Mars isn't Red/Life On Mars

Levin, a physicist now at Lockheed Martin in Phoenix, knew exactly how to tell if something was amiss. Two years earlier he had written a paper titled “Solving the Color-Calibration Problem of Martian Lander Images.” Like earlier Mars landers, each rover carries a color-calibration target—a set of primary-color squares used as a reference for its cameras. If the settings are correct the, squares seen through the rover’s cameras look about the same as matching squares on Earth. Levin tracked down Mars images that included a view of the colored squares, and what he saw confirmed his fears: “When the color-calibration target is in the same scene as the Martian surface and sky, it looks completely different. The blue panel is red. It’s as if NASA color-coded blue to be red, and green as a mustard-brown color.” The results dramatically transform Mars from an ocher planet to a red one.

The myth of a red Mars should have died in 1998, when the Pathfinder imaging team finished analyzing 17,050 images from the mission. The researchers conclusively showed that the predominant colors of Mars are yellowish brown, with only subtle variations. Subsequent “true color” images of Mars from Hubble duly show a yellow-brown planet. More recently, images from the European Space Agency’s Mars Express orbiter in January and February of 2004 present Mars as a world awash in browns, blues, golds, even olives—hence Ron Levin’s surprise and dismay at seeing the garish old red Mars resurface in the cutting-edge pictures from Spirit and Opportunity

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Most of the red Mars images resulted from using filters out of the range of human vision. Even recent rover panoramas and close-ups labeled “approximate true color” are made with infrared filters standing in for red. Olivier de Goursac, an imaging technician on the Viking Lander mission, argues that the glut of phony colors is easily avoidable. “NASA’s rovers have the capability for true-color imaging with the left camera eye, but they simply choose to use the L2 filter [infrared] as their red and the L7 filter [near-ultraviolet] for their blue,” he says. “They do this because they want to maximize the data stream by sending back to Earth images that can be readily used for stereo imaging with the widest possible range in the spectrum.”

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If the Martian atmosphere were to be completely cleansed of dust, the daytime sky would appear blue, just as our own sky, because of Rayleigh scattering by the molecules (primarily carbon dioxide molecules) which make up the atmosphere. Pictures from the Hubble Space Telescope in the early 1990s suggested that the Martian atmosphere had much less dust loading than in the Viking years. So perhaps the Martian sky was closer to blue than in the Viking years(or perhaps the Hubble Space Telescope was inaccurate on this matter until repairs were completed in February 1997). However, Mars Pathfinder pictures in 1997 showed essentially the same sky color and dust loading as the Viking landers in 1976.

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The first color image (12A006/001) of the surface of Mars was taken July 21, 1976, at the Viking 1 site, one day after the landing. Immediately displayed on color monitors at JPL, as seen in Figure 1a, the landscape awed observers with its resemblance to that of Arizona. Typical desert colorations of soil and rock, ranging from umber sand to yellowish-brown and olivine-colored rocks stood out clearly under a blue sky. Two hours later, however, the official image was changed to the monotone of orange-red (NASA P-17164), Figure 1b, that, with few exceptions, has prevailed in NASA-published images of Mars ever since, as presented by Mutch et al.[1]. However, a spectral analysis of color images of the Viking 1 site reported[2] a broader palate. The paper made the first, and perhaps only, reported use of JPL’s Image Processing Laboratory to analyze digitally the red, green and blue color channels of the images taken by the Viking 1 lander camera. In addition to studying the color images, their RGB components were transformed into saturation, hue and intensity components to enhance subtle deviations. When these components were equally amplified to produce an equal average sensitivity over the spectral bandpass, the resulting “radiometric” (closest possible approach in appearance to a human observer) images very closely resembled the first color image (12A006/001). Among the range of colors, the paper reported that some of the rocks exhibited greenish patterns that apparently changed between images taken 301 sols apart. Radiometric images of lichen-bearing terrestrial rocks taken and processed through the same system as were the Viking images showed a close resemblance of the lichen colonies to the greenish patches on the Mars rocks. Inclusion in the analysis of three near-IR channels available on the Martian images enhanced the greenness of the patches that were, to the sensitivity of the method, virtually indistinguishable from the lichen colonies on the terrestrial rocks.

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