[meteorite-list] Current Puzzles of Interplanetary Meteorites - PART 1

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 09:43:31 2004
Message-ID: <20010713052238.1413.qmail_at_web10403.mail.yahoo.com>

Hello List,

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.

- ------- Forwarded Message

Date: Wed, 11 Jul 2001 15:23:53 -0400
From: Lew Gramer <dedalus_at_latrade.com>
Subject: (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_at_atmob.org>

- ------- Original Message
------------------------------

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

Subject: CCNet SPECIAL: THE CURRENT PUZZLES OF
INTERPLANETARY METEORITES


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_at_rulpre.LeidenUniv.nl>

(2) RESPONSE TO FRED SINGER I
    Grenville Turner <gturner_at_fs1.ge.man.ac.uk>

(3) RESPONSE TO FRED SINGER II
    Oliver Morton <abq72_at_pop.dial.pipex.com>

(4) EJECTION OF ROCKS FROM MARS
    Max Wallis <wallismk_at_Cardiff.ac.uk>

(5) RESPONSE TO: MEN ARE FROM MARS, WOMEN ARE FROM
VENUS; BUT WHAT ABOUT MARTIAN METEORITES?
    Tom Van Flandern <tomvf_at_metaresearch.org>

(6) FRED SINGER REPLIES
    Fred Singer <singer_at_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_at_rulpre.LeidenUniv.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.wanadoo.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_at_fs1.ge.man.ac.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 theEarth, 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_at_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_at_Cardiff.ac.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_at_cf.ac.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_at_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>.

A 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 specially 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_at_metaresearch.org>
Meta Research <http://metaresearch.org>

=============
(6) FRED SINGER REPLIES

>From Fred Singer <singer_at_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

=============

(See next message for PART 2)



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Received on Fri 13 Jul 2001 01:22:38 AM PDT