Icy Blue Hood On Mars

The north of Mars is getting cold. It's winter and that brings temperatures as low as -140^o C. The most flamboyant sign of the season is a giant icy cloud hanging over the martian north pole--the North Polar Hood.

Amateur astronomer Dan Petersen of Racine, Wisconsin, took the picture on Oct. 7th using his 10-inch backyard telescope. The electric blue color of the Hood is a sign that many ice crystals in the cloud are tiny--smaller than the wavelength of light. Sunlight scattered from sub-wavelength particles looks blue; local examples include cigarette smoke, noctilucent clouds and a New England sky.

Recent photos hint that the Hood is in motion possibly in response to warm dusty air wafting up from temperate latitudes. To monitor developments, point your telescope at the bright red star--make that red planet--high in the eastern sky at dawn.