You've heard of a green flash on the sun. But a green flash on Jupiter? "I've never come across one before," says atmospheric optics expert Les Cowley. Until now, that is. Spanish astrophotographer Juan Manuel Perez Rayego captured the rare phenomenon on Dec. 26th:

"I was taking one last photo of the Great Conjunction between Jupiter and Saturn, just saying goodbye," says Rayego. "Suddenly, a green fragment of Jupiter split off and floated away from the planet. It was spectacular."

"I've analyzed Juan's image and conclude that it is very likely a mock mirage--the same type of mirage that can create green flashes on the sun," says Cowley.

Mock mirages are caused by atmospheric temperature inversions, in which layers of air are warmer than usual. An extra 1 or 2 degrees Celsius is all it takes. Inversion layers can be quite close to ground. Indeed, Jupiter was only 1/3rd of a degree above the horizon of Arroyo de San Serván, Spain, when Rayego recorded the flash.

The low altitude of Jupiter is why the planet looked like a rainbow-colored smear when the flash occurred. The low atmosphere acts like a prism, spreading the light of stars and planets into their R-G-B components. In this wider-angle animation, Saturn may be seen as well (lower right). It too looks like a miniature rainbow, albeit without the green flash.