Aeolian processes on Venus

Abstract

In this pre-Magellan review of aeolian processes on Venus we show that the average rate of resurfacing is less than 2 to 4 km/Ga, based on the impact crater size frequency distribution derived from Venera observations, reasonable values of the impact flux, and the assumption of steady state conditions between crater production and obliteration. Viscous relaxation of crater topography, burial by volcanic deposits, tectonic disruption, chemical and mechanical weathering and erosion, and accumulation of windblown sediments probably all contribute to resurfacing. Based on the rate of disappearance of radar-bright haloes around impact craters, the rate of removal of blocky surfaces has been estimated to be about lo-* km/Ga. Pioneer-Venus altimetry data show that the average relative permittivity (at 17 cm radar wavelength) of the surface is too high for exposure of soils 3 10 cm deep, except for -5% of the planet located primarily in tessarae terrains. The tectonically disrupted tessarae terrains may be sites of soil generation caused by tectonic disruption of bedrock and the presence of relatively steep slopes, or they may be terrains that serve as traps for windblown material. The overall impression is that Venus is a geologically active planet, but one dominated by volcanism and tectonism. On the other hand, theoretical considerations and experimental data on weathering and transport of surface materials suggest rather different conditions. Thermochemical arguments have been advanced that show: (1) CO2 and SO2 incorporate into weathering products at high elevation, (2) transport of weathered material by the wind to lower-elevation plains, and (3) re-equilibration of weathered material, releasing both CO2 and SOz. In addition, kinetic data suggest a rate of anhydrite formation of 1 km/Ga, a value comparable to the soil erosion rate on Mars, a planet with an active aeolian environment. Experiments and theoretical studies of aeolian processes show that measured surface winds are capable of moving sand and silt on Venus. Assuming that there is a ready sand supply. the flux could be as high as 2.5 X 10m5 g/cm/s, a value comparable to desert terrains on Earth. In an active aeolian abrasion environment, sand grains could have lifetimes <lo" years. In addition. comminuted debris may be cold-welded to surfaces at the same time as abrasion is occurring. Magellan altimetry and SAR observations should allow assessment of which model for venusian surface modification (active vs. inactive surftcial processes) is correct, given the global coverage, high spatial resolution. the calibrated nature of the data, and the potential during extended missions of acquiring multiple SAR views of the surface.

Extraterrestrial dunes: An introduction to the special issue on planetary dune systems

“…Only two prominent Greeley and Iversen (1985) and Lodders and Fegley (1998). Tessera (Greeley et al, 1992a(Greeley et al, , 1997. Thousands of wind streaks are seen in the Magellan data, as were some features interpreted to be yardangs, but sand dunes only cover approximately 0.004% of the surface.…”

“…The Magellan mission used Synthetic Aperture Radar (SAR) to image more than 98% of the surface of Venus (Saunders et al, 1992), and sand dunes were identified by the specular pattern they produced in some of the SAR images (Greeley et al, 1997). Only two prominent Greeley and Iversen (1985) and Lodders and Fegley (1998).…”

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

“…Such thin deposits are unlikely to significantly alter either the size or depth of a crater floor, but a number of factors might influence their distribution and development. First, the effect of atmospheric pressure on the saltation threshold [Greeley and Arvidson, 1990] indicates that eolian transport is easiest at low elevations and requires progressively more windspeed at higher elevations. Second, since wind streaks on Venus preferentially occur between -•20 ø and 30 ø north and south latitude , atmospheric circulation may affect the distribution of eolian features on Venus.…”

Wind streaks: geological and botanical effects on surface albedo contrast

“…Some martian wind streaks were observed to have changed between Viking observations (Sagan et al, 1973;Thomas and Veverka, 19791, proving that some sort of surface process was presently operating, with the wind being the only reasonable candidate. Most recently, Magellan radar observations of the venusian surface have revealed albedo features that are almost certainly a result of aeolian activity (Saunders et al, 1990;Greeley and Arvidson, 1990;Greeley et al, 1992). The general conditions necessary to generate the albedo contrast that makes the wind streak visible are the focus of this Fig.…”