Feedback
Science

Color Controversies Started With Mars, Not With #TheDress

This white-balanced view of Mount Sharp from NASA's Curiosity rover shows the Martian mountain as it would look under Earthlike lighting conditions. Because of the color adjustment, the sky looks blue rather than its natural red. NASA / JPL-Caltech / MSSS

It may seem like a trivial pursuit to argue over whether a dress is blue and black, or white and gold, but the question of color balance can apply to weightier matters as well — such as whether there’s life on Mars.

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.

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.

Image: Three views of Mars
These three versions of the same scene on Mars, captured by NASA's Curiosity rover, reflect three different choices that scientists can make in presenting the colors recorded by the camera. The left version is the raw, unprocessed color view as it is received directly from Mars. The center rendering is an estimate of the "natural" color that humans would see if they visited Mars. The right version shows the result of white-balancing, which interprets the scene as if it were viewed under Earthlike lighting conditions. NASA / JPL-Caltech / MSSS

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.

To make things even more complicated, scientists also produce "stretched-color" images that optimize color contrast to highlight compositional differences in the Martian terrain. That's why many of the color pictures from the high-resolution camera on NASA's Mars Reconnaissance Orbiter look bluish rather than reddish.

And it's not just a question of blue vs. red: Curiosity's science team was amazed to discover that beneath their reddish exterior, Martian rocks are actually gray.

Why Is Mars Red? 0:42