[meteorite-list] Planetary Meteorites - Part 1 of 2 (4KB)

From: Bernd Pauli HD <bernd.pauli_at_meteoritecentral.com>
Date: Thu Apr 22 09:55:45 2004
Message-ID: <3C406536.EEEAE22D_at_lehrer1.rz.uni-karlsruhe.de>

Allan wrote:

> There were suggestions in print that the SNCs
> were from Mars at least as early as 1979.

... and mentioned the following reference:

WASSON J.T. et al. (1979) Dynamical, chemica, and isotopic
evidence regarding the formation locations of asteroids and
meteorites (In Asteroids, ed. T. Gehrels, pp. 926-974, Univ.
Arizona Press, Tucson, Arizona).


Hello All!

Some very interesting assumptions - especially now that we can look back
on these ideas with all the scientific knowledge we have accumulated in
the meantime, but, please bear in mind w h e n this paper was written!


Best wishes,
and enjoy,

Bernd

Planetary surfaces, pp. 963-965:

It is generally thought that planetary surfaces are not likely source
regions for meteorites of any kind, because acceleration to the escape
velocity from a planetary body even as small as the moon is expected to
cause shock effects greater than those observed. In spite of this
difficulty, a lunar origin was proposed for carbonaceous chondrites
(Urey 1965), H-group ordinary chondrites (Hintenberger et al. 1964),
eucrites and howardites (Duke and Silver 1967), and it was suggested
that L-group ordinary chondrites were from Mars (Wänke 1966). The
samples returned from the moon by the Apollo and Luna programs showed
that lunar material is not present in our meteorite collections. This
reinforced arguments indicating that such material should show more
extreme shock effects than those observed in most stony meteorites, and
it seemed likely that if we were not receiving meteorites from the
nearby Moon, the probability of their coming from larger and more
distant planets was nil (e.g., Wetherill 1974).
However, this conclusion may prove hasty. Comparison of chemical and
isotopic data on tektites with lunar and terrestrial material has
persuaded almost all workers that these impactites are of terrestrial
origin. Nevertheless, the aerodynamic evidence that the Australian
tektites were accelerated to above the earth's atmosphere (i.e., to 7-11
km sec^-1) has not been seriously challenged (Baker 1958; Chapman and
Larson 1963). Thus it appears that it is possible for an object to
survive acceleration to nearly escape velocity even from a planet with
as large a gravity field and as massive an atmosphere as the Earth's. To
be sure, the tektites were melted in the process, but the tektite data
do show that, provided suitable target materials are available, some
objects can be ejected from smaller planets with smaller atmospheres,
e.g., the moon and Mars.
Several workers (Urey 1957; Lin 1966) have proposed that terrestrial
acceleration of tektites was a result of volatilization of the nucleus
of a comet upon impact with the earth. In these theories it is
hypothesized that the massive quantities of cometary water released by
the impact aerodynamically accelerate the tektite up to the escape
velocity, at the same time preserving relatively low velocities between
the tektite and the gas surrounding it. Terrestrial cratering by comets
certainly occurs, probably with a frequency ~10 % that associated with
Apollo impact, i.e., about one continental cometary crater >20 km in
diameter every 10 Myr. At typical cometary impact velocities the kinetic
energy of the impacting body is sufficient to volatilize several hundred
times its own mass. Therefore the quantity of water released by a comet
impacting the ocean could be much greater than that contained within the
nucleus of the comet itself. Little is known about the partition of
energy for impacts into an aqueous target. However, Mars is more similar
to the earth than the moon inasmuch as the Viking data have been
interpreted showing that the Martian subsurface contains water in the
form of permafrost up to a depth of ~1 km. When combined with its
relatively small atmosphere and gravity field, conditions should be more
favorable for ejection without melting from Mars than from the earth. In
contrast, the absence of a water-rich layer on the moon may be the
reason we have no meteorites from the moon.
Received on Sat 12 Jan 2002 11:32:54 AM PST