[meteorite-list] Wandering Gas Giants and Lunar Bombardment

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Aug 25 01:28:36 2006
Message-ID: <200608242233.PAA04623_at_zagami.jpl.nasa.gov>

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 Nectaris.
However, the impact melted portion in the rocks consists of very small
grains, implying rapid cooling. As the late Graham Ryder (Lunar and
Planetary Institute) argued, this means that the embedded feldspar
fragments, which come from ancient rocks, might not have been heated
enough by the impact so they retain an isotopic memory of the older
crustal igneous rocks. In other words, the formation of the impact melt
breccia did not reset the clock to the time of the impact event.

[thin section of breccia]

Photomicrograph of a thin slice of a feldspar-rich impact melt breccia
from the Apollo 16 landing site. The small, lath-shaped grains are
plagioclase, as are most of the larger, irregularly-shaped fragments.
The lathy shape is indicative of crystallization from magma, in this
case one formed by impact. The mineral fragments are unmelted pieces of
pre-existing igneous rocks. Several small rock fragments like these have
ages in the range 4.09 to 4.14 billion years, suggesting that the narrow
cluster of impact melt ages required by the cataclysm hypothesis is not
valid. However, the irregular-shaped mineral fragments might not have
been heated enough to reset their ages when mixed with the small amount
of impact melt. If so, the ages are upper limits to the age of an impact
event.

Bill Hartmann (Planetary Science Institute, Tucson) has had completely
different reasons for being skeptical. Driven by the lack of a sound
mechanism for a cataclysm, he always preferred a declining impact rate
from the time of lunar formation until 3.8 billion years ago. He
suggests that this spread-out early bombardment continually pulverized
the upper reaches of the lunar crust, systematically removing impact
melt breccias that formed early. The only survivors as large fragments
are the most recent ones, giving us the false impression that the ages
of impact melts cluster between 3.85 and 3.92 billion years. He calls it
the stone-wall effect.

The argument against Hartmann's interpretation is that we have samples
of lava flows that have ages up to 4.25 billion years. If these surface
rocks survived the declining early bombardment, surely some impact melt
breccias would have, too. Ironically, if the feldspar-rich Apollo 16
impact melt breccias are shown to be over 4.1 billion years old, it
would simultaneously weaken the cataclysm idea and Hartmann's stone wall
hypothesis against the cataclysm interpretation!

It is important to test the cataclysm hypothesis with additional lunar
samples, as discussed below. For now, let's assume that there was a
lunar cataclysm. What could have caused it?

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

Dramatic Dynamics of the Early Solar System

We normally think of the orbits of the planets as unchanging and
reliable. Oh sure, stray asteroids and comets can come close to Earth
and even hit it, maybe causing extinctions of numerous species. But not
the planets. They stay put. We can rely on them. Well, it turns out that
early in the history of the Solar System, the planets may have roamed,
especially the giants Jupiter, Saturn, Uranus, and Neptune. Jupiter
seems to have migrated in towards the Sun while the others wandered away
from the Sun. When their orbits reached certain simple relationships to
each other, serious gravitational pushing and pulling happened that
violently destabilized the orbits of Uranus and Nepture, thus flinging
millions of leftovers from planet formation (asteroid-to-moon-sized
planetesimals) throughout the Solar System. Planets moved and craters
formed, possibly in a dramatic, chaotic, messy moment of geologic history.

This is the story of the early Solar System being developed by
physicists and astronomers, including Morbidelli, Tsiganis, Gomes, and
Levison. Current theory says that migrating planets are a natural
consequence of planet formation. Such migration would be accompanied by
changes in the inclination and eccentricity of the outer planets,
increasing from nice, co-planar circular orbits to those that are
inclined to the ecliptic (Earth's orbital plane) and not exactly
circular (they follow elliptical paths).

After the planets were constructed, Levison and his colleagues say, the
Solar System still teemed with the detritus of planet
formation--planetesimals that had not accreted to a planet. These
stunted planets were indiscriminately hurled around by gravitational
interactions with the outer planets. These interactions, called
dynamical friction, caused changes in the orbits of the outer planets,
too. Jupiter moved inward toward the Sun as Saturn, Neptune, and Uranus
migrated outward. As Saturn drifted outward it eventually reached a
resonance with Jupiter, in which it orbited the Sun once for every two
orbits of Jupiter around the Sun. This is called a 1:2 resonance. This
special timing of orbits of these two giant planets caused their
gravitational interactions to pump up the motions of Neptune and Uranus.
Tsiganis and his colleagues calculate, for example, that the orbits of
Neptune and Uranus might have become highly elliptical and Neptune's
orbital distance from the Sun doubled, sending it into a zone peppered
with planetesimals. The planetesimals were scattered by Neptune's
substantial gravity field, sending them all over the place, including
into the inner Solar System to bombard the rocky planets and the Moon.
The situation is so dynamic that Neptune started closer to the Sun than
Uranus, but ended up farther away. See the animation below.

[animation of dynamics]

In this dynamical simulation of the late heavy bombardment, the Sun is
in the center, the colored circular rings represent the orbits of the
four giant planets, and the green dots represent the disk of
planetesimals between 15.5 AU and 34 AU.

Each panel represents the state of the planetary system at a different
time, starting at t=100 million years. Saturn and Jupiter migrate
slowly, reaching 2:1 resonance. This scatters Neptune and Uranus. Their
extreme migrations scatter planetesimals in a short time interval--a
cataclysm.

The four panels below correspond to four different snapshots taken from
the simulations. From left to right: The beginning of planetary
migration (100 Myr), just before the beginning of the scattering (879
Myr), just after scattering has started (882 Myr), and 200 Myr later,
when only 3% of the initial mass of the disk is left and the planets
have achieved their final orbits.

[4 frames of animation]

There is a timing problem. The lunar cataclysm (if this is the valid
interpretation of the ages of lunar impact breccias) took place between
about 3.92 and 3.85 billion years ago. This means that Saturn would have
to move into the 1:2 resonance with Jupiter 600 to 700 million years
after the formation of the Solar System. What mechanism could delay
migration of the giant planets for so long? That's the central focus of
the paper by Gomes. He points out that the time at which Jupiter and
Saturn reach their 1:2 resonance depends on three major factors. One,
logically enough, is their distance from the resonance position. The
second is the mass of materials in the planetesimal disk, particularly
near its inner edge. The third factor is the relative location of the
inner edge of the disk and the outermost ice giant (Neptune or Uranus).

The effect of varying the position of the inner edge of the planetesimal
disk is shown in the diagram below. The farther from the Sun it is, the
longer it takes for Jupiter and Saturn to reach their planet-scattering
1:2 resonance. At slightly more than 15 astronomical units (AU,
the time is in the right range of a few
hundred million years. By varying the other parameters, Gomes and his
coworkers find that Jupiter and Saturn could have reached their 1:2
resonance between 200 million and 1.1 billion years after formation of
the planets. Although not precise, it shows that the resonance mechanism
is reasonable for producing the late heavy bombardment. In fact, if the
lunar cataclysm is proven, perhaps its duration and age can be used to
set limits on the variables that affect the timing of migration.

[timing of 2:1 resonance]

Calculations by R. Gomes and his colleagues show that the time when
Jupiter and Saturn reach their 1:2 resonance depends on the distance to
the inner edge of a disk of planetesimals inhabiting the outer solar
system. If the inner edge of the disk lay beyond 15 astronomical units
(AU), then the two giant planets reached their
1:2 resonance in several hundred million years, the right timing to have
caused the lunar cataclysm.

The simulations suggest that the impactors on the Moon would be from
both comets and asteroids, but the exact percentage of each is highly
uncertain. However, the dynamicists are sure that there would be a lot
of comet impacts, adding on the order of 8 x 1021 grams of cometary
material to the Moon. It is curious that we do not see evidence of this
influx of icy objects in lunar samples, all of which are bone dry.

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

Effects on Mars--Was the Early Wet Period on Mars Triggered by the
Cataclysm?

Assuming that there was a late heavy bombardment, it must have had
dramatic effects on all the planets. The record is obscure on Earth,
although the maximum ages for Earth rocks is about 3.8 billion years, in
the right range. The late Graham Ryder (Lunar and Planetary Institute,
Houston) suggested that early bombardment of the Earth provided
environments in which life could arise, although some astrobiologists
have suggested that the bombardment would have sterilized Earth, thus
requiring life to begin multiple times. The surface of Mercury is
battered like that of the Moon. (Venus has been resurfaced by volcanism,
erasing all evidence for the early bombardment.) All interesting
possibilities, but the effects of the late heavy bombardment may be most
evident on Mars.

[Mars cratered highlands]

This topographic map of Mars shows the many craters of the martian
highlands. The huge Hellas basin (~2,000 kilometers diameter) is clearly
visible (blue is low and red is high elevation). It is one of many
basins formed early in the history of Mars. The basins and craters
dramatically record the bombardment history of the planet.

Besides forming thousands of craters larger than 20 km in diameter and
perhaps a hundred basins larger than 300 km, the effects of a
cataclysmic bombardment of Mars may be widespread. They could include
mixing of diverse rock types, as we see in samples collected from the
lunar highlands. Formation of large basins might have erased the record
of an early magnetic field in Mars, now recorded only in certain regions
of the highlands. Impacts would have formed large amounts of impact melt
and fragmental breccias, making it difficult to find large expanses of
ancient rocks.

Perhaps most interesting, a cataclysmic bombardment of Mars with icy
comets would have added a substantial amount of water to the crust. It
might have triggered the early, wet period in the history of Mars. Each
impact might have caused rainy periods, as proposed by Teresa Sigura
(University of Colorado, Boulder), leading to intense erosion of the
highlands and deposition into craters that are mostly filled with
sediment. Could the early, wet period on Mars have been caused by the
migration of Jupiter and Saturn?

[Mars craters with sediment filling]

The ancient martian crust has been greatly affected by flowing water and
perhaps rain. This image shows impact craters partly filled with
sediment. Valley networks occur in the lower left part of the image.

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

Testing an Important Concept

The idea of a short, but strong, spike in the rate of impacts 3.9
billion years ago is an extremely important concept. It is not proven,
as Joe Hahn (Saint Mary's University, Halifax) points out in a clear and
concise summary of the dynamical calculations. He says that the good
agreement with the age of lunar basin formation and the explanation for
the eccentricities and inclinations of the outer planets, although very
interesting, is not proof that the simulations are correct. Perhaps the
eccentricities and inclinations were pumped up by the presence of other
large protoplanets that eventually wound up inside a giant planet or
ejected from the Solar System.

What happened to all the water in the comets that would have whacked
into the Moon? How could all of it escape? If it did escape, did it also
escape from Mars? More important for the Moon, did all those basins
really form in a short time around 3.85 billion years ago?

Cosmochemists intend to test this idea. We can use existing Apollo
samples, doing more detailed studies of those ambiguous Apollo 16 group
4 impact melt breccias and of more fragments of impact melts inside
lunar meteorites. Even better, we can date samples returned by robotic
missions to lunar basins. One favorite among cosmochemists is the oldest
basin on the Moon, South Pole-Aitken basin. The samples could be
returned on a robotic mission or by astronauts when humans return to the
Moon, an event scheduled for somewhere around 2018. Settlement of the
Moon, a central theme of the national Vision for Space Exploration, will
enable detailed studies by astronauts living on the Moon of the ages of
many lunar basins, including Nectaris and Crisium.

[artist's concept of lunar base]

Future settlement of the Moon will allow for the extensive field work
needed to collect the best samples to use to date lunar basins.

It is critical to test the idea of the lunar cataclysm. If proven, then
the calculations done by Morbidelli, Tsiganis, Gomes, and Levison are
more likely to be correct and their portrait of processes operating
during planet formation may be accurate. This portrait of planetary
migration explains the late heavy bombardment, the orbital parameters of
the outer planets, the present orbital distribution of main-belt
asteroids, and the presence of Jupiter's Trojan asteroids. If there was
no cataclysm, they will have to modify their painting, perhaps
drastically. As often happens in planetary science, cosmochemists and
astronomers are partners trying to unravel the secrets of the early
Solar System.

------------------------------------------------------------------------
ADDITIONAL RESOURCES

    * Cohen, B. (2001) Lunar Meteorites and the Lunar Cataclysm.
      Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Jan01/lunarCataclysm.html
    * 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.
    * Hahn, J. (2005) When giants roamed: News and Views. Nature, v.
      435, p. 432-433.
    * 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.
    * Norman, M. (2004) The Oldest Moon Rocks. Planetary Science
      Research Discoveries.
      http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html
    * 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.
    * Taylor, G. J. (2001) Uranus, Neptune, and the Mountains of the
      Moon. Planetary Science Research Discoveries.
      http://www.psrd.hawaii.edu/Aug01/bombardment.html
    * Vision for Space Exploration
      <http://www.nasa.gov/mission_pages/exploration/main/index.html>
      website from NASA.
Received on Thu 24 Aug 2006 06:33:19 PM PDT