The smaller number of impact craters on the plains compared with the southern highlands indicates that they formed after the decline in impact rates between 3.8 and 3.5 billion years ago. The plains can be divided into two broad areas: the volcanic plains of Tharsis composed largely of lava flows and the northern plains. The northern plains have remarkably little relief. They encompass all the terrain within 30° of the pole except for the layered terrains immediately around the pole. Three broad lobes extend to lower latitudes. These include Chryse Planitia and Acidalia Planitia (centred on 30° W longitude), Amazonis Planitia (160° W), and Utopia Planitia (250° W). The only significant relief in this huge area is a large ancient impact basin, informally called the Utopia basin (40° N, 250° W).
Several different types of terrain have been recognized within the northern plains. In knobby terrain, numerous small hills are separated by smooth plains. The hills appear to be remnants of an ancient cratered surface now almost completely buried by younger material that forms the plains. Various plains have a polygonal fracture pattern that resembles landforms found in permafrost regions on Earth. Others have a peculiar thumbprintlike texture, possibly indicative of the former presence of stagnant ice.
The origin of the low-lying northern plains remains controversial. Parts appear to be formed of lava, like the lunar maria. But some scientists have proposed that they were formerly occupied by ocean-sized bodies of water that were fed by large floods and that the surface of the plains is composed of sediments.
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