Impact crater outflows on Venus: Morphology and emplacement mechanisms

Abstract

Many of the 932 impact craters discovered by the Magellan spacecraft at Venus are associated with lobate flows that originate at or near the crater rim. They extend for several to several hundred kilometers from the crater, and they commonly have a strong radar backscatter. A morphologic study of all identifiable crater outflows on Venus has revealed that many individual flows each consist of two areas, defined by distinct morphologic features. These two areas appear to represent two stages of deposition for each flow. The part of the flow that is generally deposited closest to the crater tends to be on the downrange side of the crater, flows in the downrange direction, and it is interpreted to be a late‐stage ejecta. In many cases, this proximal part of the flow is too thin to completely bury the large blocks in subjacent ejecta deposits. Dendritic channels, present in many proximal flows, appear to have drained liquid from the proximal part in the downhill direction, and they debouch to feed the outer part of the flows. This distal part flows downhill, fills small grabens, and is ponded by ridges, behavior that mimics that of volcanic lava flows. The meandering and dendritic channels and the relation of the distal flows to topography strongly suggest that the distal portion is the result of coalescence and slow drainage of impact melt from the proximal portion. Impact melt forms a lining to the transient crater and mixes turbulently with solid clasts, and part of this mixture may be ejected to form the proximal part of the flow during the excavation stage of crater development. A statistical study of the Venusian craters has revealed that, in general, large craters produced by impacts with relatively low incidence angles to the surface are more likely to produce flows than small craters produced by higher‐angle impacts. The greater flow production and downrange focusing of the proximal flows with decreasing incidence angle indicate a strong control of the flows by the impactor flight direction, and a high downrange velocity imparted to the proximal flow material in lower angle impacts. On the Moon, small flows interpreted to be composed of impact melt are observed atop the ejecta of large, fresh craters; on Earth, melt‐rich suevite deposits form the uppermost layer of ejecta of some fresh craters. These features, albeit much smaller, may be analogous to the flows on Venus. Numerical models have predicted that larger volumes of impact melt would be produced on Venus than on the cooler terrestrial bodies due to high atmospheric and target temperatures, perhaps 3 times the volume produced on the Moon for a given crater diameter.

Impact ejecta emplacement on terrestrial planets

“…In this case, the initial direction of the flows is preferentially downrange. Indeed some of the most notable melt outflows from complex craters on Venus are associated with craters formed from oblique impacts (Chadwick and Schaber, 1993). This also may the case for the Ries impact on Earth, where the only bona fide impact melt rocks are found outside the inner ring on the inner slope of the eastern rim, in keeping with "the preferred flow of melt downrange" (Stöffler et al, 2002).…”

“…4). Melt outflows are most common in craters resulting from oblique impacts and in larger structures (Chadwick and Schaber, 1993). Larger structures have reduced cratering efficiency relative to impact melt production compared to smaller impacts (Grieve and Cintala, 1995).…”

“…Many of the outflows have a proximal portion that have higher and more uniform radar backscatter than the continuous ejecta and a well-defined boundary between them, suggesting a discrete stratigraphic boundary. The distal portions of the outflows may extend several crater diameters, are more radar dark (suggesting less entrained clastic blocks) and flow down-slope, following the local topography (Chadwick and Schaber, 1993).…”

Embayed intermediate volcanoes on Venus: Implications for the evolution of the volcanic plains

“…In the case documented in this work, crater-related outflows, though to be the result of partial melting during impact (e.g. Chadwick and Schaber, 1993), are embaying the intermediate volcano. 1997; Basilevsky and Head, 1998a;Basilevsky and Head, 2000a (Basilevsky et al, 1997;Basilevsky and Head, 1998a;Basilevsky and Head, 2000a).…”

Impact Ejecta Characterization for Small-Sized Fresh and Degraded Lunar Craters Using Radar Data

“…In view of similar dimensions, studies on crater analysis and characterization on the gradual change in the morphology of eroding ejecta blankets were also attempted using radar data 9 . The transitions between the radar bright and low backscatter within a given flow are now possible to be attributed to a range of surface roughness changes 10 , or due to the deposition of blocky material owing to topographic obstructions 11 , or changes in features similar to lava flows that form in impact melt 12 . However, the available data (both theoretical and geological) are not large enough for detailed estimates 13 .…”