[meteorite-list] Lunar Meteorites and the Lunar Cataclysm

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:06 2004
Message-ID: <200102021935.LAA07335_at_zagami.jpl.nasa.gov>

http://www.soest.hawaii.edu/PSRdiscoveries/Jan01/lunarCataclysm.html

Lunar Meteorites and the Lunar Cataclysm
Planetary Science Research Discoveries
January 24, 2001

Written by Barbara A. Cohen
University of Tennessee, Knoxville

The Moon has been pummeled with asteroids and comets throughout its long,
4.5 billion-year history. While even a single impact can be an impressive
event, there seems to have been one particularly spectacular era about 3.9
billion years ago which saw the formation of 1700 craters 100 kilometers in
size or larger, resurfacing 80% of the Moon's crust. This intense
bombardment, known as the "Lunar Cataclysm," was first suspected nearly 30
years ago, based on the rocks returned by the Apollo astronauts. However,
because the Apollo Moon rocks all come from a relatively small region on the
Moon, many scientists worried that the effect was really just a local
pounding.

In the December 1, 2000 issue of Science, my colleagues, Tim Swindle and
David Kring, and I report that this intense bombardment is also reflected in
lunar meteorites. Because lunar meteorites are a more random sampling of the
Moon than the Apollo samples, the Lunar Cataclysm does indeed seem to have
been a Moon-wide phenomenon. The Earth would not have escaped a similar
beating during this time -- and neither would life on Earth.

     Reference:

     Cohen, B. A.,T. D. Swindle, D. A. Kring, 2000, Support for the Lunar
     Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages, Science, v.
     290, no. 5497, p. 1754-1755.

             --------------------------------------------------

The Pummeled Moon

When you look up at the moon, the first thing you notice with your naked eye
is the dark circles (maria) in a bright background (highlands.) When you
look through even a small telescope, though, craters seem to pop up
everywhere. In fact, the lunar highlands are saturated with craters, which
means that if a new crater were to form, it would have to wipe out other
craters because there is no empty space left. Craters come in all sizes,
from micrometers to hundreds of kilometers in diameter. In fact, the dark
maria are circular because they fill in the largest lunar craters, the
basins. There are ~50 lunar basins, scars from extremely large impact
events, that are each about the size of Texas.

The number of craters on a planetary surface is related to how frequently an
impactor hits and the age of the surface. Older surfaces have more craters
on them. At the beginning of the solar system, there was much more material
available to be swept up than there is now, so the frequency of impacts
might be expected to taper off with time. This would mean that very old
surfaces have many more craters than young ones. At first glance, this seems
to work out on the moon. The maria, which clearly came later than the
highlands, have very few craters compared with the highlands.

When the Apollo astronauts provided us with samples of the Moon's crust,
scientists confirmed that the mare rocks are young (3.3-3.6 billion years)
compared to rocks from the primitive lunar crust (4.5 billion years).
However, the rocks affected by impact events yielded a surprise. If there
were many more old impacts than young ones, then there should be many more
old impact rocks than young ones. Instead, virtually all the impact rocks in
the Apollo collection were roughly the same age, 3.9 billion years, and none
were older. Scientists like Fouad Tera and Grenville Turner suggested that
an unusual event must have occurred 3.9 billion years ago, a lunar cataclysm
that created most of the large basins such as Imbrium as well as many
smaller craters.

However, the Apollo collection is a small sampling of rocks from a small
area on the Moon. For the astronauts' safety, they had to land near the
equator so that the command module could pass overhead every few hours, in
case anything went wrong. Also, they had to land on the near side to
maintain radio contact with the Earth. The six landing sites (and the three
Luna landing sites from which robotic Soviet missions also returned rocks)
are from a confined area on the Moon.

Remember, the basins are huge impact craters. It seemed possible that the
Apollo samples were just pieces from these particular large basins, and the
strange 3.9-billion-year ages were not representative of what was going on
over the entire Moon. The best way to test for such a bias is to look at
rocks from other places on the Moon, like the far side. The lunar meteorites
fit this bill nicely.

             --------------------------------------------------

Lunar Meteorites

The Moon rocks collected by the Apollo astronauts and the Soviet Luna probes
are not the only rocks from the Moon that we have. We know that the Earth
collects material from space all the time in the form of meteorites,
although until we had samples we knew were from the Moon, scientists
wouldn't have been able to identify a lunar meteorite.

When scientists studied ALHA81005 in detail, it bore so many resemblances to
the Apollo and Lunar samples that they agreed virtually unanimously that it
was from the Moon. They re-examined other meteorites and found three
additional lunar meteorites already in our collections. Subsequently, more
than a dozen other lunar meteorites have been found, mostly in Antarctica
and the Saharan desert. [See listing of lunar meteorites.] These rocks are
ejected from the Moon gently enough to remain intact for a journey through
space and then a fiery passage through Earth's atmosphere.

Some of these meteorites are regolith breccias, formed when a chunk of
regolith (the "soil," including all the rock fragments in it) is compressed
into a single rock. So, these meteorites have many different pieces of the
Moon's surface assembled into a single rock.

Since some of the fragments may be impact melts, within a single meteorite,
there may be fragments of different impact rocks, dating different impact
events. If you can show that the impact rocks don't come from the part of
the Moon where the astronauts landed, this would be a good test of the Lunar
Cataclysm idea. This was first suggested by G. Jeffrey Taylor at the
University of Hawai'i when he examined the meteorite MAC88105. Jennifer
Grier at the University of Arizona saw similar potential in another
meteorite, QUE93069. Cohen saw DaG262 and DaG400 described at a meeting.
While working on my PhD at the University of Arizona, Tim Swindle (my
advisor, an expert in finding the ages of rocks), Dave Kring (an expert in
studying the rocks formed in impacts) and I requested samples from these
four meteorites to study.

To test the Lunar Cataclysm idea, the rocks had to be different from the
Apollo samples and they had to come from large craters. We know now from
Clementine and Lunar Prospector mapping missions that the Apollo sites are
chemically unique in containing high abundances of a certain elements,
including potassium (K) and phosphorous (P). The meteorites, on the other
hand, are very low in K and P. Although no one knows exactly where on the
Moon any of these meteorites come from, they don't come from the same place
as the Apollo and Luna samples. To find rocks from large impacts, the group
looked for "crystalline impact melts." It takes a fairly large impact to
melt rock, but it takes an even larger one to create so much melt that some
of it will be buried deeply and warmly enough to cool slowly enough to allow
crystals to form.

             --------------------------------------------------

Dating Tiny Rocks

The impact melts satisfying these criteria were dated with a technique known
as the 39Ar-40Ar method. It is based on radioactive decay, just as the
well-known 14C dating technique is. However, in 39Ar-40Ar dating, the
radioactive isotope, 40K, has a half-life of more than a billion years (as
opposed to the 5700 years for 14C), so it is possible to date things that
happened billions of years ago, like the Lunar Cataclysm.

The element potassium is common in many rocks. On Earth, for example, it is
found in the pink feldspar in granites. The Moon has feldspar too, and it
also has K, though in low levels. Over time, the radioactive isotope of
potassium, 40K, decays to an isotope of argon, 40Ar. Argon, like helium, is
a noble gas, which means that it does not bond chemically to the minerals.
However, an argon atom is a large atom and cannot easily leak out of a rock.
So 40Ar builds up inside. By measuring the amount of 40Ar in a rock, and the
amount of K which was available to produce it, scientists can calculate how
long the 40Ar has been building up - in other words, the age of the rock.
The "39Ar" in the name of the technique comes from using a nuclear reactor
to convert some other potassium into the argon isotope 39Ar, a technique
pioneered by Grenville Turner in the 1960s, and used in early work on
determining the ages of Apollo samples. The technique is sensitive enough
that I was able to analyze tiny rock fragments less than a tenth of a
millimeter long.

In testing the Lunar Cataclysm, we expected that if impact melt older than
3.9 billion years existed and was abundant on the Moon's surface, it was
probable that we would find a piece of it among the four meteorites.
Instead, we found no impact melts older than 3.9 billion years. This
strengthens the idea that the Moon took a severe beating at around 3.9
billion years, and that it was Moon-wide. It seems that most of the current
lunar surface acquired its visible craters in a short time, instead of
gradually since the beginning of the solar system.

What could have caused the cataclysm? No one is sure yet. The most likely
story may be that an asteroid broke up in the asteroid belt and was swept
into the inner solar system. It would have had to be a very large asteroid,
the size of the largest asteroids today. Alternatively, the gas giant
planets Uranus and Neptune might have been still working to clear out solar
system debris at this time, sending a shower of icy chunks to the Moon.
Since the Moon is, and always has been, very dry, it seems unlikely that
either the Uranus-Neptune objects or comets were responsible, since either
one would have delivered a lot of water to the Moon.

How long did the Cataclysm last? Using the same 39Ar-40Ar technique on
Apollo samples, Graham Ryder of the Lunar and Planetary Institute and G.
Brent Dalrymple of Oregon State University have suggested that the Lunar
Cataclysm may have lasted less than 100 million years.

             --------------------------------------------------

What About the Earth?

Earth is a much bigger target than the Moon, so nearly 10 times as many
impactors will hit the Earth. So, if the Moon was pummeled by 1,700 objects
in 200 million years, the Earth got more than 17,000. This is a lot compared
to the current flux. One crater the size of Chicxulub in Mexico (about 200
kilometers) is formed about every 100 million years now. The devastation and
climate change associated with one Chicxulub crater helped wipe out the
dinosaurs and many other species of life on Earth. What would it be like to
have 17,000 Chicxulub craters in the same span? Furthermore, there would
have been many craters much larger. Some would have dumped enough heat into
the Earth's atmosphere to boil the oceans away. Perhaps that is why the
oldest evidence we have for life on Earth is 3.85 billion years old -
perhaps any life that got started before that was destroyed in the
Cataclysm.

On the other hand, there are some forms of life that might not find it so
bad. Single-celled organisms called hyperthermophiles (lovers of extreme
heat) live in places like deep-sea vents and Yellowstone hot springs. These
places are what the whole Earth might have been like as it was being
pummeled, so these organisms might have survived or even have been quite
happy. In fact, all life on Earth today seems to be genetically related to a
hyperthermophile, so maybe these organisms survived and went on to populate
the whole Earth, sort of like a microbial Noah.

             --------------------------------------------------

Additional Resources

     Cohen, B.A., T. D. Swindle, D. A. Kring, 2000, Support for the Lunar
     Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages. Science, v.
     290, no. 5497, 1 p. 1754-1755.

     Dalrymple, G. B. and G. Ryder, 1996, Argon-40/argon-39 age spectra of
     Apollo 17 highlands breccia samples by laser step heating and the age
     of the Serenitatis basin. Journal of Geophysical Research, v. 101(E11),
     p. 26,069-26,084.

     Dalrymple, G. B. and G. Ryder, 1993, 40Ar/39Ar age spectra of Apollo 15
     impact melt rocks by laser step-heating and their bearing on the
     history of lunar basin formation. Journal of Geophysical Research,v.
     98(E7), p. 13,085-13,095.

     Lunar Meteorites web page by Dr. Randy Korotev, Washington University
     in St. Louis.

     Martel, Linda, "Damage by Impact." PSR Discoveries. Dec. 1997.
     <http://www.soest.hawaii.edu/PSRdiscoveries/Dec97/impactBlast.html>.

     Tera, F., et al., 1974, Isotopic evidence for a terminal lunar
     cataclysm. Earth and Planetary Science Letters, v. 22, p. 1-21.

     Turner, G. and P. H. Cadogan, 1975, The history of lunar bombardment
     inferred from 40Ar-39Ar dating of highland rocks. Proceedings of the
     Sixth Lunar Science Conference, p. 1509-1538.
Received on Fri 02 Feb 2001 02:35:27 PM PST