Wednesday, July 8, 2015

Impact Crater Ejecta Mantling on Venusian Tesserae? Earth-based Radar Seems to Say Yes


The Smithsonian's Bruce Campbell and his colleagues (Campbell et al., 2015) combined radar imagery captured in 1988 and 2012 by the Arecibo and Greenbank radio telescopes to better detect the parabola-shaped deposits of impact crater ejecta on Venus. They were looking for such deposits on the highly-deformed terrain of tessera regions, which are suspected of having formed at a time when there was still water on the surface.
Previous researchers had identified large parabolic deposits of radar-dark material extending to the west of many impact craters on Venus. Once launched into the air, the strong winds can transport the ejecta as far as 2000km from the impact site, with the fine-grained material presenting as darker on radar than the surrounding terrain. What had not been conclusively observed was mantling by such deposits on tessera.
Top: Magellan image of Stuart crater parabola
Bottom: Same area imaged by Earth radar
By combining the Earth-based radar maps from multiple observations, Campbell et al. achieved 1-2km spatial resolution. They also achieved greater sensitivity to small changes in backscatter using Same-sense Circular (SC) polarization of the transmitted radar signal.

What Did They Find?

Examining combined images of the area around the tessera region of Alpha Regio, the authors focused on a previously-identified (and fairly obvious) ejecta parabola extending to the West from Stuart crater. In an image compiled from Magellan radar data, the darker material seems to stop at the edge of the tessera terrain. In the SC-polarization image captured by Earth-based telescopes, the increased sensitivity seems to reveal a mantling of a good deal of Alpha Regio by fine-grained material that continues the parabolic shape further westward.
Other researchers had hypothesized that the dark parabolas and halos of ejecta around some craters (but not all) might be useful as a coarse dating method for impact craters, but Campbell et al. suspect varying conditions during the deposition of ejecta and during subsequent erosion may undermine the creation of a model for dating craters.

Why Is It Important?

Tesserae are areas of high interest for future lander missions to Venus, and better characterization of the surface composition will aid in the selection of landing sites.


Campbell, B., Campbell, D., Morgan, G., Carter, L., Nolan, M., & Chandler, J. (2015). Evidence for crater ejecta on Venus tessera terrain from Earth-based radar images Icarus, 250, 123-130 DOI: 10.1016/j.icarus.2014.11.025

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