Blue cloud on Mars

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

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.