Mars has the most extreme seasons of any planet. It is always cold and, at its warmest, temperatures rarely exceed freezing.
The Mars Global Surveyor spacecraft has taken a series of images showing frost moving across the Martian surface.
As spring thaws the northern hemisphere, and winter starts to grip the southern, scientists can see the frost-line migrate, changing the appearance of craters as it passes over them.
Mars has the most extreme seasons of any planet. It is always cold and, at its warmest, temperatures rarely exceed freezing.
It is currently spring in the northern hemisphere, and frost that accumulated during the recent winter has been retreating since May.
Of particular interest to experts is a frosty patch seen in an unnamed crater. In 1976, the Viking Orbiter saw a region of frost in the same place and scientists suspect that it persists throughout the Martian summer.
In the southern hemisphere it is autumn and frost was seen on the rims of some craters, such as Barnard, as early as August. Later, the frost line moved further north, and frost was seen in Lowell crater.
Seasonal variations are greater on Mars than on any other planet with the exception of Pluto. Mars has a more elliptical orbit than Earth, so its distance from the Sun varies more.
At one point during the year, the temperature drops so that carbon dioxide freezes out of the thin Martian atmosphere. The atmospheric pressure decreases by 25% when this happens.