In these views the north polar cap is tilted toward the Earth and is visible prominently at the top of each picture. The images were taken in the middle of the Martian northern summer, when the polar cap had shrunk to its smallest size. During this season the Sun shines continuously on the polar cap. These Hubble telescope snapshots reveal that substantial changes in the bright and dark markings on Mars have occurred in the 20 years since the NASA Viking spacecraft missions first mapped the planet. The Martian surface is dynamic and ever changing.
Winds move sand and dust from region to region, often in spectacular dust storms.
Over long timescales many of the larger bright and dark markings remain stable, but smaller details come and go as they are covered and then uncovered by sand and dust. Image files are available on the Internet. The upper-left image is centered near the location of the Pathfinder landing site. Dark sand dunes that surround the polar cap merge into a large, dark region called Acidalia. This area, as shown by images from the Hubble telescope and other spacecraft, is composed of dark, sand-sized grains of pulverized volcanic rock.