Monday, July 20, 2015

Quick Look: Touchdown on Venus: Analytic Wind Models and a Heuristic Approach to Estimating Landing Dispersions

I'm working through a lot of recent Venus-related papers, so here's another morsel for you that I will not have time to read in depth (I'm prioritizing articles relating to the Venusian surface). The author created a straightforward model of winds on Venus (using data from the VEGA Balloons and the Pioneer descent probes) to determine entry and descent dispersions for future Venus landers.

From the April 2015 edition of Planetary and Space Science:


Touchdown on Venus: Analytic wind models and a heuristic approach to estimating landing dispersions


The ‘landing ellipse’ or region of uncertainty within which an unguided probe to Venus may be expected to land is calculated. The region can be usefully seen as the convolution of three different factors: an initial circular delivery uncertainty which is smeared at a grazing entry angle onto the planetary sphere, an along-track uncertainty due to atmospheric density and vehicle aerodynamic variations during hypersonic entry, and a descent dispersion due to uncertain and/or variable zonal and meridional winds. This decomposition allows the various contributions to be instructively exposed and conveniently traded-off, without conducting explicit entry and descent dynamics simulations. It is seen that for descent durations and delivery errors typical of past Venus missions, the zonal wind contribution (determined with an analytic fit to Pioneer Venus tracking data) generally dominates, causing a ~200 km E–W (99%) dispersion, with meridional dispersions being about 4 times smaller. However, when entry angles become shallower than about 8°, the along-track dispersions may dominate, with the resulting ellipse becoming longer or wider depending on the entry azimuth. The analytic wind descriptions presented here may be applied to scientific problems, such as the dispersal of volcanic plumes or impact ejecta.

Full Citation:

Lorenz, R. (2015). Touchdown on Venus: Analytic wind models and a heuristic approach to estimating landing dispersions Planetary and Space Science, 108, 66-72 DOI: 10.1016/j.pss.2015.01.003

Thursday, July 9, 2015

Quick Look: Computer model shows imaging of Venus surface possible from balloon

From the April 2015 edition of Solar System Research:


Resolving the surface details on Venus in the balloon- or lander-borne images with a computer modeling method


Due to the presence of opaque clouds at high altitudes, it is difficult to survey the surface of Venus in the optical spectral range. At the same time, in the under-cloud layer, there are transparency windows at the wavelengths λ = 1.08, 0.85, and 0.65 μm. At these wavelengths, the gaseous absorption (in the whole atmosphere rather than only in the under-cloud layer) is weaker, and the atmospheric transparency is mainly determined by the scattering on molecules. The paper presents the results of the Monte-Carlo computer modeling of the imaging of the surface from a balloon or a lander. It has been shown that the imaging from the lower boundary of the clouds is possible.

Full Citation:

Ekonomov, A. (2015). Resolving the surface details on Venus in the balloon- or lander-borne images with a computer modeling method Solar System Research, 49 (2), 110-113 DOI: 10.1134/S003809461502001X

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