Pioneer Venus Radar results altimetry and surface properties

The radar altimeter carried aboard the Pioneer Venus orbiter spacecraft has yielded a topographic map covering 93% of the Venus globe, with a linear surface resolution of better than 150 km. Vertical measurement accuracy exceeds 200 m. Extremes in relief (expressed as a center‐of‐mass‐to‐surface radius) extend from a low of 6049 km to a high of 6062 km. Only about 5% of the observed surface is elevated more than 2 km above the mean radius (6051.5 ±0.1 km). Although the elevated terrain comprises a number of separated components, it is dominated by a massive equatorial region the size of South America. Of the total surface, 60% lies within 500 m, and 20% within 125 m, of the modal radius (6051.1 km). The planetary polar ellipticity is nearly zero, with an upper bound of 4 × 10−5. In addition to the surface relief, the distribution of average meter‐scale surface slopes, in the observable range from 1° to 10°, is determined for the same regions, and at the same footprint resolution, as in the altimetric observations. Elevated areas have generally higher values of average slope; most features seen in the earth‐based images are also seen in the vertical‐incidence spacecraft observations, although a few exceptions are noted. Of interest are several very long (up to nearly 5000 km, in one case), thin, and relatively straight parallel features, not hitherto reported on Venus.