[meteorite-list] impact ejecta [melt] emplacement on terrestrial planets, Gordon R Osinski et al 2011, 2 pages: Rich Murray 2011.10.19

From: Rich Murray <rmforall_at_meteoritecentral.com>
Date: Wed, 19 Oct 2011 17:00:52 -0700
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impact ejecta [melt] emplacement on terrestrial planets, Gordon R
Osinski et al 2011, 2 pages: Rich Murray 2011.10.19
http://rmforall.blogspot.com/2011/10/impact-ejecta-melt-emplacement-on.html

http://www.sciencedirect.com/science/article/pii/S0012821X11004675

http://www.lpi.usra.edu/meetings/lpsc2011/pdf/1866.pdf
2 pages

IMPACT EJECTA EMPLACEMENT ON TERRESTRIAL PLANETS.
G. R. Osinski 1,
L L. Tornabene 2, and
R. A. F. Grieve 1,


Gordon R. Osinski a,b,
Livio L. Tornabene c,
Richard A.F. Grieve a b d,
a Departments of Earth Sciences/Physics and Astronomy, University of
Western Ontario, 1151 Richmond Street, London, ON, Canada N6A 5B7
b Canadian Lunar Research Network/NASA Lunar Science Institute, Canada
c Center for Earth and Planetary Studies, National Air and Space
Museum, Smithsonian Institution, Washington, DC 20560-0315, USA
d Earth Sciences Sector, Natural Resources Canada, Ottawa, ON, Canada K1A 0E8
Received 9 February 2011; revised 4 August 2011; Accepted 8 August 2011.
Editor: T. Spohn. Available online 21 September 2011.

1 Departments of Earth Sciences/Physics and Astronomy, University of
Western Ontario, 1151
Richmond Street, London, ON, N6A 5B7, Canada,
2 Center for Earth and Planetary Studies, National Air and Space
Museum, Smithsonian Institution, Washington, DC 20560-0315, USA (
gosinski at uwo.ca )

Introduction:

Impact cratering is one of the most
fundamental processes responsible for shaping the
surfaces of solid planetary bodies. One of the principal
characteristics of impact events is the formation and
emplacement of ejecta deposits. An understanding of
impact ejecta deposits, and their components, is critical
for the results of planetary exploration; particularly
future sample return missions. Their compositional and
physical characteristics provide fundamental information
about the sub-surface of planets. Current models
of ejecta emplacement, however, do not account for
several important observations of planetary ejecta
deposits; in particular, the presence of double or
multiple layers of ejecta. Further, there is also no
universal model for the origin and emplacement of
ejecta on different planetary bodies. Here, we present a
new working model for the origin and emplacement of
ejecta on the terrestrial planets, in which ejecta is
emplaced in a multi-stage process.

Critical observations from the terrestrial planets:
It is generally acknowledged that proximal ejecta
deposits around impact craters on airless bodies, such
as the Moon and Mercury, are emplaced via the process
of ballistic sedimentation ? this results in the incorporation
of local material (secondary ejecta) in the
primary ejecta, via considerable modification and erosion
of the local external substrate [1]. A typically
overlooked, but critical, observation is that proximal
ejecta may consist of more than one layer.
Moon and Mercury. On the Moon, melt ponds on
the rim terraces of complex lunar craters and overlying
parts of continuous ejecta have been documented since
the 1970s [2] (Fig. 1a). The interpretation is that these
deposits consist of impact melt that has flowed and
pooled according to local slopes, after its initial
emplacement as ejecta. This is consistent with
observations from the Lunar Reconnaissance Orbiter Camera
(LROC) (Fig. 1b). Images recently returned by the
Messenger spacecraft show the presence of what is
interpreted as melt ponds around several Mercurian
impact craters [3]. If ballistic sedimentation followed
by radial flow accounts for the emplacement of the
continuous ballistic ejecta, it begs the question as to
the origin of this overlying melt-rich ejecta observed
around many lunar and Mercurian craters.

Venus.
The relative increase in the volume of impact melt
produced on Venus, for a given transient
crater size, compared to the Moon is manifest as
spectacular melt outflows exterior to Venusian craters [4].
Several factors complicate the interpretation of these
outflows around Venusian craters and not all may have
the same origin; however, they share many traits with
exterior lunar impact melt deposits and ponds.

Fig. 1. (a) Large impact melt (?m?) pond around King Crater
crater. Portion of Apollo 16 image 1580 (NASA).
(b) A portion of LROC NAC image pair (M106209806RE) of melt
overlying the continuous ejecta blanket at Giordano Bruno
crater (NASA/GSFC/ASU).

Earth.
Ejecta deposits are relatively rare due to
high rates of erosion on Earth; however, given the ability
to ground truth, any model must be consistent with
the interpretation of the characteristics of ejecta deposits
on Earth, if it is to be generally applicable.
The nature, lithological and stratigraphic relations
of ejecta at several buried complex structures are
known only from drill core but indicate the presence in
the proximal ejecta of a low-shock, lithic breccia overlain by
melt-bearing deposits. Some of the best preserved and
exposed ejecta deposits on Earth occur
at the Ries structure, Germany [5]. The Ries clearly
displays a distinctive two-layer ejecta deposit (Fig. 1a)
with a series of impact melt-bearing breccias
(suevites), and impact melt rocks overlying the
continuous ejecta blanket (Bunte Breccia). Studies of the
Bunte Breccia strongly support the concept of ballistic
sedimentation [6].
Early workers suggested that the upper surficial
?suevite? ejecta at the Ries was deposited sub-aerially
from an ejecta plume [5] and the term ?fallout? was
used to qualify the suevites. This is despite the fact that
the surficial suevites are unsorted to poorly sorted,
which is not predicted by subaerial deposition (i.e.,
fallout from an ejecta plume). More recently, it was
suggested that the proximal surficial suevites were
emplaced as surface flow(s), either comparable to
pyroclastic flows or as ground-hugging volatile- and
melt-rich flows [7, 8]. The most intransigent argument
used to support an airborne origin for suevites at the

42nd Lunar and Planetary Science Conference (2011) 1866.pdf

Ries are so-called ?aerodynamically shaped? glass
clasts and ?gneiss-cored glass bombs? [5]. An airborne
emplacement was offered as a possible explanation for
their shape; however, such ?morphologies? can be
found in injection dikes of impact melt-bearing breccia
that clearly cross-cut the lithologies of the crater floor
of the Mistastin Lake impact structure, Canada (Fig.
2b). In this case, the ?fladen? cannot be explained as
aerodynamically-shaped, and are likely due to shaping
during transport within a confined flow (cf., glass coatings on lithic clasts).

Fig. 2.
(a) Impact melt-bearing breccias, or ?suevites? (S)
overlying Bunte Breccia (BB) at the Ries structure. Note the
sharp contact between the two units.
(c) Elongate, ?aerodynamically-shaped? glass bombs and
gneiss-cored glass ?bombs?.
These features are widely cited as de facto evidence
for airborne emplacement but these are actually from a
dike of suevite in the crater floor of the Mistastin structure.

Mars.
The Martian impact cratering record is notably more diverse
than that of the Moon, Mercury or Venus.
In particular, approximately one-third of all
Martian craters !5 km in diameter possess discernable
ejecta blankets, with over 90% possessing so-called
layered ejecta that display single (SLE), double (DLE),
or multiple (MLE) layer morphologies [9]. In general,
MLE craters are typically larger and their ejecta are
found at greater radial distances than SLE craters,
when normalized to crater size. DLE craters remain
enigmatic. Contrasting models have been proposed to
account for these layered ejecta on Mars.
The working hypothesis: Based on the above
observations that impact melt occurs outside the rim
at both simple and complex craters, we suggest that the
current view of impact ejecta emplacement, i.e., a one
stage and strickly ballistic process that occurs during
the excavation stage, is incomplete. A multi-stage
emplacement process that intimately links the generation
of impact melt lithologies, allochthonous crater-fill
deposits and ejecta is proposed:

1) Crater excavation and ballistic emplacement ?
The initial emplacement of a continuous ejecta blanket
is via the process of ballistic sedimentation. Materials
are derived from the excavated zone of the transient
cavity and are of generally relatively low shock level.
It is suggested that several parameters will affect the
morphology and extent of this primary ejecta layer, in
particular the volatile content and cohesiveness of
surficial target materials.

2) Late excavation ? early modification and minor
flow emplacement ?
In simple craters, there is movement of
highly shocked and melted materials initially
down into the expanding transient cavity. Some of
these materials are driven up and over the transient
cavity walls and rim region, consistent with the presence
of thin melt veneers around some simple lunar
craters and impact melt rocks outside the rim at some
terrestrial craters. Impact angle and preexisting
topography can have a major influence during this stage.

3) Crater modification and ?late? flow emplacement ?
The observations of impact melt-rich deposits
overlying ballistic ejecta deposits and the sharp contact
between these units, where observed, argue for the
general late-stage emplacement of melt-rich ejecta. It
is suggested that cavity modification, in particular
uplift, imparts an additional outward momentum to the
melt- and clast-rich lining of the transient cavity during
the modification stage, resulting in flow towards and
over the collapsing crater rim and onto the proximal
ballistic ejecta blanket, forming a second thinner and
potentially discontinuous layer of non-ballistic ejecta.
For oblique impacts, an additional mechanism may be
the emplacement of external melt in a process more
akin to what is believed to occur at simple craters, i.e.,
by the late-stages of the cratering flow-field. In this
case, the initial direction of the flows is preferentially
downrange. The final resting place for the melt-rich
deposits will be controlled to a large extent by the local
topography of the target region.

4) Minor fallback ?
 Fallback of material from the
vapour-rich ejecta plume will occur during the final
stages of crater formation on bodies with an atmosphere
and will produce the very minor ?fallback? material in the
crater interior, which will be characteristically graded,
such as observed at the Ries structure in
the terrestrial environment.

Acknowledgements:
G.R.O. is support by an Natural Sciences and Engineering
Research Council of Canada (NSERC) Industrial Research
Chair sponsored by MDA Space Missions and the
Canadian Space Agency (CSA).

References:
[1] Oberbeck V.R. 1975.
Rev. Geophys. Space Phys. 13:337-362.

[2] Hawke B.R. and Head J.W. 1977
in Impact and Explosion Cratering. Pergamon Press:
New York. p. 815-841.

[3] Prockter L.M., et al. 2010.
Science 668-671.

[4] Asimow P.D. and Wood J.A. 1992.
JGR 97:13643-13665.

[5] Engelhardt W.v. 1990.
Tectonophys. 171:259-273.

[6] H?rz F., et al. 1983.
Reviews of Geophysics and Space Physics
21:1667-1725.

[7] Osinski G.R., et al. 2004.
Meteor.Planet. Sci. 39:1655-1684.

[8] Newsom H.E., et al. 1986.
JGR 91:239-251.

[9] Barlow N.G. 2005
in Large Meteorite Impacts III: GSA Special Paper 384.
Geological Society of America: Boulder. p. 433-442.

42nd Lunar and Planetary Science Conference (2011) 1866.pdf
______________________________________________


10 m broken rock hill with black glazes, W of Rancho Alegre Road, S of
Coyote Trail, W of Hwy 14, S of Santa Fe, New Mexico, tour of 50
photos 1 MB size each via DropBox: Rich Murray 2011.07.28 2011.08.03
http://rmforall.blogspot.com/2011/08/10-m-broken-rock-hill-with-black-glazes.html
http://rmforall.blogspot.com/2011/08/35479730-106085926-1865-km-el-top-10-m.html
photos 3-5 of 50
http://tech.groups.yahoo.com/group/astrodeep/message/92


ground views of over 100 .1-.5 km shallow (ice comet fragment bursts)
craters, Bajada del Diablo, Argentina (.78-.13 Ma BP) [42.87 S 67.47 W]
Rogelio D Acevedo et al, Geomorphology 2009 Sept: Rich Murray 2010.03.28
http://rmforall.blogspot.com/2010/03/ground-views-of-over-100-1-5-km-shallow.html
http://tech.groups.yahoo.com/group/astrodeep/message/47


______________________________________________


Rich Murray, MA
Boston University Graduate School 1967 psychology,
BS MIT 1964, history and physics,
1943 Otowi Road, Santa Fe, New Mexico 87505
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Received on Wed 19 Oct 2011 08:00:52 PM PDT