Venus Express Options

[tedstryk]

If all goes well, Venus Express will be a major topic for discussion in this forum a year from now. Does anyone know how good the surface coverage will be from VIRTIS and VMC? My understanding is that VIRTIS will obtain low resolution multispectral maps, and that VMC will, in addition to cloud monitoring, have one channel that can see the surface, but I don't know at what resolution or at what quality. It will be nice to have some non-radar images of Venus' surface besides the Venera snapshots and the shadowy images from Earth and Galileo's NIMS.

Ted

[edstrick]

On Cassin, the VIMS is the best instrument we could design with early 90's mid-infrared imaging technology.

We now have real mid-infrared imaging detectors. Not 10 megapixel, but still real camera chips, that can take quality images with really good resolution in the mid-IR (1 to 5 micrometers, more or less). We just @#$@# didn't back then.

[JRehling]

On Cassin, the VIMS is the best instrument we could design with early 90's mid-infrared imaging technology.

We now have real mid-infrared imaging detectors.  Not 10 megapixel, but still real camera chips, that can take quality images with really good resolution in the mid-IR  (1 to 5 micrometers, more or less).  We just @#$@# didn't back then.

It seems apparent that the 938nm wavelength, which is the best one ISS has to use on Titan, is not the best penetrator of Titan's haze, and that VIMS can thereby make up for (some of) ISS's edge in intrinsic resolution by utilizing better IR wavelengths. Not only by choosing the best possible single wavelength (the best ones are longer than 938nm, at other spectral "holes"), and taking one-wavelength images in that. Surely VIRTIS will beat ISS in that respect. On the other hand, CO2 will block different holes than CH4, and I'm not sure if it leaves us a better hole than CO2 or not.


I think the best IR products of Venus's surface will come from compiling repeated coverage of the same areas and "stacking" or somehow integrating the data to average out over varying cloud thicknesses.

Finally, note that IR is complicated terribly at Venus, but not at Titan, by thermal effects. If all we see is a thermal record of the surface, then we may not get much more than crude altimetry we already knew about. Sorting out the albedo may be impossible.

[Gsnorgathon]

I suppose it's possible, if not easy, to subtract images taken at night from daytime images. "Not easy" might be too much of an understatement.

[JRehling]

I suppose it's possible, if not easy, to subtract images taken at night from daytime images. "Not easy" might be too much of an understatement.

One problem is, can you project an image onto a map accurately? It depends upon whether you are looking at a cloud-level sphere or a surface sphere. And there is undoubtedly refraction. My guess is that you would assume that a cloud level sphere is what you're looking at, assume that when you look at a point in the clouds you are trying to collect albedo data from a diffuse area centered below that point, and use multiple images to integrate observations.

Subtracting night from day -- what does this buy? There will be a lot of thermal interference at both times, but you can't assume that it is precisely the same. You'll get some combination of thermal inertia and albedo -- messy!

When it comes right down to it, trying to observe Venus from above the clouds and learn a lot is pretty tricky. Unless some mineral of interest has a heck of a wicked IR spectrum and that happens to fall inside one of the CO2 windows, I'm just plain skeptical that we'll get good geological data from this mission. Hope I'm wrong.

Surface spectra are, IMO, going to have to come from below the clouds. That still doesn't solve the problem of atmospheric interference, but the fact that the horizons are so clear in Venera imagery provides a lot of cause for optimism. A camera-to-horizon "chord" of 5km will cut through more than or the same as atmospheric interference than a vertical line from surface to clouds (surface air is the densest, and a horizontal chord is entirely through dense air, whereas a vertical slice only goes through the densest air at the bottom). So, with clear images at the surface, I think we can get good images from several km up. A balloon could do the trick.