[meteorite-list] Hell not so hellish

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Wed, 20 May 2009 17:15:01 -0500
Message-ID: <f50915ljtl2oafco6cqgdcq0dea0g98nno_at_4ax.com>

http://www.sciencedaily.com/releases/2009/05/090520140403.htm

Asteroid Attack 3.9 Billion Years Ago May Have Enhanced Early Life On Earth

ScienceDaily (May 20, 2009) ? The bombardment of Earth nearly 4 billion years
ago by asteroids as large as Kansas would not have had the firepower to
extinguish potential early life on the planet and may even have given it a
boost, says a new University of Colorado at Boulder study.

Impact evidence from lunar samples, meteorites and the pockmarked surfaces of
the inner planets paints a picture of a violent environment in the solar system
during the Hadean Eon 4.5 to 3.8 billion years ago, particularly through a
cataclysmic event known as the Late Heavy Bombardment about 3.9 million years
ago. Although many believe the bombardment would have sterilized Earth, the new
study shows it would have melted only a fraction of Earth's crust, and that
microbes could well have survived in subsurface habitats, insulated from the
destruction.

"These new results push back the possible beginnings of life on Earth to well
before the bombardment period 3.9 billion years ago," said CU-Boulder Research
Associate Oleg Abramov. "It opens up the possibility that life emerged as far
back as 4.4 billion years ago, about the time the first oceans are thought to
have formed."

A paper on the subject by Abramov and CU-Boulder geological sciences Professor
Stephen Mojzsis appears in the May 21 issue of Nature.

Because physical evidence of Earth's early bombardment has been erased by
weathering and plate tectonics over the eons, the researchers used data from
Apollo moon rocks, impact records from the moon, Mars and Mercury, and previous
theoretical studies to build three-dimensional computer models that replicate
the bombardment. Abramov and Mojzsis plugged in asteroid size, frequency and
distribution estimates into their simulations to chart the damage to the Earth
during the Late Heavy Bombardment, which is thought to have lasted for 20
million to 200 million years.

The 3-D models allowed Abramov and Mojzsis to monitor temperatures beneath
individual craters to assess heating and cooling of the crust following large
impacts in order to evaluate habitability, said Abramov. The study indicated
that less than 25 percent of Earth's crust would have melted during such a
bombardment.

The CU-Boulder researchers even cranked up the intensity of the asteroid barrage
in their simulations by 10-fold -- an event that could have vaporized Earth's
oceans. "Even under the most extreme conditions we imposed, Earth would not have
been completely sterilized by the bombardment," said Abramov.

Instead, hydrothermal vents may have provided sanctuaries for extreme,
heat-loving microbes known as "hyperthermophilic bacteria" following
bombardments, said Mojzsis. Even if life had not emerged by 3.9 billion years
ago, such underground havens could still have provided a "crucible" for life's
origin on Earth, Mojzsis said.

The researchers concluded subterranean microbes living at temperatures ranging
from 175 degrees to 230 degrees Fahrenheit would have flourished during the Late
Heavy Bombardment. The models indicate that underground habitats for such
microbes increased in volume and duration as a result of the massive impacts.
Some extreme microbial species on Earth today -- including so-called "unboilable
bugs" discovered in hydrothermal vents in Yellowstone National Park -- thrive at
250 F.

Geologic evidence suggests that life on Earth was present at least 3.83 billion
years ago, said Mojzsis. "So it is not unreasonable to suggest there was life on
Earth before 3.9 billion years ago. We know from the geochemical record that our
planet was eminently habitable by that time, and this new study sews up a major
problem in origins of life studies by sweeping away the necessity for multiple
origins of life on Earth."

Most planetary scientists believe a rogue planet as large as Mars smacked Earth
with a glancing blow 4.5 billion years ago, vaporizing itself and part of Earth.
The collision would have created an immense vapor cloud from which moonlets, and
later our moon, coalesced, Mojzsis said. "That event, which preceded the Late
Heavy Bombardment by at least 500 million years, would have effectively hit
Earth's re-set button," he said.

"But our results strongly suggest that no events since the moon formation were
capable of destroying Earth's crust and wiping out any biosphere that was
present," Mojzsis said. "Instead of chopping down the tree of life, our view is
that the bombardment pruned it."

The results also support the potential for microbial life on other planets like
Mars and perhaps even rocky, Earth-like planets in other solar systems that may
have been resurfaced by impacts, said Abramov.

"Exactly when life originated on Earth is a hotly debated topic," says NASA's
Astrobiology Discipline Scientist Michael H. New, manager of the Exobiology and
Evolutionary Biology program. "These findings are significant because they
indicate life could have begun well before the LHB, during the so-called Hadean
Eon of Earth's history 3.8 billion to 4.5 billion years ago."

The research by Abramov and Mojzsis is sponsored by NASA Astrobiology Program's
Exobiology and Evolutionary Biology Department and the NASA Postdoctoral
Program. The Exobiology and Evolutionary Biology Program supports research into
the origin, evolution and distribution of life on Earth and the potential for
life elsewhere. Mojzsis is a member of the new NASA Lunar Science Institute
through the Center for Lunar Origin and Evolution.

For more information on CU-Boulder's Early Earth and Planetary Geology Group
visit http://isotope.colorado.edu. For more information on the NASA Lunar
Science Institute Program visit http://lunarscience.arc.nasa.gov/. For more
information on the NASA Astrobiology Institute visit
http://astrobiology.nasa.gov/nai/.
Received on Wed 20 May 2009 06:15:01 PM PDT