Mars, surface features
Mars, surface featuresImpact craters dominate Mars' mountainous southern hemisphere – and may be as old as 3500 million years – but are thinly scattered over the younger and mainly volcanic surface of most of the low-lying northern hemisphere. In addition there are white polar caps, volcanic features (see Mars, polar caps; Mars, volcanoes), and areas, particularly in equatorial regions, that show evidence of running water in the past. The existence of surface plates and ruts over an extensive area of the equatorial Elysium Planitia has led some scientists to conclude that a frozen subterranean sea lies in this region.
Many of the Martian craters show the effects of erosion by dust, which is generally of a higher albedo than uneroded surface rocks and forms bright deposits in low-lying terrain such as crater floors and the southern hemisphere impact basins of Argyre Planitia and Hellas Planitia. In places the dust is swept into ridges or dunes by the wind. Bright or dark streaks occur in the lee of some craters where the prevailing wind has piled up the dust or has scoured the surface to reveal the underlying darker bedrock. Variations in dust deposition are responsible for albedo and outline changes of the darker markings, or maria, once attributed to life processes.
Southeast of the volcanoes of the Tharsis Ridge lies the complex equatorial canyon system of Valles Marineris, which measures 4500 km from east to west and 150 to 700 km from north to south. Individual canyons are up to 200 km wide and 7 km deep, dwarfing the maximum 28 km width and 2 km depth of Earth's Grand Canyon. They appear to result from faulting and collapse of the Martian surface – a process that may be continuing today. Other fault systems are found elsewhere, all of them apparently associated with areas of volcanic activity.
Smaller valleys, or channels, meander for up to 1000 km or more across equatorial areas of Mars. They appear to have been formed by running water derived, at least in part, from rain falling on the planet's surface. Some channels begin in areas of collapsed terrain as though the water they carried was originally frozen as a subsurface layer of permafrost that was subsequently thawed and released by volcanic heating. The enigma is that neither rain nor surface water can survive on Mars given present atmospheric constraints. Possibly the channels indicate that Mars has experienced spells of milder climatic conditions when the atmosphere was thicker than it is now, and the carbon dioxide could have trapped solar energy to warm the planet. Unfortunately, there is not enough carbon dioxide locked up in the polar caps today to support this view. In Mars' past, however, there may have been fluctuation in the Sun's output, variations in the planet's axial tilt, or periods of enhanced volcanic activity, during all of which polar and subsurface ice may have been released as liquid water. Further evidence for the existence of a permafrost layer comes from the shapes of the ejecta blankets surrounding some large craters.