead the Nemesis theory: that a companion star to the sun
whose orbit takes it through the Oort Cloud every 28 million years
is responsible.
If the Nemesis theory is correct, it would mean that the pace and
direction of evolution are determined partly by an extraterrestrial
force. Survival of a species would depend as much on luck as on
fitness-living far enough away, in space or time, from a comet's
impact. Muller and his colleagues are poring over records of star
sightings to find evidence of Nemesis and are awaiting the results of
upcoming unmanned space missions to comets to see if these wanderers
have the rocky cores necessary to cause such destruction. Even if
there is a Nemesis, it's a little early to build comet shelters. The
next barrage is due in 14 million years.mmt@dciem.UUCP (Martin Taylor) (03/07/84)
There was a recent article in Science referring to the periodicity of mass
extinctions, and enquiring into the possibility of periodic large impacts.
They searched for iridium anomalies (and other exotic elements or isotopes
associated with meteorites) in sediments at extinction boundaries, but
found none except at the well-known Cretaceous boundary. This doesn't
deny the possibility of periodic massive impacts, but neither does it
lend much support to the idea. (Sorry, I didn't keep the issue, and I
don't have the reference, but it was within the last 6 weeks or so.)
--
Martin Taylor
{allegra,linus,ihnp4,uw-beaver,floyd,ubc-vision}!utzoo!dciem!mmt