Mathematics – Logic
Scientific paper
Dec 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agufm.v53c..06h&link_type=abstract
American Geophysical Union, Fall Meeting 2004, abstract #V53C-06
Mathematics
Logic
5420 Impact Phenomena (Includes Cratering), 5455 Origin And Evolution, 5475 Tectonics (8149), 5480 Volcanism (8450), 6295 Venus
Scientific paper
Venus displays hundreds of circular structures, with topographic rims 10-2,000 km in diameter, that have the morphology, cookie-cutter superposition, and log frequency/log size distribution required of, and unique to, impact craters and basins. They nevertheless are assumed to be endogenic by specialists. Many have interior central or ring uplifts or broad, low volcanic constructs. Many are multiring. Old uplands are saturated with the structures, which there are variably eroded, whereas lowland structures are partly to entirely buried. The largest (Artemis, Heng-O, and Quetzalpetlatl, rim diameters 2000, 900, and 800 km) are among the youngest. Analogy with dated large, and similarly relatively late, Imbrium impact basin on the Moon requires ages greater than 3.85 or 3.90 Ga. Venus preserves much of its surface of late-stage main planetary accretion. Early investigators of Venusian radar imagery accepted the possible impact origin and great age of the circular structures, but in the late 1980s impact was replaced, almost without analysis, by plume conjectures. Almost all specialists now assume that Venus has a thermal structure and heat loss comparable to that of Earth, and that its only impact structures are mostly-pristine small to midsize (maximum rim diameter, 270 km) craters with an assumed age of less than 0.5 or 1.0 Ga. (Ages as old as 3.9 Ga are advocated here for these young craters.) The older circular structures are rationalized as produced by mantle plumes and upwellings that deformed crust and upper mantle from beneath, with or without extrusion of subordinate lava, and that magmatically and tectonically resurfaced Venus in a brief period before the late impacts. Extrapolation of plume conjecture to Venus from Earth has little merit. Terrestrial plume speculation is based on assumptions whose predictions have been consistently falsified. Not only do plumes probably not exist on Earth, but even the least-constrained attributions of geologic and tectonic features to them do not include circular structures that in any way resemble those of Venus. Conversely, Venusian conjectures neither address nor account for circularity and superpositions. The hot-Venus assumption behind young-surface speculation also is dubious. The lack of a magnetic field on Venus (its core is likely solid), the positive correlation of its topography and geoid (outer Venus is much stiffer than Earth), and its origin close to the Sun (less potassium, so much less early radiogenic heat), and other factors indicate Venus to be much colder internally than Earth below the depth of influence of greenhouse atmosphere. The most eroded and breached, or buried, of the quasi-pristine craters are discriminated only arbitrarily from the best-preserved of the ancient, and mostly larger, circular structures. From those in turn, there are all gradations back to the deepest-eroded, or the most-buried, structures of the old family. Broad, low volcanic constructs (unlike any terrestrial volcanoes) inside impact basins likely are products of cogenetic impact melts. Other large, low volcanoes also are circular, are isolated, and may be of impact melts that buried their basins. Broad tessera-surfaced plateaus are of layered rocks, display deformation and topography indicative of outward gravitational spreading, and may have formed from ancient impact-melt lakes. Venusian lowlands are floored not by young lava plains but by ancient sediments, probably including deposits in a transient ocean, derived from uplands. The plains are speckled with mud volcanoes (not lava cones) that, like minor deformation of the sediments, are due to top-down heating by the evolving atmosphere.
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