SOLAR SYSTEM/Venus 263 appear to represent a broadly similar unit with a distinct stratigraphic position (either postdating or predating groups of other units) that can be traced continuously around the globe of Venus In the alternative model of geological history, the observed sequence of units is interpreted to be due to specific volcano-tectonic regimes that occur at different times on different parts of Venus, similar to Wilson cycles on Earth (individual plate-tectonic cycles that are repeated at different times and in different places on Earth) In this model for Venus, the local sequence of units represents only local or regional time-dependent sequential styles of endogenous activity This is a ‘non-directional’ model of geological history, implying that the individual sequences represent local conditions occurring at different times in different places Because the sequences of units and structures are almost the same in different regions on Venus, this model indicates that similar sequences of events resulted in similar stratigraphic columns occurring in these areas throughout the visible part of geological history Another aspect of this model is its ‘non-synchronous’ nature, implying that the sequences of units/events are non-synchronous globally and that similar stratigraphic columns in specific regions are shifted relative to each other in terms of their age Further Investigation The data collected during the exploration of Venus reveal the uniqueness of this planet Venus does not have the surface age dichotomy characterizing the Moon and smaller planets such as Mercury and Mars This fundamental characteristic implies that Venus, like Earth, has a prolonged history of geological activity that did not significantly decrease in intensity early in the evolution of the planet In contrast to Earth, where the global heat loss mechanism is governed by plate tectonics, vertical crust accretion/recycling appears to be the principal style of geological activity on Venus Although current knowledge of Venus is great, there are still several major issues about its geology that are open to debate and further investigation What is the evolution of the heat loss mechanisms on Venus? What are the paths of the evolution of the large-scale topographic features on the planet? How did the properties of the Venus lithosphere change as a function of time? Why are the Earth and Venus, the ‘twin’ planets, so different? What is the role of water in the evolution of both planets? Why is there little evidence on Venus for the presence of non-basaltic continental crust, which constitutes the major part of crustal material on Earth? How and when did the present atmosphere form and how has it evolved with time? How has the atmosphere interacted with the surface in recent and more ancient history of Venus? Obtaining answers to these questions requires continued exploration and key datasets, including seismic data, global high-resolution topography, in situ analysis of ancient terrains such as tesserae, and samples returned to terrestrial laboratories See Also Earth Structure and Origins Solar System: Mercury; Moon; Mars Further Reading Barsukov VL, Basilevsky AT, Burba GA, et al (1986) The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Venera 15 and 16 Journal of Geophysical Research 91: D399 D411 Bougher SW, Hunten DM, and Philips RJ (eds.) (1997) Venus II Geology, Geophysics, Atmosphere, and Solar Wind Environment Tucson: University of Arizona Press Crumpler LS and Aubele JA (2000) Volcanism on Venus In: Sigurdsson H, Houghton BF, McNutt SR, Rymer H, and Stix J (eds.) Encyclopedia of Volcanoes, pp 727 769 San Diego: Academic Press Ford PG and Pettengill GH (1992) Venus topography at kilometer scale slopes Journal of Geophysical Research 97: 13103 13114 Hansen VL, Willis JJ, and Banerdt WB (1997) Tectonic overview and synthesis In: Bougher SW, Hunten DM, and Phillips RJ (eds.) Venus II Geology, Geophysics, Atmosphere, and Solar Wind Environment, pp 797 844 Tucson: University of Arizona Press Hauck SA, Phillips RJ, and Price MH (1998) Venus: Crater distribution and plains resurfacing models Journal of Geophysical Research 103: 13 635 13 642 Head JW, Crumpler LS, Aubele JC, Guest JE, and Saunders RS (1992) Venus volcanism: classification of volcanic features and structures, associations, and global distribu tion from Magellan data Journal of Geophysical Re search 97: 13 153 13 197 Ivanov MA and Head JW (2001) Geology of Venus: map ping of a global geotraverse at 30o N latitude Journal of Geophysical Research 106: 17 515 17 566 Masursky H, Eliason E, Ford PG, et al (1980) Pioneer Venus radar results: geology from the images and altime try Journal of Geophysical Research 85: 8232 8260 McKinnon WB, Zahnle KJ, Ivanov BA, and Melosh HJ (1997) Cratering on Venus: models and observations In: Bougher SW, Hunten DM, and Phillips RJ (eds.) Venus II Geology, Geophysics, Atmosphere, and Solar Wind Environment, pp 969 1014 Tucson: University of Arizona Press Parmentier EM and Hess PC (1992) Chemical differenti ation of a convecting planetary interior: consequences for a one plate planet such as Venus Geophysical Research Letters 19: 2015 2018