Future Venus Missions Options

[Phil Stooke]

Oh well, might as well start that new topic since it's already well advanced in the Juno area...

My perspective on landers is as follows. All the landers we've had so far were dropped blind onto an essentially unknown surface. Any future landers can be targeted for specific terrains. It really is not true that we have had representative landings. Even a descent image or two, a panoramic photo plus a bit of surface composition, from a simple Venera-class lander just updated a bit, would be useful if we could put several down at well chosen targets. My choices would be:

Examples of the main plains units (smooth, fractured, ridged)

tesserae

high elevation radar-bright tesserae

large fresh lava flow unit ('fluctus')

crater dark parabola

crater ejecta outflow unit

dunes area.

And I have always assumed, rightly or wrongly, that it would be relatively easy to put these down, so they ought to be fairly inexpensive as planetary landers go.

Phil

[Guest_BruceMoomaw_*]

First: it's not the WEIGHT of imaging cameras that's the problem -- it's their huge data return, which requires a flyby or orbiter craft to serve as a com relay. Get rid of imaging, and the landers can relay all the rest of their data directly back to Earth, which greatly simplifies a mission. (There was a debate about this for giant-planet entry probes at COMPLEX, in which Tom Spilker did a very good job of shooting down Scott Bolton's enthusiasm for the idea. But the technical problems that make it hopelessly impractical for the giant planets -- mostly because of the difficulty of having the probe enter at a place that allows DTE transmissions without it having to make a destructively steep dive into the atmosphere -- don't apply to Venus, as we already know from Pioneer 13.)

As for the tesserae, they seem to be areas that have been faulted by being stretched in one direction and squeezed in another (not at the same time), but there seems to be no evidence from their radar appearance that they're actually patchworks of small areas of different materials laid down at different times. It is, however, quite possible that -- if they really are the oldest areas on Venus, and date back to a time when it still had oceans, crustal tectonics, and the start of the formation of granite continents -- they really are a complex mixture of materials. In any case, the way to start investigating this seems to be simply to land on a couple of different tesserae -- or, again, Ishtar Terra, which is a very high-altitude plateaus that seems to be geologically unique on Venus -- with simple fixed landers and see if dramatic compositional differences show up.

And as for aerial observations, keep in mind that any probe too high to illuminate the surface with a flashlamp will only be able to see reflected near-IR sunlight in the five narrow spectral bands which Venus' clouds and air allow to reach the surface at all. (It's the same kind of problem that Cassini or a balloon have for near-IR mapping of Titan.) Those five bands will provide some useful compositional information, but not a really big amount (Kevin Baines discussed this several years ago in "Icarus"), and so it might be better to put such a multi-channel photometer on a balloon in the lower clouds rather than on a lot of separate descent probes. As for descent imaging, it obviously also has its uses -- but, again, to decide where to send surface-composition landers we might get better advance reconnaissance information from a really high-resolution SAR map made by a follow-up orbiter to Magellan, such as has been talked about. (A SAR orbiter capable of obtaining 25-meter resolution images may be doable for costs on the Discovery/New Frontiers borderline.)