Microscopic imager

I was a little disappointed to learn that the microscopic imager on the Mars rovers isn't really all that microscopic; it's about as powerful as my Nikon in macro mode. I suppose that to do much better you'd really want to remove samples from their original location and put them on something like a microscope stage.

What color is the sky on Mars?

I'm sensing a job for Phil Plait, the Bad Astronomer here. Ever since 1976, when the Viking landers sent back badly color-corrected pictures of the surface of Mars, there's been an ongoing debate about the sky color (and the color of the ground) there, some of it genuine and some of it ridiculous and conspiracy-based. Over the past week the issue has exploded again, because of the Spirit rover team's willingness to publicize raw photos that haven't been color-corrected at all.

As far as I know, this page by a NASA atmospheric scientist has the best summary of the situation as currently known. The Viking 1 lander sent back pictures that seemed at first, incorrectly, to indicate a blue sky. This was because of overcompensation for a color calibration target on the lander that had its colors skewed by reddish dust (since the Vikings used a soft-landing-with-retrorockets technique, lots of dust got thrown up from the ground and got all over everything). It became difficult to figure out what the correct calibration was; later photo releases from Viking were often absurdly overcorrected and had the ground bright red and the sky Pepto-Bismol pink, which was also wrong. The consensus that emerged was that the sky was more or less tan-colored and the soil reddish-tan, which is consistent with telescopic observations.

The color is mostly because of the absorption characteristics of airborne dust rich in iron oxide. Were the dust not there, the sky would be deep blue, as the atmosphere fringe of Mars sometimes looks in Hubble photos. And it's quite possible that the sky is blue on Mars at some times and places.

Mars Pathfinder in 1997 finally provided a better color calibration opportunity, since it landed with an airbag-tetrahedron technique similar to that used by the Spirit and Opportunity rovers. Some had predicted that it would see a blue sky because of a less-dusty atmosphere, but it didn't; the sky was still tan, at least according to NASA's calibration. Interestingly, there's a blue halo around the sun near sunrise and sunset, almost the reverse of Earth (that's a Pathfinder photo; some Viking photos of Martian sunset looked much more psychedelic because of the primitive digital photo technology used). Pictures from the Spirit lander, once corrected, have shown similar colors.

Now, all this said, there's still some ambiguity involved, something that often gets lost in these discussions (as in other, similar discussions, such as the confusing arguments over whether the sun is actually yellow). The color calibration targets are illuminated by Martian ambient light, not by a characterized standard light source. Were you actually standing on Mars, your eyes and brain would probably adjust for that to some extent, and things might not look that red. Since the sky is undoubtedly less red than the ground, it might actually end up looking more neutral or even blue to you than the corrected NASA photos show. And some people argue that NASA ought to be calibrating the released images that way. (I'm not sure I buy everything in this paper and it seems to shade into the fringey stuff I'll mention below, but it's interesting reading.)




Now for the stupid part. That last paper was in part motivated by Gilbert Levin's claim that, if properly calibrated, olive-green areas show up that might be patches of chlorophyll-bearing organisms. This has, naturally, been blown up into a grandiose conspiracy theory. As far as I can tell, the most popular version goes something like this: In 1976, Viking 1 immediately discovered life on Mars. Once JPL realized that the secret was out, they realized that the discovery of a Mars teeming with vegetation would jeopardize the power and job security inherent in their spearheading the search for life on Mars. So the pictures showing plant life were claimed to be in error, and "corrected" in such a manner that the sky became pink; and since then NASA has been living the lie, bolstering incorrect claims about the color of the sky on Mars to cover up their initial deception.

Speaking personally here, this rings false for anyone who actually knows any scientists. Scientists want money and job security like anyone else, but if that were all they wanted, they'd be in a different line of work, because a career in science sucks as a means of getting these things. Scientists are glory-driven creatures. When they go bad and fake data, it's always a positive result that gets faked. Given the incredible eagerness of NASA to turn any fugitive indication of a once-wet, life-bearing Mars into an opportunity for a shouting press release, it's hard for me to believe that proof positive of life on Mars would be taken as anything other than a bonanza.

So why has the blue-sky-on-Mars business exploded over the past couple of weeks? Well, the uncorrected images coming down from the Spirit pancam, with the channels mapped linearly to RGB, show a blue-gray sky (the best way to see these uncorrected is probably via the Maestro application, which is apparently what the people at JPL use when controlling the rover). The conspiracists saw some of these pictures in photos of JPL press conferences, and concluded that NASA had either dropped its guard, or was distancing itself from the conspiracy because "the jig is up" (because of other countries' Mars exploration plans, perhaps).

But considering that the same photos show the ground littered with sky-blue rocks, I'd be a bit reluctant to take that color balance as authoritative. Maybe they're all turquoises. Or maybe they're covered with blue-green algae.

MarsDial in action

Here's (more or less) what I wanted to see, on the Planetary Society's kids page: Bill Nye's MarsDial in action, with some of the pictures in color. (One caveat: I don't know what filters were actually used to take these pictures; sometimes JPL uses a near-infrared filter in place of the visible red one.)

Anyway, this thing has mirrors on it that reflect the sort of tan-beige sky color (they are at about 3 o'clock and 9 o'clock on the circular rim), and bright primary-colored swatches painted on the four corners. Unfortunately the pictures are a little dark, but messing around with the color balance of these pictures in the photo editor of your choice makes it pretty clear that you can't make the sky blue without screwing up all the other colors to a ridiculous extent. (Clarification: The really crucial thing is that the rings in the center are painted a neutral gray color, and they're less red than the mirror images of the sky are. You can pretty easily make the sky images blue by messing with channel curves in an image editor, but then the gray rings get even bluer than that. I am, therefore, pretty confident that the sky on Mars has a tan cast relative to the total incoming light that is illuminating the target.)

Unless, of course, the conspiracy has already gotten to these pictures and messed with them, or secretly swapped the MarsDial with a mis-painted version to fool everybody. For all you know, I'm part of the plot.