Tuesday, October 20, 2009
Venus New Frontiers Radar Mapping Mission
A number of previous blogs have looked at possible NASA missions to explore Venus in the coming decade. (See this blog entry for links to the full set.) As discussed in those blogs, exploration of our sister world is hampered by the lack of funds for a Flagship (>$1B) mission given likely commitments of Flagship class missions to Mars and Jupiter-Europa. A number of missions ideas to study the planet from within the atmosphere or on the surface have been proposed. More recently, a proposal for a New Frontiers (~$650M) mission to continue the radar mapping of Venus has been published in a Decadal Survey White Paper.
Until this paper, I had been under the assumption that radar mapping missions that would meaningfully exceed the Magellan resolution (more on that below) would require a Flagship class mission. In fact, an orbiter estimated at over a $1B to do this has been proposed as part of a Flagship mission for the 2020s. This White Paper and an abstract for the upcoming Fall American Geophysical Union (AGU) meeting makes the case that a capable mission could be done in the New Frontiers program. (It's not clear whether the AGU abstract is discussing a Flagship or New Frontiers class mission, but the science rational remains the same either way. There doesn't appear to be any overlap in the authors of the White Paper and the AGU abstract, so different classes of missions may be being proposed.)
Venus has previously been mapped by NASA's Magellan spacecraft in the early 1990s. The resolutions were crude: 100 - 150 meters horizontal resolution in the mapping mode and 80 meter vertical topography measured every 8 - 10 km. Imaging radar mapped the terrain at 100 - 150 m. As the AGU abstract states, "Our state of knowledge about Venus is currently analogous to our knowledge of Mars in the post-Viking era, and a high-resolution imaging radar mission to Venus could revolutionize our understanding of Venus in the way that the Mars Global Surveyor mission did for Mars."
The crux of the proposed New Frontiers mission is to improve topographic resolution dramatically. Several schemes are discussed that would return different resolutions, presumably at different mission costs. One to two meter vertical resolution at 1 km spacings is given as an example of a reasonable implementation. A synthetic aperture radar (SAR) could also image selected targets at ~10 m for a few percent of the planet. (This is much the same strategy that the very high resolution cameras on Mars orbiters have employed to revolutionize our understanding of that world.) The paper also briefly discusses how the Venus Express instruments have mapped albedo differences in the surface of Venus in the near IR band, with the suggestion that enhanced mapping using this approach might be an additional study for this orbiter.
The White Paper gives a number of examples of how the enhanced radar mapping would advanced our understanding of Venus. I'll give a summary of one example here. Roughly 20% of Venus is covered by highland "continents" which are crossed by linear features known as tesseras. Currently, there are two theories about the origin of the highlands: They represent ancient crustal units, or they have been gradually built up over the history of Venus. The proposed mission would be able to determine if the surrounding plains intersect the tesseras at a sharp angle (indicating ancient origin for the tessaras) or a gradual angle (indicating gradual formation).
The AGU abstract gives a long summary of what might be learned. (I normally don't like to quote this much material from other websites, but the AGU abstracts can't be linked with simple URLs.) The abstract states, "Such a mission would substantially further our understanding of Venus by means of: (1) assessing the fundamental framework of the planet's geologic history (e.g., catastrophic change, slow evolution, uniformitarian) by imaging key stratigraphic contacts; (2) expanding the global framework of geomorphic unit types and relative stratigraphy with reconnaissance surveys of large geographic provinces; (3) directly detecting volcanic and tectonic activity through imaging of flows and fault-related activities (e.g., landslides) that occur between imaging passes; (4) monitoring present-day volcanic and tectonic activity with repeat-pass InSAR deformation studies; (5) constraining the nature of Venusian geologic volcanic and tectonic processes, and their relationship to mantle convective processes; (6) understanding the role of eolian processes in modifying the surface and the use of eolian features as stratigraphic markers (e.g., parabolic features) through detailed examination; (7) constraining Venusian impact processes, particularly the role of the atmosphere in the ejecta emplacement process; (8) constraining the processes responsible for the abrupt decrease in emissivity at high altitudes; (9) selecting landing sites for future missions; and (10) identifying past landers/probes to place them in geologic context."
Past blog entries on Venus mission concepts
White Paper http://www.lpi.usra.edu/decadal/vexag/venusGeoFinal.pdf
AGU website http://agu-fm09.abstractcentral.com/login Search for abstract, P33A-1281, "The Rationale for a New High-resolution Imaging Radar Mission to Venus."