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(meteorobs) Fwd: CCNet SPECIAL: CURRENT PUZZLES OF INTERPLANETARY METEORITES
This whole thread is slightly off-topic for 'meteorobs', but is close
enough, and of sufficient interest (obviously!) to our readers, that I
am forwarding this entire issue unedited to our list. Enjoy!
Lew Gramer <owner-meteorobs@atmob.org>
------- Forwarded Message
From: Peiser Benny <B.J.Peiser@livjm.acdot uk>
To: cambridge-conference <cambridge-conference@livjm.acdot uk>
Subject: CCNet SPECIAL: THE CURRENT PUZZLES OF INTERPLANETARY METEORITES
Date: Wed, 11 Jul 2001 10:57:11 +0100
CCNet SPECIAL: THE CURRENT PUZZLES OF INTERPLANETARY METEORITES
RESPONSES TO FRED SINGER'S LETTER (CCNet 10 July 2001)
---------------------------------------------------------------------
[...]
(1) MARCO LANGBROEK ON MARS METEORITES
Marco Langbroek <m.langbroek@rulpre.LeidenUnivdot nl>
(2) RESPONSE TO FRED SINGER I
Grenville Turner <gturner@fs1.ge.man.acdot uk>
(3) RESPONSE TO FRED SINGER II
Oliver Morton <abq72@pop.dial.pipex.com>
(4) EJECTION OF ROCKS FROM MARS
Max Wallis <wallismk@Cardiff.acdot uk>
(5) RESPONSE TO: MEN ARE FROM MARS, WOMEN ARE FROM VENUS; BUT WHAT ABOUT
MARTIAN METEORITES?
Tom Van Flandern <tomvf@metaresearch.org>
(6) FRED SINGER REPLIES
Fred Singer <singer@sepp.org>
(7) MARS METEORITES-SWAPPING ROCKS: EXCHANGE OF SURFACE MATERIAL AMONG THE
PLANETS
H. Jay Melosh
(8) TOWARDS A SELF-CONSISTENT MODEL OF LUNAR AND MARTIAN METEORITE DELIVERY
Brett Gladman and Joseph A. Burns; Department of Astronomy, Cornell
University
==========
(1) MARCO LANGBROEK ON MARS METEORITES
>From Marco Langbroek <m.langbroek@rulpre.LeidenUnivdot nl>
"So -- my question to the chemists: Could the 'Martian' meteorites
have come from Deimos? Or must they originate from the Mars surface, in
which case we may need to find some mechanism for a more sustained and
gentle acceleration?"
Dear Prof. Singer, dear Benny,
Shergottite, Nahklite, and Chassignite meteorites and ALH 84001, those
suspected to be from Mars, are rocks of magmatic origin - basalts and
dunites. They must originate from a large and differentiated parent body.
Moreover, given their crystallization ages their parent body must have seen
volcanism rather recently - some crystallization ages for SNC meteorites are
as young as 330 million years. Thus, it is out of the question that they
come from a small parent body like Deimos. They have to come from a large
differentiated parent body which relatively recently still displayed
volcanic activity. There is not much of that kind in our solar system
besides Venus, Earth and Mars.
Sincerely,
Marco Langbroek
Dutch Meteor Society (DMS) - meteorite section
http://home.wanadoodot nl/marco.langbroek/dutchmet.html
P.S. Although some SNC meteorites show severe shock signs, a surprising
number of them show only weak evidence for shock. This is perhaps a bit
surprising if they were launched by a large impact. Goes to show that we
probably still understand very little of impact phenomena.
- ---
Drs Marco Langbroek
Faculty of Archaeology
Leiden University
P.O. Box 9515
NL-2300 RA Leiden
The Netherlands
===========
(2) RESPONSE TO FRED SINGER
>From Grenville Turner <gturner@fs1.ge.man.acdot uk>
Response to S. Fred Singer,
The reasons for concluding that SNC meteorites come from the surface of Mars
are well rehearsed and seem to me to preclude an origin on Deimos.
The crystallisation ages of all but ALH84001 are very young (165 - 1300 Ma),
implying an origin on a geologically active body. The generation of heat
from the decay of radioactive species (235U, 238U, 232Th and 40K) is
inadequate to sustain melting in an object the size of Deimos. The heat flow
from radioactive decay and the resulting temperature gradient scale roughly
with radius, so for Deimos (r ~ 7km) must be of order 1/2000 that of the
Earth, i.e. negligible.
Generation of igneous melts by impact is not very effective on a small body
and tidal heating can probably be ruled out also.
The clinching argument in favour of a martian origin is the similarity
between the elemental and isotopic composition of the martian atmosphere, as
measured by Viking, and the gases (CO2, N2,
and the noble gases) trapped in impact glass in the SNC meteorite, EET79001.
Evidence collected in the last decade indicates that other SNCs have trapped
this same 'atmospheric' component
(129Xe/132Xe is a key fingerprint). Nevertheless there are relatively small
systematic differences between the meteorite data and Viking. Given that
the meteorite analyses are much more precise, the broad agreement between
SNCs and Viking is taken to imply that the differences are the result of
systematic errors in the very difficult Viking analyses.
The chemical arguments for a martian origin are based on correlations
between pairs of elements which 'stay together' during igneous melting (e.g.
elements with large ionic radii such as K and U, which concentrate in the
melt), but behaved differently in the pre-planetary solar accretion disk
(e.g. as a result of differences in volatility). Consequently the ratios of
these
elements may differ between different planetary bodies but are relatively
uniform in igneous rocks from a given object, in spite of several orders of
magnitude variation in concentration (e.g. on Earth K/U ~ 12,000, on the
Moon K/U ~1,000 - currently explained in terms of the giant impact theory
for the origin of the Moon). Fe/Mn for SNC meteorites is ~39, identical to
the Pathfinder value. The corresponding ratios for Earth and Moon are ~60
and ~70. Since the
value for Deimos is not known this observation cannot rule out Phobos as a
source but does support Mars as a possible source.
A fourth line of evidence, from oxygen isotopes, indicates that SNCs are
from the same body but not necessarily Mars. 18O/16O and 17O/16O ratios lie
on a common 'fractionation trend' with a precision of better than 20ppm.
This trend, which results from igneous fractionation, is distinct from the
Earth and the Moon, but we don't know anything of the oxygen isotopes on
Deimos.
A final point that is usually made is that the discovery of Lunar meteorites
in Antarctica (and the Sahara) proves that ejection from the Moon is
possible, so why not Mars. Ejection from Mars is more difficult,
particularly given the presence of an atmosphere, but the evidence that SNCs
are from Mars is convincing. I would bet money on it but not my life!
Grenville Turner
________________________
Grenville Turner
Professor of Isotope Geochemistry
Dept of Earth Sciences
University of Manchester
Manchester, M13 9PL, UK
Tel +44 (0)161 275 3800
Fax +44 (0)161 275 3947
=============
(3) RESPONSE TO FRED SINGER
>From Oliver Morton <abq72@pop.dial.pipex.com>
While I am ignorant of the physics of launch, anything other than a planet
seems a highly unlikely alternative source. The SNCs are basalts, and thus
come from partial melting in a differentiated source; they are also of
differing ages. ALH is about 4.4 billion years old, with alterations perhaps
a billion years later. The Nakhlites are about 1.3 billion years old, and
the Shergottites about 170m years old. So the source needs to have been
active enough to produce fresh basalts in recent geological history. That
seems a lot to ask of anything less than a full blown planet...
best, oliver
============
(4) EJECTION OF ROCKS FROM MARS
>From Max Wallis <wallismk@Cardiff.acdot uk>
Fred Singer questions the scientifically established fact that meteorites
come from Mars, saying
"I find it difficult to visualize a scenario that can impart a
velocity of the order of 10 km/sec to a rock coming from such an impact
without the accelerating force exceeding the crushing strength of the
rock"
and that
"John Michael Williams seems to have demonstrated that a gentle
acceleration of the rock by a gas cloud is physically impossible."
The "scenario" is a hypervelocity impact (impact speed >> speed of sound) on
which there is a lot of experimental evidence. Dimensionally,the
acceleration is V*V/L so only 100g for V=10km/s and a 100km crater (10km
impactor). It's no "gas cloud" but a high pressure fluid far above the
critical point.
Experiments, computations and theory (Melosh etc.) show ejection of surface
material as solid spall. I find the visualising is easy, though the physics
may seem challenging!
Max Wallis
Cardiff Centre for Astrobiology wallismk@cf.acdot uk
67 Park Place tel. 029 2087 6426
Cardiff University CF10 3AS fax 029 2087 6425
=============
(5) RESPONSE TO: MEN ARE FROM MARS, WOMEN ARE FROM VENUS; BUT WHAT ABOUT
MARTIAN METEORITES?
>From Tom Van Flandern <tomvf@metaresearch.org>
Dear Benny,
Fred Singer makes the very good point that it is almost impossible to launch
meteorites intact from Mars. He then asks: "Could the 'Martian' meteorites
have come from Deimos? Or must they originate from the Mars surface, in
which case we may need to find some mechanism for a more sustained and
gentle acceleration."
We have no surface samples from Deimos. But from spectroscopy and Viking
imagery, Deimos bears the characteristics of C-type asteroids originally
associated with chondritic meteorites; whereas the so-called "Mars
meteorites" are classified as achondritic. Moreover, the primary indicators
that Mars meteorites are from a planet rather than an asteroidal parent body
are:
The "Mars" meteorites show water erosion and weathering, cooling
rates, oxygen isotope ratios, and other geological evidence from their
pre-Earth existence that requires an origin on a major planet parent
body, not of asteroidal, cometary, or terrestrial origin.
Mars is the only known existing parent body that meets most of the
necessary constraints.
Therefore, Deimos can be ruled out as a source for "Martian
meteorites".
Earlier, I addressed the problems associated with determining the source of
"Martian meteorites" in an article: "Are the Mars meteorites really from
Mars?", Meta Research Bulletin, v. 5, pp. 33-38 (1996); see a web version at
<http://www.planetarymysteries.com/mars/marsmeteorites.html>. I few of the
points made in that article seem relevant to the issue raised by Fred
Singer.
To be from Mars, "Mars meteorites" must first escape the Martian gravity
field. This implies a launch speed greater than 5 km/s to exceed escape
velocity. A projectile velocity that high can result only from the largest
of asteroidal impacts on Mars. It cannot arise from even the largest
volcanoes, or any other known acceleration mechanism. The meteorite-to-be
must be suddenly accelerated from rest to at least 5 km/s as the impact
blast wave passes, but without
vaporizing. It is easy to compute the amount of energy that must be
transferred to the meteorite, and the short time it has for its acceleration
to escape speed. Small bodies the size of Mars meteorites found on Earth
would normally be completely vaporized by such a shock wave transferring
that much energy that quickly, and any surviving fragments of a rock barely
big enough to partially survive vaporization would themselves be heavily
shocked. Meteorites associated with a lunar origin, for example, apparently
all had ejection velocities under 3 km/s, with survival rate decreasing
sharply at the higher ejection speeds. [B.J. Gladman, J.A. Burns et al.,
"The exchange of impact ejecta between terrestrial planets", Science, v.
271, pp. 1387-1392 (1996).]
"Mars meteorites" were neither vaporized nor heavily shocked. So the rock
initially ejected from Mars by an impact must have been huge compared with
the surviving fragments. Those fragments must themselves have been well
shielded deep in the interior of the larger rock. The requirements to eject
relatively large rocks at speeds of at least 5 km/s with minimal shock, and
the other physical and chemical constraints for Mars meteorites, place a
lower limit on the size of the crater on Mars produced by the responsible
Mars-impacting asteroid: at least 175 km in diameter. [A.M. Vickery and H.J.
Melosh, "The large crater origin of SNC meteorites", Science, v. 237,
738-743 (1987).] Scenarios for ejection during the formation of smaller
craters are all problematical.
The only craters that large on the surface of Mars are on the "old terrain",
dated at least 200 million years (My) old. So the launching impacts must
have been at least that long ago, and the Mars meteorite parent rocks must
have been orbiting in space for at least that long. Objects in
Earth-crossing or near-Earth-crossing orbits have a half-life of just 30 My
before collision with the Earth or gravitational elimination. (Common types
of gravitational elimination: ejection from solar system; ejection into
Jupiter-crossing orbit, collision with Jupiter; or falling into the Sun.)
Almost nothing that orbits near the Earth can survive for 200 My. So the
initial ejection orbit must not have come especially close to Earth.
Cosmic rays exposure ages of Martian meteorites are typically just some few
millions of years. This appears to contradict the previous requirement. But
a consistent picture can be patched together by assuming that the parent
rocks had to be at least 12 meters in diameter to shield a
potential Mars meteorite deep in its interior from cosmic rays for most of
its life. This is also consistent with the need to have a large parent body
to prevent vaporization and shield the future meteorite from shock. This
larger parent rock presumably had an orbit that did not venture too close to
the Earth, but perhaps took it into the main asteroid belt.
Then the parent rock must have been shattered some millions of years ago in
a collision with another sizable asteroid, exposing the future meteorite
fragment directly to cosmic rays thereafter, and altering its orbit to an
Earth-crossing one. Finally, the meteorite must have collided with the Earth
and fell probably within the last 15,000 years to be discovered today. This
entire scenario must occur more often than chips off the Moon reach Earth
because "Mars"
meteorites outnumber "Moon" meteorites. Despite these problems, and with no
better alternative explanations acceptable to the mainstream available, the
Martian origin scenario continues to go largely unchallenged expect by
careful thinkers such as Fred Singer.
However, a viable alternative source has been proposed. Extensive evidence
exists for the explosion of one or more bodies in or near the asteroid belt
during the past half billion years of solar system history. [T. Van
Flandern, "Dark Matter, Missing Planets and New Comets", North
Atlantic Books, Berkeley, Ch. 11 (1993); see also "A revision of the
exploded planet hypothesis", Meta Research Bulletin, v. 4, 33-42 (1995),
reprinted at <http://metaresearch.org/>, "Solar System" tab, "EPH" sub-tab.]
Such an explosive break-up of a larger body solves all the dynamical
problems involved in delivery of the life-bearing meteorites to Earth in
recent times. Even with
high shock-wave speeds, planetary explosions take place over many minutes,
not seconds; so accelerations of fragments are relatively gentle. This
provides the "sustained and gentle acceleration" mechanism Fred Singer calls
for. Moreover, it is not an idea invented to solve this problem (an ad hoc
theory). The exploded planet hypothesis exists because of extensive but
unrelated evidence, and just happens to solve the problem at hand nicely
too.
Much evidence also exists to suggest that Mars was a moon of the most recent
exploded planet. For example, only one hemisphere of Mars is heavily
cratered and has a thick crust, the pole is known to have shifted suddenly
relative to the crust, much of the original Martian atmosphere was lost,
Mars has excess Xenon-129 (an explosion by-product), etc., etc. [See "The
exploded planet hypothesis - 2000", preprint available at
<http://metaresearch.org>, "Solar System" tab, "EPH" sub-tab.] The
present-day Martian atmosphere would then be a mixture of its original
atmosphere and gases from Planet V, accounting for the rough similarities
seen for those gases in the Mars meteorites. Possible planetary explosion
mechanisms are also covered briefly in this last reference.
Tom Van Flandern <tomvf@metaresearch.org>
Meta Research <http://metaresearch.org>
=============
(6) FRED SINGER REPLIES
>From Fred Singer <singer@sepp.org>
Dear Benny,
My letter seems to have produced many responses -- as I hoped it would. I am
particularly grateful for the detailed letter from Grenville Turner.
In reply, I would say:
Pls don't take the suggestion of Deimos literally. I merely wanted to make
the point that we need a source with a low gravity potential.
For we seem to have a dilemma. The chemistry suggests origin from a large
body (Mars) while physical arguments seem to call for modest ejection
velocities (below escape velocity) to avoid destruction.
I suspect that we need to think of a more sophisticated ejection mechanism
that supplies a gentler acceleration to a final velocity of more than about
10 km/sec.
Any suggestions?
Best Fred
S. Fred Singer, President
Science & Environmental Policy Project
http://www.sepp.org
=============
(7) MARS METEORITES - SWAPPING ROCKS: EXCHANGE OF SURFACE MATERIAL AMONG THE
PLANETS
http://calspace.ucsddot edu/marsnow/library/science/mars_meteorites3.html
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.
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 damage.
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 destination.
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
survive.
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 with
components, 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 molecules.
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
===============
(8) TOWARDS A SELF-CONSISTENT MODEL OF LUNAR AND MARTIAN METEORITE DELIVERY
Brett Gladman and Joseph A. Burns; Department of Astronomy, Cornell
University, Ithaca NY, 14853, USA.
http://cass.jsc.nasadot gov/pub/lpi/meteorites/glaxxvii.html
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 almost all of the meteorites
originate from different, small, source craters.
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 unity.
References:
[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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