Rhinocrisy

One of my favorite conspiracy theories is that NASA is deliberately false-coloring images so that Mars appears to have a red sky, to cover up the fact that it’s actually blue there. I’m not exactly sure why NASA would be doing this, but I’ll admit I’ve been disappointed by the color of the sky (and the ground) in Mars photos, so I’m on board! Let’s deconvolve:

First, for the uninitiated, our a priori expectation should be a blue sky. The sky everywhere should be blue, because the color of the sky is the product of Rayleigh scattering - which is basically to say, the atmosphere tends to scatter higher-frequency light more than it scatters lower-frequency light. If you look up at any random part of the sky that isn’t the sun, the light you’re seeing is light that has scattered off some part of the atmosphere in that direction; in space, that light would have gone straight to its target, and you would see no light coming from that direction. Since the atmosphere scatters violet light more than red light, this, combined with our human visual system’s spectral bias (weak in the blue end of the spectrum), results in our seeing the sky as blue. This simple phenomenon should apply equally well to all atmospheres.

So it’s a bit glum when we’re told that Mars has a red sky - red because it’s full of thick, choking dust. Well, poop. But maybe it’s not so! Maybe NASA is attempting to pull the dust over our eyes, and Mars is really true-blue. Fortunately, we can verify for ourselves. NASA puts up “raw” JPEGs of all the data the rovers send back. The relevant images are the “PANCAM” ones, which apply a series of fairly narrow bandpass filters before the CCD capture (that is, each filter captures an image of the scene at a specific wavelength of light).

The NASA “true color” images are generated by the PANCAM group at Cornell; their methodology is quite rigorous. The naive method (employed by most conspiracy theorists, notably Keith Laney) is simply to use the three filters that closest approximate the human visual peaks - 600, 530 and 480 nm, for Red, Green and Blue - and slap them together with Ye Olde Photoshop (or in my case, Perl) to make a full-color image. This produces very satisfying images. Check it out!

However, this method has a flaw: the human visual system is additive, meaning that single-wavelengths don’t give the whole picture - each color opsin in your eye is stimulated by the whole spectrum, meaning that what you see as “red” might actually be an amalagam of two individually non-red peaks.^*

Unfortunately, we don’t have data from the whole spectrum available to us - we only have eight wavelengths, six of them in the visual range. PANCAM takes this data and fits a third-order polynomial to it to generate an approximation of the true spectrum. This spectral data is then converted into the XYZ color space (a standard color space) by convolving it with the XYZ standard observer functions that (more or less) define the primary colors of that space.^† Those XYZ values are then mapped to the familiar sRGB space and slapped together to produce a “true color” image. The result, side by side with the “naive” method:

Egad! There’s a world of difference there. So who’s got it right? Hard to tell… unfortunately PANCAM doesn’t have any pictures of the color calibration target that sits on top of the rovers posted in their collection of true color images, so it’s difficult to be sure. However, the average spectra values in the data for the above image for the sky (blue line) vs. the ground (red line) look like so:

This seems to suggest there’s something off about the PANCAM results - the sky should be white-tending-to-blue, and the ground should be red. Of course, it’s possible our visual system is so heavily red-skewed that we’d still see the spectrum on the left as reddish, but I’m inclined to disbelieve it could see the deep, dusty red shown in the PANCAM image. It seems more likely the reconstruction method is flawed somewhere. Furthermore, the naive method should be pretty good at telling us the color of the sky, since the sky color is composed of all wavelengths in varying intensities (which may not be true of rocks on the ground, meaning those colors are more likely to be wrong in the naive method). I’m staying aboard this conspiracy ship!

^* Leading me to wonder if we will, one day, encounter a fully spectrographic visual system - one that sees spectra instead of colors. You’d recognize the chemical composition of everything!

^† Sloth prevents me from recapitulating their method - all the necessary data is linked from here, if you feel inclined to do it for yourself.