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Originally posted by Phage
If it weren't for the fine dust carried throughout the Martian atmosphere, the sky on Mars would be a dark blue due to Raleigh scattering.
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.
calspace.ucsd.edu...
But because of that dust the Martian sky is a butterscotch color. Earth's atmosphere does not have the dust content that dry, dry Mars does.
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
mars.spherix.com...
Using the calibration target on each rover, scientists have a good knowledge of how to adjust colors according to the circumstances found by the rover cameras on Mars. Scientists saved half of the material for the silicon colored chips on the calibration targets, so they can make comparisons and accurately measure how the rings and chips on Earth are reflecting light on
Before sending cameras to Mars, the team took more than 100,000 pictures in a vacuum chamber on Earth that simulates martian conditions. They experimented with different levels of light entering the lens, depending on the angle of sun or amount of dust in the air. They then fine-tuned the cameras to respond correctly to temperature changes or anything else that might cause the instrument to vary.
"It's crazy how many pictures we took with the Pancam on Earth, but those images were essential to finding the answers on Mars," Jim recounted. Using a "calibration target" with green, red, blue, and yellow silicon swatches and rings of varying shades of gray, the team monitored how those colors changed under different lighting and environmental conditions in the laboratory.
Each rover carries its own calibration target, and is regularly instructed to take pictures of it. With their earth-bound experience, scientists have a good knowledge of how to adjust colors according to the circumstances found by the rover cameras on Mars. They know how much sunlight the three rings of gray, black, and white reflect on the calibration target in the different filters. They also saved half of the material for the silicon colored chips on the calibration targets, so they can make comparisons and accurately measure how the rings and chips are reflecting light on Mars.
marsrovers.nasa.gov...
Originally posted by Phage
The rovers can't provide true color images because they aren't designed to. You can make the color wheel look right but there are still a lot of missing wavelengths.
Originally posted by Astyanax
I think I may be missing something here.
StellarX, you spoke of 'political consequences'. As I understand them, political consequences are things like riots, revolutions, the fall of governments or,
at the very least, the destruction of the careers of leading statesmen. Surely you aren't saying that the discovery of life on another planet is thought so likely to lead to such outcomes that it has to be suppressed?
To me, Lunarminer seems to be saying the same thing too, citing for evidence a half-century-old document called the Brookings Report. Plus the possibility that some leading scientists and bureaucrats might lose their jobs or their reputations.
Surely you guys don't mean that? I mean, that's ridiculous, completely bananas. There must be something more you're hinting at here. What am I not getting?
Originally posted by Phage
Yeah, the link to the sky color article doesn't work anymore. I paraphrased from memory. I would have liked to quote and link it.
The rovers can't provide true color images because they aren't designed to.
Two views of a sundial called the MarsDial can be seen in this image taken on Mars by the Mars Exploration Rover Spirit's panoramic camera. These calibration instruments, positioned on the solar panels of both Spirit and the Mars Exploration Rover Opportunity, are tools for both scientists and educators. Scientists use the sundial to adjust the rovers' panoramic cameras, while students participating in NASA's Red Rover Goes to Mars program will monitor the dial to track time on Mars. Students worldwide will also have the opportunity to build their own Earth sundial and compare it to that on Mars.
The left image was captured near martian noon when the Sun was very high in the sky. The right image was acquired later in the afternoon when the Sun was lower in sky, casting longer shadows. The colored blocks in the corners of the sundial are used to fine-tune the panoramic camera's sense of color. Shadows cast on the sundial help scientists adjust the brightness of images.
marsrovers.jpl.nasa.gov...
So far, however, the images produced are only approximate martian colors. That's because many of the pictures are taken with set of camera filters that include near-infrared or ultraviolet wavelengths, which our eyes do not perceive. Overall, there are 14 "geology" filters (two additional camera filters were designed exclusively to observe the sun). Scientists find these geology filters extremely useful because they provide maximum contrast for analyzing some of the most interesting geological features at the landing site.
"We almost never choose to take the images in natural color, because that's not as helpful to us scientifically," said Eric. "However, we're able to approximate what humans might see because Jim's team lived and breathed with this camera for many years, experimenting to get the colors in the camera models just right."
marsrovers.nasa.gov...
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.”
discovermagazine.com...:int=1&-C=
The filters they use are limited to very narrow sections of the visible spectrum so no matter how they are processed they end up being "false color".
You can make the color wheel look right but there are still a lot of missing wavelengths.
I try to avoid non-working links. I get a 404 error when I click yours and a google search for the text yields me no results.
Originally posted by StellarX
It doesn't take THAT much time to defend what you say with actual sources.
It depends on what photos we see.
Originally posted by lunarminer
On the subject of the sky color on Mars. From what I have seen, the photos that we see from Mars are red saturated. When they are balanced for natural sunlight, the sky is in fact blue.
Originally posted by Phage
Yeah, the link to the sky color article doesn't work anymore. I paraphrased from memory. I would have liked to quote and link it.
Originally posted by Phage
You're right though, if it weren't for the dust. The sky would be blue.
Originally posted by Phage
Or maybe Mars is covered in turquoise.
You're right though, if it weren't for the dust. The sky would be blue.
"Airborne dust on Mars is about as fine as cigarette smoke,"
These fine grains reflect 20% to 25% of the sunlight that hits them; that's why the clouds look bright. (For comparison, the reflectivity of typical martian terrain is 10% to 15%.)