[meteorite-list] Current Puzzles of Interplanetary Meteorites - PART 2

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:31 2004
Message-ID: <20010713004104.48785.qmail_at_web10407.mail.yahoo.com>

Hello List,

*** Read ahead if you are continuing from PART 1 ***

There has been a good deal of discusion generated on
this Subject. Just thought I would get this whole
thread encapsulated and entered into our List Archives
for future reference. For additional reading, there
is an interesting article in the current issue of
DISCOVER magazine.

Bob V.

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From: Peiser Benny <B.J.Peiser_at_livjm.ac.uk>
To: cambridge-conference
Date: Wed, 11 Jul 2001 10:57:11 +0100


(CCNet 10 July 2001)


    Marco Langbroek <m.langbroek_at_rulpre.LeidenUniv.nl>

    Grenville Turner <gturner_at_fs1.ge.man.ac.uk>

    Oliver Morton <abq72_at_pop.dial.pipex.com>

    Max Wallis <wallismk_at_Cardiff.ac.uk>

    Tom Van Flandern <tomvf_at_metaresearch.org>

    Fred Singer <singer_at_sepp.org>

    H. Jay Melosh

    Brett Gladman and Joseph A. Burns; Department of
Astronomy, Cornell University


*** Continued from PART 1 ***



by H. Jay Melosh

The returning Apollo 11 astronauts' triumphal
reception in July 1969 was somewhat delayed by a
strict and lengthy biological quarantine. In those
days, no one was certain that the Moon was entirely
sterile. No one knew whether the lunar rocks might
harbor deadly microorganisms. One wonders whether the
level of concern would have been as high if scientists
had known that dozens of lunar rocks had been lying in
the Antarctic ice for thousands of years, or that
about 10 small fragments of the Moon must fall onto
Earth's surface every year. Unfortunately for the
astronauts, the first lunar meteorite was not
recognized until 1982. Before that time, no one
seriously believed that nearly unaltered rocks could
be blasted off the surface of one planet and later
fall onto the surface of another.

Now, however, not only do we know that lunar rocks
occasionally fall to Earth, but we are also reasonably
certain that a group of nine meteorites, the so-called
SNCs (named after the sites where they landed,
Shergotty, Nakhla and Chassigny), originated on the
planet Mars. Although all of the lunar meteorites were
collected long after they fell, four of the SNCs were
observed dropping from the sky. In 1911, a piece of
Nakhla, which fell near Alexandria, Egypt, killed a
dog, scoring the only known mammalian fatality
caused by a meteorite. [Sorry Kevin, this "unedited"]

The total flux of Martian material falling onto Earth
has been estimated at about half a ton per year. Under
these circumstances, it may seem silly to worry about
hypothetical Martian organisms contaminating Earth,
since Martian material has evidently been raining on
our planet throughout its history. Although a good
case can be made for limiting modern biological
contamination of Mars by terrestrial spacecraft, the
discovery of Mars rocks on Earth brings up the
immediate question of whether Earth rocks have been
ejected into space, eventually to fall onto Mars, thus
closing the circle of potential contamination.

Blasting Rocks off Planets

Only a few years ago, the question, "Can rocks be
launched from the surface of a major planet or
satellite by natural processes?" would have been
answered with a resounding no by experts on both
impact and volcanism, the only geological processes
known to eject solid material at high velocities. The
existence of the lunar and SNC meteorites has,
however, forced these experts to rethink the mechanics
of ejection. Although volcanic eruptions still seem
incapable of achieving planetary escape
velocity[Although volcanic eruptions on Io, a Jovian
sattelite, often do exceed escape velocity-Ed.], the
ejecta from large impacts are not so limited.

Older work on the maximum velocities achieved by
impact ejecta focused on the relationship between the
pressure in the shock wave generated by the impact and
the velocity of material just behind the shock.
Measured directly in laboratory experiments, the shock
pressure needed to accelerate material to planetary
escape velocities, 2.4 kilometers per second (about
5,000 miles per hour) for the Moon and 5.0 kilometers
per second (about 11,000 miles per hour) for Mars,
implying pressures of 0.44 and 1.5 megabars (a megabar
equals 1 million times Earth's atmospheric pressure at
sea level) for lunar and Martian basalts,
respectively, would have been high enough to melt or
even vaporize the ejected rock. Yet study of the lunar
meteorites indicates that their ejection was
accompanied by no more than about 0.2 megabar of
shock, and the most highly shocked Martian meteorites
(which contain pockets of once-melted glass) still
indicate only about 0.4 megabar.

The problem with the pressure-velocity relationship is
that it applies only to material completely engulfed
by the shock wave. Very close to the target surface,
however, the ambient pressure is zero. No matter how
strong the impinging shock wave, the free surface can
never be raised to a pressure higher than zero. This
effectively shields surface rocks from strong
compression. However, the pressure increases very
rapidly with depth below the surface, which translates
into a powerful acceleration which throws lightly
shocked surface rocks out at speeds comparable to the
original impactor's speed.

An experiment performed several years ago by Andy
Gratz and colleagues at the Lawrence Livermore
Laboratory has verified the general correctness of
this model. An aluminum projectile about the size of a
penny was fired at a granite block at about 4
kilometers per second (9,000 miles per hour). Material
from the face of the block was ejected at about 1
kilometer per second (2,000 miles per hour). This
material was caught in a foam cylinder and, upon
analysis, proved to be composed of millimeter-size,
lightly shocked fragments of granite.

Furthermore, blocks up to a meter in diameter from the
uppermost limestone layer surrounding the
24-kilometer-diameter (15-mile) Ries impact crater in
southern Germany have been found nearly 200 kilometers
away in Switzerland. Although they were not actually
ejected from Earth, these blocks again show a
combination of low shock damage (less than 10
kilobars, 10,000 times Earth's atmospheric pressure at
sea level) and high ejection velocity (1.4 kilometers
per second or about 3,000 miles per hour). Thus,
current theory, experiment and observation all agree
in indicating that a small quantity of material near
the surface surrounding the site of an impact is
ejected at high speed while suffering little shock

Impacts such as the one which created the
180-kilometer-diameter (110-mile) Chicxulub crater in
Yucatan 65 million years ago (and incidentally wiped
out the dinosaurs, among other species) may have
launched millions of rock fragments, 10 meters (30
feet) or more in diameter, into interplanetary space.
Of these fragments, a small fraction, perhaps 1 in
500, would have been so lightly shocked that internal
temperatures remained below 100 degrees Celsius (212
degrees Fahrenheit). Higher temperatures would
presumably kill any microorganisms present in the
rock, but a few thousand of the ejected rocks, those
originating nearest the free surface, could have
carried viable organisms into interplanetary space.
Although such impacts are rare at the present time
(the only comparable craters known are the
1.85-billion-year-old Sudbury crater in Ontario and
the 1.97-billion-year-old Vredefort crater in South
Africa), the much higher cratering rate early in solar
system history during the period of heavy bombardment
which lasted up to about 3.8 billion years ago would
have made ejection of microorganisms a much more
common occurrence at that time.

The most lightly shocked rocks ejected at high speed
are necessarily those closest to the free surface. The
surface is also the place where biological activity is
highest, thus a large impact on Earth, or on an
earlier life-harboring Mars, would be very likely to
throw rocks which might contain microorganisms into
interplanetary space. Larger organisms, even if
present, would be unlikely to survive the 10,000 g
accelerations accompanying the launch process.

Current cratering calculations indicate that large
impacts on Venus, despite its dense atmosphere, could
eject surface rocks into interplanetary space.
Meteorites from Venus have not yet been discovered,
but there appears to be no reason why they might not
someday be found on Earth. Large impacts on all of the
terrestrial planets are thus capable of ejecting
lightly shocked surface rocks into interplanetary
space. If there should be microorganisms on the
surfaces of these planets, then they too have a chance
of journeying to another planet.

Between the Planets

Ejecta from even the largest, fastest impacts do not
travel fast enough to make a direct trip from one
planet to another. In general, the quantity of ejecta
is largest at the lowest ejection velocities, so most
planetary ejecta move relatively slowly with respect
to the planet they escape (naturally, a much larger
quantity of ejecta moves still more
slowly and ends up falling back onto the planet of
origin). The way that an ejecta fragment from, say,
Mars eventually reaches Earth is by a series of
encounters with Mars as it and the fragment orbit the
Sun. Occasionally such a fragment comes too close to
Mars and ends up falling back onto the planet after
some time in space. However, it is much more likely to
miss Mars and recede into interplanetary space, but
not before Mars' gravity has deflected the fragment
and changed its orbit.

After a long series of such encounters, a few
fragments' orbits get "pumped up" sufficiently to
cross Earth's orbit. Then the more massive Earth takes
over this cosmic volleyball game, changing the orbit
still more, until the fragment may become Venus
crossing. Sometimes the fragment is deflected all the
way out to Jupiter or Saturn, which themselves may
eject it from the solar system entirely. At any stage
of this random walk through the solar system, the
fragment may actually hit one of the planets, ending
its journey.

Natural orbital perturbations thus supply the means
for rocks ejected from one planet to spread throughout
the solar system and eventually fall onto another
planet (or leave the solar system entirely). This is
presumably how the SNC meteorites reached Earth. Any
microorganism contained in these rocks would thus have
an opportunity to colonize the new planet, if it was
able to survive both the journey and the fall to its

Surviving the Journey

Can microorganisms survive long exposure to the space
environment? This question is of paramount importance
for the transfer of viable microorganisms from one
planet to another, since even dormant organisms might
not be able to survive a long trip. Furthermore,
cosmic rays, ultraviolet light or even radiation from
the enclosing rocks might kill
the organisms along the way.

Many microorganisms stand up surprisingly well to the
space environment. Subjected to high vacuum, some
bacteria quickly dehydrate and enter a state of
suspended animation from which they are readily
revived by contact with water and nutrients. Medical
laboratories routinely use high vacuums for
preservation of bacteria. Viable microorganisms were
recovered from parts of the Surveyor 3 camera system
after three years of exposure to the lunar
environment. However, these instances of preservation
have only been tested over times approaching decades,
not over the tens to hundreds of millions of years
necessary for interplanetary travel.

Nature, however, has been kind enough to give us
several instances of long-term preservation of viable
microorganisms. Chris McKay of NASA Ames Research
Center has extracted microorganisms preserved for
perhaps as long as 3 million years from deep cores in
the Siberian permafrost. Even more impressive is the
discovery of bacteria which were preserved for some
255 million years in salt beds of Permian age at a
site in New Mexico. Dehydrated by contact with salt
and protected from radiation by the
salt's low content of radioactive elements, these
ancient bacteria demonstrated their viability by
causing the decay of fish which had been packed with
the salt.

Living bacteria can tolerate extremely high radiation
doses, far higher than any multicellular organism can
withstand. They can resist the effects of radiation
largely because of active DNA repair systems. It is
less clear that a dormant bacterium could tolerate
large amounts of radiation. However, if the
microorganisms happened to be living in cracks or
pores of rocks which were ejected as large blocks, the
rock itself might provide adequate shielding against
both cosmic rays and ultraviolet light. Since
shielding against high-energy galactic cosmic rays
requires about 3 meters of rock, if the impact event
were to throw out rock fragments of about 10 meters
(30 feet) in diameter or larger, a
significant interior volume would be protected against
this radiation. Ultraviolet light can be screened by
only a few microns of silicate dust, so the interiors
of large ejecta blocks might be excellent havens for
spacefaring bacteria.

Entering a New World

When a meteorite strikes the surface of an airless
body like the Moon at high speed, it creates a shock
wave in both the target rocks and in the meteorite
which converts most of its initial kinetic energy into
heat, melting or even vaporizing the original
meteorite. Organisms inside such a meteorite would
have little chance of surviving the impact. However,
if the planet has an atmosphere, the meteorite might
be slowed sufficiently so that it strikes the ground
at terminal velocity, perhaps only a few hundred
meters per second, which microorganisms could easily

The fate of a meteorite entering a planetary
atmosphere depends largely upon its initial size and
speed. Small meteorites, smaller than a few
centimeters, burn up in Earth's atmosphere. Very large
ones, a kilometer or more in diameter, traverse it
without slowing and make craters. Meteorites of
intermediate sizes, a few meters to tens of meters,
however, are significantly slowed by the atmosphere.
Buffeted by kilobars of aerodynamic pressure, they
break up in the atmosphere (as did the famous
Peekskill meteorite which disintegrated over the
eastern United States on October 9, 1992) and may
eventually fall to the ground in a shower of small
fragments. Even on the modern Mars, with its thin
atmosphere, meter-size meteorites are greatly slowed
before striking the surface.

This scenario of slowing and breakup of
intermediate-size meteorites is nearly ideal for the
dispersion of microorganisms onto a new planet.
Whether or not these organisms can survive and
multiply depends, of course, on conditions at their
new home. It seems unlikely that terrestrial organisms
arriving on the modern Mars or Venus would survive.
However, in the past, conditions may have been much
more hospitable on Mars and perhaps at that time
microorganisms from Earth found a home on Mars, or
vice versa.

The current impact-exchange rates among the
terrestrial planets are relatively low. However,
during the era of heavy bombardment, when most of the
visible craters on the Moon and Mars formed, cratering
rates were thousands of times higher than current
rates. Blue-green algae were apparently present on
Earth as early as 3.5 billion years ago and life may
have been present even earlier, overlapping the period
of heavy bombardment. Given the possibility of
exchange of life among the planets by large impacts,
we may have to regard the terrestrial planets not as
biologically isolated, but rather as a single
ecological system withcomponents, like islands in the
sea, which occasionally communicate with one another.

Although this scenario is highly speculative, it may
be testable: If sample returns from former lake
deposits on Mars should contain evidence of the
existence of a microbiota, it may be possible to
extract organic molecules from the samples. If
familiar terrestrial molecules such as DNA, RNA and
proteins are discovered, and especially if a genetic
code similar to that of terrestrial organisms is
found, then it would provide very strong verification
of the idea that Earth and Mars have exchanged
microorganisms in the past. Naturally, any such test
requires that we be very careful not to contaminate
the samples beforehand with terrestrial organic

H. Jay Melosh is a professor of planetary science at
the Lunar and Planetary Laboratory at the University
of Arizona. His latest book, Impact Cratering:
A Geologic Process, has been published by Oxford
University Press.

Copyright 1999-2000 Mars Now Team and the California
Space Institute



Brett Gladman and Joseph A. Burns; Department of
Astronomy, Cornell University, Ithaca NY, 14853, USA.

Published at the Lunar and Planetary Science
Conference XXVII, Lunar and Planetary Institute,
Houston, Texas.

The lunar and martian meteorites present several
puzzles: (1) the equal numbers of each group, (2) the
much larger average mass of the martian meteorites,(3)
the inferred shallow prelaunch depths of the lunar
meteorites vs. the deep ones of the martian
meteorites, (4) the prevalence of geologically young
rocks amongst the SNCs (even though such terrain is
relatively rare on Mars), and (5) the 4-pi age
spectrum of the martian objects terminates at ~15 Myr.
We have undertaken detailed numerical studies of the
orbital history of meteoroids liberated from these
bodies. By comparing these results with the age
spectrum obtained from cosmic ray exposure studies of
the meteorites, we develop a self-consistent model
that can explain the above features, although not
uniquely since surface properties of the two targets
appear to play a major role.

At the start of 1995, 11 lunar and 12 martian
meteorites had been recovered, with all but one of the
lunar meteorites, and half of the martian meteorites,
from Antarctica. This presents a problem, since the
transfer efficiency (the fraction of escaping
meteoroids that reach the Earth) is much larger for
the Moon than for Mars (~40% as opposed to
~3-6%) [1,2]. Moreover, the lower escape velocity
from the Moon suggests that more lunar meteoroids
should be liberated in any impact of a given size. The
total mass of recovered martian material is ~38 times
that of the lunar meteorites; this difference, along
with the cosmic ray exposure (CRE) data indicating
deep (>several m) prelaunch depths, has suggested [3]
that the martian originate in larger impacts than the
lunar meteorites. The fact that, of all 12 of the
martian meteorites, only ALH 84001 appears
to come from geologically old terrain, even though
only ~10% of the martian surface is "young", indicates
that the surface properties of Mars are a major factor
in determining its meteorite launch rate.

We have approached the problem by trying to understand
the orbital dynamics of the transfer of the escaped
meteoroids from their launch sites to the Earth. We
launch thousands of particles off the body of interest
in random directions and track the resulting particles
in full N-body simulations of the solar system.
Particles are removed when they impact a planet, cross
the orbit of Jupiter, or have their perihelion lowered
below the solar radius.

We find [4] that the absolute delivery efficiency of
lunar material is between 25% and 50%, depending on
the launch velocity. A comparison of the arrival time
spectrum of the simulated deliveries to the Earth with
the CRE data of the meteorites implies that few
meteoroids were launched from the Moon at speeds in
excess of 3 km/s, indicating that the velocity
spectrum of the escaping ejecta must be quite steep. A
steep spectrum implies that the lunar delivery
efficiency is about 40% (integrated over the 10-Myr
lifetime of the oldest lunar meteorite). The time
spectrum of the Earth-arrivals is consistent with a
purely gravitational delivery in which collisional
effects in space are minor, and most all of the
meteorites originate from different, small, source

We now have similar numerical studies of the martian
problem which yield an expected delivery spectrum. We
find that the secular resonances in the martian region
are absolutely crucial to the delivery dynamics [1].
First, the action of such resonances increases the
transfer efficiency since more particles are quickly
placed on Earth-crossing orbits. Second, they shorten
the available time scale for delivery: a large
fraction (more than one-third) of the launched
meteoroids are driven into the Sun on 50-Myr
time scales. The last process helps deplete the
meteoroid population, preventing the existence of
long-lived meteoroids (which are not observed). Among
the simulated martian meteoroids that spend longer
than 15 Myr in space, most reside for many Myr with
their aphelia in the asteroid belt (while those that
arrive in <15 Myr do not); this should result in their
collisional destruction. Our best model, assuming a
collisional half-life of 2 Myr in the main belt (for
decimeter-sized particles), is shown in Fig. 1. The
model is consistent with all of the martian meteorites
spending their entire 4-pi exposure ages in space as
small bodies. The model is insensitive to
source-crater pairing,
since all that is relevant is the length of time spent
in space as small bodies (>1 m) from Mars is unlikely
to reproduce the observed CRE spectrum.
The issue of the equal numbers of lunar and martian
meteorites can be alleviated by realizing that the
Antarctic ice sheet has a finite age (almost all lunar
and martian meteorites have terrestrial ages <0.1
Myr). This results in our sampling different portions
of the time spectrum of each lunar or martian impact.
Also, we presume that larger impacts will generate
more meteoroids. Since most lunar meteoroids are
delivered very quickly (<50 kyr), only recent impacts
(or ancient larger ones) will be delivering
meteorites to the ice sheet today. The impact rate
onto Mars (for impactors of a given diameter) should
be larger than the Moon's by at least the ratio of the
surface areas (~3.8). Our preliminary modeling shows
that, if these effects are taken into account and the
correct delivery spectra are included, the
lunar/martian meteorite ratio can be reduced to order

[1] Gladman B., Burns J. A., Duncan M., Lee P., and
Levison H. (1996)
The exchange of impact ejecta between terrestrial
planets, Science, submitted.
[2] Wetherill G. W. (1984) Meteoritics, 19, 1-12.
[3] Warren P. (1994) Icarus, 111, 338-363.
[4] Gladman B., Burns J. A., Duncan M., and Levison H.
(1995) Icarus, 118, 302-321.

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