31 July 2009

New Comet Modeling




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


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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