Search

The replacement of semidesert grassland by woody shrubland is a widespread form of desertification. This change in physiognomy and species composition tends to sharply reduce the productivity of the land for grazing by domestic livestock, increase soil erosion and reduce soil fertility, and greatly alter many other aspects of ecosystem structure and functioning. Remote sensing methods are needed to assess and monitor shrubland encroachment. Detection of woody shrubs at low density would provide a particularly useful baseline on which to access changes, because an initially low shrub density often tends to increase even after cessation of the disturbance (e.g., overgrazing, drought, or fire suppression) responsible for triggering the initial stages of the invasion (Grover and Musick, 1990). Limited success has been achieved using optical remote sensing. In contrast to other forms of desertification, biomass does not consistently decrease with a shift from grassland to shrubland. Estimation of green vegetation amount (e.g., by NDVI) is thus of limited utility, unless the shrubs and herbaceous plants differ consistently in phenology and the area can be viewed during a season when only one of these is green. The objective of this study was to determine if the potential sensitivity of active microwave remote sensing to vegetation structure could be used to assess the degree of shrub invasion of grassland. Polarimetric Airborne Synthetic Aperture Radar (AIRSAR) data were acquired for a semiarid site containing varied mixtures of shrubs and herbaceous vegetation and compared with ground observations of vegetation type and other landsurface characteristics. In this preliminary report we examine the response of radar backscatter intensity to shrub density. The response of other multipolarization parameters will be examined in future work.

A total of 912 impact craters between 1.5 and 280 km in diameter have now been identified on about 98% of Venus' surface. This total includes an additional 71 craters (mostly in the southern hemisphere) not in the crater inventory given in Schaber et al. for 89% of the planet. (Extrapolated to 100% of the surface, the inventory should consist of 930 craters greater than 1.5 km in diameter.) The total crater inventory for 98% of the planet is only slightly larger (912 versus 882) than that extrapolated from the inventory of 135 craters when only 15% of the planet had been mapped by Magellan. The remarkably consistent density of about 2.02 impact craters per 10(exp 6) sq km emphasizes the completely random nature of the crater population's distribution. About 400 craters recognized from Magellan mapping have been assigned names.

Schaber et al. compiled a database of 841 craters on Venus, based on Magellan coverage of 89 percent of the planet's surface. That database, derived from coverage of approximately 98 percent of Venus' surface, has been expanded to 912 craters, ranging in diameter from 1.5 to 280 km. About 150 of the larger craters were previously identified by Pioneer Venus and Soviet Venera projects and subsequently formally named by the International Astronomical Union (IAU). Altogether, the crater names submitted to the IAU for approval to date number about 550, a little more than half of the number of craters identified on Magellan images. The IAU will consider more names as they are submitted for approval. Anyone--planetary scientist or layman--may submit names; however, candidate names must conform to IAU rules. The person to be honored must be deceased for at least three years, must not be a religious figure or a military or political figure of the 19th or 20th century, and, for Venus, must be a woman. All formally and provisionally approved names for Venusian impact craters, along with their latitude, longitude, size, and origin of their name, will be presented at LPSC and will be available as handouts.

It is apparent that few, if any, craters as old, or highly modified as Imbrian craters on the surface of the moon are present on the Venusian highlands, or indeed anywhere on the planet's surface. Degraded craters such as those seen on the Moon, Mercury, or Mars are conspicuously absent. Furthermore, virtually all the impact craters on the Venusian surface show modification only by extensional tectonics, whereas the Venusian highlands show modification by compression, strike-slip movement, and finally by extension. Presumably at an earlier time, the surface of Venus resembled the surfaces of the other inner planets. The relatively recent resurfacing event that produced the plains units may be a mechanism for covering the older craters in the plains, but these vast outpourings of lava cannot be invoked as a mechanism for covering the craters on the Venusian highlands. If the plains units had covered the highlands, the entire Venusian surface would appear to be as smooth as the plains units that are embayed by and, therefore, postdate the highlands. The last major tectonic events to affect the highlands such as those seen on Ovda Regio appear to be thrust faulting with consequent folding, followed by at least three episodes of strike-slip faulting and finally extensional faulting. To test whether such tectonic movements could have destroyed highlands crater, a preliminary experiment was conducted by using a Lunar Orbiter mosaic of the lunar uplands as an analog for the ancient Venusian highlands.

Lobate features with a strong backscatter are associated with 43 percent of the impact craters cataloged in Magellan's cycle 1. Their apparent thinness and great lengths are consistent with a low-viscosity material. The longest outflow yet identified is about 600 km in length and flows from the 90-km-diameter crater Addams. There is strong evidence that the outflows are largely composed of impact melt, although the mechanisms of their emplacement are not clearly understood. High temperatures and pressures of target rocks on Venus allow for more melt to be produced than on other terrestrial planets because lower shock pressures are required for melting. The percentage of impact craters with outflows increases with increasing crater diameter. The mean diameter of craters without outflows is 14.4 km, compared with 27.8 km for craters with outflows. No craters smaller than 3 km, 43 percent of craters in the 10- to 30-km-diameter range, and 90 percent in the 80- to 100-km-diameter range have associated bright outflows. More melt is produced in the more energetic impact events that produce larger craters. However, three of the four largest craters have no outflows. We present four possible mechanisms for the emplacement of bright outflows. We believe this 'shotgun' approach is justified because all four mechanisms may indeed have operated to some degree.