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CRASHING COMETS NOT LIKELY THE
CAUSE OF EARTH'S MASS EXTINCTIONS
Scientists have debated how many mass extinction events in Earth's
history were triggered by a space body crashing into the planet's surface.
Most agree that an asteroid collision 65 million years ago brought an end to
the age of dinosaurs, but there is uncertainty about how many other
extinctions might have resulted from asteroid or comet collisions with
Earth.
In fact, astronomers know the inner solar system has been protected
at least to some degree by Saturn and Jupiter, whose gravitational fields
can eject comets into interstellar space or sometimes send them crashing
into the giant planets. That point was reinforced last week (July 20) when a
huge scar appeared on Jupiter's surface, likely evidence of a comet impact.
New University of Washington research indicates it is highly
unlikely that comets have caused any mass extinctions or have been
responsible for more than one minor extinction event. The work also shows
that many long-period comets that end up in Earth-crossing orbits likely
originate from a region astronomers have long believed could not produce
observable comets. A long-period comet takes from 200 years to tens of
millions of years to make a single orbit of the sun.
"It was thought the long-period comets we see just tell us about the
outer Oort Cloud, but they really give us a murky picture of the entire Oort
Cloud," said Nathan Kaib, a University of Washington doctoral student in
astronomy and lead author of a paper on the work being published July 30 in
Science Express, the online edition of the journal Science. NASA and the
National Science Foundation funded the work.
The Oort Cloud is a remnant of the nebula from which the solar
system formed 4.5 billion years ago. It begins about 93 billion miles from
the sun (1,000 times Earth's distance from the sun) and stretches to about
three light years away (a light year is about 5.9 trillion miles). The Oort
Cloud could contain billions of comets, most so small and distant as to
never be observed.
There are about 3,200 known long-period comets. Among the
best-remembered is Hale-Bopp, which was easily visible to the naked eye for
much of 1996 and 1997 and was one of the brightest comets of the 20th
century. By comparison, Halley's comet, which reappears about every 75
years, is perhaps the best-known comet, but it is a short-period comet, most
of which are believed to originate in a different part of the solar system
called the Kuiper Belt.
It has been believed that nearly all long-period comets that move
inside Jupiter to Earth-crossing trajectories originated in the outer Oort
Cloud. Their orbits can change when they are nudged by the gravity of a
neighboring star as it passes close to the solar system, and it was thought
such encounters only affect very distant outer Oort Cloud bodies.
It also was believed that inner Oort Cloud bodies could reach
Earth-crossing orbits only during the rare close passage of a star, which
would cause a comet shower. But it turns out that even without a star
encounter, long-period comets from the inner Oort Cloud can slip past the
protective barrier posed by the presence of Jupiter and Saturn and travel a
path that crosses Earth's orbit.
In the new research, Kaib and co-author Thomas Quinn, a UW astronomy
professor and Kaib's doctoral adviser, used computer models to simulate the
evolution of comet clouds in the solar system for 1.2 billion years. They
found that even outside the periods of comet showers, the inner Oort Cloud
was a major source of long-period comets that eventually cross E arth's path.
By assuming the inner Oort Cloud as the only source of long-period
comets, they were able to estimate the highest possible number of comets in
the inner Oort Cloud. The actual number is not known. But by using the
maximum number possible, they determined that no more than two or three
comets could have struck Earth during what is believed to be the most
powerful comet shower of the last 500 million years.
"For the past 25 years, the inner Oort Cloud has been considered a
mysterious, unobserved region of the solar system capable of providing
bursts of bodies that occasionally wipe out life on Earth," Quinn said. "We
have shown that comets already discovered can actually be used to estimate
an upper limit on the number of bodies in this reservoir."
With three major impacts taking place nearly simultaneously, it had
been proposed that the minor extinction event about 40 million years ago
resulted from a comet shower. Kaib and Quinn's research implies that if that
relatively minor extinction event was caused by a comet shower, then that
was probably the most-intense comet shower since the fossil record began.
"That tells you that the most powerful comet showers caused minor
extinctions and other showers should have been less severe, so comet showers
are probably not likely causes of mass extinction events," Kaib said.
He noted that the work assumes the area surrounding the solar system
has remained relatively unchanged for the last 500 million years, but it is
unclear whether that is really the case. It is clear, though, that Earth has
benefitted from having Jupiter and Saturn standing guard like giant catchers
mitts, deflecting or absorbing comets that might otherwise strike Earth.
"We show that Jupiter and Saturn are not perfect and some of the
comets from the inner Oort Cloud are able to leak through. But most don't,"
Kaib said.
For more information, contact Kaib at 1-206-616-4549, 1-206-375-1048 or
kaib@astro.washington.edu; or Quinn at 1-206-685-9009 or
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