A Comparison of the Regional Slope Characteristics of Venus and Earth: Implications for Geologic Processes on Venus

The range of 3 ø by 3 ø regional slopes of the earth and Venus is similar (approximately 0.0ø-2.4ø); however, the surface distribution of these values differs significantly. On earth, cratonic and abyssal plains form extensive regions of 0.0 ø slope. Within these regions a variety of features (mid-ocean ridges, volcanic island chains, subduction zones, and folded mountains) have regional slope characteristics influenced by seafloor spreading and plate recycling, as well as an active weathering regime. Continental margins form laterally continuous zones of relatively high slope (passive margins up to 1.9ø; active margins up to 2.4ø). The plains provinces of Venus are much more rugged than earth's plains and are marked by numerous closely spaced circular and linear features (0.1ø-0.2 ø regional slope) concentrated into broad linear zones of global extent. Although Venus highlands are bounded by narrow zones of relatively steep slope, the margins of Aphrodite Terra and Beta Regio are not as steep as earth's continental margins and appear to be best developed parallel to the trends of major chasmata within these regions. Linear trends formed by some highland margin segments extend into the plains provinces as rugged belts densely populated with circular and linear features. Ishtar Terra's margins are significantly steeper and more continuous than other highland margins and are comparable to passive margins on earth. The Venus highlands do not contain appreciable smooth, flat interior regions, implying that highland topography is not significantly modified by erosion or deposition. Systematic variations in the density of plains features, elongate planitia, highland margin trends, and aligned highland topography form several major great-circle-like patterns oriented at generally less than 45 ø to the equator and differing in character from both the mosaiclike patterns of terrestrial lithospheric plates, and the subdued tectonic fracture grids of the smaller terrestrial one-plate planets (the moon and Mercury).