Dark areas on surface, what are they?

I've been looking at some mars photos and found these dark areas in some places. This is apartcularly dark one. What is this, do you have any idea?

[img]https://www.flickr.com/photos/92172119@N02/14335446481/[/img]

I don't know if the image is displaying correctly...

Dark spots are probably Bromine Geyser.