[meteorite-list] Cosmic Smack Encourages Life To Go Forth and Multiply

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:44:36 2004
Message-ID: <200103081926.LAA18974_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/solarsystem/impact_rebound_010309.html

Cosmic Smack Encourages Life To Go Forth and Multiply
By Robert Roy Britt
space.com
08 March 2001

A discovery that earthly flora and fauna rebound and advance rapidly after
catastrophic collisions by space rocks highlights the resourcefulness of
life and bolsters a growing suspicion that evolution does its best work in
extreme crises.

When a comet or asteroid smacked Earth 65 million years ago, the dinosaurs
bid farewell. They could not adapt to a world remade by the extreme
ecological violence. They were not alone. Roughly two-thirds of all life on
Earth vanished.

But new species sprang forth from the catastrophe, evolving quickly to fill
new niches of climate, water and soil conditions. A new study examined
extraterrestrial dust to pin down just how quickly.

In a mere 10,000 years -- the blink of an eye on a geologic time scale --
the planet was repopulated by newly evolved versions of tiny critters that
had been mostly wiped out by the impact, according to a study published in
the March 9, 2001 issue of the journal Science.

Around the globe, layers of sediment deposited by ancient seas hold a record
of what has lived and died on Earth going back billions of years. And when
scientists dig, they usually find a layer less than an inch (2 centimeters)
thick that contains virtually no fossils and is thought to correspond to the
time of the mass extinction. Careful analysis recently yielded a
surprisingly short estimate for the period of time represented by those
fossil-free layers, and the new thinking is that new species arise and
evolve much more quickly than scientists had imagined.

The mass extinction came, the thinking goes, after a cosmic impact in what
is now Mexico's Yucatan Peninsula shook the planet and sent up a choking mix
of dust and chemicals that darkened the global skies for possibly decades.
Most plankton and many tropical marine invertebrates, especially
reef-dwellers, perished in the event, along with many species of land
animals and plants.

Until now, some researchers have supposed it took hundreds of thousands of
years for many species to die out as the impact combined with other killing
mechanisms, such as long-term volcanic activity or changing sea levels.

Clues in space dust

The new research measured space dust laid down in the ancient sediment.
Packed inside the dust, and revealing its extraterrestrial origin, was
helium 3, an isotope of helium that is rarely found on Earth except when it
originated from space.

The dust, deposited at a steady rate over millions of years, revealed how
much time passed -- just 10,000 years -- between the demise of vast numbers
of marine organisms and the rebirth of new ones.

"These types of severe mass extinctions seem to pave the way for life to
more or less reinvent itself and come back even stronger and more capable of
adapting to extreme environmental change," said the University of
Washington's Luann Becker, who was not involved in the study. "We simply
don't get significant evolutionary change without an extraterrestrial
cause."

Becker, who in February provided evidence that an earlier mass extinction
was caused by a space rock, said the new study is consistent with what
paleontologists have been suggesting in recent years: The worst extinctions
seem to be rapid events. And she said the ability of organisms to re-emerge
quickly might shed light on how life evolved on Earth in the beginning,
billions of years ago, when the planet was regularly pummeled by comets and
asteroids.

Ruling out other causes of extinction

The evidence for the short recovery is recorded in exposed layers of Earth
that formed at the bottom of ancient seas about 100 miles (160 kilometers)
north of what is now Rome.

Prior to the impact, marine organisms lived and died in droves, and their
corpses helped create a global coffin of limestone that built up over the
eons. Abruptly, the limestone layers stop, covered over by thin layer of
clay that formed in the virtual absence of marine life. Above that clay the
limestone sharply resumes, which means marine organisms had repopulated the
Earth, and had done so in a rather sudden manner.

Researchers have long known the thickness of this relatively fossil-free
layer of clay, but they have argued over how much time passed during its
creation. The clay layer marks the boundary between the Cretaceous and
Tertiary Eras of geologic time.

"And now we know how far apart they are in time," said Kenneth Farley, a
Caltech geochemist and coauthor of the Science paper discussing the
research.

The finding implies that the impact itself, along with the severe climatic
and geologic effects it generated, was the primary reason the dinosaurs and
other species perished.

Others have speculated that ongoing volcanic activity might have been a
significant contributor, but Farley and others say that volcanism would have
been a slower killing mechanism, requiring 100,000 years or more to deal the
amount of death now thought to have occurred in less than 10,000 years.

"I think it rules out long-term progressive kinds of [extinction] processes
like sea-level change or volcanism," Farley said in a telephone interview.

The finding would also rule out multiple impacts as the cause of the
extinction event.

"There's been a longstanding debate whether the mass extinctions at the
[Cretaceous-Tertiary] boundary were caused by a single impact or maybe a
swarm of millions of comets," said Sujoy Mukhopadhyay, a graduate student at
Caltech and lead author of the paper.

Alessandro Montanari of the Geological Observatory in Apiro, Italy, also
worked on the study.

Impacts accelerate evolution

The fossils of critters that emerged, survived and thrived in just 10,000
years now speak volumes about the tight relationship between life and cosmic
catastrophe.

"It's clear when you look at the species before and after, that there are
new [species] after the impact," Farley said. "Surely they've been culled
from whatever population existed before the event. They haven't re-evolved
from scratch. It shows how fast new species can populate a niche."

This rapid diversification of species is "particularly exciting because it
implies that impacts might have accelerated the evolution of life on Earth,"
said Matthew Genge, a researcher at the London Natural History Museum.

Details in the dust

The chronology of death and rebirth was created by a steady supply of space
dust that works its way to Earth from farther out in the solar system.

Asteroids located mostly between Mars and Jupiter, in a region known as the
Asteroid Belt, bump and grind over time and create large amounts of dust.
And comets that pass through the inner solar system release spurts of dust,
which is spread through space over time.

These processes have been generating relatively consistent amounts of dust
for many millions of years. Drawn toward the Sun by gravity, this dust
captures and carries a high concentration of helium 3 that is riding outward
from the Sun on the solar wind, a stream of energized particles.

The dust brings the helium 3 to Earth in miniscule but near-constant
amounts. Some of it is stripped from the dust during entry into the
atmosphere, but a steady fraction makes it to the surface. Genge, who
studies the sources and effects of cosmic dust and meteorites, said using
the dust to create a chronology is a very reliable method.

Comet or asteroid?

The new research might also shed light on the origin of the space rock that
caused the extinction.

Genge said the findings suggest the impactor was not a "short-period" comet
(like Comet Halley that regularly orbits the Sun). Periodic comets rarely
spend more than a few tens of thousands of years in the inner solar system,
explains Genge, before being swallowed by the Sun.

"We should see a gradual increase in the amount of helium 3 brought in by
dust just before the impact and a decrease afterwards if it was a periodic
comet," he told SPACE.com. "This is, therefore, evidence that the
[Cretaceous-Tertiary] impactor was either an asteroid or a long-period
comet."

Long-period comets originate as far as a third of the way to the nearest
star in a halo of objects called the Oort Cloud, which surrounds our solar
system. They can be tossed our way by the gravitational effects of passing
stars.

More controversial claims have been made that these comets might be hurled
Earthward by gravitational changes when our solar system passes through the
dense plane of the Milky Way Galaxy, which it does periodically as we orbit
around the galactic center.

Researchers have tried to tie this periodicity to an apparent cycle of mass
extinctions occurring every 26 million years or so, though there is no
agreement on the issue.

"It is thus interesting to ponder that if the position and motion of a
planetary system controls the rate at which impacts occur, then it may also
influence the biological evolution in habitable planetary environments,"
Genge said. "Too many impacts, and evolution may be stalled; too few, and it
may only crawl along."
Received on Thu 08 Mar 2001 02:26:06 PM PST