Magellan observations of extended impact crater related features on the surface of Venus

Abstract

A total of 57 parabolic‐shaped and 9 approximately circular extended, impact crater related features have been found in Magellan synthetic aperture radar (SAR) and thermal emissivity data covering 92% of the surface of Venus. The parabolic features are, with seven exceptions, oriented E‐W with the apex to the east and the impact crater located just west of the apex. They were first identified in the surface emissivity data derived from Magellan radiometry measurements, but the great majority are only clearly visible in the SAR imagery. The overall sizes of both the parabolic and circular features range from several hundred to about two thousand kilometers and are loosely correlated with the diameters of the “parent” craters. The floors of almost all these craters have high specific radar backscatter cross sections (i.e., they are bright in the SAR imagery) relative to their surroundings and tend to have low emissivities. Approximately one‐third of the impact craters with diameters ≥15 km appear to have bright floors and about half of these have an associated parabolic feature which can be observed in the SAR or emissivity data. No features have been found which overlie the parabolic features, indicating that they are among the youngest features on the surface of the planet. This suggests that radar‐bright floors characterize the freshest impact craters and that modification processes subsequently darken their radar signature. A model for the formation of the parabolic features is developed based on the injection of small particles into the upper atmosphere at the time of impact and their transport to the west by the E‐W zonal winds. Fitting of a small perturbation scattering model to the measured average scattering law for the parabolic features placed an upper limit of about 0.6 cm on the wavelength scale (12.6 cm) surface roughness and, hence, of 1 to 2 cm on the largest particle sizes of interest. Fallout times from 50 km in the Venus atmosphere for particles of this size are about 2 hours, allowing westerly drifts of several hundred kilometers for zonal winds of 50 to 100 m s−1. Measurements of the change in backscatter cross section of features overlaid by these extended ejecta deposits, are consistent with deposit depths of a few centimeters to 1 or 2m.

Geologic units on Venus: evidence for their global correlation

“…Radar-dark parabolas, observed around about 50 craters, are believed to be relatively ®ne-grained ejecta from the craters with which they are associated. Being injected into the upper atmosphere, these small particles are transported to the west by the E±W zonal wind, thus forming parabolas with their apex to the east (Campbell et al, 1992). Radardark parabolas occupy a total of about 8% of Venus' surface, while the areal distribution of other components of this unit is signi®cantly smaller.…”

“…Radardark parabolas occupy a total of about 8% of Venus' surface, while the areal distribution of other components of this unit is signi®cantly smaller. The discoverers of the dark parabola craters found that these craters are among the youngest features of the Venusian surface (Campbell et al, 1992). A special Fig.…”

“…These were distinguished in early stage of the Magellan data analysis as features associated with the youngest part of the crater population (Campbell et al, 1992). There are about 55 craters larger than 12± 13 km in diameter which have associated parabolas.…”

“…The Aurelia Group is composed of materials of parabolic radar-dark mantles typical of the youngest 10% or so of impact craters (Cdp) and other crater materials of this crater subpopulation (Basilevsky, 1993;Campbell et al, 1992;Strom, 1993). Also included in this group are some debris accumulations in the form of radar patches (Sp) in local topographic lows against or behind positive topographic obstacles, as well as some radar-dark wind streaks (Ss) .…”

Internal crater modification on Venus: Recognizing crater‐centered volcanism by changes in floor morphometry and floor brightness

“…Further, while crater-filling dust deposits can potentially change floor sizes by partially burying crater walls, these deposits should affect floor brightness long before they achieve sufficient thickness to alter the floor morphometry. If the parabolic ejecta units on Venus can darken surfaces with layer thicknesses of only -•10-100 cm Campbell et al, 1992], then even a few meters of eolian dust should be enough to darken the brightest crater floor. Since thin deposits should form much faster than deposits over 100 m thick, we would therefore expect significant changes in floor brightness to be largely independent of the observed floor enlargements if the eolian mechanism dominated floor darkening.…”