[meteorite-list] Treasures from the Lunar Attic

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:48 2004
Message-ID: <200303141712.JAA10826_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=401&mode=thread&order=0&thold=0

Treasures from the Lunar Attic
Astrobiology Magazine
March 14, 2003

Summary: Computer simulations of what part of Earth,
Mars and Venus might be found on the moon point to new
methods for extraterrestrial sample return. Because the
moon is lifeless, its sterile condition gives astrobiologists
a very rare laboratory for collecting what may be as high as
3 grams of Earth's past, from the half-ton of lunar rocks
and soil Apollo returned for study.

Treasures from the Lunar Attic

Because our moon is lifeless, it is one of the most appealing places
to look for the preserved records of life elsewhere. At least according
to recent estimates for the amount of ejected rocks that might survive
there, the Moon may hold clues from the early history of Mars, Venus and
Earth.

Prior to the work of John Armstrong and colleagues from the University of
Washington and Iowa State, there were no published "estimates for the
abundance of Terran, Martian, and Venusian meteorites on the Moon." To
fill this gap, the team undertook computer simulations based on the
impact events from 3.9 billion years ago during what is called the Late
Heavy Bombardment--the last time that the inner solar system was pelted
with asteroid debris. The simulations must take account of the gravity
and escape velocities for each inner planet, the orbital paths of debris
trails, and finally the Moon's ability to capture strategic samples.

One of the reasons such meteorites might be so valuable is "to substantiate
or extend a contested fossil record that begins 3.5" billion years
(3.5 Ga) ago", writes Armstrong, thus filling in what early Earth life
might have offered. Even shorter spans are available for Venus, where its
surface records were catastrophically erased 700 million years ago.

The Washington study indicates that if such meteorites reached Earth- and
could be recovered even from the ice plains of Antarctica- a lunar sample
would still be preserved as the best recorded history lesson. Whether
from wind (atmosphere), water, or fire (volcanism), the Moon's lack of
erosion might provide a unique collection compared to anywhere else in
our solar system. The authors note: "Most significantly, the Moon lacks
the water capable of carrying contaminants into the interior of rocks
through cracks."

Boomerang from the Past

Tracking the debris spray from a heavy bombardment proved challenging.
Three cases were considered: 1) "direct transfer", where the ejected rocks
liftoff the Earth, Mars or Venus with medium velocity, but not too high
that the Moon could not have captured them; 2) "orbital transfer", where
the meteorite debris leaves at high speed, but comes back later to land
on the lunar surface; and finally, 3) "lucky strikes", where the rocks
cross paths with the Moon directly.

In the Earth's case, at least, the incoming asteroids or fragments during
Late Heavy Bombardment average a whopping 14 kilometers per second (or
around 31,000 miles per hour [MPH]). To escape the Earth's gravity
(or reach escape velocity), the outgoing rocks also must have a relatively
high speed, around 11.5 km/s (37,000 MPH). To complete the lunar capture, a
final high-speed event must include an impact on the Moon, or a shock that
would complete the sample's journey after a relatively hard-landing at
around 2-5 km/s (~10,000 MPH).

One discovery from computer simulations was that the second method of
capturing rocks on the moon--orbital transfer-- is dominant. Most (58%
by mass) of the terrestrial samples (called Terran) that would be preserved
today on the Moon, would have left Earth in all directions, but then later
have come back to visit on a centuries-old, boomerang pattern that depends
on its orbit and lunar crossing points.

One startling feature of the Moon's pockmarked surface is the cumulative
destruction that asteroid and meteor impacts have had already. For
instance on the Moon's South Pole (called the Aitken Basin), an impactor
weighing 10 quadrillion (10^15) tons (10^19 kb) left a crater nearly 2200
km (1320 miles), which is at least 100 times the thickness of the Earth's
atmosphere. So whatever the source of the lunar South Pole impact was, it
had little chance of coming from anything terrestrial, at least not
without leaving a similar gash in the Earth's crust.

The Washington study sums up their findings: "The amount of Terran material
on the surface of the Moon will depend largely on the age of the surface
that is searched. Assuming the regolith (soil) is well mixed, we estimate
the total surface abundance of Terran to lunar material to be 7 (parts per
million) ppm. This corresponds to ~ 20,000 kg of Terran material over a
10 x 10 square km area."

For the other inner planets, Venusian chunks would from 1000 to 30,000 times
less likely on the Moon, but "an area of 10 x 10 square km should still
yield almost 1 kg of Venusian material, if it can be identified as such,"
as a lower bound, and as high as 30 kg.

Finally, for Mars, approximately fifteen (100-gram) Mars rocks today reach
and impact the Earth each year. So if identifiable on the Moon, this
translates "to about 180 kg in the same 10 x 10 square km area."

Rummaging for Life in the Lunar Attic

But what kind of evidence would prove a rock's origins? To unravel such a
history for a three and half billion year old rock, the researchers
considered a cadre of tools to look for what would be evidence of the
rock's origins. To estimate the possibilities, these would be "isotopes,
significant volatile inventories, organic carbon, and molecular
fossils (biomarkers)", according to their study. Could the evidence of
another planet survive the high pressures and temperatures of impact
and capture?

The chances of tracing back such a complex
history have improved dramatically in recent
years, and spawned new investigations within
the larger meteorite and astrobiology community.
The authors note that estimated survival
likelihood has risen dramatically: "Until recently,
the prospect that material could escape a planet
via a natural process was considered extremely
unlikely, much less that the material could do so
without being heavily shocked. Experimental and
observational evidence has forced a revision of
this opinion...In fact, (the Allen Hills, Martian
meteorite) ALH84001 apparently traveled from
the surface of Mars to Earth without ever
exceeding 40 C"--or a mild 104 F.

"Terran materials are abundant and near the surface,"
they conclude, "with a significant fraction retaining their
geochemical and biological signatures for detailed
analysis. In addition, since the majority of Terran
samples date from the end of the Late Heavy
Bombardment, the samples in the lunar 'attic' are a
unique probe of the early conditions on Earth, and
potentially contain clues to the earliest forms of life."

What's Next

What the lunar attic might hold of the Earth's past is not entirely a
theoretical argument, given that nearly a half-ton of the Moon was
brought back during the Apollo missions. As first steps, Armstrong and
his coauthors propose looking at what scientists already have vaulted.
"Before any such [lunar] mission is attempted, the current stock of
lunar material (approximately 400 kg worth) should be searched for
Terran material. Given a concentration of 7 ppm, there should be
roughly 3 grams of Earth material in the current lunar samples."

A tell-tale sign of what might have originated terrestrially would be
what is known as 'hydrated silicates': a remnant of the Earth's watery
composition compared to the dry moon. As the authors write: "While this
is not likely to yield much in the way of information about the early
Earth, it would act as a proof of concept and a baseline for
future missions."

Collaborators include Llyd E. Wells (U. Wash.) and Guillermo Gonzalez
(Iowa State). This research was supported by the National Science
Foundation (NSF-IGERT) training-ship in Astrobiology, an NDSEG fellowship,
and the NASA Astrobiology Institute.
Received on Fri 14 Mar 2003 12:12:12 PM PST