The truth lies buried in the digital charge-coupled-device cameras used on the recent Mars rovers and orbiters, which do not record color directly. Rather, they collect black-and-white images through various filters that each let through only a narrow band of wavelengths, or colors, some of which may be invisible to the eye. To make a true-color image, the cameras must take three separate images of the same scene, each through a different primary-color filter: red, green, and blue. When all three are put together they can render a true-color composite. Even then, balancing the colors so they closely match what the eye would see has to take into account the effects of dust, changing light levels, and other variables.
WHICH IS THE REAL COLOR?
Courtesy of NASA/JPL/Olivier de Goursac
Courtesy of NASA/JPL
Ironically, NASA’s dedication to the needs of the scientific community may have encouraged the release of the miscolored Mars images. The Spirit and Opportunity rover cameras have two “eyes,” each with eight color filters. The left eye includes the red, green, and blue color filters; the right is dedicated entirely to ultraviolet and infrared. Planetary geologists rely on ultraviolet and infrared data to identify rocks and minerals—the primary scientific goal of the current Mars rover mission—so mission planners try to use those filters as often as possible. Anytime they add invisible wavelengths into the mix, however, they necessarily get a false-color image.
WHICH IS THE REAL COLOR?
Two global views of Mars, both taken by the Hubble Space Telescope, show how subjective the planet’s color is. A composite image created in 1997 (far left) reflects the red-Mars aesthetic of the time. Six years later, Hubble researchers made a true-color, red-green-blue picture (left). The updated Mars has lost most of its red and looks similar to what people see when they look at the planet through a telescope. Surface snapshots should have a similar palette.
(Left: Courtesy of NASA/JPL/MSSS. Right: courtesy of D. Crisp (JPL), the WFPC2 science team, and NASA)
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.”
Using infrared and ultraviolet filters in both of the rover’s eyes allows the imaging team to create three-dimensional panoramas, which are important for guiding the rovers over uneven Martian terrain. Such panoramas are geologically meaningful at the same time. True-color imaging gets much less attention. “The priority is to obtain the stereo coverage so that NASA can have the most accurate information for driving the rovers and making mechanical-
arm placement decisions,” Bell says. “One of my important goals was to try to present the planet in as natural a view as possible. Unfortunately, some of my colleagues do not share this desire for accurate color renderings, and so sometimes planetary images can get quite garish, with no explanation of what is being shown.”
Copyright ESA/DLR/FU Berlin (G. Neukum)
So what does Mars really look like? Finding the answer requires decoding NASA’s imaging systems, isolating information from the red-green-blue filters, and correcting for the exact nature of those filters. A few colleagues and I have done just that for several Mars shots by making a grayscale image from a NASA color image and applying red-green-blue filters to make corrections. Then I run the result through color-rendering software developed for Europe’s Beagle 2 Mars Lander. The corrected views show a planet of mud-brown hills and skies that range from yellow to pink to white to bluish. It is at once more Earth-like and more alien than before—a world to marvel at all over again.
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