Steven Hamilton Williams

Windblown sand on Venus: Preliminary results of laboratory simulations

Abstract Small particles and winds of sufficient strength to move them have been detected from Venera and Pioneer-Venus data and suggest the existence of aeolian processes on Venus. The Venus wind tunnel (VWT) was fabricated in order to investigate the behavior of windblown particles in a simulated Venusian environment. Preliminary results show that sand-size material is readily entrained at the wind speeds detected on Venus and that saltating grains achieve velocities closely matching those of the wind. Measurements of saltation threshold and particle flux for various particle sizes have been compared with theoretical models which were developed by extrapolation of findings from Martian and terrestial simulations. Results are in general agreement with theory, although certain discrepancies are apparent which may be attributed to experimental and/or theoretical-modeling procedures. Present findings enable a better understanding of Venusian surface processes and suggest that aeolian processes are important in the geological evolution of Venus.

Sand Transport Paths in the Mojave Desert, Southwestern United States

Remote sensing and field evidence are used to describe sand deposits found in associated pathways of emplacement in the eastern Mojave Desert. Two separate pathways are identified here: one extending eastward from the Bristol Playa through the Cadiz and Danby Playas and Rice Valley to the Colorado River, and a second parallel path extending eastward from Dale Playa through the Palen and Ford Playas to the Mule Mountains near the Colorado River. The preferential location of sand ramps on the west slopes of mountains along each path suggests that the eastward moving, wind-driven sand was not confined by topographic divides between separate drainage basins around the individual playas and valleys. Sediment analysis of selected samples shows that there are discreet associations of sand characteristics along the sand pathways, with an inferred similarity between the stabilized (vegetated) sands in Rice Valley, west of the Colorado River, and stabilized sand dunes on Cactus Plain and La Posa Plain in Arizona, east of the Colorado River. Sand transport along the paths appears to have been episodic, based on multiple paleosols present in several dissected sand ramps. Future testing of the sand transport path hypothesis will require additional sediment analyses, spectral studies of remote sensing data, and obtaining dates for selected soil horizons along the sand paths.

White Rock: An eroded Martian lacustrine deposit(?)

The existence and location of ancient lake sites and sediments have important implications for Martian paleoclimate and exobiology. White Rock, an enigmatic crater interior deposit, may be the eroded remnant of such a lacustrine deposit. Stereogrammetric analysis of newly processed Viking images allows better determination of the dimensions of White Rock (12.5 x 15 km, thickness 180 to 540 m, volume ∼40 km 3 ), reveals differences in erosion patterns that may reflect differences in depositional environment, and allows the identification in or on the crater wall of a possible source region of the high-albedo White Rock material. If White Rock is the remnant of a once-larger deposit, then open- system circulation may have been required to deliver the required quantity of evaporites.

Desert Pavement Evolution: An Example of the Role of Sheetflood

Patches of young, well-developed desert pavement were found atop bare rock on the Pisgah basalt flow, California. These particular stone mosaics formed directly atop the flow; no soil is present, or ever has been. Therefore, soil expansion and aeolian processes played no significant role in their creation or maintenance. The unusual setting at Pisgah allows sheetflood to be the sole agent responsible for the lateral movement of surface stones into these mosaics. The Pisgah mosaics thus represent an end-member case of desert pavement types, and they may represent a previously unrecognized initial substage of accretionary mantle formation.

Wind streaks: geological and botanical effects on surface albedo contrast

Abstract Two wind streaks in the eastern Mojave Desert of California were examined to gain insight into the origin of the surface brightness contrast that makes them visible, both on the ground and in remote sensing data. The two localities are: a 4-km-long dark streak oriented S43E from the Amboy cinder cone (34°32′N, 115°46′W), located on a Quaternary basalt flow covered with aeolian sand, and a 2-km-long dark streak oriented S22E from a low hill near the southwestern base of Sleeping Beauty Mountain (34°48′N, 116°20′W), located on a sand-covered alluvial surface. In both cases, the dark streaks have enhanced rock abundances on the streak surface, relative to the surroundings. At the Amboy streak, slope wash likely contributed to the rock concentration on the streak surface, shielded from burial under aeolian sand by the cinder cone. At the Sleeping Beauty streak, the relative albedo contrast is strongly emphasized by the presence of Big Galleta grass only outside of the streak. The albedo contrast of the Sleeping Beauty streak can be effectively eliminated by the seasonal presence of annual grass preferentially within the streak. Some plants may have reflectances that are strongly dependent upon viewing and illumination geometry, raising the possibility that certain terrestrial aeolian features may appear variable on a diurnal basis. Alluvial processes appear to have been important at both localities for redistributing surface materials, even given the infrequent rain conditions present in the Mojave Desert.

Aeolian saltation transport rate: an example of the effect of sediment supply

The channel of the Whitewater River near Palm Springs, California, is an excellent location to assess the effect of sand supply and mobility on the quantity of material moved by aeolian saltation. Saltation fluxes at the site taken by Sharp (1964) were compared to the fluxes that would have been produced under ideal conditions for the spectrum of winds inferred for the site. The discrepancy between the two fluxes is, in general, consistent with whose observed elsewhere for similar surfaces. Variations are linked to runoff in the Whitewater River and, hence, the supply of fresh sand.

Windblown sand on Venus: The effect of high atmospheric density

The high density of the venusian atmosphere significantly affects aeolian saltation transport and may also allow rapid alterations in the radar backscatter cross-section of the surface. Saltation flux experiments were conducted under terrestrial and simulated venusian conditions to assess the effect of atmospheric density on aeolian transport of sediment. Total lane flux, the vertical distribution of saltating particles, and the average horizontal component of particle speed were measured and used to estimate the volumetric concentration of particles above the surface. Results show that so many particles are set into motion when wind speeds exceed a critical value that mid-air collisions are a common result, a condition termed “choked” saltation. When choking occurs, there is a reduction in the rate at which the total saltation flux increases with increasing wind speed at least in part due to a reduction in the mean horizontal speed of the particles. Choked saltation on Venus may result in a reduction of the local aeolian erosion rate, the obliteration of aeolian bedforms, and rapid alteration of surface radar backscatter characteristics.

The Geology of Mars: Eolian dunes and deposits in the western United States as analogs to wind-related features on Mars

Eolian processes produce distinctive features and deposits on planetary surfaces where the atmosphere is sufficiently dense to allow interactions between the wind and sediments on the surface (Greeley and I versen. 1985). Arid and semi-arid regions on Earth contain abundant evidence of windsurface interactions (e.g., Lancaster, 1995a; Thomas, 1997), and the Martian surface shows a diverse array of eolian features across the planet (e.g.. Greeley ct al.. 1992). The characteristics of several eolian localities (primarily sand dunes) in the western part of the United States have been used previously as analogs to features seen on Mars in data obtained from several spacecraft (e.g., Greeley et al., 1978; Greeley and Iversen, 1987; Golombek et al., 1995). yet the analog potential of other western eolian sites is relatively underutilized. Rather than attempting a comprehen.sive survey of all eolian features in the United States, this chapter will focus on several examples illustrative of a variety of dune forms and their potential applicability as analogs to eolian features observed on Mars. Dunes in the Great Plains, east of the Rocky Mountains, and all coastal dunes are excluded from this survey in order to concentrate on discrete sand accumulations in arid or semi-arid environments. Both traditional publications and selected internet sites (cited here as W#) are referenced throughout the text. Eolian features in the western United States reflect varying climatic and drainage conditions that have directly contributed to the formation of the individual deposits. Cold conditions during the last glacial period gave way to warmer and more arid conditions during the ensuing interglacial