form. Called the polar hood, the clouds drop precipitation which thickens the cap. The north polar cap is symmetrical around the pole and covers the surface down to about 60 degrees latitude. High resolution images taken with NASA's Mars Global Surveyor show that the northern polar cap is covered mainly by pits, cracks, small bumps and knobs that give it a cottage cheese look. The pits are spaced close together relative to the very different depressions in the south polar cap. Both polar caps show layered features that result from seasonal melting and deposition of ice together with dust from Martian dust storms. These polar layered deposits lie under the permanent polar caps. Information about the past climate of Mars may be eventually revealed in these layers, just as tree ring patterns and ice core data do on Earth. Both polar caps also display grooved features, probably caused by wind flow patterns and sun angles, although there are several theories that have been advanced. The grooves are also influenced by the amount of dust. The more dust, the darker the surface. The darker the surface, the more melting. Dark surfaces absorb more light energy. One large valley, Chasma Boreale runs halfway across the cap. It is about 100 km wide and up to 2 km deep—that's deeper than Earth's Grand Canyon. When the tilt or obliquity changes the size of the polar caps change. When the tilt is at its highest, the poles receive far more sunlight and for more hours each day. The extra sunlight causes the ice to melt, so much so that it could cover parts of the surface in 10 m of ice. Much evidence has been found for glaciers that probably formed when this tilt-induced climate change occurred. Research reported in 2009 shows that the ice rich layers of the ice cap match models for Martian climate swings. NASA's Mars Reconnaissance Orbiter's radar instrument can measure the contrast in electrical properties between layers. The pattern of reflectivity reveals the pattern of material variations within the layers. Radar produced a cross-sectional view of the north-polar layered deposits of Mars. High-reflectivity zones, with multiple contrasting layers, alternate with zones of lower reflectivity. Patterns of how these two types of zones alternate can be correlated to models of changes in the tilt of Mars. Since the top zone of the north-polar layered deposits—the most recently deposited portion—is strongly