Laboratory experiments allow examining the consequences of complex processes operating over a wide range of scales (both temporal and spatial) and frequently reveal effects that are obvious only in hindsight. Even though all processes may not scale directly, isolation of the controlling variables allows assessing first-order effects through analytical approximations. This approach can be illustrated by the systematic sequence of ballistic ejection, the response of an atmosphere to a strong energy source, the scaling of ejecta thickness, and the role of secondary cratering. Here it is proposed that the effects of atmospheric pressure and density on crater growth (hence, scaling) observed in laboratory experiments has particular relevance for craters on Venus

Schultz, Peter H.

English

NASA Technical Reports Server

LPI Contribution No. 789 101
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The depth/diameter ratios for fresh, large craters on Venus may
be more akin to those on the Moon, Mercury, and Mars than to those
on Earth. The reasons for this are presently unclear.
Lobate features partly surrounding a crater with a strong back-
scatter emanate from 43% of the impact sites on Venus. The flow-
like features (outflows) extend tens or hundreds of kilometers from
their crater rims and have a morphology consistent with a low-
viscosity material. There is strong evidence that the outflows are
composed primarily of impact melt, although the mechanism of
their emplacement is not clearly understood. High temperatures and
pressures of target rocks on Venus allow more melt to be produced
than on the cooler terrestrial planets, because lower shock pressures
are required for meldng [21]. In addition, Venus' high atmospheric
temperature may allow the melt to remain molten longer by about
an order of magnitude than on the cooler planets [22]. The percent-
age of impact craters with outflows increases with increasing crater
diameter. However, three of the largest craters, Mead, Kelenova,
and Meimcr, have no recognized outflows. Outflow occurrence is
also correlative with impact incidence angle and the degree of
asymmetry in the ejecta. Of craters with asymmetric ejecta, those
with outflows are more numerous than those without above about
15 krn in diameter. Forty-eight percent of asymmetric-ejecta craters
have outflows, compared with only 34% of those with symmetric
ejecta.
"Splotches" or "shadows" (features with low-backscatter cen-
ters surrounded by higher backscatter) are common on the surface
of Venus. They range in diameter from 10 to 70 krn with a mean of
about 20 kin. A variety of arguments suggest that if the splotches
were produced by stony asteroidal objects traveling about 10 km/s,
the bolides would have been several hundred meters in diameter
with energies of order 10ts-]9j (1025-26 ergs or roughly 100 mega-
tons). A small fraction of the bolides that would have produced
2-10-kin craters on an airless Venus (hut were filtered in the
atmosphere) are thought to have produced the observed splotches.
Bolides <100 m in diameter are not thought to affect the surface.
Heavily fractured craters and lava-embayed craters are found to
have higher than average densities along the major fracture belts and
rifted uplands connecting Aphrodite Terra and Atla, Beta, Themis,
and Phoebe Regiones [23], thus providing physical evidence for
recent (or ongoing) low-level volcanic and tectonic activity in these
regions.
References: [1] Schaber G. G. et al. (1992) LPSC XXIII,
1213-1214. [2] Schaber G. G. et al., JGR, special Magellan issue,
in press. [3] Campbell D. B. et al.,JGR, special Magellan issue, in
press. [4] Phillips R. J. et al. (1992) LPSC XXilI, 1065-1066.
[5] Phillips R. J. etal., JGR, specialMagellan issue, in press.
[6] Head J. W. etal. (1992)LPSCXXIII, 517-518. [7] Head L W.
etal., JGR, special Magellan issue, in press. [8] Arksni-Hamed I.
and Toksoz N. (1984) PEPL 34, 232-250. [9] Strum R. G. etal.
(1992) LPSC XXIII, 1379-1380. [ 101 Arkani-H amed J. etal., this
volume. [1 I] Arkani-Hamed J. et al,, GRL, in press. [12] Pike R. J.
(1983) JGR, 88, 2500-2504. [13] Hartrnann W. K. (1982) Icarus,
17, 707-713. [14] Pike R. J. (1988) Mercury (F. Vilas etal., eds.),
165-273, Univ. of Arizona, Tucson. [15] Melosh H. J. (1989)
Impact Cratering-A Geologic Process, Oxford, New York, 245 pp.
[16] Wood C. A. (1980) LPSCXI, 1271-1273. [17] Harmaann W. K.
and Kuiper G. P. (1962) Lunar Planet. Lab., 1, 51-56, Univ. of
Arizona, Tucson. [18] Hartmann W. K. and Wood C. A. (1971)
Moon, 3, 3-78. [19] Wood C. A. and Head J. W. (1976) Proc. LSC
7th, 3629-3651. [20] Pike R. J. and Spudis P. D. (1987) EarthMoon
Planets, 39, 129-194. [21] Vickery A. M. and Melosh H. J. (1992)
LPSC XXIII, 1443-1444. [22] Ivanov B. A. et al., JGR, special
Magellan issue, in press. [23] Schaber G. G. (1982) GRL, 9,
499-502.
N93-14370 "37- 
ATMOSPHERIC EFFECTS ON CRATER GR()WTIt ON
VENUS. Peter H. Schultz, Brown University, Department of
Geological Sciences, Box 1846, Providence RI 02912, USA.
Laboratory experiments allow examining the consequences of
complex processes operating over a wide range of scales (both
temporal and spatial) and frequendy reveal effects that are obvious
only in hindsight. Even though all processes may not scale direcdy,
isolation of the controlling variables allows assessing first-order
effects through analytical approximations. This approach can be
illustrated by the systematic sequence of ballistic ejection [ 1], the
response of an atmosphere to a strong energy source [2], the scaling
of ejecta thickness [3], and the role of secondary eratering [4]. Here
it is proposed that the effects of atmospheric pressure and density on
cratergrowth (hence,scaling)observed inlaboratoryexperiments
[5,6]has particularelevanceforcraterson Venus.
Crater Growth: Both static(ambient)and dynamic (viscous
drag)pressurereduce cratcringefficiency(displacedmass/irnpac-
for mass) for cratersproduced in particulatetargct[5].Targct
strength(i.e.,internalangleoffriction,¢) isshown tohave minimal
cffcct;in fact,similarreductionin crateringefficiencyoccurs for
craters formed in compacted pumice (¢ - 85°), loose sand (¢ - 33°),
and low-density microspheres (0 < 20°). Rather, it is found that
particle size plays the most important role: The smaller the constitu-
ent particle sizes, the greater the reduction in cratering efficiency.
This result can be interpreted as the effect of aerodynamic drag
acting on both individual particles and the ensemble of these
particles comprising the ejecta curtain. By using a helium atmo-
sphere (hence low density at high pressure), the role of static
pressure can be separated from the role of dynamic pressure as
clearly illustrated by the contrasting evolution of the ejecta curtain
[7].
The principal effect of internal angle of friction is in the
preservation of the transient crater. Craters in t'me sand and
microspheres with low internal cohesion collapse, whereas craters
in compacted pumice retain their shape. Nevertheless, crater growth
and the evolution of the ejecta curtain during growth are essentially
the same for the same value of the ratio of drag to gravitational
forces. The role of the atmosphere is to choke off crater growth.
Since craters first grow downward and then outward [8.9], arresting
https://ntrs.nasa.gov/search.jsp?R=19930005182 2020-03-17T08:42:35+00:00Z
102 /nternational Colloquiumon Venus
crater growth changes the profile of the transient crater. As shown
in Fig. 1, increasing atmospheric pressure reduces crater diameter
in compacted pumice while maintaining a nearly constant depth.
Quarter-space experiments using sand and mierospheres clearly
reveal the same growth, but the evidence is erased by rim/wall
collapse and consequent floor uplift.
Aerodynamic drag affects crater growth at two scales. At broad
scales, the ejecta curtain is an extension of the material flow field in
the targeL The advancing curtain impinges on the atmosphere; or,
in the flame of reference of the curtain, the atmosphere impinges on
the curtain [7,10]. The force exerted on the curtain in a unit area
changes with time because the velocity of the curtain (and its
constituent ejecta), as well as the mass behind this unit area, changes
with time. At small scales, the redirected air flow created in front of
O0
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Fig. 1. (a) Effect of atmospheric pressure P on craterdiameter and depth
scaled to impactor diameter 2r for impacts into compacted pumice. Atmo-
spheric pressure is expressed as a dimensionless ratio involving target density
times specific energy of impactor. For a given value of it2 (the gravity scaling
parameter), impact velocity is constant; hence, this plot principally shows the
effect of atmospheric pressure. As atmospheric pressure increases, crater
growth is stopped prematurely. The identical process is observed for impacts
into sand and microspheres with very low strengths, but the craters are
unstable and cellapse. (b) Schematic represenation of atmospheric effects on
crater depth and diameter for craters without rim/wall collapse (compacted
pumice) and with collapse (sand). Dotted line represents constant ratio of
diameter to depth for vacuum conditions. If the atmosphere affects crater
growth (hence scaling) on Venus, it may he revealed in the observed relation
between diameter and depth.
the inclined curtain induces aerodynamic drag on individual eject&
These two effects of drag are expressed by steepening angles of the
ejecta curtain before the crater has finished forming and nonballistic
ejecta emplacement after formation [7].
Application to Venus: The laboratory results cannot be di-
re,ctly applied to Venus without assessing both the role of the
disturbed atmosphere surrounding the impact and the velocity of
crater growth. First, the time required for atmospheric density to
recover from a strong shock is long compared to the time for crater
growth [11]. The evolution of the impact-coupled shock is com-
monly assumed to resemble a stationary point source [e.g., 12], but
laboratory experiments and surface features on Venus indicate that
the early-time shock develops from a moving hypervelocity source:
consequently, the effects on the atmosphere are displaced down-
range, largely decoupled from later-stage crater excavation [I 3]
since most impacts are oblique (i.e., impact angles less than 600).
Interference between the downrange-c.entered fireball and late-
stage ejecta emplacement is clearly recorded around craters on
Venus [13]. Second. supersonic advance of the ejecta curtain (and
its ejecta) occurs for only a small fraction of crater growth at
laboratory scales, yet creates a distinctive turbulent kink in the
plume due to shear drag [14]. At the scale of craters on Venus. the
advancing ejecta curtain represents a significant fraction of crater
growth, but scaling of ejection velocities reveal that even for a 60-
kin-diameter crater, about 80% of the ejected mass occurs at
subsonic velocities [ 14].
Because passage of the shock wave and shock comminutlon in
the target precedes development of the cratering flow field, it is
assumed that the late-stage ejection process is basically the same
whether in a vacuum or under the dense atmosphere of Venus. With
this assumption, gravity should ultimately limit growth when the
ejection velocity falls below a critical value, q_g, necessary for
escaping the cavity. If only static pressure limits growth, then the
effects will resemble a strength term [5]. But dynamic pressure
acting on the ejecta curtain can also limit growth, since the flow field
(including its extension forming the base of the ejecta curtain) is
essentially incompressible and hydrostatic. Hence, scaling will be
controlled by drag force, d, which replaces gravity g [5]. If drag
forces reduce the outward advance of the ejecta curtain(tied to
crater growth) to a value below q3g, then completion of the transient
cavity occurs at an earlier stage of growth just as observed in
laboratory experiments. In such experiments, the ratio of d/g was
found to be the controlling parameter because the ejecta curtain is
relatively thin. An alternative approach considers the advancing
curtain analogous to a vertical plate [5,7,10]. Deceleration of such
a plate of thickness w and unit area A c from a velocity v0 to v is
simply expressed as
In vlvo = -I/2 C D pAcL/M c
= -]/2 C D (plSc)(L/w)
(la)
(Ib)
where C D is the drag coefficient (=2 for a fiat plate), p is atmospheric
density, L is the distance over which the force acts, and M e is the
mass of curtain. Because the curtain width per unit area will map on
the surface as ejecta thickness per unit area, w can be given by ejecta
thickness tc at a given scaled range from the crater had it formed in
a vacuum. For purposes of illustration, values typical for laboratory
impact craters (L- 15 cm, re~ 0.05 cm, p = 1.3 x 10 -3. and _ie= 1.5)
result in a velocity of reduction of 0.77. Since gravity-limited
growth varies as R v _ [ 15], this is equivalent to a crater only 0.60 as
large, comparable to observations [5]. For a 40-kin-diameter Venus,
LPI Contribution No. 789 103
the same equation predicts arresting crater growth when it had
advanced to only about 68% of its size in a vacuum (with the added
assumption that the length scale, L, over which the forces act begin
after the crater has grown to 50% of its final size). It is important to
recognize that equation (1)predicts that atmospheric deceleration
on the curtain increases with increasing crater size because L ~ Rv
and t, ~ Rvta; consegluently, ha (V/Vo) ~ Rv ta.
Tests: Several observations are consistent with the inferences
drawn from the laboratory experiments and the simple analogy.
First, nonballistie ejecta emplacement near the rim reflects decel-
eration and collapse of the cjeeta curtain. Craters 70 km in diameter
on Venus exhibit this transition within 0.25 crater radii of the rim.
Second, as atmospheric effects become extreme, the combined roles
of rim/wall collapse and decreased ejecta run-out should result in
increasing collapse of the uplifted rim and inner ejecta facies with
increasing size. Third, diameter-toMepth relations for complex
craters on Venus should parallel simple craters on other planets
(Fig. 1).
References: [ 1] Gault D. E. et al. (1968) In Shock and Meta-
morphism ofNaturalMaterials (B. M. French et al., eds.), 87-100,
Mono, Baltimore. [2] Schultz P. H. and Gault D. E. (1990) In
GSA Spec. Pap. 247 (V. Sharpton and P. Ward, eds.), 239-261.
[3] McGetchin T. R. et al, (1973) EPSL, 20. 226-236. [4] Oberbeck
V. R. et al. (1975) Moon, 12, 19-54. [5] Sehultz P. H. (1992) JGR,
97, 975-1006. [6] Sehultz P. H. (1990) LPSC XXI, 1097-1098.
[7] Schultz P. H. (1992)JGR, in press. [8] Orphal D. etal. (1980)
Proc. LPSC l l th, 230°_--.2.32.3. _.
N93-14371
EFFECT OF IMPACT ANGLE ON CENTRAL-PEAK/PEAK-
RING FORMATION AND CRATER COLLAPSE ON VENUS.
Peter H. Sehultz, Brown University, Department of Geological
Sciences, Providence, RI 02912, USA.
Although asymmetry in ejecta patterns and crater shape-in-plan
are commonly cited as diagnostic features of impact angle [1,2], the
early-time transfer of energy from impactor to target also creates
distinctive asymmetries in crater profile with the greatest depth
uprange [ l ]. In order to simulate gravity-controlled crater growth,
laboratory experiments use loose particulate targets as analogs for
low-strength material properties following passage of the shock. As
a result, impact crater diameter D in laboratory experiments gener-
ally is many times greater than the impactor diameter 2r (factor of
40), and early-time asymmetries in energy transfer from oblique
impacts are consumed by subsequent symmetrical crater growth,
except at the lowest angles (<2.5°), Such asymmetry is evident for
oblique (<60 ° from horizontal) impacts into aluminum where D/2r
is only 2 to 4. Because cratering efficiency decreases with increas-
ing crater size [3,4] and decreasing impact angle [I ], large-scale
planetary craters (40--80 kin) should have transient excavation
diameters only 6-10 times larger than the impactor [5]. At basin
scales, D/2r is predicted to be only 3-5, i.e., approaching values for
impacts into aluminum in laboratory experiments. As a result,
evidence for early-time asymmetry in impactor energy transfer
should become evident on planetary surfaces, yet craters generally
retain a circular outline for all but the lowest impact angles.
Evidence for energy-transfer effects in fact occurs on the Moon
and Mercury but depends on scale. For simple craters (Messier,
Toricelli), crater depth is greatest uprange with a steep uprange and
shallow downrange wall slope. For complex craters (Buys-Ballot,
Tycho, King), the centlal peak is offset uprange (corresponding to
the greatest depth) but the wall exhibits greater failure uprange
(corresponding to higher slope). Moreover, the central peak in King
Crater is breached downrange. For two-ringed basins (Bach on
Mercury), the interior ring is breached downrange with evidence for
greater rim/wall failure uprartge, observations also consistent with
the oblong Crlsium Basin on the Moon [6]. The cratering record on
Venus allows extending such observations where D/2r should be
further reduced because of the greater gravity and perhaps effects of
the atmosphere [71.
Craters on Venus: Figure 1 illustrates a 42 kin-diameter crater
with central peak offset uprange, a steep (narrow) uprange inner
wall slope, and a broad but gently sloping downrange wall. Since the
radar look direction is nearly transverse to impact direction, the
observed asymmetry reflects the impact process and not imaging
perspective. Figure 2aillustrates a similar uprange offset of acentral
peak ring and a similar contrast in the uprange/dowrtrange wall.
Figure 2b, however, reveals a reversal in this pattern for a larger
crater:, a downrange offset of the inner ring. It is proposed that this
reversal reflects more extensive rim/wall failure as crater depth and
uprange slope exceexts a critical value. This proposal is consistent
with the concentric scarps within the crater, transform faults cross-
ing the peak ring, and step faulting beyond the rim. The examples
in Figs. 1 and 2 are typical for Venus. Exceptions occur only where
topography also plays a role or where the impactor was clearly
multiple.
If the central massifs (peaks and peak rings) reflect the region of
maximum depth, then the size of this disruption may reflect the size
of the impactor [7,8]. As a test, crater diameter referenced to peak-
ring diameter should increase with decreasing impact angle (judged
from the missing sector uprange and the overall degree of ejects
asymmetry) as cratering efficiency decreases. If peak-ring diameter
reflects a response to impactor kinetic energy or potential energy
(depth), then this ratio should decrease with decreasing impact
angle. As shown in Fig. 3, peaking diameter comprises a greater
fraction of crater diameter as impact angle decreases; consequently,
it is suggested that peak rings indeed may provide markers of
impactor size. This marker most likely reflects a limiting (but
common) value of peak stress created during penetration [8].
\
Fig. 1. Crater (42 km in diameter) with central peak offset uprange and
exhibiting contrast between steep, narrow and shallow, broad downrange
wall. Arrows indicate crater rim. C1-15 S009. Radar look direction from the
left; arrow indicates impact direction.


Atmospheric effects on crater growth on Venus

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