THE rolling landscape of red soil is strewn with dark rocks. NASA鈥檚 brace of rovers have certainly confirmed the picture we have come to expect of Mars. But take a closer look at the images they have been sending back. Is the Red Planet really, well, quite this red?
Welcome to the latest space conspiracy theory. The soft version of the story claims that in a bid to make an ordinary-looking, brownish Mars live up to its billing, NASA has been naughtily tweaking the colours in Spirit鈥檚 digital images. The hard version has the evil NASA doctoring the colours so the rest of us won鈥檛 notice evidence of life, such as patches of green.
Leaving aside the question of why NASA would want to hide such a momentous find, has it been taking liberties with the colours? Talk to NASA鈥檚 image experts and you discover that getting the colours right is a surprisingly difficult 鈥� and, despite the technical wizardry involved, subjective 鈥� job. In fact, truly accurate results, the specialists agree, are not going to happen until people have been to Mars and seen its colours first-hand.
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That said, there are problems related to these Mars rover images, some of them preventable. And in failing to make it clear just what we are seeing, NASA has naively allowed conspiracy theorists a field day.
Although there are standard red, green and blue (RGB) filters on board that can produce a fair approximation of 鈥渢rue鈥� colour, these have hardly been used. Instead, most of the colour images displayed so far have been taken through green, blue and infrared filters (IR-GB). When the infrared gets rendered as red, the results are pretty close to true, but with some really glaring exceptions. Blue and green, in particular, just don鈥檛 come out right. As far as we know, those colours don鈥檛 exist anywhere on the surface of Mars. If they did, we would have noticed them in the few images that have been produced using a normal red filter.
But they are to be found on the spacecraft itself 鈥� hence the conspiracy theories. Standard blue and green paints, it turns out, are extremely reflective in the infrared, even though they hardly reflect any red light at all. So the red-yellow-green-blue colour targets installed on each rover, as well as the bright blue NASA logo, look very strange indeed. So does the blue insulation around much of the wiring. The blue paint reflects more than three times as strongly in infrared as it does in blue. So when the pictures taken with IR-GB filters are printed as RGB, the result is that the red pigment overwhelms the blue and you see a deep burgundy or even, with the insulation, a hot pink. Similarly, the green reflects more than twice as much infrared as it does green, so the green colour patch ends up a sort of mustard colour.
Why have they been doing this? Jim Bell of NASA tells me that it鈥檚 because the important thing is to get the information the geologists need to distinguish rock types, and to tell dusty rocks from clean ones. And for that, infrared is much more useful than red; hence its use for the main panoramic images we have been seeing.
That鈥檚 just part of it, though. Because of the reddish dust that is always in the air, the light falling on the surface of Mars is red to begin with; the effect is likely to be rather like terrestrial lighting close to sunset, when hills take on a pink or magenta hue. And the quality of Mar鈥檚 red light will depend greatly on the level of dust as well as the time of day. That鈥檚 a problem for NASA because the panoramas it has been showing us are mosaics assembled from dozens of separate frames. It may take months of fine-tuning to get the colours consistent between frames.
Still, Bell says, compared to the initial images from Viking, which were way too red, even the initial images from this mission have been closer to what things would really look like there. Better still for Bell, nobody is in a position to argue with him. Until of course someone goes there to check.
