The determination of the timing of impact events and the ages of cratered planetary surfaces is a complex and challenging undertaking. A powerful approach to this endeavor is a multidisciplinary study; understanding and using data from both rock samples and remote sensing. Shocked material (especially melt material) found in meteorites like Orvinio provides critical material dating impact craters. Orvinio, in spite of a complex degassing history shows evidence for multiple impacts at 4.2 Ga, 7.5 Ma, and possibly 330 Ma. Correlating impact histories for inner solar system bodies and the asteroid belt will constrain the genesis of impactor populations. Determining the recent cratering history of the Earth, however, is complicated by surface processes which erode and destroy impact craters. The Gardnos impact structure, for example, while possessing samples suitable for dating, has suffered substantial post-impact degassing due to metamorphism in the Caledonian orogeny ∼385 Ma. We must therefore look to the Moon to unravel the recent cratering history of the Earth-Moon system The Clementine mission data set provides an excellent resource for research into the bright rayed craters on the lunar surface. Studies of large rayed craters using the OMAT (optical maturity parameter) technique of Lucey and colleagues has revealed much information on the maturation of the crater ejecta. Profiles of OMAT values for the ejecta of large craters as a group show no evidence for an increase in the cratering rate during the Copernican era as advocated by Shoemaker. Future studies of both remote sensing and sample data will allow a better understanding of meteorite parent body impact histories and their implications for widespread epochs of increased impactor flux; the age-size correlation in lunar craters; the calibration of a large crater relative age scheme based on optical maturity with implications for the nature of the impactor flux in recent history; the nature of the recent small impactors on the lunar surface; implications for impact hazards on Earth today; and the best target sites for future landings and sample acquisition on the lunar surface.

No description available.

This thesis is concerned with the dynamics of the middle atmosphere over the southern-most continent on our planet, Antarctica. Building on previous observational and theoretical efforts to understand the dynamics of this region of the atmosphere, the work encompasses instrument improvements as well as data analysis studies of gravity waves in the lower and middle stratosphere, tides in the upper mesosphere and lower thermosphere, and short-period planetary waves in the stratosphere and mesosphere. An upgrade of the Medium-Frequency Spaced Antenna (MFSA) radar at Scott Base, Antarctica, was carried out in 2004/5 in order to ensure continued operation and enhance its capabilities to measure gravity waves. As a result, the quality of the wind measurements was greatly enhanced and the amount of data collected is now greater by a factor of approximately 15 compared with before the upgrade. Analysis of over two decades of wind velocity data from Scott Base yields a reliable climatology of the dynamics of the upper mesosphere and lower thermosphere in this area, namely the tidal oscillations and the background winds. In addition, interannual variability is assessed and periodicities of approximately 11 years and strong positive trends in tidal amplitudes are reported. Mechanisms that could explain the observed behaviour are proposed. The data are then combined with wind measurements from Halley, the longitudinal conjugate site, in order to study the zonal character of the semi-diurnal tide. Zonal wavenumber 1 and 2 waves are both found and mechanisms that could explain the generation of a wavenumber 1 component are suggested. Two further sets of MFSA radar wind measurements are used to investigate the behaviour of planetary waves with periods of between two and four days in the Antarctic middle atmosphere. Satellite temperature measurements further help to create a more complete picture of these waves. Baroclinic and barotropic instabilities, which result from shears of the zonal wind, appear to be responsible for much of the observed wave activity. In addition, a quasi-to day wave event in mid-May 2005 with unusually large amplitudes is examined and suggested to be linked to a solar proton event. Gravity wave activity over Antarctica is studied using temperature profiles obtained through the satellite radio occultation technique. Although the measurements are restricted to below 35 km altitude, high-resolution temperature profiles allow conclusions to be drawn about the seasonal, geographical, and height distribution of gravity wave activity. Mountain waves are found to be important over the Antarctic Peninsula and the Transantarctic mountains where they contribute more than 20% of the observed wave activity in the lower stratosphere. In addition, the analysis indicates the importance of critical-level filtering and Doppler-shifting.

A dynamic spatial model of deforestation and land-use change was developed from remotely sensed data for the Fouta Djallon mountain range in the Republic of Guinea, West Africa. The objective was to simulate patterns of land clearing for shifting cultivation in terms of farmers' selection behavior for new fields based on topography and proximity to villages. Data describing the current and historic condition of the vegetation cover, land use, and erosion for a watershed in Guinea were derived from aerial photography and ground sampling. Maps of these conditions were prepared and entered in a geographic information system (GIS) together with topographic data. From these data, maps of secondary variables (slope, village proximity, site productivity, and labor) were derived using the spatial operators contained in the GIS. These variables were ranked for agricultural preference and combined following a pair-wise hierarchy to generate a composite agricultural site-preference surface. This ranking was done in iterations, using a two-year time increment, which corresponds to the typical duration of cultivation for any one field. Different variable combinations and underlying assumptions of model logic were tested to determine influence on simulation results. To validate the model, the projected landscape was compared with land-use data collected in 1989. Although the model did not simulate the farmers' selection behavior for topography and village proximity successfully, test results with individual variables suggest that site productivity as determined by the length of fallow is a critical variable in the site selection process. The addition of site quality data should improve model results. The watershed in which this study was performed is the focus of a development initiative supported by the U.S. Agency for International Development (USAID), in which viable options are being sought for regional application. Thus, aside from documenting the dynamics of shifting cultivation, this model allows planners to evaluate alternative strategies of land-use conversion with a graphic display of zones of potential hazards. Finally, the data contained in the GIS serve as a structure for monitoring long-term change in the region.

Decades of overgrazing have left many rangelands in northwestern Mexico in poor condition. This has led to the practice of converting native rangeland plant communities to buffelgrass pastures. Buffelgrass (Pennisetum ciliare) is a perennial bunchgrass native to Africa. Both the extent of buffelgrass pastures within Mexico and the impacts of land conversion on biodiversity and ecosystem functioning. In the present study I address the effects of land conversion on the productivity and diversity of rangelands in the southern Sonoran Desert in the state of Sonora, Mexico. First, using satellite imagery from the Landsat mission, I found that rates of land conversion in the most heavily affected region of Sonora have continued to accelerate over the past three decades and that productivity of buffelgrass pastures is lower than that of native rangeland. Next, I examined the impacts of land conversion on the diversity and structure of plant communities and ant assemblages across a rainfall driven gradient of productivity in central Sonora. The regional extent of this land use change allowed me to explore the interaction between site productivity and land conversion. Within native rangeland I detected strong positive relationships between productivity and the species richness of perennial plant communities, but only weak positive relationships between productivity and species richness of ant assemblages. These results were discussed in the context of species diversity theory. Land conversion reduced the species richness of perennial plant communities by approximately 50% at both local and regional scales, whereas the species richness of ant assemblages was reduced by 17% at the local scale and only 8% at the regional scale. I found no evidence for an interaction between site productivity and land conversion in either plant communities or ant assemblages. The implications of these findings for long-term trajectories of biodiversity in the southern Sonoran Desert are discussed.

No description available.

Reflectance and temperature of bare field soil were measured to determine the relationship between soil water content and spectral reflectance and temperature. Reflectance in the six reflective Thematic Mapper (TM) wavebands plus a 1.15 to 1.30 μm waveband (referred to as MMR5) was measured using a ground-based radiometer across a soil water gradient provided by a line source sprinkler system. Temperature was measured using a hand-held infrared thermometer. The results of this study show that TM band 7 reflectance (2.05-2.30 μm) provided improved estimates of surface soil water content (0-0.5 cm depth) over estimates using reflectance information from all seven TM bands. Good correlations were found between band ratio spectral indices of TM5/TM7, MMR5/TM7, and MMR5/TM5 and surface soil water content. A soil temperature normalization utilizing daily values of maximum vapor pressure deficit was found to provide improved estimates of soil water content to depths greater than 0.5 cm over estimates using other temperature normalizations.

The absolute calibration of remote sensing systems is based upon radiometric standards. The irradiance standards are currently 1000-watt quartz tungsten-halogen lamps that are traced to National Institute of Standards and Technology standards by a middle party at a significant cost. Lamp instability, short lifetime and calibration inaccuracy present problems. A detector-based absolute calibration technique is described here to replace the lamp calibration in the visible portion of the spectrum (approximately 400 to 700 nm). The system utilizes a quantum efficient QED-200 trap detector in an automated facility to absolutely characterize lamp spectral irradiances or reflectance panel spectral radiances. The measured irradiance of a directly-viewed standard lamp compared favorably (within 0.8%) to the original calibration. Standard-reflectance-panel radiance measurements could not be accurately compared (within 5%) to the illuminating standard lamp irradiance because of excessive stray light in the calibration facility.

Remote sensing and geographic information system techniques have proved to be effective tools to detect, analyze, and evaluate land cover and land use changes over time. In this research project, changes in land cover and land use were detected in northwestern Sonora, Mexico between 1972 and 1992 using Landsat MSS imagery. About 40% of the entire land cover in the study area changed during that period of time. Of the six classes assigned to the imagery, cropland had the highest rate of change being modified into riparian areas by more than 60%, more than 20% into plains vegetation, and about 8% into bajadas with vegetation. From the two classification methods utilized in this study, the seeding pixels method yielded an over all accuracy over 96%, while the seeking polygons method generated overall accuracy values smaller than 82% probably to user's error.

No description available.