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GREEN FLASH ON THE RED PLANET Occasionally, when the sun sets behind the waves of the ocean, observers witness an emerald pulse of light just above the vanishing solar disk. Once thought to be a fable, the "green flash" is now known to be real, and is a regular entry on bucket lists. The phenomenon, however, is not limited to the sun. Yesterday, Peter Rosén of Stockholm, Sweden, observed a type of green flash on Mars.    "I was observing Mars through my 9-inch Celestron telescope," says Rosén. "It was setting, only 3° above the horizon, and the atmospheric turbulence was extreme. Suddenly I saw the upper part of the planet getting distorted and turning green." "It looked just like a green flash on the sun," he says. The phenomenon Rosén photographed is related to the ordinary twinkling of stars. Near the horizon, blobs of turbulent air refract and distort light, some colors more so than others, separating red from green and blue. The puff of green rising from the Martian disk is a result of this type of chromatic turbulence. Green flashes on the sun happen for different, but related reasons: According to Les Cowley's atmospheric optics web site: "The classical green flash relies on a mirage to magnify small differences in refraction between red and green light. The mirage occurs when there is warm air immediately over the ocean and the air temperature gradient changes rapidly with height." "I have observed and photographed solar green flashes on multiple occasions," says Rosén, "but this was the first time I caught a green flash on the red planet!" |
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