|
Valles Marineris (Latin for ''Mariner Valleys'', named after the ''Mariner 9'' Mars orbiter of 1971–72 which discovered it) is a system of canyons that runs along the Martian surface east of the Tharsis region. At more than long, wide and up to deep,〔(Vallis.Marineris )〕〔(WTP: Mars: Valles Marineris )〕 the Valles Marineris rift system is one of the largest canyons of the Solar System, surpassed only by the rift valleys of Earth. Valles Marineris is located along the equator of Mars, on the east side of the Tharsis Bulge, and stretches for nearly a quarter of the planet’s circumference. The Valles Marineris system starts in the west with Noctis Labyrinthus; proceeding to the east are Tithonium and Ius chasmata, then Melas, Candor and Ophir chasmata, then Coprates Chasma, then Ganges, Capri and Eos chasmata; finally it empties into an outflow channel region containing chaotic terrain that ends in the basin of Chryse Planitia. It has been recently suggested that Valles Marineris is a large tectonic "crack" in the Martian crust. Most researchers agree that this formed as the crust thickened in the Tharsis region to the west, and was subsequently widened by erosion. However, near the eastern flanks of the rift, there appear to be some channels that may have been formed by water or carbon dioxide. ==Formation== There have been many different theories about the formation of Valles Marineris that have changed over the years.〔Cabrol, N. and E. Grin (eds.). 2010. Lakes on Mars. Elsevier. NY〕 Ideas in the 1970s were erosion by water or thermokarst activity, which is the melting of permafrost in glacial climes. Thermokarst activity may contribute, but erosion by water is a problematic mechanism because liquid water cannot exist in most current Martian surface conditions, which typically experience about 1% of Earth’s atmospheric pressure and a temperature range of to . However, scientists agree that there was liquid water flowing on the Martian surface in the past. Valles Marineris may have been formed by flowing water at this time. Another hypothesis by McCauley in 1972 was that the canyons formed by withdrawal of subsurface magma. Around 1989 Tanaka and Golombek proposed a theory of formation by tensional fracturing. The most agreed upon theory today is that Valles Marineris was formed by rift faults like the East African Rift, later made bigger by erosion and collapsing of the rift walls. Because Valles Marineris is thought to be a large rift valley, its formation is closely tied with the formation of the Tharsis Bulge. The Tharsis Bulge was formed from the Noachian to Late Hesperian period of Mars, in three stages. The first stage consisted of a combination of volcanism and isostatic uplift; soon, however, the volcanism loaded the crust to a point at which the crust could no longer support the added weight of Tharsis, leading to widespread graben formation in the elevated regions of Tharsis. Stage two consisted of more volcanism and a loss of isostatic equilibrium; the source regions of the volcanism no longer resided underneath Tharsis, creating a very large load. Finally, the crust failed to hold up Tharsis and radial fractures, like Valles Marineris, formed. Stage three mainly consisted of more volcanism and asteroid impacts. The crust, having already reached its failure point, just stayed in place and younger volcanoes formed. Tharsis volcanism involved very low viscosity magma, forming shield volcanoes similar to those of the Hawaiian Island chain, but, because there is minor or no current active plate tectonics on Mars, the hotspot activity led to very long histories of repeated volcanic eruptions at the same spots, creating some of the largest volcanoes in the solar system, including the biggest, Olympus Mons.〔Cattermole, Peter; Mars: The Mystery Unfolds; Terra Publishing; 2001. p. 103-104〕 Landslides have left numerous deposits on the floor of Valles Marineris and contributed to widening it. Possible triggers of landslides are quakes caused by tectonic activity or impact events. Both types of events release seismic waves that accelerate the ground at and below the surface. Mars is much less tectonically active than Earth, and Mars-quakes are unlikely to have provided seismic waves of the required magnitude. Most sizable craters on Mars date to the Late Heavy Bombardment, 4.1 to 3.8 billion years ago (the Noachian period), and are older than the landslide deposits in Valles Marineris. However, three craters (including the crater Oudemans) have been identified, on the basis of their proximity and later dates, as ones whose formation may have caused some of the landslides.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Valles Marineris」の詳細全文を読む スポンサード リンク
|