SETI bioastro: Fw: Geophysicist Studies Life In The Early Solar System

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Date: Sat Dec 15 2001 - 18:49:56 PST

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Subject: Geophysicist Studies Life In The Early Solar System

News Service
Stanford University
Stanford, California

Mark Shwartz, News Service
(650) 723-9296; e-mail:


Geophysicist studies life in the early solar system
By Etienne Benson

Between the cataclysmic impact that created the Moon around 4.5 billion
years ago and the first evidence of life 3.8 billion years ago, there
may have been long periods during which life repeatedly spread across
the globe, only to be nearly annihilated by the impact of large

The early Earth, in other words, may have been an interrupted Eden --
a planet where life repeatedly evolved and diversified, only to be sent
back to square one by asteroids 10 or 20 times wider than the one that
hastened the dinosaurs' demise. When the surface of the Earth finally
became inhabitable again, thousands of years after each asteroid
impact, the survivors would have emerged from their hiding places and
spread across the planet -- until another asteroid struck and the
whole cycle was repeated.

"We know that large asteroid impacts can sterilize or partially
sterilize planets," says Norman Sleep, a professor of geophysics at
Stanford who will present the theory at the fall meeting of the
American Geophysical Union in San Francisco on Friday, Dec. 14.

"An asteroid a few hundred kilometers in diameter will boil off much
of the ocean and leave the rest of the ocean very hot, so all that
will survive will be high-temperature organisms living deep in the
subsurface," he says. Rock vapor and water would fill the atmosphere,
killing off any life on the surface with temperatures upwards of
1,000 C (1,800 F).

The only organisms that could survive such an impact are
thermophiles -- heat-loving microbes -- buried half a mile or
more below the Earth's surface, where the effects of the burning
atmosphere would have been muted to a survivable 100 C (212 F).
Those organisms may have given rise to much of the life on today's

Sleep calls the region where those organisms would have lived the
"Goldilocks Zone" -- deep enough for microbes to avoid the heat of
the burning atmosphere, but not so deep that they ran afoul of the
Earth's internal heat.

Since there are no records of life before 3.8 billion years ago,
there is no direct proof that Sleep's theory is correct. But several
strands of evidence are highly suggestive.

The first is that two of the three major branches of life that
exist on Earth today -- Archaea, Bacteria and Eukarya -- began with
organisms that were designed to live in extremely hot environments,
the kinds that would have existed for millions of years after the
impact of a large asteroid.

A glance at the names of modern members of the Archaea and Bacteria
branches turns up an overwhelming number of "thermos" -- Thermococcus,
Thermotoga, Thermoproteus and others. All of them thrive at
temperatures above 80 C (176 F), with some managing to eke out an
existence in conditions that would literally boil most organisms
alive. (The current record-holder can survive in environments above
115 C [239 F], says Sleep.)

"The roots of these two branches of the tree are clearly thermophile,
which is exactly what's going to survive in a large impact," says

Where Eukarya -- the branch that includes yeast, worms, corn and
humans -- fits into the story is less certain. "It's unclear whether
Eukarya, which we are, has a thermophile root or not," says Sleep.
"We may never have had a high-temperature-organism ancestor. But
clearly two of the three branches look like asteroid survivors:
very complex, highly-evolved organisms that are thermophile."

The second strand of evidence is geophysical. Although it has long
been thought that early Earth would have been rendered lifeless by
continual asteroid bombardment, there are now good reasons to believe
that our planet was struck by fewer than 20 large asteroids between
the time of the Moon-forming impact and the first fossil signs of
life. That would leave hundreds of millions of years between each
asteroid strike, during which complex organisms -- and life
itself -- would be free to evolve.

When asteroids did strike, only those organisms that could find some
kind of shelter would have survived. The most obvious refuge is deep
within the Earth itself, but Sleep believes there may have been
another, more exotic way for early organisms to survive such
Earth-shattering catastrophes.

Martian invaders

Perhaps, says Sleep, some of the asteroids that struck the early
Earth were large enough to destroy all life on the planet, even those
organisms hidden deep within the crust. There was still at least one
other place where life could have survived, even flourished, before
returning to Earth: Mars. Although Mars is now a frigid desert, four
billion years ago it may have been a warm, water-filled oasis as
friendly to life as early Earth.

But could a microorganism really have survived the trip from Earth to
Mars? To successfully complete the interplanetary journey, a microbe
first would have to survive an asteroid impact powerful enough to
free a chunk of rock from the grip of gravity. Once in space, the
traveler would be faced with conditions harsher than anything found
on Mars or Earth: total vacuum, subzero temperatures, harmful
radiation and the passage of perhaps thousands of years before the
interplanetary dart hit its target. Even then, the colonizing microbe
would have to hope that some of its descendants were buried deep
enough in the rock to avoid burning up in Earth's atmosphere.

Sleep says these factors make the trip difficult, but not impossible.
Models have shown that the initial shock of ejection from a planet
isn't necessarily deadly, especially for the hardiest microbes, and
especially from a small planet like Mars where the atmosphere is
thin and gravity is relatively weak. "You don't sterilize a milk
bottle by throwing it off your roof," he explains.

And laboratory experiments have shown that earthly microbes,
especially if hidden in cracks deep within a meteorite, can survive
the harsh conditions of space at least for a few years. Of course,
no one has tested whether they can survive for thousands of years,
but there's no reason to think they can't, notes Sleep. "Conditions
are not good for microorganisms, but they're not bad," he adds.

So it is possible that life came from another planet -- but did it
really happen? So far there is no direct evidence of life on other
planets or asteroids, although it is becoming clear that conditions
exist, at least on Mars and Europa -- one of Jupiter's inner
moons -- where microbes that live comfortably in Earth's harsher
climates would have felt at home. As Sleep put its, Mars "is no
more uninhabitable than Antarctica" -- uncomfortable for humans,
but perfect for some microbes.

Conclusive evidence for or against the theory only will come when
scientists can examine samples from other planets and asteroids,
something that is still a long way off. But Sleep says he's not
frustrated by the sometimes slow pace of studying early life.

"The origin of life is one of the fundamental problems of science,
and it always has been. Living at a time when you can do that, it's
not something I'm going to pass up," he says.


Norman Sleep, Geophysics, (650) 723-0882,

EDITORS: This press release was written by science writing intern
Etienne Benson. The American Geophysical Union will hold its annual
fall meeting Dec. 10 to 14 at the Moscone Convention Center, 747
Howard Street, San Francisco, CA 94103. Prof. Norman Sleep will give
the opening talk at AGU Session U51A, "Origin and Early Evolution of
the Earth I," on Fri., Dec. 14, 8:30 a.m. PT in Room 134. For more
information, visit the AGU website at .

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