Magellan's global data on Venus have been acquired as part of a series of spacecraft investigations of the inner planets (Mercury, Venus, Earth, and Mars). The data are more complete than similar data for Earth (whose largely unmapped ocean floors make up 70 percent of its surface). The Magellan images, altimetry, and data on the physical-electrical properties of the surface of Venus were acquired at a wavelength of 12.6 cm and are very different from images and data acquired at optical wavelengths that have been employed for planetary geologic mapping and studies. The properties of surfaces that influence reflection of radio waves are not the same as those that influence the reflection at optical wavelengths. This factor must be considered when interpreting synthetic aperture radar (SAR) images and surface properties measured at radio wavelengths and when preparing geologic maps.
In this section we briefly discuss (1) the Magellan mission, (2) the radar instruments, (3) data acquisition and processing, and (4) data products that are or will become available. For detailed information, please see the Recommended Reading.
The Magellan radar mapping mission to Venus has provided SAR images of 98.3% of the planet's surface, as well as data on topography, small-scale surface roughness, radar reflectivity, and thermal emissivity--all of which cover similar percentages of the surface (Saunders and others, 1992). The images and the ancillary data are the resource materials for the VMAP program.
The Magellan spacecraft went into orbit around Venus with an orbit period of 3 hr. 25 min. and inclination of 85. This corresponds to about 7.5 orbits per day and a total of 1,790 orbits during one 243-day rotation period (a 'cycle' in Magellan parlance). Periapsis of the orbit was at lat 9.9 N. and the motion of the spacecraft was from north to south during data acquisition. The altitude of the spacecraft was 294 km at periapsis and 2,100 km over the north pole. The 85o inclination angle for the orbit was chosen to allow imaging of the polar regions.
After orbit insertion, the first four weeks of the mission were devoted to engineering tests, but some test images were acquired as well. Systematic data collection began on 15 September 1990. Coverage was completed in cycle 1 on 15 May 1991, and in cycle 2 on 14 January 1992. Cycle 3, the last cycle in which systematic SAR data were obtained, ended on 14 September 1992. Cycle 4 mission operations focused on the collection of gravity data. Continuation of the Magellan mission beyond cycle 4 involved aerobraking to a low-altitude (250 km) circular orbit, acquiring high-resolution gravity data, and special experimenting.
Images of 83.7% of the planet's surface were obtained during cycle 1 with the SAR antenna
looking to the east of the orbit plane (left-looking) (see Michaels, 1992, fig. 2); 54.5% was
covered during cycle 2 with right-looking images and some left-looking images (Michaels, 1992,
fig. 3); and 22.8% was covered during cycle 3 with left-looking images (chiefly for stereometric
use with cycle 1 images). Coverage of Venus with SAR images is summarized in Ford and
others (1993; figs. 2.2, 2.4, 2.5, and 2.6).
Many geologists are familiar with the techniques of photogeologic mapping and have a working knowledge of standard aerial photography, satellite images, and stereoscopy. Magellan SAR images are basically similar to photographs and images acquired in the visible wavelengths in that they faithfully portray landforms and topography. However, there are important differences between the SAR images and images acquired at visible wavelengths because of the source of illumination, the manner in which the radar echoes are translated into map distances, and the large wavelength of the radar. The surface is illuminated by the Sun for aerial photography and satellite imagery, whereas the SAR provides its own illumination. Geometric distortions or relief displacements in aerial photographs and satellite images are radial to the nadir, whereas those in SAR images are in the direction of the radar antenna. The large wavelength (12.6 cm) of the Magellan SAR results in scattering properties of the surface that are much different than those at visual wavelengths; these scattering properties vary with several parameters such as wavelength, polarization, and incidence angle of the transmitted radar waves and the fine- to coarse-scale roughness and physical-electrical properties of the surface. Thus, geologic mappers will need to be familiar with the characteristics of the Magellan radar system, how the data were acquired and processed, and responses of natural surfaces to incident radar energy so that they can intelligently analyze the data.
The Magellan spacecraft carries a single scientific instrument-a multi-mode radar sensor that
operates in burst cycles. Each burst cycle consists of sequential synthetic aperture radar (SAR),
altimetry, and radiometry measurements (Saunders and others, 1990, fig. 2; Saunders and
Pettengill, 1991; Ford and Pettengill, 1992; Michaels, 1992, table 1 and fig. 1). The SAR and
altimeter are active sensors that transmit radar waves and measure the radar echoes from the
surface (backscatter or reflectivity) in oblique and vertical (near-nadir) viewing geometries,
respectively (Pettengill and others, 1991). The radiometer passively measures the thermal
emission from the surface at 12.6-cm wavelength by sampling the SAR receiver between radar
echoes. The SAR and the radiometer operate through a narrow bandwidth, high-gain antenna
(HGA), while the altimeter uses a separate, smaller horn antenna (ALTA) positioned to the side
of the HGA. Magellan's SAR operates at angles of incidence between about 17o and 45.7o from
the local surface normal. Incidence angles are smallest toward the poles and largest at periapsis
(Pettengill and others, 1991; Saunders and others, 1992; Tyler and others, 1992). During each
orbit the SAR imaged a strip about 20 km wide and 17,000 km long, alternately from the north
pole to 57o S (intermediate swaths) and from 70o N to 70o S (delayed swaths). The three
Magellan radar modes are briefly discussed below.
Synthetic aperture radar (SAR)
Figure 2. Method of data acquisition by the Magellan synthetic aperture radar (SAR) system; see text for discussion. (Reprinted from Engineering & Science, Spring 1991, v. LIV, no. 3, p. 16)
Several parameters control Magellan SAR image quality. The most important are spatial resolution, the number of "looks," amplitude resolution, signal-to-noise ratio, and incidence angle. The SAR was designed to produce a 120-m along-track or azimuth resolution. Cross-track or range resolution is governed by normal techniques of pulse encoding common to most radars and is a function of radar bandwidth and incidence angle. Range resolution varies from about 120 m near periapsis to about 280 m near the poles (Michaels, 1992, figs. 16, 17). These two resolution dimensions constitute a resolution cell. The backscattered echoes were resampled at a pixel spacing of 75 m for the construction of images. Each measurement of the power backscattered from a resolution cell is a single estimate of a random variable whose mean is the backscatter radar cross section. For Magellan, the number of looks (N) ranged from 5 near the equator to 15 near the poles (Saunders and others, 1992, table 3). If these estimates or looks are averaged to obtain a mean, then the standard deviation of this measurement of the mean is equal to the mean divided by N1/2. The angle between the incident radar waves and the surface normal was varied to maintain an acceptable ratio of signal-to-noise as the altitude of the spacecraft above the surface varied. Decreasing the angle of incidence increases the returned power because of Venus' scattering law at 12.6-cm wavelength, and thus the decreased angle compensates for the increased signal loss as the distance to the surface increases (Pettengill and others, 1991).
Image pixel values (DN) are normalized backscatter cross sections and equal to the ratio of the measured mean backscatter cross section to the value predicted by the Magellan project scattering law (Saunders and others, 1992). Predicted values closely match empirically derived average backscatter behavior of the surface of Venus as a function of incidence angle (Muhleman, 1964). The pixel (DN) values range between 1 and 251, and each increment of 1 DN corresponds to an increment of 0.2 decibels (dB). Relative accuracy of the radar cross section is less than 1 dB in the cross-track direction and 2 decibels (dB) in the along-track direction. Performance checks during the mission verified that the radar sensor was operating according to sensitivity specifications.
The backscatter coefficient (also called backscatter cross section, sigma-zero, or 
) is a measure
of the power of an echo from the surface. Because backscatter coefficients are of interest to
mappers (as discussed later), a procedure for calculating them is given here. Normalized
backscatter cross sections ( 
) are ratios of backscatter coefficients to the expected backscatter
cross section ( 
)) for a horizontal, level surface at the incidence angle ( 
) of the Magellan
SAR image:
Normaized backscatter cross sections ( 
) are related to DN values by:
,
where 
. The normalizing equation used by the Magellan project for the
expected backscatter cross section is given by:
.
Sigma-zero ( 
) is the product of 
and 
(i.e., 
) and 
gives the
value of 
in decibels.
The key parameters of Magellan's radar imaging system, as well as those for previously acquired
radar data, are given in Table 1 of Michaels (1992).
Altimeter
Shapes of the time-dispersed and (or) frequency-dispersed echoes and their amplitudes for each
transmitted pulse from the altimeter horn antenna are dependent on the surface roughness at both
wavelength and larger scales and on the surface reflectivities of the areas within the altimeter
footprint. The shapes of the echoes (that is, the amplitudes as a function of time or frequency)
are measures of the meter-scale surface roughness within the antenna footprint and can be fitted
to several theoretical scattering functions (the scattering laws); delays or frequencies correspond
to incidence angles from zero to about 10 or 30 (Tyler and others, 1992). The integrated power
in the echoes provides an estimate of the backscatter cross sections at normal incidence.
Radiometer Analysis of Magellan Radar Data SAR images
Magellan SAR echoes are thought to result primarily from two scattering mechanisms: quasi-specular and diffuse. Quasi-specular echoes are produced by mirrorlike reflections from facets oriented perpendicular to the SAR antenna (Hagfors, 1964; Tyler and others, 1991) that are much larger than the wavelength of the radar (12.6 cm). Quasi-specular scattering dominates echoes at small incidence angles, but echo strengths decrease rapidly with increasing incidence angle and become weak to nonexistent relative to the diffuse echo at incidence angles near 10 to 30 or so (depending on the root-mean-square slope). Quasi-specular echoes are rare in Magellan SAR images because of the large incidence angles. Diffuse scattering is produced by wavelength-size roughness elements at and near the surface, which scatter the incident radar energy in all directions so that diffuse echoes are received at all incidence angles. Diffuse scattering dominates echoes at large incidence angles that are typically greater than ~30, and the echoes also tend to become weaker with increasing incidence angle. However, this tendency is a function of the attributes of the wavelength-size scatterers on the surface (such as concentration, size-probability distribution, and dielectric properties). Diffuse echoes dominate the Magellan SAR images because of the large incidence angles.
Other applications and aspects of SAR data are discussed in the following numbered sections.
This discussion highlights the derivation of several parameters and measurements and their
relevance to geologic mapping, as well as important topographic effects, so that the mapper can
make fullest use of the data.
1. Backscatter coefficient.
, of the transmitted radar energy with the surface and the physical-electrical properties of the
surface materials. Backscatter coefficients of planar level surfaces tend to be larger at small
incidence angles than at larger incidence angles; similarly, surfaces tilted toward the radar tend to
have large backscatter coefficients than surfaces tilted away from the radar. Concentrations of
wavelength-size roughness elements at and near the surface also affect the backscatter
coefficient. Roughness elements include rocks, surfaces of aa lava flows, near-surface voids, and
other wavelength-size discontinuities. Large concentrations of roughness elements produce
larger coefficients than smaller concentrations. Magellan SAR images contain quantitative
information on backscatter coefficients because the DN values, as noted previously, are
normalized by values of backscatter (Saunders and others, 1992). Of importance to the mapper is
the information that can be gained about the properties of geologic map units. Some terrestrial
data illustrating this application are given in figure 3; the literature should be consulted for
further possibilities.
Figure 3. Backscatter coefficient (also known as sigma-zero or 
) as a function of incidence
angle for the average Venus surface and four terrestrial surfaces at 12.6-cm wavelength (after
Plaut, 1991). Note that the aa flow, a strong diffuse scatterer, has large coefficients that are
nearly independent of incidence angle, and the playa, a weak diffuse scatterer, has small
coefficients that are a strong function of incidence angle.
2. Identification resolution
. The identification resolution of a SAR image is larger than the resolution stated in terms of pixel or cell size (120 m x 120 to 280 m) and is a function of landform shape, size, and backscatter properties and SAR incidence angles. Analyses of lunar radar images suggest that an observer begins to identify crater landforms when they are about four times the cell size of the radar (Moore and Thompson, 1988). Experience with spacecraft images at visual wavelengths yields similar results (Dial and Schaber, 1981). For Magellan SAR images, geologic mappers will learn by experience the sizes of landforms that they can identify.The mapper's ability to identify linear features also depends on their orientations. Linear
landforms that are transverse to the illumination or look direction are often prominent, but those
that are parallel to the illumination or look direction may not be evident (see Wise, 1969;
Yamaguchi, 1985). Other asymmetrical landforms, such as sand dunes, may be more evident in
one look direction than they are in the opposite look direction because of their shapes. Similar
phenomena may occur at scales that are finer than the resolution of the images (Plaut and others,
1992).
3. Geometric distortion.
Although a minor nuisance to geologic mappers, geometric distortions in single images
(monoscopic) can be used advantageously to estimate the relief and slopes of landforms that are
symmetrical in the cross-track direction (Michaels, 1992, fig. 15). Foreshortening and elongation
due to relief of landforms in different images of the same scene acquired with different incidence
angles also provide a means of estimating with parallax measurements the relief of the
landforms.
4. Radar shadows. 5. Stereoscopy.
Nearly all cycle 3 images are left-looking with smaller incidence angles than those of cycle 1 and
can be paired with images of the same scenes for stereoscopic viewing. For most of these image
pairs with north up, the image with the smaller incidence angle (cycle 3) should be viewed with
the right eye and the image with the larger incidence angle (cycle 1) should be viewed with the
left eye in order to attain the correct sense of relief. A reversed viewing arrangement is necessary
for images of Maxwell Montes, where cycle 3 images have larger incidence angles than those of
cycle 1. Mountains, hills, and domes appear to lean toward the antenna in the look direction. In
attempts to view stereoscopically opposite-side images with north up, the left-looking image
should be viewed with the right eye and the right-looking image with the left eye.
6. Parallax relief.
Geometric relations and equations for making such estimates are illustrated for same-side or left-looking image pairs in figure 4 and opposite-side or right- and left-looking image pairs in figure 5. Of particular importance is the identification of "conjugate-image" points (the same points on the surface in each image of the pair). This identification is more readily achieved with same-side image pairs than with opposite-side image pairs. Measurements and calculations on digital displays in terms of pixels with subsequent translations to meters are recommended, but such measurements can be made with stereoscopes, parallax bars, and suitably oriented hardcopy images or their equivalents. It is also important to demonstrate that the look directions of the two images are roughly parallel, say within 10 of each other, because relief displacements are cross-track in the direction of the antenna.
Figure 4. Vertical profile of surface in the cross-track plane illustrating geometry of relief
displacement and parallax for left-looking image pairs. A, a point on the surface at scale of
image; As, point A in image with smaller incidence angle (usually cycle 3); Al , point A in image
with larger incidence angle (usually cycle 1); B, a second point on the surface at scale of the
image; Bs, point B in image with smaller incidence angle (usually cycle 3); Bl, point B in image
with larger incidence angle (usually cycle 1); 
, smaller incidence angle; 
, larger incidence
angle; P, parallax; h, relief from A to B.
Figure 5. Vertical profile of surface in the cross-track plane illustrating geometry of relief
displacement and parallax for left- and right-looking image pairs. A, a point on the surface; Ar,
point A in image with right-looking incidence angle (cycle 2); Al, point A in image with left-looking incidence angle (usually cycle 1 or 3); B, a second point on the surface; Br, point B in
right-looking image (cycle 2); Bl, point B in left-looking (usually cycle 1 or cycle 3); 
,
incidence angle of right-looking image; 
, incidence angle of left-looking image; P, parallax; h,
relief from A to B.
Parallax measurements have been used to estimate the relief of lava flows (Moore and others, 1992), of crater rims above their floors (Schaber and others, 1992; Moore and others, 1993), and of volcanic edifices (Moore and others, 1993; Plaut, 1993). An excellent discussion of the method is given in Ford and others (1993).
Some caution must be exercised in applying parallax measurements, because there may be
significant image displacements related to navigational boundaries (differences in orbital
parameters between uploads or each group of eight swaths). These displacements can be
recognized by rotating the image pairs 90 and viewing them stereoscopically. Because of this
problem, we recommend that parallax measurements be confined to small landforms or images
that have been corrected for spurious image displacements. There are plans for correcting the
Magellan images, but usable products may not be available in time for use in The Venus Geologic
Mapping Program.
Altimetry
Altimetry data suitable for most geologic mapping are furnished to the mapper of each
quadrangle in two forms: (1) in the same projection (Mercator, Lambert Conformal, or Polar
Stereographic) as the mapper's quadrangle at 1:5,000,000 scale, and (2) a reduced color-transparency version of (1). A reduced-scale black and white transparency of the SAR image
with the same scale and projection as the altimetry color transparency is also furnished to the
mapper of each quadrangle to facilitate correlations and comparisons. The data are also available
on CD-ROMs, which include global data sets in both sinusoidal and Mercator projections
(GTDR.SINU and GTDR.MERC files) and ancillary data for those wishing more quantitative
information; pixel size is about 4.6 km at the equator.
Reflectivity
Normal reflectivity (for brevity, simply "reflectivity") is the ratio of quasi-specular echo power
received from a surface at normal incidence and the power transmitted to the same surface. For
Magellan, reflectivities are estimated by using the equation of Hagfors (1964; see for example,
Tyler and others, 1992):
where 
is the total cross section, 
is the normal reflectivity, C is a surface-roughness
parameter, and 
is the incidence angle. C-1/2 is interpreted as root-mean-square slope (see
below). Diffuse scatterers on and at the surface effectively reduce the surface area available for
quasi-specular echo scattering (Evans and Hagfors, 1964; Pettengill and others, 1988); some
Magellan reflectivity data sets have been corrected for this effect by using SAR backscatter data
(see below).
Of particular importance to geologic mappers are the relations between reflectivity and dielectric
constant and model-dependent relations between relative dielectric constant and the physical-electrical properties of the reflecting materials. The relation between reflectivity ( 
) and
relative dielectric constant ( 
) is given by the Fresnel reflection coefficient:
Relative dielectric constant (also called dielectric permittivity) is the ratio of the dielectric constant of a material to that of free space or a vacuum.
There are a variety of model-dependent relations between the relative dielectric constant and physical-electrical properties of natural materials. Olhoeft and Strangway (1975) advocated the Lichtenecker mixing formula (see Saint-Amant and Strangway, 1970) and showed its applicability to lunar rocks and regoliths and their bulk densities; Garvin and others (1985) offered a modest modification to the equation of Olhoeft and Strangway. Campbell and Ulrichs (1969) applied the Rayleigh mixing formula to their data on dielectric constants and porosities of rocks, meteorites, and dry powders of these materials and cited references for other models. Selected relations are illustrated in figure 6 to give the mapper an appreciation for the significance of reflectivity.
Figure 6. Relations between normal reflectivity and bulk density of rocks and dry rock powders or regoliths. Rayleigh mixing curve assumes a "parent rock" with a relative dielectric constant of 7.7 and a bulk density of 3,100 kg/m3 (see Campbell and Ulrichs, 1969); curve extrapolated to a reflectivity of 0.25. Olhoeft and Strangway (1975) curve is based on experimental measurements of lunar rocks, aggregates, and regolith samples; light curves indicate one standard deviation. Relative dielectric constants (permittivities) calculated with the Fresnel reflection coefficient (see text). Conducting inclusions are not considered in these calculations.
Inclusions of electrical conductors, such as the mineral pyrite, in rocks and regoliths may have a profound effect on their dielectric constants (Pettengill and others, 1988). Rocks and regoliths containing such inclusions have been called "loaded dielectrics." This phenomenon may be an important factor contributing to the large radar reflectivities and low emissivities of Venusian surfaces at high elevations. The effects of conducting inclusions are presented by Pettengill and others (1988) and illustrated in figure 7.
Figure 7. Relation between ratio of dielectric constants of a "loaded" dielectric and host material and the volume fraction of electrical conducting inclusions in the "loaded" dielectric (after Pettengill and others, 1988). For a volume fraction of conducting inclusions of 0.1, if the dielectric constant of the host is 5.0, the ratio is 4.87 and the dielectric constant of the "loaded" dielectric is 24.4.
Reflectivity data suitable for most geologic mapping purposes are furnished to the mapper of
each quadrangle in a color-transparency with the same scale and projection as the SAR-image
and altimetry transparencies mentioned above. The data are also available on CD-ROMs, which
include global data sets in both sinusoidal and Mercator projections (GREDR.SINU and
GREDR.MERC files) and ancillary data for those wishing more quantitative information; pixel
size is about 4.6 km at the equator. It is important to realize that only the GREDR.MERC files
have been corrected for both the systematic errors and diffuse scattering mentioned above (P.G.
Ford, personal communication, 1992).
Root-mean-square (RMS) slope.
Figure 8. Slope-probability distributions of part of Mare Serenitatis obtained by using Apollo 15
panoramic camera photography and bistatic radar; (a) photogrammetry, 25-m slope length, and
(b) bistatic radar, 13-cm wavelength. Incremental probability shown by bars, cumulative
probability shown by solid line; hypothetical Gaussian cumulative probability for algebraic
standard (
) shown by dashed line. Note that actual cumulative curves are larger than Gaussian
curves at large slope angles (reprinted from Moore and others, 1976, upper part of fig. 43).
Inversions of the delay and Doppler-frequency spectra show that the probability distributions of slopes of Venusian surfaces between 0 and 10 can be described with exponential, Hagfors, and Gaussian distribution functions (Tyler and others, 1992). In the analyses, contributions of diffuse echoes are justifiably assumed to be negligible. For nominal analyses (Ford and Pettengill, 1992), the echo spectra are fit with the Hagfors' scattering function noted above to obtain RMS slopes.
RMS-slope data from nominal analyses that are suitable for most geologic mapping purposes are
furnished to the mapper of each quadrangle in a color transparency with the same scale and
projection as the SAR-image, altimetry, and reflectivity transparencies mentioned above. The
data are also available on CD-ROMs, which include global data sets in both sinusoidal and
Mercator projections (GSDR.SINU and GSDR.MERC files) and ancillary data for those wishing
more quantitative information; pixel size is about 4.6 km at the equator. The results of Tyler and
others (1992) will be available on CD-ROMs in the future.
Emissivity
) is
the complement of the emissivity (e) (see Pettengill and others, 1988, 1991):
.
This approximate relation between reflectivity and emissivity should be viewed with some caution, because the incidence angles for reflectivities (altimeter antenna) and emission angles for emissivities (SAR antenna) are not the same. Both reflectivity and emissivity are affected by surface roughness, and emissivity varies with incidence angle (Pettengill and others, 1992).
Emissivity data suitable for most geologic mapping purposes are furnished to the mapper of each
quadrangle in a color transparency with the same scale and projection as the SAR-image,
altimetry, reflectivity, and RMS-slope transparencies mentioned above. The data are also
available on CD-ROMs, which include global data sets in both sinusoidal and Mercator
projections (GEDR.SINU and GEDR.MERC files) and ancillary data for those wishing more
quantitative information; pixel size is about 4.6 km at the equator.
Magellan Data Products SAR image data records
One Full-Resolution Basic Image Data Record (F-BIDR) was produced for each Magellan orbit; 1,790 orbits complete one mapping cycle (360 of longitude). (For the primary mission, solar conjunction prevented the return of SAR data for 110 orbits.) An additional 10 percent was lost during the nominal mission because of solar heating of electronic components (the spacecraft's solar reflectors had lost much of their efficiency), failure of one of the spacecraft's two tape recorders, and problems affecting spacecraft attitude. One F-BIDR contains nearly 60 Mbytes of 8-bit image data and less than 10 Mbytes of ancillary data. The F-BIDR format is 75 m/pixel resampled from the original 120-m cross-track and along-track resolutions and transformed into Sinusoidal Equal-Area projection; within 10 of the poles, the data are in oblique Sinusoidal projection (the F-PIDR format). Each F-BIDR forms a strip about 300 pixels wide by about 220,000 pixels long. In turn, about 30 F-BIDRs (or more at higher latitudes) are used to generate a mosaic, which has 7,168 lines by 8,192 samples. The mosaics are called Full-Resolution Mosaicked Image Data Records (F-MIDRs). Each F-MIDR is identified by center latitude and longitude and covers 5 of latitude and >5 of longitude (depending on latitude; see Michaels, 1992, figure 4). A "venetian blind" effect is commonly evident in mosaicked F-BIDRs because of border mismatches in backscatter intensity caused by minor errors in spacecraft pointing and navigation, in the topography model, and in processing.
To provide image-mosaic coverage for larger areas, the data are compressed in successive operations in which nine pixels are averaged and replaced by one pixel and then reprojected. The Once-Compressed Mosaicked Image Data Record (C1-MIDR) consequently has a resolution of 225 m/pixel and covers 15 of latitude and >15 of longitude (see Michaels, 1992, figure 5). In turn, successive compressions result in the 675 m/pixel C2-MIDRs (45 of latitude by >45 of longitude; Michaels, 1992, figure 6) and the 2,025 m/pixel C3-MIDRs (80 of latitude by >120 of longitude). Polar data records (F-PIDRs) and projections (P-MIDRs) portray areas at >80 latitude.
The various MIDR products are being distributed as photographic prints and digital files on
CD-ROM disks. The CD-ROMs released by the Magellan Project are available to the planetary
geoscience community through the National Space Science Data Center in Greenbelt, Maryland.
Synthetic stereopairs and merged databases
In addition, perspective views can be generated from the merged topography and SAR image
mosaics. Other radar databases (emissivity, reflectivity, rms slope, etc.) can be colorscaled and
combined with black-and-white SAR mosaics for normal or perspective views or for synthetic
stereo analysis.
Map projections
Other map series that are being produced for Venus contain 340 sheets (Sinusoidal) at
1:1,500,000 scale, 8 sheets (6 Mercator and 2 Polar) at 1:10,000,000 scale, 3 sheets (2 Mercator
and 1 Polar) at 1:25,000,000 scale, and 1 sheet at 1:50,000,000 scale. For more information, refer
to Batson (1990).
Non-Magellan Radar and Other Data
Since the early 1960s, Venus has become one of the planets most visited by spacecraft. Fifteen Soviet and six U.S. missions have probed its sulfurous clouds to measure atmospheric structure and composition. Other investigations disclosed a lack of water vapor and the absence of a magnetic field. Seven of the Soviet craft were landers that conducted chemical analyses of rocks, which indicated that some have compositions similar to basalt and thus may be of volcanic origin. One of the landers, Venera 9, gave us our first glimpse of the surface when, in 1975, it relayed a panoramic view. These early explorations were enhanced by observations in the field of radio astronomy that indicated that Venus is a perpetual furnace; surface temperatures reach 482 C (~900 F), and the atmospheric pressure is 90 times that of Earth's (Young, 1990).
The 1978 U.S. Pioneer Venus Orbiter (PVO) was the first spacecraft to carry a radar (SAR-type) sensor to Venus, and the altimetry of 92 percent of the surface was mapped at a resolution of 50 to 140 km. For the first time, planetary scientists had a global map of Venus. Continent-size highlands, hilly plains, large mountains looking like volcanoes, and flat lowlands were revealed (Masursky and others, 1980). Much of the initial scientific results from the Pioneer Venus mission were published in a special issue of the Journal of Geophysical Research dedicated to that mission (December 1980, v. 85, no. A13, p. 7575-8337).
Five years after Pioneer Venus went into orbit, the Soviet Venera 15 and 16 spacecraft used radar to map about 25 percent of Venus (lat 25-90 N.) at a resolution of 1.2 to 2.4 km. These images revealed evidence of abundant volcanism, impact craters, and complex tectonic deformation, including coronae--large, oval features of apparent volcanotectonic origin previously unrecognized on other bodies in the Solar System (Kotelnikov, 1989). The geoscience investigations of Venera 15 and 16 are described in many publications (see Recommended Reading).
Earth-based radar observations of some regions of Venus have been made primarily from the Arecibo Radio Observatory and the Goldstone receiving station since the mid-1960s (see, for example, Jurgens and others, 1980, 1988a, b; Burns and Campbell, 1985; Campbell and others, 1989, 1990; Campbell and Campbell, 1992; Plaut and Arvidson, 1992). The Goldstone radar coverage is restricted to lat 23 S. to 23 N. and long 260 to 32 E. (Plaut and Arvidson, 1992), while that from the Arecibo Observatory is limited to most of the area between lat 67 and long 265 to 35 E., about 23 percent of the surface (Campbell and others, 1990). In all, about 40 percent of Venus has been mapped from these radar stations (Campbell and others, 1990).
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