The contacts between 34 geological/geomorphic terrain units in the northern quarter of Venus mapped from Venera 15/16 data were digitized and converted to a Sinusoidal Equal-Area projection. The result was then registered with a merged Pioneer Venus/Venera 15/16 altimetric database, root mean square (rms) slope values, and radar reflectivity values derived from Pioneer Venus. The resulting information includes comparisons among individual terrain units and terrain groups to which they are assigned in regard to percentage of map area covered, elevation, rms slopes, distribution of suspected craters greater than 10 km in diameter

Schaber, G. G.

English

NASA Technical Reports Server

N92-10854
VENUS IMPACT CRATERS: IMPLICATIONS FOR ATMOSPHERIC AND
RESURFACING PROCESSES FROM MAGELLAN OBSERVATIONS
R.J. Phillips, Department of Geological Sciences, Southem Methodist University, Dallas,
Texas 75275 and other members of the Magellan Science Team
Introduction. Observations of impact craters on Venus by Magellan yield important
insights into (i) atmospheric effects on the formation of impact craters and their attendant
ejecta deposits and (ii) the resurfacing history of the planet [1].
Atmospheric Processes. Most craters smaller than 15 km are classified as "irregular";
they possess irregularly shaped rims, and multiple hummocky floors. The irregular nature of
these craters is interpreted to be the consequence of breakup and dispersion of incoming
meteoroids by the dense atmosphere [2].
Two major ejecta facies of venusian impact craters are "hummocky ejecta" and "outer
ejecta". The latter is akin to "continuous ejecta" seen on other planets, except that on Venus
azimuthal sectors of ejecta are often missing, and this is attributed to the effects of oblique
impact amplified by an atmospheric cushion. The distal edge of the outer ejecta of complex
craters on Venus is sharp and lobate to slightly pointed, often with a petal-like appearance
suggesting emplacement by flow. Schultz and Gault [3,4] have described a mechanism that
may explain this observation. They showed that below a critical size, the ballistic paths of
ejecta material are significantly modified by an atmosphere. In particular, small particles are
decelerated and form an ejecta cloud that is deposited as turbulent ejecta and flows outward
as a base surge from the crater rim.
A number of craters documented in the Magellan images possess often non-radial,
"flow-like ejecta" indicative of a low viscosity material. Typically the flows extend up to
two crater diameters from either beneath or within the crater's hummocky and outer ejecta
deposits. The origin of the flow-like ejecta is problematical, and there is more than one
form of this unit (e.g., uniformly radar bright, or bright just along the boundaries), and mul-
tiple origins are possible. Three hypotheses for the origin of flow-like ejecta are: (i) tur-
bidity flows of very-fine-grained ejecta, (ii) flows consisting of a mixture of ejecta and
magma released during the impact event, and (iii) flows of impact melt.
Approximately half of the impact craters observed with the Magellan radar are partially
or wholly surrounded by areas with low radar backscatter cross sections, o0; we term these
areas "dark margins". In most cases these regions are irregularly shaped and extend up to 3
or 4 crater diameters from the crater center; in a few cases the dark areas are much more ex-
tensive. The interface between this dark area and the surrounding brighter terrain occurs
over a relatively short distance but, in general, it is not a sharp, well-defined boundary. The
crater Stephania, for example, has dark margins that extend from about 25 km west of the
crater to approximately 60 km to the east. The dark margins appear to be areas that are very
smooth, with little wavelength-sized roughness to diffusely scatter the incident radar signal.
Hypotheses for the origin of dark margins include ablated meteoroid material, ejecta
sorting, seismic shaking, and surface pulverization by the shock/pressure wave associated
with the incoming meteoroid. Energy considerations show that the last mechanism is quite
feasible [ 1].
Resurfacing Processes. Venera 15/16 imaging data showed that impact craters are not
uniformly distributed on the surface of Venus [5,6,7], and this is evident in the Magellan
data. Particularly apparent are regions that do not have any impact craters at all, for in-
stance, the Sappho Patera region in central Eistla Regio. The hypothesis that the observed
areal distribution of craters is random can readily be tested. The area around Sappho that is
devoid of impact craters comprises approximately 5 x 106 km 2. We consider 135 (the total
number of impact craters observed in the ftrst 277 mapping orbits) Bemoulli trials for im-
pact into the Sappho region. A "success" occurs when a crater is formed at Sappho and the
probability of success for a spatially random process must be the ratio of the Sappho area to
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the total area surveyed, or 1/14. The resulting probability distribution must be binomial,
b(x,135,1/14), with _t = 9.6 and q = 3.0. This meets the criterion, _t :> 3(_, that ensures the
sampling area is large enough to have at least 1 crater [6]• In fact, the compliment of this
outcome, the nrohahilitv of observin2 no craters at Savvho under a random spatial distribu-
t-i0n is 4.5 x 1(3-5 (b(0,1_35,1/14)). Titus it is highly unlikely that the "event" of no craters at
Sappho can occur. However, this does not reject the hypothesis that the null occurrence of
craters can belong to a random process because the Sappho area is part of a larger sampling
population. If this experiment is repeated a sufficient number of times (i.e., other 5 million
square kilometer areas are examined), the probability that at least one such area will be
found with no craters will approach unity. The maximum number of experiments possible is
92, the surface area of the planet divided by 5 x 106. The expected number of 5 million
square kilometer areas with no craters is approximately 92xb(0,135,1/14) = .004. Therefore,
if Venus truly has a spatially random distribution of impact craters, the probability of finding
a region the size of the Sappho area with no craters is essentially nil.
Those areas on Venus with few or no impact craters must have young surfaces undergo-
ing rapid resurfacing on a geological time scale. At Sappho, volcanism is playing a major
role; elsewhere, tectonic processes may also be important. A hypothesis that might explain
the impact crater distribution on Venus is that cratering occurs randomly in space and time,
whereas resurfacing has a spatial and temporal dependence [8]. In this case, craters are
preserved in relatively pristine form in tectonically and volcanically quiescent regions•
Areas of recent volcanism and tectonism have completely removed craters because resurfac-
ing rates have been so high. This hypothesis appears to account for the seemingly con-
tradictory observations that: (i) very few impact craters are observed to be in the process of
removal by resurfacing, yet (ii) there are areas of the planet where no impact craters are ob-
served, and thus there must be processes removing craters on a regional basis.
A simple, end-member model for this hypothesis is one of "regional resurfacing". This
model is an end-member construction because it is binary: either craters are pristine, or they
are completely removed; there are no craters in the process of removal. In a production
model, for a surface of age x, the cumulative size-frequency distribution, Cv, is given ap-
proximately by C_ = R.,xD -(z, where R,, is the present cratering rate, D is c_ater diameter,
• 1J 1-' oP ......
and (z is the power-law exponent that determmes size distribution. For a regional resurfac-
ing model, the information that can be gleaned from cumulative crater statistics is not sur-
face age (production model) or retention age/resurfacing rate (equilibrium model), but in-
stead the areal resurfaeing rate of the planet• With the binary assumption, the cumulative
size-frequency distribution, C r, is given by C r = 0.5(Rv/fr)D-(Z, where fr is the fraction of the
planet resurfaced in one year. The reciprocal of fr is'I?, the average time it takes to resur-
face the planet once• The portion of the cumulative size-frequency distribution curve un-
affected by the atmosphere (D > 25 km) is well matched by Tr = 0.8 Go. In this model there
is no one unique age of the surface; surface ages span the range from 0 to 800 Ma, with Sap-
pho, for example, representative of one of the youngest regions on the planet. We consider
that this is the most realistic interpretation of the crater data, in terms of both the cumulative
statistics and the appearance of individual craters.
References. [1] R.L Phillips et al., submitted to Science (1990); [2] G.G. Schaber et al., this volume; [3]
P.H. Schultz and D.E. Gault, J. Geophys. Res. 84, 7669 (1979); [4] P.H. Schultz and D.E. Gault, in Geological
Implications of Impacts of Large Asteroids and Comets on the Earth, GSA Special Paper 190, 153 (1982); [5]
G.A. Burba, Lunar and Planet. Sci. XX, 123 (1989); [6] J.J. Plaut and R.E. Arvidson, d.Geophys. Res. 93,
15339 (1988); [7] G.G. Schaber, U.S. Geological Survey Open-File Report 90.468 (1990); [8] R.E. Arvidson
et al., Geophys. Res. Lett. 17, 1385 (1990).
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Quantitative analysis of terrain units mapped in the northern quarter of Venus from Venera 15/16 data

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