Publications

The nature of the Venusian surface was revealed by Pioneer-Venus (PV) observations to be diverse at scales from tens to hundreds of kilometers. Spatial correlations of elevation, surface roughness, and radar reflectivity were investigated as a means of assessing the degree of homogeneity of surface radar properties within topographic provinces and develop a map of possible geologic boundaries. Correlations were performed in a supervised fashion whereby unit boundaries were selected on the basis of statistical and empirical studies of the individual data sets. Interpretations of these units in terms of geologic characteristics are based on the determination of physical units which are model dependent.

A large population of discrete volcanic edifices on Venus has been identified and cataloged by means of Magellan SAR images, and an extensive database describing thousands of such features is in final preparation. Those volcanoes categorized as Intermediate to Large in scale, while relatively small in number (approx. 400), nonetheless constitute a significant volumetric component (approx. 13 x 10(exp 6) cu km) of the total apparent crustal volume of Venus. For this reason, we have focused attention on the morphometry of a representative suite of the larger edifices on Venus and, in particular, on ways of constraining the eruptive histories of these possibly geologically youthful landforms. Our approach has been to determine a series of reproducible morphometric parameters for as many of the discrete volcanoes on Venus that have an obvious expression within the global altimetry data acquired by Magellan. In addition, we have attempted to objectively and systematically define the mathematical essence of the shapes of these larger volcanoes using a polynomial cross-section approximation involving only parameters easily measured from digital topography, as well as with simple surface cylindrical harmonic expansions. The goal is to reduce the topological complexities of the larger edifices to a few simple parameters which can then be related to similar expressions for well-studied terrestrial and martian features.

A laser ranging instrument has been developed for altimetry measurements of the Earth's surface along the nadir track of the Space Shuttle. The Shuttle Laser Altimeter is designed for transmission of a short laser pulse and reception of the backscattered laser radiation from the Earth's surface. The source laser is a Q-switched Nd:YAG operating at its fundamental wavelength of 1.064 μm. A reflector telescope and silicon avalanche photodiode are the basis of the altimeter receiver. Laser, telescope, detector, data processing and storage electronics, and dc power supplies are packaged for spaceflight into two adjacent Get-Away-Special canisters. This provides the basis for a very compact and low-cost interface to the Space Shuttle.

Simple impact craters are known to occur on all of the terrestrial planets and the morphologic expression of their ejecta blankets is a reliable indicator of their relative ages on the Moon, Mars, Mercury, and most recently for Venus. It will be crucial for the interpretation of the geology of Venus to develop a reliable means of distinguishing smaller impact landforms from volcanic collapse and explosion craters, and further to use the observed SAR characteristics of crater ejecta blankets (CEB) as a means of relative age estimation. With these concepts in mind, a study was initiated of the quantitative SAR textural characteristics of the ejecta blanket preserved at Meteor Crater, Arizona, the well studied 1.2 km diameter simple crater that formed approx. 49,000 years ago from the impact of an octahedrite bolide. While Meteor Crater was formed as the result of an impact into wind and water lain sediments and has undergone recognizable water and wind related erosion, it nonetheless represents the only well studied simple impact crater on Earth with a reasonably preserved CEB. Whether the scattering behavior of the CEB can provide an independent perspective on its preservation state and style of erosion is explored. Finally, airborne laser altimeter profiles of the microtopography of the Meteor Crater CEB were used to further quantify the subradar pizel scale topographic slopes and RMS height variations for comparisons with the scattering mechanisms computed from SAR polarimetry. A preliminary assessment was summarized of the L-band radar scattering mechanisms within the Meteor Crater CEB as derived from a NASA/JPL DC-8 SAR Polarimetry dataset acquired in 1988, and the dominant scattering behavior was compared with microtopographic data (laser altimeter profiles and 1:10,000 scale topographic maps).

We compared the target types and the morphologies and morphometries of various features within fresh complex craters on Mars to assess target dependence. The wide scatter in depth-diameter data from Martian craters is more pronounced than for lunar or Mercurian craters. This was previously assumed to be predominantly due to significant degrees of denudation and secondary infilling of the Martian craters. However, our data for fresh craters still exhibit a wide variation, which we interpret to be the result of comparatively higher target heterogeneity on Mars. Complex central peaks exhibit some crater diameter dependence, preferentially occurring in craters >50 km. Neither peak complexity nor geometry shows any statistical correlation with target type. Although central peak heights and aspect ratios do not exhibit any clear target dependence, they do appear to be correlated—higher peaks possess narrower aspect ratios. Floor and summit pits appear to be more common on lava targets than sedimentary targets, contrary to earlier studies with smaller sample sizes. This observation imposes additional constraints on models proposed for the origin of pits, especially those models that require the presence of volatiles in the target.

Spacecraft landings on Mars and Venus provide an opportunity to seek and discover transient dust phenomena with various instruments, and to make inferences about local-scale surface modification processes. The present study is concerned with such dust phenomena. In addition, an attempt is made to relate all of the landing sites on Venus to each other in terms of dust phenomena. On Mars a difference in the magnitude of dust effects was observed between the Viking Lander 1 (VL-1) and Viking Lander 2 (VL-2) sites, which is due to either a particle size variation, or the degree of surface blockiness. It appears that the blocks at VL-2 have somewhat protected the surface fines from being raised by retroengine exhaust. The Venera 10 panorama contains a zone of darkening similar to that in the VL-1 zone.

All‐weather spaceborne imaging of isolated oceanic islands is opening the door to some new, previously unfeasible studies of global environmental change.The first orbital synthetic aperture radar (SAR) views of some relatively small,yet important islands have provided icecover and other data that until recently have been impossible to gather.These islands, whose geologic histories are dominated by the interactions of volcanism, glacier dynamics, and marine erosion, are often cloud enshrouded but can be monitored by SAR regardless of cloud cover or solar illumination. Oceanic islands are often overlooked components of the global Earth system. Yet their landscapes and ecosystems provide natural laboratories for investigating the effects of various natural and anthropogenic forcing factors in an essentially closed, controlled environment.