Fluid outflows from Venus impact craters: Analysis from Magellan data

Abstract

Many impact craters on Venus have unusual outflow features originating in or under the continuous ejecta blankets and continuing downhill into the surrounding terrain. These features clearly resulted from flow of lowviscosity fluids, but the identity of those fluids is not clear. In particular, it should not be assumed a priori that the fluid is an impact melt. A number of candidate processes by which impact events might generate the observed features are considered, and predictions are made concerning the theological character of flows produced by each mechanism. A sample of outflows was analyzed using Magellan images and a model of unconstrained Bingham plastic flow on inclined planes, leading to estimates of viscosity and yield strength for the flow materials. It is argued that at least two different mechanisms have produced outflows on Venus: an erosive, channel-forming process and a depositional process. The erosive fluid is probably an impact melt, but the depositional fluid may consist of fluidized solid debris, vaporized material, and/or melt.

Impact ejecta emplacement on terrestrial planets

“…In addition, any entrained clastic debris in the impact melt is hotter than on the Moon, resulting in higher thermal equilibrium temperatures, lower viscosities, and longer cooling time for the impact melt deposits (Grieve and Cintala, 1995). This relative increase in the amount (a factor of greater than 6) and nature of impact melt lithologies produced for a given transient crater size compared to the Moon is manifest as spectacular melt outflows exterior to Venusian craters (Asimow and Wood, 1992) (Fig. 4).…”

Two Different Supply Styles of Crater Outflow Materials on Venus Inferred from Numerical Simulations over DEMs

“…They may result from segregation and drainage of melted or vaporized materials from within ejecta materials via channels (Baker et al 1992, Chadwick andSchaber 1993), or they may be emplaced by a fluid which consists of fluidized solid debris, vaporized material, and/or melt (Asimow and Wood 1992), by a combination of both processes Baker 1994, Johnson andGaddis 1996), or by materials fluidized by interaction of the ejecta with the dense venusian atmosphere (Schultz 1992).…”

“…They are called crater outflows (e.g., Asimow andWood 1992, Chadwick andSchaber 1993), fluidized ejecta blankets (FEBs) (e.g., Komatsu et al 1 Current address: Department of Geosystem Engineering, University of Tokyo,Japan. 1991, Johnson and Baker 1994), or run-out flows (e.g., Sugita and Schultz 1996). These different terms are defined according to the different ideas about their origin.…”

“…There have been several attempts to estimate rheological properties of crater outflows. Asimow and Wood (1992) tried to analyze some outflows using a simple model of isothermal Bingham plastic flow on inclined planes. This model was first proposed by Hulme (1974) and was used for estimations of viscosity and yield strength of lava flows.…”

Crater Outflow (Venus)

“…Ejecta materials may be fluidized by the interaction of ejecta with the dense atmosphere (Schultz 1992). Erosive, channel forming outflows may be emplaced by fluids composed of fluidized solid debris, vaporized material, and melts (Asimow and Wood 1992). (3) Impact fluidization of melting of subsurface.…”