[meteorite-list] Lunar Surface May Hold Evidence That Asteroids Crashed Into Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:50:29 2004
Message-ID: <200204231840.LAA18733_at_zagami.jpl.nasa.gov>

http://www.sfgate.com/cgi-bin/article.cgi?f=/c/a/2002/04/22/MN108065.DTL

Shooting for the moon
Lunar surface may hold evidence that asteroids crashed into Earth

Keay Davidson
San Francisco Chronicle
April 22, 2002
  
Clues to Earth's earliest days and first microbial inhabitants may survive
in an unexpected place: the moon.

Scientists have long debated what happened on the primordial Earth almost 4
billion years ago. Did primitive microbes wriggle within volcano-heated
pools of water? Did falling asteroids vaporize oceans and gouge craters the
size of small states?

Such questions are terribly hard to answer. The clues have been largely
erased by erosion -- by rain, wind, tides, plate tectonics, and other
natural forces.

But some clues might still exist a quarter of a million miles away, on the
frigid, airless surface of the moon. Long ignored by mainstream scientists,
the idea has begun to attract some serious attention, including the first
serious proposals to go looking for hard evidence.

New calculations by a youthful team of researchers at the University of
Washington and Iowa State suggest a strong probability that asteroid impacts
could have splashed substantial amounts of terrestrial rock toward the moon,
like mud sprayed by a car racing down a dirt road.

Clouds of what the researchers call "terran meteorites" might have sprinkled
across the lunar surface. There, in a much less erosive environment than
exists on Earth -- no wind ever blew and no water ever flowed on the moon --
the rocky relics of Earth's primeval days may endure, awaiting discovery by
future astronauts or remotely controlled robotic vehicles.

Hence the three researchers dub the moon "Earth's attic": a deep-freeze
repository for relics of the terrestrial dawn.

The researchers have outlined a plan to test the hypothesis as part of some
future lunar-prospecting mission. Details were presented for the first time
at a recent astrobiology science conference held at NASA's Ames Research
Center in Mountain View.

After a large impact, the terrestrial rocks "could just fly off Earth and
get scooped up by the moon, or go into orbit around the Sun and then later
on land on the moon," explained John C. Armstrong of the Center for
Astrobiology and Early Evolution at the University of Washington at Seattle.

Perhaps 20 tons of terrestrial rock could be buried over a typical lunar
area of about 40 square miles, according to calculations by Armstrong and
Llyd E. Wells, also at the Seattle center, and Guillermo Gonzalez, assistant
professor in the physics and astronomy department at Iowa State University
in Ames, Iowa.

Armstrong is a graduate student in astronomy who expects to receive his
doctorate at year's end. Wells is a biologist and graduate student in
oceanography.

The surface of the moon is not completely free of erosion: It is pelted by a
steady rain of "micrometeorites" and cosmic rays. The most intact terran
rocks are likely to survive within a few feet of the lunar surface, shielded
by the overlying rock.

Armstrong said the three men got the idea while "stuck in traffic" near the
Ames center in early 2000. Armstrong says they began batting around ideas
for space exploration, "and Guillermo said, 'Say, have you ever thought
about what would happen if an asteroid could blast stuff off the Earth and
onto the moon?' "

A similar question was asked in the 1960s by a famous chemist, Harold C.
Urey, a top adviser to the U.S. space program. His idea drew little
attention, though. One reason: It was hard to imagine how material could be
violently transferred from one world to another without being destroyed in
the process. (To escape Earth gravity, an object must be accelerated to a
speed of 7 miles per second or 25,000 miles per hour.)

In recent years, though, scientists have grown accustomed to finding
fragments of the moon and Mars on Earth, especially in Antarctica. There,
they pluck lunar and Martian meteorites out of the polar ice like kids
plucking raisins from raisin pudding.

They know the Mars rocks come from that planet because they contain small
pockets of gas whose isotopic contents match those recorded in the Martian
atmosphere by the twin Viking robots, which landed on Mars in 1976. Mars
meteorites are clear evidence that chunks of one planet can survive a voyage
to another.

The most controversial Martian meteorite is known as ALH84001 (ALH stands
for the Allan Hills region of Antarctica, where it was found). A few
scientists suspect it contains fossils of Martian microbes.

"The moon is strategically located within the inner solar system as a
collector of debris," Armstrong said. 'It has, potentially, collected
material from all the terrestrial planets," including Earth, Mars and Venus.

"The Earth meteorites on the moon could provide a geological record of early
Earth not available anywhere else in the solar system. . . . While there
isn't a whole lot of Earth stuff up there, some of the Earth material may
contain geochemical and biological information such as isotopic signatures,
organic carbon, biologically derived molecules and minerals, and maybe even
microbial fossils."

Skepticism is expressed by NASA-Ames scientist Dale Cruikshank, a leading
figure in the search for organic molecules in space.

"Earth materials probably exist on the moon," he acknowledged, but cautioned
that they are probably "hugely diluted in the vast and thick dusty layers
that mantle every square inch of our neighbor in space."

Also, any Earth rocks that reached the moon about 4 billion years ago should
have been altered by lunar volcanic activity or changed "chemically and
mechanically beyond recognition" by other natural means, he said in an
e-mail to The Chronicle.

In response, Armstrong agreed that terrestrial materials might be diluted to
a scarcity of one to 10 parts per million. Still, even such scarce particles
are "not insignificant" and could be identified and studied with advanced
scientific methods.

He pointed out that in recent years, scientists have learned a great deal
about the evolution of the solar system by studying interstellar dust
particles (IDPs), which are literally dust grains that drop to Earth from
space. As for lunar vulcanism, Armstrong says it might help, not harm, their
proposal because lava "could actually help protect the material from the
Earth" from lunar erosive processes such as micrometeorites.

If robots or astronauts return to the moon, how could they distinguish
terrestrial meteorites from native lunar rocks? Armstrong's team is now
investigating that question, using small samples of lunar rocks from NASA's
Johnson Space Center in Houston.

One way, they suspect, is by analyzing the rocks' reaction to ultraviolet
light. Ultraviolet light could expose carbonates typically formed in the
presence of liquid water, which has long been abundant on Earth.

Also, future explorers might keep their eyes peeled for rocks with burned or
"ablated" surfaces. Ablation is a clue that they experienced high friction
while shooting through the atmosphere of another planet.

One of the most exciting questions facing space scientists is: Did the inner
solar system experience a horrendous "late heavy bombardment" of asteroids
3.8 billion to 4.1 billion years ago? Scientists have debated this question
for years.

Terrestrial rocks on the moon might "shed a lot of light on the question of
whether there really was a (late) heavy bombardment" at that time, Armstrong
said.

If the late heavy bombardment really happened, might it have wiped out any
early life? Possibly so, some scientists say.

However, Wells speculates that terrestrial life might have survived the
bombardment via an unusual route: brief sojourns in space.

To be specific, asteroid impacts might have hurled rocks with microbes into
space. After thousands of years in the deep-freeze of orbit, the rocks might
have fallen back to Earth and "re-seeded" the planet with life, Wells says.

If he's right, then Earth's first "astronauts" were not Yuri Gagarin and
Alan Shepard but, rather, microbes. Knowing that, maybe you'll show a little
more respect for the greenish mold on your shower wall: It looks humble, but
its ancestors might have boldly gone where no microbe went before.

E-mail Keay Davidson at kdavidson_at_sfchronicle.com.
Received on Tue 23 Apr 2002 02:40:34 PM PDT