Volcanic and Tectonic Processes    ===============================      Magellan images of the Venus surface show widespread evidence      for volcanic activity.  A major goal of the Magellan mission was      to provide a detailed global characterization of volcanic      landforms on Venus and an understanding of the mechanics of      volcanism in the Venus context.  Of particular interest was the      role of volcanism in transporting heat through the lithosphere.      While this goal will largely be accomplished by a careful      analysis of images of volcanic features and of the geological      relationships of these features to tectonic and impact      structures, an essential aspect of characterization will be an      integration of image data with altimetry and other measurements      of surface properties.       Explosive pyroclastic volcanism should not occur in the present      Venus environment, unless the magma contains amounts of      volatiles that are large by terrestrial experience.  Thus,      evidence for extensive pyroclastic deposits would imply the      presence of large amounts of volatiles or, if the deposits are      old, may suggest historic changes in atmospheric density.  Such      ideas can be tested using SAR and altimetry data, combined with      knowledge of the local geopotential field and may shed light on      magma dynamics.  Measurements of longitudinal and transverse      slope, flow margin relief, and flow surface relief also provide      powerful constraints on flow models, as well as on the      rheological properties and physical state of the lava.       A parallel goal was the global characterization of tectonic      features on Venus and an appreciation of the tectonic evolution      of the planet.  This goal addressed issues on several scales.      On the scale of individual tectonic features is the mechanical      nature of the faulting process, the documentation of geometry      and sense of fault slip, and the relationship between mechanical      and thermal properties of the lithosphere.  On a somewhat      broader scale is linking groups of features to specific      processes (e.g., uplift, orogeny, gravity sliding, flexure,      compression or extension of the lithosphere) and testing      quantitative models for these processes with SAR images and      supporting topographic, gravitational, and surface compositional      data.  On a global scale is the question of whether spatially      coherent, large-scale patterns in tectonic behavior are      discernible, patterns that might be related to an organized      system of plates or to mantle convective flow       For more information on volcanic and tectonic investigations see      papers by [HEADETAL1992] and [SOLOMONETAL1992], respectively.      Impact Processes    ================      The final physical form of an impact crater has meaning only      when the effects of the cratering event and any subsequent      modification of the crater can be distinguished.  To this end, a      careful search of the SAR images can identify and characterize      both relatively pristine and degraded impact craters, together      with their ejecta deposits (in each size range) as well as      distinguishing impact craters from those of volcanic origin.      The topographic measures of depth-to-diameter ratio, ejecta      thickness distribution as a function of distance from the      crater, and the relief of central peaks contribute to this      documentation.       It is expected that several time-dependent processes influence      the change in appearance of craters with increasing crater age,      including continued bombardment of the surface, variations in      the mechanical properties of the lithosphere (as a result of      cooling or loss of near-surface volatiles), horizontal      deformation of the lithosphere, possible variations in the mass      of the atmosphere, volcanism, and finally, surface erosion and      deposition.  Distinguishing and understanding these processes      constitute important components of the study of crater      morphology.       Beyond their intrinsic interest in providing a record of impact      and deformational processes, craters provide a tool for the      relative dating of surface geological units.  Relative ages can      be established from a comparison of the variations in the areal      density of craters of a given size as well as from a comparison      of the maximum extent to which different craters are degraded.      Together with superpositional relationships (a lava flow that      covers an older fault) and transectional relationships (a graben      that cuts through an older volcano), the relative temporal      evolution of large areas of the Venus surface can be      reconstructed.       For more information on investigations of impact processes see      [SCHABERETAL1992].      Erosional, Depositional, and Chemical Processes    ===============================================      The nature of erosional and depositional processes on Venus is      poorly known, primarily because the diagnostic landforms      typically occur at a scale too small to have been resolved in      Earth-based or Venera 15/16 radar images.  Magellan images show      wind eroded terrains, landforms produced by deposition (dune      fields), possible landslides and other down slope movements, as      well as aeolian features such as radar bright or dark streaks      'downwind' from prominent topographic anomalies.  One measure of      weathering, erosion, and deposition is provided by the extent to      which soil covers the surface (for Venus, the term soil is used      for porous material, as implied by its relatively low value of      bulk dielectric constant).  The existence of such material, and      its dependence on elevation and geologic setting, provide      important insights into the interactions that have taken place      between the atmosphere and the lithosphere.       Because of the inference drawn from the deuterium-to-hydrogen      ratio of the present atmosphere for the past existence of      substantial amounts of water on Venus, radar images continue to      be searched for evidence of past episodes of fluvial activity      (drainage systems) and for lake beds and coastal signatures      (strandlines).       The existence of a thick and cloudy atmosphere precludes      infrared, visual, ultraviolet, x-ray, or gamma-ray observation      of the Venus surface from orbit.  Thus it is impossible to      obtain information on a global basis about the surface      composition or mineralogy using remote-sensing techniques at      these wavelengths.  Pioneer Venus and Magellan have disclosed      that very often the surfaces of elevated regions possess both      anomalously high values of normal-incidence radar reflectivity,      occasionally exceeding 0.43, and associated low values of radio      emissivity, reaching as low as 0.50.  In the absence of liquid      water, which is known from a variety of evidence not to be      present today on Venus, it is necessary to assume a surface      composition that would be unusual in terrestrial experience to      explain values of dielectric constant implied by these      observations.  The most acceptable of the current hypotheses      requires a significant number of electrically conducting      elements in surface materials.  If these are iron sulfides, as      some chemical evidence suggests, they may possibly be brought to      the surface by volcanic activity.  The good spatial resolution      of the Magellan instrumentation, both in determining the surface      reflectivity from the altimetric observations and in measuring      the emissivity from radiometric observations, promises to      outline the structure of these regions and may shed light on      their origin.  Results will be applied to testing hypotheses for      regional and global buffering of atmospheric composition by      reactions with crustal materials.       For more information on erosional, depositional, and chemical      processes see papers by [ARVIDSONETAL1992], [GREELEYETAL1992],      and [GREELEYETAL1994].      Isostatic and Convective Processes    ==================================      Topography and gravity are intimately and inextricably related,      and must be jointly examined when undertaking geophysical      investigations of the interior of a planet, where isostatic and      convective processes dominate.  Topography provides a surface      boundary condition for modeling the interior density of Venus.       Modeling of the interior density using gravity data is, of      course, nonunique.  Meaningful interpretation rests on      integrating other data sets and/or incorporating specific      mechanical models of the interior.  For example, a single      density interface underlying the known topography can be found      that exactly matches any observed gravity field.  The interface      can be at any depth; the greater the depth, the larger the      density contrast needed.       The thickness of the elastic lithosphere of Venus, i.e., the      outer region of the planet that behaves elastically over      geologically long periods of time, is of special interest.  The      base of this zone is likely to be defined by a specific isotherm      whose location depends on the particular temperature-dependent      flow or creep properties of the material underneath.  If this      isotherm can be mapped in space and time, then models for the      thermal evolution of the planet can be developed.       The key to determining lithospheric thickness variations in      space and time is through flexure studies.  If a mass load,      e.g., a shield volcano or a mascon, is placed on the planetary      surface, then the elastic lithosphere will flex under the load.      The controlling parameter is the flexural rigidity, which is      dependent on the elastic constants and lithospheric thickness.       Crucial to applying estimates of flexural rigidity to the task      of unraveling the thermal history is an estimate of when the      load was emplaced.  Thus age determinations derived by various      geologic techniques are essential to this scheme.       For more information on topography and gravity see papers by      [FORD&PETTENGILL1992], [ICARUSMGN1994], [KONOPLIVETAL1993],      and [MCNAMEEETAL1993].