Today's Image of Mars shows frosted ground in the Southern Hemisphere of Mars during late fall, as imaged by HiRISE. The frost in this image is primarily carbon dioxide frost that began to accumulate as average temperatures dropped. Most of the frost is carbon dioxide frost because the Martian atmosphere is primarily carbon dioxide, which condenses onto the surface, not like the nitrogen that makes up most of the Earth's atmosphere and remains vapor at all Earthly temperatures.
In this image the frost is blueish and primarily on south-facing slopes. The slopes facing north receive more sunlight so they contain less frost and appear red. During the winter months the frost will accumulate more heavily as the Sun becomes dimmer and less heat reaches Mars. Come spring the frost will sublimate until the next fall, and thus is the cycle on much of Mars.
Clicking on this image will take you to the original high resolution image from HiRISE. [See the HiRISE caption for this image]