The correct geologic interpretation of radar images depends critically on a knowledge of how radar waves interact with natural surfaces. There are significant differences between the microwave (12.6 centimeters wavelength) and more familiar optical wavelengths (less than a millionth of a meter) in the mechanics of imaging and in the measured characteristics of the target. Because of the side-looking illumination geometry, all radar images are distorted to some extent. In addition, the longer wavelength of radar waves relative to light waves makes radar most sensitive to surface roughness at scales near the radar wavelength. Of secondary importance are variations in the electrical properties of the target; this factor is similar for most dry geologic materials except metallic compounds, which may be present at high elevations on Venus.
Brightness variations in Magellan radar images result primarily from three different surface variables: (1) topographic effects, (2) roughness, and (3) electrical properties, each of which is influenced to some extent by variations in the angle at which the radar strikes the surface (radar incidence angle).
(1) Topographic Effects. Topography may have a pronounced effect on image brightness. Terrain that slopes toward the imaging sensor appears bright while terrain that slopes away appears dark. Where the radar pulse strikes at an angle less than 20 degrees, small changes in slope give large changes in backscatter. [Backscatter is when radar waves, after striking the ground, go in a direction generally opposite to the direction they were moving before striking the ground. To be more precise, it's when the radar "bounces" off at angles greater than 90 degrees relative to the direction it was originally going.] In cases where broad regional slopes are too small to provide brightness contrast, surface altitudes from altimetry data are needed to distinguish large-scale positive and negative relief.
(2) Roughness. Surface roughness the size of the radar wavelength may dominate the backscatter where the radar strikes the planet's surface at angles greater than 20 degrees but less than 60 degrees. Rough surfaces have a notably higher backscatter than smooth surfaces. Surface roughness can strongly influence image brightness.
Radar waves impinging on a surface scatter from the surface and near-surface material. Very smooth surfaces scatter the radar in a mirrorlike reflection often called "specular reflection." Rougher surfaces cause more of the radar energy to be scattered randomly. For a smooth surface the resulting image tone would be black. With increasing roughness, more energy is scattered back to the antenna, resulting in lighter image tones for rougher surfaces.
(3) Electrical properties. Intrinsic electrical properties of the surface can affect radar reflectivity and can have a major influence on image brightness. Electrical properties are characterized by the "dielectric constant" of the material.
It has been well established that the highlands of Venus exhibit
strong backscatter in radar images and evidence of high dielectric constant material
[Pettengill et al., 1988]. Most models at this time favor the presence, either as
inclusions or weathering products, of free metals or metallic compounds [Klose et
al., 1992]. Since liquid water is not stable on Venus' surface, metals are the only
known material that could produce the inferred high dielectric constants.
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