Note: the images reside on the Mars Global Surveyor website:

Mars Global Surveyor Mars Orbiter Camera (MOC)

A Look at “Active” and “Inactive” Martian Sand Dunes

MGS MOC Releases MOC2-199 to MOC2-204, 31 January 2000

Mars has been known for centuries to experience large dust storms, but it wasn’t until the Mariner 9 mission in
1971-1972 that pictures were returned from the planet that showed sand dunes. Dunes are also good evidence
that the martian atmosphere is presently capable of transporting loose sediment and depositing it elsewhere.
Dunes are, in fact, deposits of sand that have accumulated usually over periods measured in thousands of
years. Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) images obtained since September 1997 provide
new clues about martian dune activity. MOC has found that some dunes appear to be moderately active today,
but they do not appear to have moved downwind since the Viking missions more than 20 years ago. MOC also
finds some sand dune fields that appear to be inactive because the dunes are either covered by dust and
landslide debris, or cemented and eroded into ridged and grooved terrain, or because they have many old
impact craters on them. The six sets of pictures presented here show the variety of dunes from recently-active
to solidified and cratered. These pictures were taken at various times throughout the MGS MOC mission.