[meteorite-list] Wandering Gas Giants and Lunar Bombardment

From: Rob McCafferty <rob_mccafferty_at_meteoritecentral.com>
Date: Sat Aug 26 07:59:39 2006
Message-ID: <20060826115936.36846.qmail_at_web50906.mail.yahoo.com>

This is another wonderful link from Ron. A remarkable
theory which I would never have considered or even
believed had I not read it.

Thanks once again to you Ron for providing us with
these real gems.

Rob McC

--- Ron Baalke <baalke_at_zagami.jpl.nasa.gov> wrote:

>
>
http://www.psrd.hawaii.edu/Aug06/cataclysmDynamics.html
>
> Wandering Gas Giants and Lunar Bombardment
> Planetary Science Research Discoveries
> August 24, 2006
>
> --- Outward migration of Saturn might have triggered
> a dramatic increase
> in the bombardment rate on the Moon 3.9 billion
> years ago, an idea
> testable with lunar samples.
>
> Written by G. Jeffrey Taylor
> Hawai'i Institute of Geophysics and Planetology
>
> There may have been a dramatic event early in the
> history of the Solar
> System--the intense bombardment of the inner planets
> and the Moon by
> planetesimals during a narrow interval between 3.92
> and 3.85 billion
> years ago, called the late heavy bombardment, but
> also nicknamed the
> lunar cataclysm. The evidence for this event comes
> from Apollo lunar
> samples and lunar meteorites. While not proven, it
> makes for an
> interesting working hypothesis. If correct, what
> caused it to happen?
>
> A group of physicists from the Observatoire de la
> C??te d'Azur (Nice,
> France), GEA/OV/Universidade Federal do Rio de
> Janeiro and Observat??rio
> Nacional/MTC (Rio de Janeiro, Brazil), and the
> Southwest Research
> Institute (Boulder, Colorado) conducted a series of
> studies of the
> dynamics of the early Solar System. Alessandro
> Morbidelli, Kleomenis
> Tsiganis, Rodney Gomes, and Harold Levison simulated
> the migration of
> Saturn and Jupiter. When the orbits of these giant
> planets reached the
> special condition of Saturn making one trip around
> the Sun for every two
> trips by Jupiter (called the 1:2 resonance), violent
> gravitational
> shoves made the orbits of Neptune and Uranus
> unstable, causing them to
> migrate rapidly and scatter countless planetesimals
> throughout the Solar
> System. This dramatic event could have happened in a
> short interval,
> anywhere from 200 million years to a billion years
> after planet
> formation, causing the lunar cataclysm, which would
> have affected all
> the inner planets.
>
> References:
>
> * Tsiganis, K., R. Gomes, A. Morbidelli, and H.
> F. Levison (2005)
> Origin of the orbital architecture of the
> giant planets of the
> Solar System. Nature, v. 435, p. 459-461.
> * Morbidelli, A., H. F. Levison, K. Tsiganis,
> and R. Gomes (2005)
> Chaotic capture of Jupiter's Trojan asteroids
> in the early Solar
> System. Nature, v. 435, p. 462-465.
> * Gomes, R., H. F. Levison, K. Tsiganis, and A.
> Morbidelli (2005)
> Origin of the cataclysmic Late Heavy
> Bombardment period of the
> terrestrial planets. Nature, v. 435, p.
> 466-469.
>
>
------------------------------------------------------------------------
>
> The Lunar Cataclysm
>
> There are lots of really old lunar rocks. Ferroan
> anorthosites, which
> were the first to accumulate from the ocean of magma
> surrounding the
> Moon when it formed, crystallized 4.45 billion years
> ago (see PSRD
> article The Oldest Moon Rocks
>
<http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html>.)
> However, many, many rocks formed by melting during
> huge impact events,
> which we call "impact melt breccias," have ages that
> fall into a narrow
> time interval, between 3.92 and 3.85 billion years.
> This apparent
> clustering of ages was first noticed in the
> mid-1970s by Faroud Tera,
> Dimitri Papanastassiou, and Gerald Wasserburg
> (Caltech) who concluded
> that the ages record an intense bombardment of the
> Moon. They called it
> the "lunar cataclysm" and proposed that it
> represented a dramatic
> increase in the rate of bombardment of the Moon
> around 3.9 billion years
> ago. More recent work on lunar samples and lunar
> meteorites generally
> confirms that there is a dearth of ages for impact
> melts older than 3.9
> billion years (see PSRD article Lunar Meteorites and
> the Lunar Cataclysm
>
<http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html>.)
>
> [lunar basins with ages]
>
> The ages of five basins on the Moon have been
> determined. Other basins
> are known to be younger than Nectaris and older than
> Orientale, so at
> least 12 basins formed between >3.80 and 3.90
> billion years ago.
> Possibly almost all 45 lunar basins formed during
> this time period.
>
>
------------------------------------------------------------------------
>
> The Cataclysm Skeptics Club
>
> The lunar cataclysm is an established, solid idea.
> Or is it? No, say the
> voices from the critics' corner. Randy Korotev
> (Washington University in
> St. Louis) is skeptical of the whole idea, as was
> his late colleague
> Larry Haskin. Korotev thinks we have a hideous
> sampling problem, and
> that the Apollo sites were all too close to the
> Imbrium impact basin.
> Imbrium is 1300 kilometers in diameter and tossed
> its continuous ejecta
> over an area twice that size; see image below. (The
> basalt flows
> composing Mare Imbrium make up a thin veneer that
> covers only part of
> the impact basin.) They say that all the impact melt
> breccias we have
> are associated with the Imbrium impact. No wonder
> they all have the same
> age--they were all made by one gigantic event.
>
> [map of Imbrium ejecta]
>
> The dark blue area surrounding Imbrium basin on this
> map shows Don
> Wilhelms' interpretation of the extent of primary
> ejecta for the Imbrium
> basin. The Apollo 16 landing site marked with a "+"
> is at the edge of
> this geologic unit. Apollo 15 site is inside the
> unit and the Apollo 17
> landing site is just outside the boundary. There are
> some uncertainties
> in the positions of the boundaries of the units.
>
> Most lunar scientists do not agree with this
> hardnosed interpretation.
> They point out that many of the samples of impact
> melts cluster into
> geochemical groups that have distinctive ages.
> Although the ages do not
> vary much from cluster to cluster, they do differ
> beyond experimental
> uncertainties. Nevertheless, it is difficult to
> prove the Imbrium-only
> hypothesis wrong... and really hard to convince
> Randy Korotev that he
> should abandon the idea and embrace the cataclysm
> interpretation!
>
> The skeptics do have some rock data on their side. A
> group of
> feldspar-rich impact melt breccias from the Apollo
> 16 landing site have
> ages between 4.09 and 4.14 billion years, averaging
> 4.12 billion years.
> This is substantially older than the narrow
> cataclysm range. If these
> ages represent the age of an impact, it shows that
> impacts certainly
> took place before 3.9 billion years. And if the ages
> represent the age
> of a basin, such as the Nectaris basin a few hundred
> kilometers to the
> east, then it casts great doubt on the cataclysm
> hypothesis. The
> feldspar-rich composition of these rocks is
> consistent with remote
> sensing observations of the lunar highlands
> surrounding
=== message truncated ===


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Received on Sat 26 Aug 2006 07:59:36 AM PDT