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, 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 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  
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]
  ------------------------------------------------------------------------ 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

  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: 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: 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

and doing a search on keywords "spallation tektite mars impact".


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


   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.


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


   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

   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.


Title:              Impact ejection, spallation, and the origin of
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


   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.


Michael Richmond                   "This is the heart that broke my finger."