This close-up of a 70-kilometer-wide region of the north polar cap in summer shows some of the internal structure of this ice cap. The white layer at the top of the image is the surface of the polar ice cap. Below this, the orange region is a cliff that is about 500 meters high, containing a series of layers that average about 50 meters thick and appear to be composed of dust and ice. Additional layering with smaller thicknesses could be present, but would not be detectable at the resolution of this image. The regularity of the layering may result from periodic variations in the orbit of Mars (on Earth, such changes may be partially responsible for the ice ages). These orbit changes may affect the frequency and strength of storms that deposit the dust in these layers. The cliff has apparently been created by erosion, possibly by wind. The dark, ripple-textured areas are believed to be dunes that formed from material eroded out of the layered cliffs. The maximum thickness of the ice cap is unknown, but is estimated to be between 4 and 6 kilometers.
Viking Orbiter mosaic P-18459