That was the debate in 1976 when NASA's Viking 1 lander became the first spacecraft to touch down on the Red Planet. Did Mars have a blue sky, like Earth's, or was it a sky of a different color? The first pictures to be sent back showed an Earthlike blue sky, leading some to hope that Mars could sustain life at the surface.
Viking team member Carl Sagan dashed those hopes at a news conference the following day. "Despite the impression on these images, the sky is not blue," he announced.
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A round of groans swept through the press corps. "Typical Earth chauvinist response," Sagan said, turning the groans to laughter. "The sky is in fact pink, which is an OK color," he added.
The problem was that Viking's filtered images had to be calibrated to get the right color balance. The first try didn't quite hit it right. Ever since then, pictures from Mars have been adjusted to reflect the reddish tinge provided by iron-rich dust suspended in the thin atmosphere. However, some conspiracy theorists —and some scientists as well — insist that the Red Planet is more bluish-green than NASA's pictures suggest.
The Martian color question has figured into the search for life on Mars for decades: On several occasions, scientists reported finding evidence for the spectral signature of chlorophyll. Such evidence might hint at the presence of lichen on Mars, but the tentative reports have never been confirmed.
Jim Bell, an astronomer at Arizona State University who is the lead scientist for the Opportunity rover's panoramic camera, has acknowledged that getting the colors right "is not an exact science." To make the job easier, color calibration targets have been installed on Opportunity as well as the Curiosity rover. Pictures from Mars are adjusted so that they reflect the colors and brightness as seen on the targets.
NASA provides several flavors of Mars imagery — including the raw, unprocessed color pictures; true-color versions that aim to duplicate what astronauts would see if they were on Mars; and white-balanced versions that show what a Martian landscape would look like under Earthlike lighting conditions.