The pressure of the dense atmosphere of Venus significantly changes the appearance of ejecta deposits relative to craters on the Moon and Mercury. Conversely, specific styles and sequences of ejecta emplacement can be inferred to represent different intensities of atmospheric response winds acting over different timescales. Three characteristic timescales can be inferred from the geologic record: surface scouring and impactor-controlled (angle and direction) initiation of the long fluidized run-out flows; nonballistic emplacement of inner, radar-bright ejecta facies and radar-dark outer facies; and very late reworking of surface materials. These three timescales roughly correspond to processes observed in laboratory experiments that can be scaled to conditions on Venus (with appropriate assumptions): coupling between the atmosphere and earlytime vapor/melt (target and impactor) that produces an intense shock that subsequently evolves into blast/response winds; less energetic dynamic response of the atmosphere to the outward-moving ballistic ejecta curtain that generates nonthermal turbulent eddies; and late recovery of the atmosphere to impact-generated thermal and pressure gradients expressed as low-energy but long-lived winds. These different timescales and processes can be viewed as the atmosphere equivalent of shock melting, material motion, and far-field seismic response in the target. The three processes (early Processes, Atmospheric Processes, and Late Recovery Winds) are discussed at length

Schultz, Pater H.

English

NASA Technical Reports Server

104 lnternationalColloquiumon Vetum
Fig, 2. (a) Crater (50 km diameter) exhibiting s partial central peak ring
offset uprange (from lower right) CI-60N 263. Radar look direction is from
upper left. (b) Larger enter (103 km diameter) exhibiting central peak ring
offset downrange from present rim, opposite to occ_rmnr._ in (a) and Fig. 1.
Reversal in position is related to _ahanced tim/wall collapse upnmge that
widens and circularizes the crater around the deepest portion of the transient
crater cavity, which occurs uprange. Further crater widening follows pre-
existing structural grain. CI-30N 135.
"2
D
O
_d
1.0
0.5
IMPACT ANGLE
20 ° 30 °
• lufbulent run-out flows
xlaminar run-out flows
50 ° 70 °
0,0 Llo 05 oo
Log (sin2e)
Fig. 3. Effect of impact angle (from horizontal) on the transverse diameter
of the central peak ring dep referenced to crater diameter. As impact angle
decreases (based on degree of eject_ symmetry), size of central peak ring
becomes larger relative to crater diameter. Such a trend is expected if central
peak ring reflects the size of impactor and cratering efficiency decreases
impact angle.
The enhanced uprange rim/wall collapse illustrated in Fig. 2b
(and numerous other large oblique impacts on Venus) provides
insight for why most craters exhibit a circular outline even though
early-time energy transfer comprises a larger fraction of crater
growth. Failure of the uprange rim/wall in response to the
over,steepened wall and greater floor depth circularizes crater
outlines. The rectilinear and conjugate scarp on the pattern uprange
rim, however, indicates failure along prcimpact stresses. Hence, a
corollary is that peak shock levels and particle motion may be
reduced uprange during oblique impacts due to the downrange
motion of the impactor, analogous to time dilation.
References: [1] Gault D. E. and Wedekind J. A. (1978) Proc.
LPSC9th, 3843-3875. [2] Moore H. J. (1979) U.S. Geol. Surv. Prof.
Pap. 812-13, 47 pp. [3] Gauh D. E. and Wedekind J. A. (1977) In
Impact and Explosion Cratering (D. Roddy et al., eds.), 123 I-1244,
Pergamon, New York. [4] Holsapple K. A. and Sehmidt R. M,
(1987) JGR, 92,6350-6376. [5] Schultz P. H, and Oault D. E. (1991)
Meteoritics, 26. [6] Wichman R. W. and Schuhz P. I4. (1992) LPSC
XXIII, 1521-1522. [7] Schultz P. H. (1988) In Mercury (F. Vilas
et al., eds.), 274--335, Univ. of Arizona, Tucson. [8] Schultz P. H.
(1992)JGR, inpress. /"
i N93-14372 i i, 3/L)
IMPACT-GENERATED WINDS ON VENUS: CAUSES AND
EFFECTS. Peter H. Schultz, Brown University, Department of
Geological Sciences, Box 1846, Providence RI 02912, USA.
The pressure of the dense atmosphere of Venus significantly
changes the appearance of ejecta deposits relative to craters on the
Moon and Mercury. Conversely, specific styles and sequences of
ejeeta emplacement can be inferred to represent different intensities
of atmospheric response winds acting over different timescales.
Three characteristic timescales can be inferred from the geologic
record: surface scouring and impactor-controlled (angle and direc-
tion) initiation of the long fluidized run-out flows; nonballistie
emplacement of inner, radar-bright ejecta facies and radar-dark
outer facies; and very late reworking of surface malerials. These
three timescales roughly correspond to processes observed in labo-
ratory experiments that can be scaled to conditions on Venus (with
appropriate assumptions): coupling between the atmosphere and
earlytime vapor/melt (target and impactor) that produces an intense
shock that subsequently evolves into blast/response winds; less
energetic dynamic response of the atmosphere to the outward-
moving ballistic ejecta curtain that generates nonthermal turbulent
eddies; and late recovery of the atmosphere to impact-generated
thermal and pressure gradients expressed as low-energy but long-
lived winds. These different timescales and processes can be viewed
as the atmosphere equivalent o f shock reel ting, material motion, and
far-field seismic response in the target.
Early Processes (Direct EffecL_ of Blast and Fireball): Under
vacuum conditions, the fate of the impactor is generally lost; even
on the Earth, most impact melt sheets exhibit little trace of the
impactor. The dense atmosphere of Venus, however, prevents
escape of the impactor through rapid deceleration of ricochet debris
and containment of the vapor cloud [1,2]. Figure la illustrates the
time required for the atmospheric blast front to decelerate to the
speed of sound as a function of crater size, where k is the fraction the
initial impactor energy (KEi) coupled to the atmosphere (EA). On
Venus, the shock front dissipates before the crater finishes forming.
If the blast is created by deceleration and containment of early high-
speed ejecta (downrange jetting and ricochet/vapor), then it will
precede ejecta emplacement and should exhibit a source area offset
https://ntrs.nasa.gov/search.jsp?R=19930005184 2020-03-17T08:43:02+00:00Z
LPI Contribution No. 789 105
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Fig. 1. (a) Recovery time for atmospheric blast to reduce to the speed of sound on Venus scaled to crater formation dine. The value of
k reprer_nU the fraction of initial impactor kindle energ 7 (KE._ coupled to the atmosphere tEA). On Venus the blast effec_ should precede
crater formation. Co) Recovery time for atmospheric pressure behind the shock front to return to ambient condition_ on Venus scaled to
crater formation time,. Although atmospheric pressure has mcovem,.A,high temperatures (die fireball) result in low denzltites. Thermal
gradienu and motion of fireball induce strong recovery winds that rework ejects it late times.
(a)
,mlhmal,_
h_ty
*tic,'kktilii nil
Illi [I/11IIIIIIII
_ui_ _por enltid__wC,_elmt_ ...
/')'_i'l---- / I I I I II I I i
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(b) _: '
._ "%
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=and-off _/ = :;';_ :
/,.-,. _7_/ _/'/i 7----
frz,_%om,elt r_i*s
do,,m ntnge run d_p_s_o_
Fig. 2. (a) Scenario for sequenceand style of ejects emplacement at early times based on inferences drawn from laboratory impact
phenomena sealed to Venus and surface features revealed by Magellan. At early times, kinetic energy and momentum in the vapor cloud
evolves into a downrange-moving fireball that creates strong winds downrange. (b) Scenario for late processes. Winds and turlmlence
created by the outward-moving ejects curtain entrain coarse fractions to produce an avalanchelike flow of coarse inner ejecra deposits.
Such deposits persist as radar-bright ejects deposits because of the toughness and low ambient sudace winds. Finer fractions entrained
in sustained turbulence result in turbidity flows with potentially much greater ran-out distmces. Deposits from these flows will be more
suceptibleto subsequenterosion.
downrange from the mater [2,3]. The downrange offset of the center
of origin of the shock is observed in laboratory impact experiments.
Features consistent with this interpretation can also be found around
venusian craters and include [2] topographic barriers shadowing
surface disruption from the blast; radar-dark/-bright striations con-
verging on the downrange rim rather than the crater; and diffuse
haloes at the base of small hills again focusing on a downrange rim
"'source" (shock-dislodged debris drawn back into the rarefied,
rising fireball). Moreover, radar-dark parabola patterns commonly
center on a point downrange and not the crater [2]. In contrast, the
time for recovery of the atmosphere to ambient pressure (or density
and temperature) is much longer than the time for crater formation
(Fig. lb). If all craters were formed by vertical impacts, this would
mean that the craters form within a f'treball or behind the wake
characterized by low density (high temperature, low pressure) as
postulated in [4]. But most craters are formed by oblique impacts
(i.e., 75% formed at angles of 60 ° or less). Consequently, the early
fireball moves away from crater excavation initially at
hypervelocities. At low impact angles (<300), the energy coupled to
the atmosphere resembles a rolling fireball containing vapor and
dispersed melt moving downrange until decelerated. Because the
vapor/meh is higher in density than conditions in the fireball, it
collapses within the fireball to form long run-out density flows
controlled by local topography, well in advance of ejecta emplace-
106 lntemationalColloquiumon Venus
merit as observed on Venus [5,6]. Such a process accounts for the
long run-out flows eonsistendy originating downrange in oblique
impacts (i.e., opposite the missing ¢jecta sector) even if'uphill from
the crater rim. A_nosphcric turbulence and recovery winds decoupled
from the gradient-controlled basal run-out flow continues down-
range and produces wind streaks in the lee of topographic highs.
Turbulence accompanying the basal density flows may also produce
wind streak patterns. Uprange the atmosphere is drawn in behind the
fireball (and enhanced by the impinging impactor wake), resulting
in strong winds that will last at least as long as the time for crater
formation (i.e., minutes). Such winds can entrain and saltate surface
materials as observed in laboratory experiments [2,3] and inferred
from large transverse dunes uprange on Venus [2].
Atmospheric Effects on Ballistic EJeeta: Even on Venus,
target debris will be ballistically ejected and form a conical ejects
curtain until its outward advance is decelerated by the atmosphere.
The well-defined, radial ejecta delineating the uprange missing
ejecta sector of craters formed by oblique impacts demonstrate
ballistic control of ejection. As the inclined ejects curtain advances
outward, however, it creates turbulent vortices, which have been
observed in the laboratory experiments [2] and modeled theoreti-
cally [7]. The ejects curtain gradually becomes more vertical ia
response to atmospheric resistance. The atmospheric density is
sufficient to decelerate meter-sized ejecta to terminal velocities [8]
that will be entrained in and driven by response winds induced by the
outward-moving curtain. While larger ejecta are deposited, smaller
size fractions become entrained in an outward ejects flow. Based on
diversion of such flows by low-relief barriers near the rims of
craters, the transition from ballistic to nonballistic emplacement
occurs within about 0.5 crater radii of the rim. This observation
underscores the fact that dynamic atmospheric pressure signifi-
cantly restricts outward advance of the ejecta curtain. The scaled
run-out distance (distance from the crater rim scaled to crater
diameter. D) of the ejecta flow should decrease on Venus as D -°.s.
unless consumed by crater rim collapse. Because of the high
atmospheric density, collapse of near-rim ejecta into a flow crudely
rescmbles an avalanche comprised of coarsc debris and blocks. But
high winds and turbulence created by the outward-moving curtain
separate during terminal emplacement of the inner flow, thereby
winnowing the freer fractions and creating an overrunning turbidity
flow that continues outward.
Turbidity flows containing freer fractions can extend to much
larger distances until turbulence supporting entrained debris no
longer can support thc load. Because turbulent wind velocitics
gready exceed ambient surface winds, such vortices are also capable
of mobilizing surface materials. It is suggcsted that the radar-dark
lobes cxtcnding beyond the inner radar-bright ejecta [2,6] reflect
this process. In addition, many craters arc surrounded by a very
diffuse boundary that masks low-relief ridges and fractures; this
boundary may indicate thc limits of a third stage of flow scparafion
and deposition. The observed radar-dark signature requires such
ejecta to be less than a few centimeters. In contrast with the coarse,
radar-bright inner facies, the outer radar-dark facies will be more
susceptible to later erosion by ambient or other impact-generated
winds because the size fractions were sorted by a similar process.
This is consistent with observed removal or reworking of craters
believed to be old, based on superposed tectonic features.
Late Recovery Winds (Secondary Effects of Atmospheric
Turbulence): On planets without atmospheres, the effects of
early, high-speed ejecta and impactor are typically lost. On Venus,
however, the dense atmosphere not only contains this energy
fraction, but the long recovery time of the atmosphere (Fig. 1b)
results in late-stsge reworking, if not self-destruction, of ejects
facies emplaced earlier. Surface expression should include bed forms
(e.g., meter-scale dunes and deeicentirneter-scale ripples) reflect-
ing eddies created in the boundary layer at the surface. Because
radar imaging indicates small-scale surface roughness (as well as
resolved surface features), regions affected by such long-lived low-
energy processes can extend to enormous dislances. Such areas are
not directly related to ejecta emplacement but reflect the atmo-
spheric equivalent to distant seismic waves in the target. Late-stage
atmospheric processes also include interactions with upper-level
winds. Deflection of the winds around the advancing/expanding
FLrebali creates a parabolic-shaped interface aloft. This is preserved
in the fall-out of finer debris for impacts directed into the winds aloft
(from the west) but self-destructs if the impact is directed with the
wind. Exception to this rule occurs for larger crater (>60 kin)
sufficient to interrupt the flow pattern not only by the fireball but
also by the ejects curtain.
References: [1] Schultz P. H. and Gauh D. E. (1990) In GSA
Spec. Pap. 247 (V. L. Sharpton and P. D. Ward, eds.), 239-261.
[2] Schultz P. H. (1992) JGR, in press. [3 ] Schultz P. H. (1992) JGR,
in press. 1.4] Ivanov B. A. eta]. (1986)Proc.LPSC16th, inJGR, 91,
D413--IM30. [5] Schultz P. H. (1991) Eos, 73, 288. [6] Phillips
R. J. et al. (1991) Science, 252, 288-296. [71 Barnouin O. and
Schultz P. H. (1992)LPSCXXIII, 65--66. [8] Schultz P. H. and Gault
D. E. (1979)JGR, 84, 7669-7687. [9] Schultz P. H. et al. (1981)
In Multi-Ring Basins, Proc. LPS 12A (P. H. Schultz et al., eds.),
181-195, Pergamon, New York. [10] B arnouin O. and Schultz P. H.
(1992) LPSC XXIII, 65-66. [11] Schultz P. H., this volume.
[ 12] Jones E. M. and San ford M. T. I1 (1982) In GSA Spec. Pap. 190
(L. Silver and P. Schultz, eds.), 175-186. [13] Sehultz P. H. (1992)
JGR, in press. [14] Schultz P. H. and Gault D. E. (1982) In GSA
Spec. Pap. 190 (L. Silver and P. Schultz, eds.), 153-174. [15] Post
R. L. (1974)AtzWL-TR-74-51. _ ^
,;N93-14373
MAGELLAN PROJECT PROGRESS REPORT. J.F. Scott,
D. G. Griffith, J. M. Gunn, R. G. Piereson, J. M. Stewart, A. M.
Tavormina, and T. W. Thompson, Jet Propulsion Laboratory,
California Institute of Technology, Pasadena CA 91109, USA.
The Magellan spacecraft was placed into orbit around Venus on
August 10, 1990 and started radar data acquisition on September 15,
1990. Since then, Magellan has completed mapping over 2.75
rotations of the planet (as of mid-July 1992). Synthetic aperture
radar (SAR), altimetry, and radiometry observations have covered
84% of the surface during the first mission cycle from mid-
September 1990 through mid-May 1991.
Operations in the second mission cycle from mid-May 1991
through mid-January 1992 emphasized filling the larger gaps (the
south polar region and a superior conjunction) from that f'wst cycle.
An Orbit Trim Maneuver (OTM) was performed at the beginning of
cycle 2 in order to interleave altimeter footprints at periapsis. This
yielded better altimetric sampling of the equatorial regions of
Venus. Some 94% of the planet was mapped at the end of mission
cycle 2.
Observations in the third mission cycle from mid-January to
mid-September 1992 emphasized reimaging of areas covered in
cycle 1 and cycle 2 such that digital stereo and digital terrain data
products can be produced. A transponder anomaly in January 1992
(just before mission cycle 3 started) forced the project to use a radar
data downlink of 115 Kbs instead of 268 Kbs. Although data
acquisition is curtailed, some 30--40% of the planet will be mapped


Impact-generated winds on Venus: Causes and effects

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