Abstract. Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coetficient cr ø are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between cr ø and z0 for L band (X: 24 cm) HV (H, vertically and V,vertically polarized modes) L band HH, and C band (X -5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems. IntroductionAeolian processes are prominent in arid and coastal regions but can occur wherever loose particles are available for movemcnt by the wind. In addition to hazards created by sand and dust storms, the migration of sand over roads, agricultural fields, and other cultural features has negative economic impacts. Moreover, because aeolian processes involve the interaction of the atmosphere and Earth's surface. they can influence the climate. Aeolian processes can occur frequently and on short timescales, resulting in rapid surface changes. Remote sensing affords the opportunity to monitor these changes and, in some cases, allows the prediction of potential aeolian activity. Remote sensing in the visible spectrum has been used to map sand dune fields [e.g., Breed et al.

From orbital, aviation and geologic documents, four circular depressions on the

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We recovered Australasian tektites in place throughout a 40X130 km region in northeast Thailand extending from the Laotian border westward to a line connecting Na Pho Klang in the northeast through Det Udom to Nam Yun in the south. With two exceptions, in sites near the western edge of this region, all fragments are layered (muong-Nong-type) tektites. It appears that large layered tektites are mainly found by rice farmers in fields that were forested until the recent past. The presence of layered tektites in this 40X130 km area implies that impact melt that fell in these areas was hot enough to flow if it was deposited on a sloping surface. The absence of splashform tektites from the region indicates that the layer was still molten when masses having shapes (teardrops, dumbbells, etc.) produced by spinning reached the ground. To account for this and to allow time for the melt to flow a few tens of centimeters requires that the atmosphere remained hot (>2300 K) for a few minutes. Tektites that are in place are almost always associated with a widespread 10-cm to meter-thick layer of laterite. In two cases (one involving layered, one involving splash-form tektites), where accurate stratigraphic control demonstrated that the tektites were still in section, they were sited on top of the laterite layer just below a layer of aeolian sand. Introduction Tektites are glasses having chemical and isotopic compositions indicating that they formed by melting well-mixed continental sediments. Four fields are known; common names and approximate ages are North American, 35 Ma; Central European, 15 Ma; Ivory Coast, 1.1 Ma and Australasian, 0.77 Ma (Australasian age by lzett and Obradovich, [ 1992]; other data are from summary by Wasson and Heins, [1993]). We investigated a portion of the youngest strewn field, the Australasian. The Australasian tektites can be divided into two fundamentally different classes: (1) splash-form [more accurately, spinform] that obtained their shapes by solidifying while spinning in a low-pressure environment; and (2) layered (also called Muong-Nong-type) tektites that seem to be fragments of melt sheets, puddles or more complex flow structures that solidified on the Earth's surface [Barnes and ?itakpaivan, 1962]. The largest splash-form tektites (from the Philippines) reach 1 kg in size; the largest splash-form tektites from SE Asia have masses of ~350 g. Splash-form tektites are found on land from Guangxi, China, to Tasmania and as microtektites in sea-sediment cores from Taiwan to Madagascar [Glass, 1982]. Layered tektites have been recovered throughout a 1100-km-long band stretching from Hainan, China [Futrell and Wasson, 1993] to Cambodia [Wasson, 1991; Schnetzler, 1992]. Because those with masses >1 kg are too large Copyfight 1995 by the American Geophysical Union. Paper number 95JE01504. 0148-0227/95/95JE-01504505.00 to have undergone appreciable alluvial transport in low-gradient terrains, discovery sites are inferred to be quite close to the original formation locations. Because most Southeast...

Abstract— The site of an impact event that spread ejecta in the form of tektites and microtektites over ∼5 × 107 km2 of the southern Pacific and Indian Ocean area has not yet been discovered. A number of lines of evidence point toward a source in eastern Indochina. From an examination of a digital topographic data set and Landsat imagery, we identified four candidate structures in southern Laos, and we visited these sites in 1995 February. No evidence of impact origin of these structures could be found; flat‐lying, undisturbed Mesozoic sedimentary rocks similar to those on Thailand's Khorat Plateau were found over the region. Small layered tektite fragments are relatively common in a lateritic horizon that is characterized by the presence of quartz pebbles. This scene is identical to the situation found several hundred kilometers to the southeast in Thailand. New tektite sites identified on this trip support a previous suggestion that there is a large region in southern NE Thailand and Laos that is rich in Muong Nong‐type (layered) tektites but seemingly devoid of the splash‐form type tektites.