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Mission Information

MISSION_NAME MAGELLAN
MISSION_NAME_OR_ALIAS VENUS RADAR MAPPER (VRM)
MISSION_START_DATE 1989-05-04
MISSION_STOP_DATE 1994-10-12
MISSION_DESCRIPTION
 
    Mission Overview
    ================
      The Magellan spacecraft was launched from the Kennedy Space
      Center on 4 May 1989.  The spacecraft was deployed from the
      Shuttle cargo bay after the Shuttle achieved parking orbit.
      Magellan, using an inertial upper stage rocket, was then placed
      into a Type IV transfer orbit to Venus where it carried out
      radar mapping and gravity studies starting in August 1990.  The
      Mission has been described in many papers including two special
      issues of the Journal of Geophysical Research [VRMPP1983;
      SAUNDERSETAL1990; JGRMGN1992].  The radar system is also
      described in [JOHNSON1990].
 
      Magellan was powered by single degree of freedom, sun-tracking,
      solar panels.  The spacecraft was 3-axis stabilized by reaction
      wheels using gyros and a star sensor for attitude reference.
      The spacecraft carried a solid rocket motor for Venus orbit
      insertion.  A small hydrazine system was used for trajectory
      corrections and certain attitude control functions.  Earth
      communication with the Deep Space Network (DSN) was by means of
      S- and X-band channels.  The high-gain antenna also functioned
      as the SAR mapping antenna during orbital operations.
 
      The interplanetary cruise phase lasted until 10 August 1990.
      During the cruise phase there were small trajectory correction
      maneuvers to ensure proper approach geometry.  Using the solid
      rocket motor, the spacecraft was placed into an elliptical orbit
      around the planet, with a periapsis latitude of approximately 10
      degrees north, a periapsis altitude of 295 km, a period of 3.263
      hours, and an apoapsis altitude of approximately 7762 km.
 
      After orbit insertion, the radar system acquired test data.
      Then, unexpectedly, the signal from the spacecraft was lost
      twice.  Following an intense recovery process, commands were sent
      to avoid further communication interruptions, and the spacecraft
      resumed mapping operations on 15 September 1990.
 
      Each mapping cycle lasted 243 days, which was the time required
      for Venus to make one rotation under the spacecraft orbit.  The
      first mapping cycle ended on 15 May 1991.  Typical activities
      during a single mapping pass on Cycle 1 were as follows.  As the
      spacecraft neared periapsis, it was oriented so the high-gain
      antenna pointed slightly to the side of the ground track.  At a
      true anomaly of -59 degrees, the radar was commanded on.  The
      radar continued to take data to a true anomaly of 80 degrees and
      then the radar was commanded off.  On the next pass the swath
      started at -80 degrees and went to 59 degrees.  Alternating
      north and south swaths were repeated throughout Cycle 1.
 
      The range of latitudes covered by the synthetic aperture radar
      (SAR) during Cycle 1 was 67 degrees S to 90 degrees N.  The
      range of SAR incidence angles was from just under 20 to just
      over 40 degrees.  The SAR data were taken at a data rate of 750
      kilobits/second and were stored in the spacecraft tape recorder.
      Altimeter and radiometer data were also taken when SAR data were
      acquired.  The altimeter data were taken using a small fan beam
      antenna at a data rate of 30 kb/s.  As the spacecraft moved away
      from the planet toward apoapsis, the spacecraft reoriented the
      high-gain antenna towards Earth and the stored radar data were
      transmitted to DSN stations.  This data taking- and
      transmitting-cycle was repeated for every orbit.  By 15 May
      1991, the planet had been completely mapped except for the area
      near the South Pole and a few regions which had been missed
      because of temporary equipment failures.
 
      Cycle 2 observations focused on filling the gaps in Cycle 1
      coverage (including the south pole area), acquiring SAR data at
      a constant incidence angle (25 degrees), and conducting a suite
      of ad hoc experiments, including high resolution imaging and
      radar stereo.  To observe the south pole the spacecraft was
      rotated 180 degrees about its nadir-pointing axis so as to
      conduct right-looking SAR observations.  Gaps in the Cycle 1
      coverage were filled by rotating the spacecraft back to its
      initial left-looking direction.  The orbit plane was adjusted
      slightly at the beginning of Cycle 2 so that altimetry tracks
      would be offset by about 10 km at the equator, bisecting the
      orbit-to-orbit offset of altimetry tracks in Cycle 1.  The
      spacecraft was rotated 90 deg about the HGA boresight on orbits
      3716-3719 to obtain SAR and radiometry data with VV
      polarization.  Radio occultation measurements were made on
      orbits 3212-3214.
 
      The principal objective of Cycle 3 was to perform radar stereo
      mapping of the Venusian surface.  About 30 percent of the Cycle
      1 coverage was remapped in this cycle with a different,
      left-looking incidence angle on the surface.  Gravity data were
      collected over Artemis Chasma.  In addition, high resolution
      altimetry data were collected by pointing the high gain antenna
      straight down during orbits 4919 to 4921.  Transmission of
      acquired radar data to Earth became nearly impossible after
      spacecraft equipment failures late in Cycle 3, and the radar was
      not used for science purposes after that.
 
      Cycle 4 was used for full (360 degree) longitudinal collection
      of gravity data because of favorable planetary and spacecraft
      geometry.  The cycle was extended by about ten days to
      compensate for passage of the radio ray through the Venus
      atmosphere during the first ten days.  To improve sensitivity to
      gravity features, orbit periapsis was lowered on orbit 5752.
      Radio occultation measurements were made on orbits 6369, 6370,
      6471, and 6472.
 
      The aerobraking phase of the mission was designed to change the
      Magellan orbit from eccentric to nearly circular.  This was
      accomplished by dropping periapsis to less than 150 km above the
      surface and using atmospheric drag to reduce the energy in the
      orbit.  Aerobraking ended on 3 August 1993, and periapsis was
      boosted above the atmosphere leaving the spacecraft in an orbit
      that was 540 km above the surface at apoapsis and 197 km above
      the surface at periapsis.  The orbit period was 94 minutes.  The
      spacecraft remained on its medium-gain antenna in this orbit
      until Cycle 5 began officially on 16 August 1993.
 
      During Cycles 5 and 6 the orbit was low and approximately
      circular.  The emphasis was on collecting high-resolution
      gravity data.  Two bistatic surface scattering experiments were
      conducted, one on 6 October (orbits 9331, 9335, and 9336) and
      the second on 9 November (orbits 9846-9848).
 
 
    Mission Phases
    ==============
      Mission phases were defined for significant spacecraft activity
      periods.  During orbital operations a 'cycle' was approximately
      the time required for Venus to rotate once under the spacecraft
      (about 243 days).  But there were orbit adjustments and other
      activities that made some mapping cycles not strictly contiguous
      and slightly longer or shorter than the rotation period.
 
 
      PRELAUNCH
      ---------
        The prelaunch phase extended from delivery of the spacecraft
        to Kennedy Space Center until the start of the launch
        countdown.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1988-09-01
        Mission Phase Stop Time        : 1989-05-04
        Spacecraft Operations Type     : ORBITER
 
 
      LAUNCH
      ------
        The launch phase extended from the start of launch countdown
        until completion of the injection into the Earth-Venus
        trajectory.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1989-05-04
        Mission Phase Stop Time        : 1989-05-04
        Spacecraft Operations Type     : ORBITER
 
 
      CRUISE
      ------
        The cruise phase extended from injection into the Earth-Venus
        trajectory until 10 days before Venus orbit insertion.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1989-05-04
        Mission Phase Stop Time        : 1990-08-01
        Spacecraft Operations Type     : ORBITER
 
 
      ORBIT INSERTION
      ---------------
        The Venus orbit insertion phase extended from 10 days before
        Venus orbit insertion until burnout of the solid rocket
        injection motor.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1990-08-01
        Mission Phase Stop Time        : 1990-08-10
        Spacecraft Operations Type     : ORBITER
 
 
      ORBIT CHECKOUT
      --------------
        The orbit trim and checkout phase extended from burnout of the
        solid rocket injection motor until the beginning of radar
        mapping.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1990-08-10
        Mission Phase Stop Time        : 1990-09-15
        Spacecraft Operations Type     : ORBITER
 
 
      MAPPING CYCLE 1
      ---------------
        The first mapping cycle extended from completion of the orbit
        trim and checkout phase until completion of one cycle of radar
        mapping (approximately 243 days).  Mapping orbits included in
        the first cycle were 373 through 2165.  Orbits 2159-2171 were
        used for an interferometry test, and orbits 2172-2175 were used
        to conduct an orbit trim maneuver (OTM).
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1990-09-15
        Mission Phase Stop Time        : 1991-05-15
        Spacecraft Operations Type     : ORBITER
 
 
      MAPPING CYCLE 2
      ---------------
        The second mapping cycle extended from completion of the first
        mapping cycle through an additional cycle of mapping.
        Acquisition of 'right-looking' SAR data was emphasized.
        Orbits included in the second cycle were 2176 through 3976.
        Radio occultation measurements were first carried out
        on orbits 3212-3214.  A period of battery reconditioning
        followed completion of Cycle 2.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1991-05-16
        Mission Phase Stop Time        : 1992-01-17
        Spacecraft Operations Type     : ORBITER
 
 
      MAPPING CYCLE 3
      ---------------
        The third mapping cycle extended from completion of battery
        reconditioning through an additional cycle of mapping
        (approximately 243 days).  Acquisition of 'stereo' SAR data
        was emphasized.  Orbits included in the third cycle were
        4031 through 5747.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1992-01-24
        Mission Phase Stop Time        : 1992-09-14
        Spacecraft Operations Type     : ORBITER
 
 
      MAPPING CYCLE 4
      ---------------
        The fourth mapping cycle extended from completion of the third
        mapping cycle through an additional cycle of mapping.
        Acquisition of radio tracking data for gravity studies was
        emphasized.  Radio occultation measurements were carried out
        on orbits 6369, 6370, 6471, and 6472.  Because of poor
        observing geometry for gravity data collection at the
        beginning of the cycle, this cycle was extended 10 days beyond
        the nominal 243 days.  Orbits included within the fourth cycle
        were 5748 through 7626.  Periapsis was lowered on orbit 5752
        to improve sensitivity to gravity features in Cycle 4.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1992-09-14
        Mission Phase Stop Time        : 1993-05-25
        Spacecraft Operations Type     : ORBITER
 
 
      AEROBRAKING
      -----------
        The aerobraking phase extended from completion of the fourth
        mapping cycle through achievement of a near-circular orbit.
        Circularization was achieved more quickly than expected; the
        first gravity data collection in the circular orbit was not
        scheduled until 11 days later.  Orbits included within the
        aerobraking phase were 7627 through 8392.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1993-05-26
        Mission Phase Stop Time        : 1993-08-05
        Spacecraft Operations Type     : ORBITER
 
 
      MAPPING CYCLE 5
      ---------------
        The fifth mapping cycle extended from completion of the
        aerobraking phase through an additional cycle of mapping
        (approximately 243 days).  Acquisition of radio tracking data
        for gravity studies was emphasized.  The first orbit in the
        fifth cycle was orbit 8393, and the last was orbit 12248.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1993-08-16
        Mission Phase Stop Time        : 1994-04-15
        Spacecraft Operations Type     : ORBITER
 
 
      MAPPING CYCLE 6
      ---------------
        The sixth mapping cycle extended from completion of the fifth
        mapping cycle through an additional cycle of mapping
        (approximately 180 days).  Acquisition of radio tracking data
        for gravity studies was emphasized.  The first orbit in the
        sixth cycle was orbit 12249, and the last was orbit 15032.
        The sixth cycle ended when radio contact was lost as the
        spacecraft entered the atmosphere and was destroyed in a
        'terminal windmill' experiment.
 
        Spacecraft Id                  : MGN
        Target Name                    : VENUS
        Mission Phase Start Time       : 1994-04-16
        Mission Phase Stop Time        : 1994-10-12
        Spacecraft Operations Type     : ORBITER
MISSION_OBJECTIVES_SUMMARY
 
    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], [KONOPLIVETAL1993], and [MCNAMEEETAL1993].
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