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Global free-air gravity is here shown as color with SAR imagery as intensity. Perturbations in the venusian gravity field caused slight perturbations in Magellan’s orbit that in turn caused slight changes in the wavelength of the spacecraft’s tracking signal. For roughly the last two years of the Magellan mission, the spacecraft was pointed toward Earth continuously so the tracking signal could be monitored for gravity data collection; for the last year, the orbit was circularized at a low altitude to improve data quality. After data processing, a 120-degree and order-spherical harmonic model of the planet’s gravity field was produced and is shown here with a rainbow color scale. Horizontal resolution of the field is ~300 kilometers, and the range of values is from -90 to +314 milligals. The gravity data are the only existing data that provide direct information about the venusian interior. For example, the high correlation between gravity and topography on Venus compared to Earth has been used to infer that Venus lacks a low-viscosity layer known as the asthenosphere, a layer that occurs at about 200 kilometers depth on Earth and may act as a lubricating layer for terrestrial plate tectonics. Click here to view a high-resolution version of the image (4.12 MB)
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