Lobate features with a strong backscatter are associated with 43 percent of the impact craters cataloged in Magellan's cycle 1. Their apparent thinness and great lengths are consistent with a low-viscosity material. The longest outflow yet identified is about 600 km in length and flows from the 90-km-diameter crater Addams. There is strong evidence that the outflows are largely composed of impact melt, although the mechanisms of their emplacement are not clearly understood. High temperatures and pressures of target rocks on Venus allow for more melt to be produced than on other terrestrial planets because lower shock pressures are required for melting. The percentage of impact craters with outflows increases with increasing crater diameter. The mean diameter of craters without outflows is 14.4 km, compared with 27.8 km for craters with outflows. No craters smaller than 3 km, 43 percent of craters in the 10- to 30-km-diameter range, and 90 percent in the 80- to 100-km-diameter range have associated bright outflows. More melt is produced in the more energetic impact events that produce larger craters. However, three of the four largest craters have no outflows. We present four possible mechanisms for the emplacement of bright outflows. We believe this 'shotgun' approach is justified because all four mechanisms may indeed have operated to some degree

Chadwick, D. John

Duval, Darla M.

Schaber, Gerald G.

Strom, Robert G.

English

NASA Technical Reports Server

20 International Colloquium on Venus
Maxwell Montes. At the then achievable spatial resolutions of about
100 kin, the measured ratios for Alpha and Beta were approximately
0.5 while that for Maxwell Montes was closer to unity. More recent
measurements with the Arecibo system in 1983 and 1988 indicate
that the ratios for parts of Maxwell Montes are very close to unity
and may exceed unity in some areas. Very high cross sections and
inverted polarization ratios have been observed for the icy Galilean
satellites and the south polar region on Mars. All models attempting
to explain the phenomenon invoke internal scattering in a very low-
loss ice medium.
Results from the Pioneer-Venus [2] and Magellan missions [3]
showed that Theia and Rhea Montes in Beta Regio and Maxwell
Montes are areas with low thermal emissivities and high Fresnel
reflectivities. Pettengill et al. [3] invoked the presence of very high
dielectric constant material and Muhlcman [4] has pointed out that
volume scattering could play an important role. However, if further
analysis of the current data and future measurements confirm that
the polarization ratio exceeds unity, then the current models to
explain the low emissivities and high reflectivities will need to be
revised.
With no new missions currendy planned, the only method to
obtain additional data for the surface of Venus in the post-Magellan
era will be with groundbased telescopes. Improvements to the
Arecibo radar system currently underway will increase its sensitiv-
ity for radar mapping of Venus by about a factor ofT, allowing high-
quality im aging at 1 - to 2-kin resolution over approx imately 40% of
the surface. This resolution and sensitivity are more than adequate
for mapping of the polarization properties of selected areas aimed
at studies such as the wavelength-scale roughness properties of
volcanic flows for comparison with terrestrial data and the under-
standing of the high-emissivity,tlow-reflectivity areas.
References: [11 Campbell B. A. (1992) JGR, in press.
[2] Pettengill G. H. et al. (1988) JGR, 93, 14881-14892. [3] Tyler
G. L. et al. (1991) Science, 252, 265-270. [4] Muhleman D. O.,
personal communica
BRIGHT CRATER OUTFLOWS: POSSIBLE EMPLACE-
MENTMECHANISMS. D.JohnChadwickt,GeraldG.Schaber t.
Robert G. Strom x, and Darla M. DuvaP, IU.S. Geological Survey,
Flagstaff AZ 86001, USA, 2Department of Planetary Sciences,
University of Arizona, Tucson AZ 85721, USA, 3University of
North Dakota, Grand Forks ND 58202, USA.
Lobate features with a strong backscatter are associated with
439'0 of the impact craters cataloged in Magellan's cycle 1 (Fig. 1).
Their apparent thinness and great lengths are consistent with a
low-viscosity material. The longest outflow yet identified is about
600 km in length and flows from the90-km-diameter crater Addams.
There is strong evidence that the outflows are largely composed of
impact melt, although the mechanisms of their emplacement are not
clearly understood. High temperatures and pressures of target rocks
on Venus allow for more melt to be produced than on other
terrestrial planets because lower shock pressures are required for
melting [1].
The percentage of impact craters with outflows increases with
increasing crater diameter. The mean diameter of craters without
outflows is 14.4 kin, compared with 27.8 km for craters with
outflows. No craters smaller than 3 kin, 43% of craters in the 10- to
30-kin-diameter range, and 909'0 in the 80- to 100-kin-diameter
range have associated bright outflows. Morc melt is prod uced in the
Fig. 1. Hellman, 35 km in diameter. Note position of outflows downrange of
missing segment of ejects-
more energetic impact events that produce larger craters [2]. How-
ever, three of the four largest craters have no outflows.
Outflow occurrence is also correlative with impact incidence
angle. Fifty-nine percent of Venus' craters have bilaterally symmet-
ric ejecta: a large concentration on one side and little or none on the
other. Such craters were probably formed by oblique impacts: more
pronounced asymmetry in the ejecta reflecting a lowering impact
angle [3]. Of craters with asymmetric ejecta, those with outflows are
more numerous than those without above about 15 km in diameter.
This transition occurs in craters with symmetric ejecta patterns
above about 35 kin, which suggests that oblique impacts are much
more efficient at producing outflows than those with higher-angle
trajectories. Forty-eight percent of asymmetric-ejecta craters have
outflows, compared with only 34% of those with symmetric ejecta.
Schuhz [3] observed a relation between decreasing impact angle and
increasing length and areal coverage of out.flows. More impact melt
is not expected to be generated in oblique impacts reladve to high-
angle impacts due to lower energy deposition in the target [4].
Rather, oblique impacts may be more efficient at removing the
outflow materials from the crater and may also incorporate a higher
percentage of the projectile in the outflows.
Of the 227 venusian craters with outflows and asymmetric
ejecta, about 26% have outflow sources centered on the downrange
axis, and 87% have outflows originating from within 90 ° in azimuth
of this axis (Fig. 1). These relations suggest that the outflows are
transported downrange during the impact. Popigai, a 100-kin-
diameter Asian impact crater, has an asymmetric distribution of
melt glass attributed to oblique impact [5]. This downrange distri-
bution is evident in flows from fresh lunar craters as well [6].
In the following paragraphs we present four possible mecha-
nisms for the emplacement of bright outflows. We believe this
"shotgun" approach is justified because all four mechanisms may
indeed have operated to some degree.
Model I: Emplacement by Jetting: There are apparently two
types of bright outflows: those deposited after emplacement of the
crater rim materials and a rarer form that appears to have been
deposited before. Jetting is a very early process that occurs before
ejecta emplacement, and jets have velocity usually higher than that
of the projectiIe [4]. Therefore, any jetted materials that remain in
the vicinity of the crater would lie stratlgraphically below the rest
https://ntrs.nasa.gov/search.jsp?R=19930005117 2020-03-17T08:47:14+00:00Z
LPI Contribution No. 789 21
of the ejecta, Impact velocities as low as 1-2 km per second may be
sufficient to produce jetting [7].
As impact angle decreases, the speed of the jet increases and the
amount of jetted material decreases to conserve momentum [4]. It
is unclear whether atmospheric drag could slow the jet enough for
local deposition.
Model 2: Emplacement as Lavalike Flows: In this model,
impact melt and solid fragments are transported out of the transient
craterwithin a turbulent, radially expanding gas cloud and deposited
in a discrete layer atop the crater rim deposits. This same mechanism
was proposed to explain the aerodynamic sculpturing and lack of
sorting of melt bombs in suevite deposits at Ries Crater [8]. At Ries,
glass constitutes about 15% of the suevite volume. Calculations by
Basilevsky and lvanov [9] and Ivanov et at. [10] suggest that a 26-
kin-diameter crater such as Ries would produce 20% to 50% more
melt on Venus; thus a venusian suevite layer may be composed of
over 20% melt.
With high melt volumes and inefficient cooling of the melt by
hot incorporated clasts, venusian suevites may remobilize after
emplacement to form laminar, lavalike outflows. Many outflows
are observed to contain sinuous, leveed channels that commonly
have complex tributary and distributary systems. Channels in
outflows are usually best developed close to the crater, giving way
to broad, unehannelized distal lobes. Similar transitions occur in
Hawaiian lava flows, apparently due to increasing viscosity of the
flows with distance from the source.
Additional evidence for a slow, lavalike emplacement of the
outflows is their common inability to surmount topographic ob-
stacles (exceptions are described below). Small volcanic cones
surrounded by flows have commonly retained their circular shape;
flows of higher velocity would be expected to submerge part of one
side of these cones. Alternatively, it is not yet clear whether the melt
could remain molten long enough to produce the more lengthy
flows.
Model 3" Emplacement by Turbulent Flow: This model
requires a low-viscosity fluid similar to that of model l, but
emplacement is at a much higher velocity in a turbulent gas/solid/
melt mixture, similar to terrestrial pyroclastic surges. This model
would best explain the outflows if the channels observed within
many of them are erosional, although it is not clear whether a
surgelike mechanism would produce the intricate networks of
leveed tributaries and distributaries that are observed.
Some outflows show a marked ability to climb slopes. At
Cochran, flow directions follow topographic contours except on the
northwestern side of the crater. Here the flow appears to climb as
much as 300 m into an elevated tessera region [I I]. The northwest-
ern side of the crater is interpreted to be in the downrange direction,
so this observation is consistent with a turbulent flow being driven
forward along the impac tor trajectory. The driving force may be the
momentum of the impact itself or a strong drafting created by the
impactor wake as suggested by Schultz [3].
Terrestrial pyroclastic flows have been analyzed by using the
energy-line model that gives the potential head of a flow along its
runout distance [12,13]. For the gravitational collapse of a debris
column, H/L is the ratio of column height to runout distance and
determines the slope of the energy line. H represents the initial
potential energy of the material converted to kinetic energy, which
is dissipated as the flow moves along the surface. The vertical
distance between the energy line and the subjacent topography is
used to calculate the velocity at a given distance from the crater. It
is possible for the flows to ta-avel up slopes ff they lie below the
energy line [12].
Values of H/L for large terrestrial pyroclastic deposits fall in the
range of 0.1 to 0.2. These values, coupled with the longest L value
(measured at Addams), result in column heights of 60 to 120 kin.
Using the height of origin for Addams and neglecting the subjacent
topography, we derive an irtidal velocity of the 600-kin-long
outflow at the crater edge of about 1 to 1,5 kra/s. This value exe, eeds
the sortie velocity of the venusian atmosphere. However, the values
of H/L in Venus' dense atmosphere are probably quite different
from those on Earth. Values of H/L of less than 0.02 are required if
the initial velocity of the outflow at Addams were to be below the
sortie velocity of Venus' amaosphere.
Kinetic energy may be imparted to the flow by the energy of the
impact in lieu of a collapsing column. This is possibly a more likely
scenario because of the tendency of the flows to be directed
downrange.
Model 4: Slow Ejection of Molten Transient-Crater Lining:
Features similar to the venusian outflows, albeit much smaller, are
the small flows and ponds on ejecta near the rims of relatlvely fresh
lunar craters. Hawke and Head [6] interpreted these materials to be
impact melt that originally lined the transient crater and w as ejected
at low velocities from the crater interior by rebound and wall
slumping during the modification stage of crater formation. This
model is supported by the great number of outflows that appear to
originate very high on the ejecta blankets of Venus' craters, com-
monly at the rims. This relation was also noted by Edmunds and
Sharpton [ 14]. The backscatter tone of the floors of many outflow
craters indicates that the floors are much smoother than the flows
themselves, however, possibly indicating that they are not the same
material.
It is possible that a combination of models 2 and 3 may best
explain the longer outflows. Schultz [3] observed a significant
population of outflows (38 q,, of 253 craters) in which a combination
of laminar and turbulent emplacement styles was evident. Thus
these flows may have been given initially high velocities by the
impact and may have behaved as turbulent, ground-hugging surges.
Upon degassing, the solid-inch fraction of the flow would have
settled to the surface and continued to flow in a slow, laminar
fashion. Secondary flowage of this type has been observed in
terrestrial rheomorphie turfs [15], which probably have a much
lower melt fraction than the outflows.
References: [ 1 ] Vickery A. M. and Melosh H+ J. (1991) LPSC
XXIL 1443--1444. [2] Grieve R. A. F. et al. (1977) In Impact and
Explosion Cratering (D. I. Roddy et al., eds.), 79 I--814, Pergamon,
New York. [31Schultz P. H. (1992) JGR, submitted. [4] Melosh H.
J. (1989) Impact Cratering: A Geologic Process, Oxford, New
York. [5] Masaitis V. L. and Mashchak M. S. (I980) LPSC XI,
674--675. [6] Hawke B. R. and Head I. W. (1977) In Impact and
Explosion Cratering (D. I. Roddy et at., eds.), 815--841, Pergamon,
New York. [7] Kieffer S. W. (1977) In Impact and Explosion
Craterlng (D. J. Roddy et al., eds.), 751-769, Pergamon, New York.
[8} Hrrz. F. (1982) GSA Spec. Pap. 190, 39-54.19] Basilevsky A. T.
and Iv ahoy B. A. (1990) GRL, 17, 175-178. [ l O] Ivanov B. A. et al.
(1992) JGR, submitted. [11] Moore H. J. (1992)LPSCXXIII,
929-930. [ 12] Sheridan M. F. (1979) GSA Spec. Pap. 180, 125-136.
[ 13] Malin M. C. and Sheridan M. F. ( 1982) Science, 217, 637-640.
[14] Edmunds M+ S. and Sharpton V. L. (1992) LPSC XXIII,
329-330. [15] Wolff J. A. and Wright J. V. (1981) J. Vole.
Geotherm. Res., 10, 13-34.


Bright crater outflows: Possible emplacement mechanisms

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