Today's Mars Photo is a comparison of two photos taken by the Hubble Space Telescope in 2001. In the June 26 2011 image a dust storm has begun in Hellas Basin (the oval at the 4 o'clock position of the image) and at the northern polar cap. The image from September 4 2001 shows the entire surface obscured by the dust storm. This comparison image shows the largest dust storm seen on Mars in decades and illustrates how quickly these legendary Martian dust storms form. There is no doubt that if we ever send a manned mission to Mars we will have to prepare for these huge storms, which seem to be a annual occurrence during the summer.
I found this image while looking at an old article on Space Today Online, so the image will link to the article, which has more details on Martian sand storms.