Information on ejection of rocks from Mars
During the months of February and March, 1999, I noticed a
persistent subject of discussion on sci.astro: the ejection
of material from the surface of Mars, and recovery of such
material as meteorites on Earth.
One of the leading posters was
jwill@pacbell.net,
who repeatedly cast doubt that current theories
of ejection of rocks from Mars could be true.
He has posted several messages in which he
uses vague references to simple physics to support
his position;
he does not accept the work of scientists who
model the process in detail.
I have asked
jwill@pacbell.net
to refer us to his technical writings on the subject;
so far (as of March 18, 1999), he has not done so.
Below is a post I sent to sci.astro on March 14, 1999.
Information on ejection of rocks from Mars computerliteracy.com
Author: Stupendous Man Find books on
Date: 1999/03/14 tektite mars
Forum: sci.astro impact melosh
more headers author posting history [post reply] [next]
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jwill@pacbell.net repeatedly has stated his belief that
intact macroscopic fragments of rock cannot be ejected
from Mars due to an impact. One example of his argument
is shown below.
Following jwill's statements, I will append sections of
the abstracts of several papers published in the astronomical
literature on this topic. Two of them are written by
H. J. Melosh, who has been studying and experimenting with
spallation for over ten years.
I ask readers to make their own decisions on the likelihood
that jwill, or the quoted astronomers, is correct. Those who
are very interested should seek out and read the papers
from which I quote. If jwill has written any papers on the
subject, I ask him to let readers know where they may find
them.
So, first, jwill:
jw: I'd like to point out that spallation is not tenable as an
jw: explanation. The referenced text seems garbled, too: Shock waves
jw: ARE NOT waves; they propagate aerodynamically, by direct material
jw: transport. What the author seems to be describing in the spallation
jw: section are SOUND WAVES of high amplitude.
jw:
jw: Once material has been accelerated to the speed of sound
jw: by a sound wave, there is nothing moving faster, any more,
jw: to accelerate it further. So, speed of a wave-accelerated
jw: object is limited to about the speed of sound.
jw:
jw: Objects accelerated by shock waves can remain intact only if
jw: they are accelerated at or below their internal speed of sound.
jw: Otherwise, they fail and turn to fluid or dust (microscopic,
jw: locally high-strength domains).
Now, for the other point of view. Below, I present abstracts
from two recent papers, by Miller and Melosh, and one "old"
paper by Melosh. I found these papers by going to the Astrophysics
Data Service's WWW site
http://adsabs.harvard.edu/abstract_service.html
and doing a search on keywords "spallation tektite mars impact".
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Title: Conditions for single and multimaterial jets
Authors: MILLER, G. H.
Affiliation: AA(Chicago, Univ.)
Journal: Conference Paper, 28th Annual Lunar and Planetary
Science Conference, p. 957.
Publication Date: 03/1997
Abstract
When materials collide obliquely a jet may for whose velocity and peak
pressure-temperature conditions greatly exceed those that would occur
in a plane impact with the same velocity. Jetting has been proposed as
a mechanism for the formation of certain chondrules, the formation of
tektites, and it has been suggested that jetting played a role in the
formation of the moon in a giant impact and in the ejection of the SNC
meteorites off the Martian surface. An experimentally verified theory
exists for the symmetric collision of thin plates that describes the
conditions under which jets form, and gives the mass and momentum
fluxes of the jets. The entire theory does not apply to asymmetric
collisions, thick plates, or spheres. Experiments involving asymmetric
collisions of thin and thick plates give very poor agreement with thin
plate theory. It has been argued, based on thin plate theory, that
jets formed when spheres collide ought to contain both impactor and
target materials. Here, a new theory is presented that argues for
common conditions leading to single material jets. Preliminary
experiments aimed at testing this theory are presented.
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Title: Cratering Dynamics and the Delivery of Meteorites to
the Earth
Authors: MELOSH, H. J.
Affiliation: Lunar and Planetary Laboratory, University of
Arizona, Tucson, AZ 85721- 0092
Journal: Meteoritics, vol. 30, no. 5, page 545
Publication Date: 09/1995
Abstract
In the past decade it has become clear that meteorites falling onto
the Earth's surface do not only originate on asteroids or comets, but
have also come from the surface of the Moon, Mars, and potentially
other major planets or moons in the solar system. One of the most
puzzling aspects of a large-planet origin for some meteorites is the
relative lack of shock damage in rocks that must have been ejected at
speeds of 2.4 km/sec (moon) to 5.0 km/sec (Mars). Older work equated
the ejection velocity to the particle velocity behind a shock wave
(or, in more sophisticated analyses, to half the particle velocity
because of velocity-doubling at the free surface). The known hugoniot
relations for, say, basalt, translate these particle velocities to
enormous shock pressures: 44 GPa for lunar ejection and 150 Gpa for
Mars, which should have pulverized, melted or even partially vaporized
the ejected rocks. Although several of the Martian meteorites show
moderate degrees of shock (30-40 GPa), some show no detectable signs
of shock compression, nor do the lunar meteorites show much evidence
for shock upon ejection.
Ten years ago I proposed that this situation could be resolved if the
process of spallation is important in impact crater ejection [1]. In
this process near-surface rocks are protected from high shock
pressures simply by virtue of being near the surface. A free surface
is, by definition, a surface of zero pressure, and the encroachment of
a shock wave cannot change that fact: The shock pressure may rise
rapidly with increasing depth, but a near-surface zone will always be
present from which material is ejected at high speeds but with little
shock damage. This prediction has now been verified directly by
laboratory experiments [2], as well as by the discovery of
sub-ballistic ejecta from the Ries crater that is composed of
lightly-shocked near-surface rocks that were thrown nearly 200 km from
the impact site [3]. It also seems that the secondary craters commonly
observed in the vicinity of large fresh impacts on the terrestrial
planets and satellites may also have been ejected by the spall
process, and are potential sources of information about the
size-velocity relation of crater ejecta [4, 5], although naturally
they pertain to material ejected at much less than escape velocity.
Another process that was suggested some time ago, but which has not
received much attention until recently, is the role of the
rapidly-expanding impact vapor plume in entraining and accelerating
surface rocks or lower-velocity spalls [6]. The widespread occurrence
of shocked quartz grains in the Chicxulub ejecta suggests some such
process and invites further studies, although vapor plume formation
itself requires rather high impact velocities that are not likely to
be realized in the asteroid belt, but may be important on the moon or
Mars.
In summary, the ejection of lightly shocked rock debris from the
surface of a planet into interplanetary space no longer seems as
difficult as it once did. This process is currently supported by the
rather strong triad of (1) observation of meteorites from the moon and
Mars, (2) theoretical studies of the spall mechanism and perhaps hints
of vapor plume acceleration, and (3) experimental observations of
fast, lightly shocked ejecta from rock targets. Future work will
hopefully flesh out details of the process and gives us hope that we
may someday find meteorites from Venus and perhaps the Earth itself.
References: [1] Melosh H. J. (1984) Icarus, 59, 234-260. [2] Gratz A.
J. et al. (1993) Nature, 363, 522-524. [3] Hofmann B. and Hofmann F.
(1992) Eclogae Geol. Helv., 85, 788-789. [4] Vickery A. M. (1986)
Icarus, 67, 224-236. [5] Vickery A. M. (1987) GRL, 14, 726-729. [6]
Vickery A. M. (1986) JGR, 91, 14139-14160.
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Title: Impact ejection, spallation, and the origin of
meteorites
Authors: MELOCH, H.J.
Affiliation: AA(Arizona, University, Tucson, AZ)
Journal: Icarus (ISSN 0019-1035), vol. 59, Aug. 1984, p.
234-260. Research supported by the Los Alamos
National Laboratory.
Publication Date: 08/1984
Abstract
A model for the ejection of material from an impact crater which links
ejection velocity, fragment size, and shock pressure through a
simplified stress-wave propagation and reflection scheme is presented.
It is shown that a small amount of material (0.01 to 0.05 projectile
mass) may be ejected at high velocity without suffering petrologically
detectable shock pressures. The largest fragments ejected at any
velocity are spalls that originate from the target planet's surface.
The spall size is proportional to the radius of the primary impactor
and the target tensile strength and inversely proportional to ejection
velocity. The shock level in the spalls is low, typically half of the
dynamic crushing strength of the rock. The model also predicts the
aspect ratio of the spalled fragments, the angle of ejection, and the
sizes and shock level of other fragments originating deeper in the
target. Comparison with observational and experimental data shows
generally good agreement.
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Michael Richmond "This is the heart that broke my finger."
mwrsps@rit.edu http://a188-L009.rit.edu/richmond/