Today's Image of Mars shows frosted gullies in a North Polar region crater. This image was taken in the summer months on Mars. The frost has survived because it lies within the North facing alcoves of the gully and is constantly shrouded in shadow. There are similar gullies on the opposite side of the crater, but because they are facing south the frost inside them has melted.
The gullies depicted in this image have the large alcove at the top of the crater wall and channels that flow down into the crater. Some of these gullies show current activity, but it seems that the most activity is at the height of the winter season when the temperatures are the lowest, which leads scientists to believe that the activity is caused by falling loose material and not liquid water.
This image was taken by HiRISE. Clicking on the image will take you to the original captioned image from them.
If you like the HiRISE images showcased in the Mars Photo of the Day posts I encourage you to enter for a free 2012 HiRISE Calendar