'Mars red' is open to interpretation

By Alexandra Witze
The Dallas Morning News

	AP A high-resolution image taken by the Opportunity rover's panoramic camera shows a rock outcropping that scientists are planning to investigate. The 4-inch-tall layered rocks are thought to be either volcanic ash deposits or sediments carried by water or wind.
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DALLAS — Depending on how you see it, Mars is the Red Planet or the Pink Planet — or, for that matter, the Orange Planet, the Salmon Planet, or the Butterscotch Planet.

No one can say for certain what color Mars is. With digital photographs now flooding from NASA's Mars rovers, scientists are trying to translate the strings of ones and zeros into images that convey the planet's true hues. Compounding the challenge is the fact that no two people see color — and no two computer monitors display color — in precisely the same way.

"What Mars looks like is very much in the eye of the beholder," said Michael Malin of Malin Space Science Systems in San Diego.

Still, scientists have come a long way from the time when, as late as the 1960s, they hand-colored maps of Mars using crayons.

To calibrate Mars' colors, the twin Spirit and Opportunity rovers rely on a small color-coded key carried on their backs. This square "Mars dial" displays in each corner a chip, similar to a paint store's color sample, of known shades of red, green, blue or yellow.

Where to look for Mars To see Mars for yourself, look for the planet high in the southwestern sky in early evening. Mars will appear as a faint orangish — or is that reddish? — "star" three fist-widths to the upper left of the bright planet Venus. — The Dallas Morning News

Each dial, known as a calibration target, also contains three concentric rings in varying shades of gray, which allows scientists to understand what Mars' black-and-white tones should look like.

The entire target "is essentially a photographer's color chart," said Jim Bell of Cornell University, lead scientist for Pancam, the panoramic camera aboard each rover.

Before the rovers went to Mars, each Pancam photographed the calibration targets again and again, working to understand how different lighting, dust or other conditions could affect the appearance of Martian colors.

Pancam's two "eyes" allow the rovers to see in stereo vision. Electronics have been tested rigorously for flaws, down to correcting for tiny variations in how each picture element responds to particles of light, or photons.

"The goal is to get back this image that represents the real number of photons which have come off of that rock or that piece of soil that we're looking at," said Jonathan Joseph, a computer programmer at Cornell.

To gather as much information as possible, each rover's Pancam carries 14 interchangeable filters, seven for each of its two "eyes." Each filter is designed to study one wavelength of light. By changing filters or combining them, geologists can photograph the surface in many ways, extracting as much information as possible.

Pancam can't take photographs in "true color"; like any digital camera, it can only approximate what the human eye sees. For the most realistic images, Pancam scientists combine images taken through three filters — red, green and blue — into a simulation of what a person might see.

The biggest problem is getting the red exactly right. Scientists tend to prefer to use Pancam's infrared red filter, which reveals more information about a rock's mineralogical makeup, than the filter sensitive to the red closer to what people on Mars might see.

"It's sort of a Catch-22," Joseph said. "To make the nicest pictures for the general public you want to take a certain set of filters. But for the scientists to get back the most scientific information, that's probably not the filters they want to choose."

Some of the rovers' pictures have been taken using an infrared filter (called L2), combined with the blue and green filters, rather than a red filter (L4) combined with blue and green. But colors look quite different when seen in the infrared.

"If your eyes worked the same way, you'd see blue as bright white," Bell said.

When the pictures are printed, the blue essentially vanishes and the red pigment takes over, tinting everything dark red or even pinkish. This explains why some colors in some Mars photos occasionally look a bit funny.

Pancam's results also change if images are taken during various times, when the angle of the sun is different, or if a lot of dust is in the atmosphere. And the landing sites differ: Opportunity's surroundings are much darker than Spirit's.

"Mars is often depicted as this bright red planet," Bell said. "It's not fire-engine red. ... Look for more muted Earthy, or 'Marsy,' kinds of colors."

The Martian sky also can be a challenge. When the first Viking mission landed in 1976, the space agency released its first photo showing a blue sky — the way people on Earth would expect sky to look. After more color calibration, scientists changed it to a pink sky, only to settle on a butterscotch tint — the way people on Mars likely would see sky.

NASA continues to release new pictures from Spirit and Opportunity. Sometimes, this means that the color calibration is a bit off, or that mosaic images aren't patched together smoothly, or there is a black square of missing data in the middle of a photograph.

More information can only improve the photos, Bell said.

The European Space Agency may have learned this lesson the hard way. Last month, it released an image taken by the orbiting Mars Express spacecraft of Spirit's landing site. Because the colors weren't calibrated properly, a great swath of volcanic rock that is in fact gray in color appears green.

Some people have misinterpreted this as evidence for vegetation on Mars, of which there is none.

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