Engineering topics

Williams, S. H.

Publications and source records attributed to Williams, S. H..

8 records

NASA NTRS2004

Lake Shorelines: Earth Analogs for Hypothesized Martian Coastal Features

The possibility of oceans on Mars has generated a lot of interest in the science community, but conclusive evidence supporting or refuting the ocean hypothesis has remained somewhat elusive. Precise topographic measurements of fresh-appearing shorelines from glacial Lake Lahontan were collected recently in an effort to obtain well-constrained data for comparison with the hypothesized Martian shorelines. This report summarizes the first results of the on-going research project.

Zimbelman, J. R.↗

NASA NTRS2002

Large Ripples on Earth and Mars

Aeolian ripples on Earth with wavelengths greater than 50 cm have distinctive attributes, that should be helpful in interpreting ripple-like features on Mars. Additional information is contained in the original extended abstract.

Williams, S. H.↗

NASA NTRS1990

Possible Aeolian megaripples on Mars

Viking orbiter image frames 442B01-10, at 8 m/pxl resolution show that valley floors are not smooth at all, but rather are covered with mounds of material interpreted as dunes. Striations oriented perpendicular to the valley axis can be seen in several locations. The striations are here interpreted to be aeolian megaripples formed from debris weathered from the yardangs. Terrestrial aeolian megaripples have wavelengths up to 25 m; it is not unreasonable that larger megaripples might form under favorable Martian conditions, given the wind speeds available and the lower Martian gravity. If the megaripple interpretation is correct, then by terrestrial analog the deposit in which they occur has a bimodal particle size distribution. One size will undergo saltation; the other, concentrated at the crests of the megaripples, is too large/and or too dense to be put into saltation. For Mars the former is sand-sized, the latter gravel-sized, provided the materials have typical densities.

Williams, S. H.↗

NASA NTRS1989

Concentric crater fill on Mars - An aeolian alternative to ice-rich mass wasting

Concentric crater fill, a distinctive martian landform represented by a concentric pattern of surface undulations confined within a crater rim, has been interpreted as an example of ice-enhanced regolith creep at midlatitudes (e.g., Squyres and Carr, 1986). Theoretical constraints on the stability and mobility of ground ice limit the applicability of an ice-rich soil in effectively mobilizing downslope movement at latitudes poleward of + or - 30 deg, where concentric crater fill is observed. High-resolution images of concentric crater fill material in the Utopia Planitia region (45 deg N, 271 deg W) show it to be an eroded, multiple-layer deposit. Layering should not be preserved if the crater fill material moved by slow deformation throughout its thickness, as envisioned in the ice-enhanced creep model. Multiple layers are also exposed in the plains material surrounding the craters, indicating a recurrent depositional process that was at least regional in extent. Mantling layers are observed in high-resolution images of many other locations around Mars, suggesting that deposition occurred on a global scale and was not limited to the Utopia Planitia region. It is suggested that an aeolian interpretation for the origin and modification of concentric crater fill material is most consistent with morphologic and theoretical constraints.

Zimbelman, J. R.↗

NASA NTRS1987

Mass transport by Aeolian saltation on Earth, Mars and Venus: The effects of full saltation cloud development and choking

Wind tunnel tests were conducted to study the characteristics of particle motion and the quantity transported by wind under a variety of planetary environmental conditions. Analysis of the results indicates that, on Earth, saltation fully develops at near-threshold wind speeds and rarely chokes (retarding of saltation due to high particle concentration and subsequent increase of mid-air collisions). On Mars, saltation becomes fully developed only at favorable locations and at wind speeds well above threshold. The Martian saltation cloud has an insufficient particle concentration to cause choking, particularly over a partially rocky surface. On Venus, full saltation occurs at wind speeds very near threshold, but saltation flux increases less rapidly than expected at relatively modest wind speeds due to choking.

Williams, S. H.↗

NASA NTRS1985

The Effect of Particle Density on Aeolian Transport

A set of experiments using a wide range of particle densities was performed in an open-circuit, terrestrial-atmospheric-pressure wind tunnel at Arizona State University. The results show that saltation flux equations derived for typical geologic material overpredict the flux of low-density particles. Walnut shells (approximately 1.1 g/cc) were used in the experiment and correspond to volcanic ash or ice. Less mass is transported by the wind in the case of low particle density because the style of transport is different. There is a direct, counter-intuitive relationship between particle density and transport height. Measurements of the vertical distribution of material show that the low-density walnut shells travel in a zone within 10 cm of the surface while high-density (approximately 4.5 g/cc) chromite particles travel as 50 cm. Furthermore, the overall saltation rate of the chromite is approximately four times greater than the walnut shells at the same freestream wind speed, even though the wind is much further above threshold for the walnut shells.

Williams, S. H.↗

NASA NTRS1985

A Comparison of Terrestrial Saltation Flux in the Laboratory and the Field

Aeolian transport of surficial material is an important part of the terrestrial geologic cycle. In many areas of geological interest, sand is not transported by the wind at the rate predicted by studies made in idealized conditions. Furthermore, calculated surface-abrasion rates based on laboratory saltation fluxes are often unrealistically high. This work is an attempt to quantify and discuss discrepancies between saltation rates under ideal conditions and in the field under natural conditions. The saltation-flux data persented by Sharp (1) will be used as the field data. Sharp measured saltation flux during four periods in the early 1950's in the Whitewater River drainage. Meteorological conditions at the experiment site were extrapolated from the airport records and from wind speeds measured simultaneously by Hunter. Wind speeds above threshold were sorted into 4-MPH bins and the frequency of observation over each experimental period was calculated. A saltation flux for each bin was calculated and summed to yield a predicted saltation rate for that period. When the predicted fluxes were compared with those measured by Sharp, large discrepancies were found.

Williams, S. H.↗

NASA NTRS1982

Rate of wind abrasion on Mars

A brief description is given of the experiments performed to obtain data on windblown particles and abrasion of rocks in a simulated Martian environment. Preliminary results are presented and combined with Viking meteorological data in estimating rates of wind abrasion at the VL-1 site on Mars. Attention is also given to the implications that the results have for Martian surface history. Calculations of the present rates of abrasion by windblown particles on Mars yield values ranging from 0.021 cm/yr to nearly zero, depending on the target, the agent of abrasion, and the availability of windblown particles.

Greeley, R.↗