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Cold, plumeless Venus has an ancient accretionary surface

Warren B. Hamilton

Department of Geophysics, Colorado School of Mines, Golden CO 80401

The Venus of conventional interpretation is currently inactive but was wholly resurfaced, mostly by plume-driven processes, no earlier than 1 Ga. This speculation is extrapolated from terrestrial plume conjectures, and ignores voluminous contrary evidence from Venus itself. Venus displays thousands of old circular structures, with topographic rims 5-2000 km in diameter, that have the morphology, cookie-cutter superposition, and frequency/size distribution required of, and unique to, impact craters and basins. Many structures have interior central- or ring uplifts or broad, low volcanic constructs. Many are multi-ring. At least 700 of these exposed circular structures have topographic-rim diameters > 100 km. Old uplands are saturated with these structures, which are variably eroded, whereas lowland structures are partly to entirely buried. The youngest include three of the largest (inside rim diameters 2000, 900, and 800 km). Analogy with the dated, large, and similarly young, Imbrium impact basin on the Moon requires ages of 3.85 or 3.90 Ga for these youngest megabasins. Venus preserves much of its surface of late-stage main planetary accretion and is, by comparison to Earth, cold and dead.

Figure 1a

Figure 1b

Figure1: Ancient circular structures with impact-crater morphology.

(a) Magellan radar mosaic of multiring Aramaiti “corona” (left, location 26°S, 82°E) has a rim steeper on the inside than the outside; basin floor lower than the surrounding plains; a central uplift; an erosion-smoothed conical ejecta apron sloping gently from rim into the outer syncline; and, almost imperceptible here, a broad outer rise with an outside diameter of 400 km. Multiring Ohogetsu “corona” (right) also has a rim, central uplift, and ejecta apron. These coronae are in the plains, where most structures are buried by sediments.

(b) pseudoperspective view northeastward, vertical exaggeration 3:1, made by Trent Hare by draping a radar-brightness image on a digital topographic model.

Conventional analysis assumes these thousands of circular structures to be entirely young and endogenic. Interpreters of early Venusian radar imagery accepted their possible impact origin and great age, but in the late 1980s impact explanations were replaced, almost without analysis, by plume conjectures. Nearly 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 1000 “pristine” small to midsize craters (maximum rim diameter, 270 km) with a presumed age of < 1.0 Ga. (Ages as old as 3.9 Ga are advocated here for them.) The conventional logic is false: all true impact structures are “pristine”, so therefore all modified structures are endogenic. The older circular structures are rationalized, in these terms, as produced by mantle plumes and upwellings that deform 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.
Figure 2: Cookie-cutter superposition of ancient circular impact structures. The rim of Ved-Aua “corona” (impact basin, upper right; topographic rim is approximately the outer ring of continuous concentric fractures) is 200 km in diameter and stands 1-2 km above the irregular floor of the enclosed basin. It is superimposed across a subdued and partly buried impact-crater rim 250 km in diameter. The common assumption that both are products of plumes is disproved by this geometry. The area is centered on -32.5°N, 141.5°E. East-looking radar mosaic by U.S. Geological Survey.

Figure 3: Terrain saturated with ancient impact structures. All circular structures in this view are here regarded as of impact origin. Impact-basin Ki “corona” (E-center) has 300-km-diameter rim, steeper on the inside than the outside and 1 km high, that retains an ejecta blanket to E and S and is cut by 65-km crater in the N. The many other old impact structures, with rims from 15 to 200 km in diameter, show varying superpositions and degrees of preservation. The only structure conventionally attributed to impact in this view, tiny Yerguk Crater, shows as a 2-mm light spot just inside the WSW rim of Ki; many more small, subdued ancient craters are present. Area extends from 40° to 47°N and is centered on 222°E. Radar mosaic by the Jet Propulsion Laboratory.

Transfer 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 most fanciful attributions of geologic and tectonic features to them do not include circular structures that in any way resemble those of Venus. Venusian speculations are unconstrained and yet neither address nor account for circularity and superpositions. The hot-Venus assumption behind young-surface conjectures also is dubious. Venus’ lack of a magnetic field (her core is likely solid), positive correlation of topography and geoid (outer Venus is much stiffer than Earth), 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 its greenhouse atmosphere.

The quasi-pristine craters can be discriminated only arbitrarily from the best-preserved of the ancient 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 impact melts. Other broad, low volcanoes also are circular, 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, possibly including deposits in a transient ocean, derived from uplands by processes still poorly defined. 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.

Figure 4a

Figure 4b

Figure 4: Radar mosaic and pseudoperspective view of the huge Artemis impact basin.

(a) inward-facing rim, 2000 km in diameter, is approximately at the inner edge of a radar-bright concentrically fractured zone. NW part of the structure is obscured by erosion and younger constructs. Mosaic, by the USGS, extends from 15° to 45°S and from 115° to 150°E.

(b) pseudo-perspective view NE over eastern Artemis. Inner rim (passes through lower-left corner, and bright spot right of center) stands 1 km above the interior. Ring syncline (most of radar-bright grooved terrain, plus half that width more into encircling radar-dull terrain, to inconspicuous crest of outer rise) is typically 200 km wide and 2 km deep. Outer slope of outer rise (mostly out of view) is a very gentle cone (an erosion-modified ejecta blanket?) that merges, in huge lobes and with an outer diameter of 4000 km, with the plain to the south. Vertical exaggeration 3:1; prepared by Trent Hare, USGS.

last updated June 15th, 2006