The potential is described of a candidate Mars Observer altimeter for determining dielectric properties of Mars regolith. It is pointed out that it is straightforward to use the time between altimeter pulse trains for passive radiometry (hence dielectric properties) and roughness can be derived. Given the mission plan the whole surface can be mapped at least three times, yielding data on seasonal variability.
Analysis of a time series of European Remote Sensing Satellite (ERS)-1 and -2, RADARSAT ScanSAR synthetic aperture radar (SAR) and Landsat images from 1973 to 1998, shows daily to interannual changes in Hofsjokull, a 923 sq km ice cap in central Iceland. A digital elevation model of Hofsjokull was constructed using interferometry, and then SAR backscatter coefficient (d) was plotted with elevation, and air temperature along a transect across the ice cap. Most of the a' changes measured along the transect are caused by a change in the state (frozen or thawed) of the surficial snow or ice when air temperature rises above or below about -5 to O C. Seasonal (sigma)deg patterns are identified in a 4-year time series of 57 ERS-1 and -2 images. In addition, June 1997 ScanSAR images display rapid changes in brightness that are tied closely to daily meteorological events. SAR and Landsat data were also used to measure changes in the areal extent of Hofsjokull, from 1973 to 1997, and to locate (sigma)deg and reflectance boundaries that relate to the glacier facies. Late-summer 1997 (sigma)deg and reflectance boundaries agree and are coincident with the approximate location of the fim line, and the January 1998 position of the equilibrium line as determined from ERS-2 data.
The three primary data sets for the Pioneer Venus orbiter radar experiment (topography, roughness, and reflectivity) contain important information about the geological and textural characteristics of the surface of Venus. We have subdivided the range of roughness and reflectivity values into three categories as follows: roughness, in degrees rms slope: relatively smooth (1°–2.5°), transitional from smooth to rough (2.5°–5°), and relatively rough (>5°); and Fresnel reflectivity: surfaces dominated by soil or porous material (<0.1), surfaces dominated by rock material (0.1–0.2), and surfaces with a significant percentage of anomalously high dielectric material (>0.2). We have analyzed each of these data sets and their relationships to each other in order to define areas of the surface that are characterized by distinctive properties (e.g., rough rocky surfaces, smooth soil surfaces). We then describe the abundance and areal distribution of such areas and locally calibrate the geological significance of some of the surface types by examining high‐resolution images from spacecraft and earth‐based observatories. We find that the majority of Venus is covered by regionally contiguous rock and bedrock surfaces. Many of the smooth surfaces we interpret to be of volcanic origin, most likely lava flows, while rougher surfaces are locally characterized by tectonic deformation of several types. Soil surfaces cover less than about 27% of the planet and are generally patchy in their distribution. On the basis of the distribution of these surfaces we see no evidence for the extensive preservation of an ancient global regolith or for widespread, topographically controlled erosion, lateral transport, and sedimentation. The small percentage of the surface of Venus characterized by high‐dielectric material appears to originate from several processes including primary lava flows probably containing enrichments of high‐dielectric materials, such as metal or metal oxides (e.g., Theia Mons in Beta Regio), and exposure of high‐dielectric materials by tectonic deformation (e.g., Maxwell Montes in Ishtar Terra). These global data set correlations provide a fundamental frame‐work for understanding the nature of the surface of Venus and will permit extrapolation of local and regional findings from future geochemical and imaging experiments to a global context.
Near-polar craterforms have been observed on Mars since Mariner 9. Hodges and Moore [1,2] described the implications of these apparently volcanic features and suggested that several were formed as a consequence of magma-ice or magma-water interactions. Indeed, they proposed that the feature illustrated in Fig. I represents a hydromagmatic explosion crater or maar, indicating interaction of ground-ice and/or water with magma in the higher Northern hemisphere latitudes of Mars. The advent of Mars Orbiter Laser Altimeter (MOLA) topographic information 161 for several of the putative volcanic craterforms described in [1,2] allows a reconsideration of their formation mechanisms. We have exploited MOLA's near-centerline cross-sections from the Science Phasing Orbit phase of the Mars Global Surveyor (MGS) mission to analyze the geometric properties of these features as constraints on their modes of origin and subsequent modification histories. From our preliminary analyses, we believe there is evidence these features may have been formed by effusive, lava shield building eruptions, and not hydromagmatic events. Furthermore, geometrical constraints indicate that some of the near-polar craterforms are geologically youthful.
The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI) mission described herein has been selected for flight to Venus as part of the NASA Discovery Program. DAVINCI will be the first mission to Venus to incorporate science-driven flybys and an instrumented descent sphere into a unified architecture. The anticipated scientific outcome will be a new understanding of the atmosphere, surface, and evolutionary path of Venus as a possibly once-habitable planet and analog to hot terrestrial exoplanets. The primary mission design for DAVINCI as selected features a preferred launch in summer/fall 2029, two flybys in 2030, and descent sphere atmospheric entry by the end of 2031. The in situ atmospheric descent phase subsequently delivers definitive chemical and isotopic composition of the Venus atmosphere during a cloud-top to surface transect above Alpha Regio. These in situ investigations of the atmosphere and near infrared descent imaging of the surface will complement remote flyby observations of the dynamic atmosphere, cloud deck, and surface near infrared emissivity. The overall mission yield will be at least 60 Gbits (compressed) new data about the atmosphere and near surface, as well as first unique characterization of the deep atmosphere environment and chemistry, including trace gases, key stable isotopes, oxygen fugacity, constraints on local rock compositions, and topography of a tessera.
