Hi Ed,

Happy New Year....

The Viking data sets languished from, seems like, the mid 80s until MGS arrived at Mars, when the new MGS images created a need to use Viking images for context and for comparisons over time. During the interregnum advances in computer imaging weren't much applied to the old data.

Yeah, dealing with this data is a tough job, and NASA needed to pay someone money to work with these images, but they never did.

Sure they did. Washington U. in St. Louis was the primary center, I think, and they've ended up the primary curator of the data set. After an initial flurry of activity with a lot of PIs looking at the new data, Washington U. continued working with it through the 80s.

I rather like working with Viking Lander images, because they're geometrically stable, and the broad array of filters makes it relatively easy to create color...some kind of color...

...but there is the problem of calibration. The Landers were equipped with color reference charts that were supposed to provide onsite calibration, but after arrival they were found to have been faded by ultraviolet exposure, and covered with a fine sheen of dust. They're partially in shadow, too, messing up the balance between color channels. The same problems affect the painted-on flag. All in all, it's proved impossible to use them to calibrate the images for color, and that's lead inevitably to controversy.

There's a persistent story that the first image to be received showed a blue sky, and NASA officials hastily ordered the color jiggered to show the expected red sky, providing much fodder for conspiracy theorists. My theory (which has gotten me denounced regularly by the conspiracy nuts as an "apologist for JPL") about what happened is that the first image was automatically balanced against the recorded color calibration data, which was determined later to be way off, and the second version with the day-glo orange sky was the uncalibrated version. The Viking filters were heavily biased toward red, and in calibration red must be heavily suppressed. The truth, as is usually the case, lies somewhere in the middle. Reasoning from first principles, Rayleigh scattering will make the sky bluish when clear, and dust will shift it toward orange-red. At different times, images with brown skies and images with blue skies are probably correct. Bright orange, though...nahhh.

Okay, given what we have, and the resources available, what next? How about just taking the images over to a white sky, which should give true geological colors as they would be seen in a laboratory?

I don't know as a white sky would be a good reference point, for the reasons I gave in the last paragraph. Pathfinder images are based on some fairly well-calibrated filters, and I think the most promising approach to "retro-calibrating" Viking images is to apply what we learned from Pathfinder. The sites are different and so the details of the rocks and things will be different, but perhaps the coating of surface dust is a global constant?

This seems representative of the (current) consensus about the "true" color of Mars, based on Pathfinder images:

Caveat: The text says "The color balance was adjusted to approximate the true color of Mars.". Sounds like there's still a bit of human taste influencing the results.

And if you try to compare one total Mars mission cost with enhancement of an earlier data set, you have to use logarithms for the scales.

Definitely. MGS and Odyssey haven't made the old data obsolete. Even Mariner 9 data comes in handy from time to time, checking for changes to large scale features over a three decade time frame. That element of time is particularly valuable.

About the lack of a panorama--the Viking Landers had two cameras in a wide binocular arrangement, and neither one had a full 360 degree range. It was more like 210, if I remember right. Between them they covered 360, but because the cameras were about 3 feet apart, the perspective each shows is different, and it's not possible to stitch images together from different cameras.

--Robert