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Hellas Planitia is a plain located within the huge, roughly circular impact basin Hellas located in the southern hemisphere of the planet Mars. Hellas is the second or third largest impact crater and the largest visible impact crater known in the Solar System. The basin floor is about deep, deeper than the Moon's South Pole-Aitken basin, and extends about east to west.〔The part below zero datum, see Geography of Mars#Zero elevation〕〔(Remote Sensing Tutorial Page 19-12 ), NASA〕 It is centered at Hellas Planitia is in the Hellas quadrangle and the Noachis quadrangle. ==Description== With a diameter of about , it is the largest unambiguous impact structure on the planet, though a distant second if the Borealis Basin proves to be an impact crater. Hellas Planitia is thought to have been formed during the Late Heavy Bombardment period of the Solar System, approximately 4.1 to 3.8 billion years ago, when a large asteroid hit the surface. The altitude difference between the rim and the bottom is . The depth of the crater (〔 ( ) below the standard topographic datum of Mars) explains the atmospheric pressure at the bottom: 12.4 mbar (0.012 bar) during the northern summer .〔"...the maximum surface pressure in the baseline simulation is only 12.4 mbar. This occurs in the bottom of the Hellas basin during northern summer", JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. El0, PAGES 23,317-23,326, OCTOBER 25, 2001, On the possibility of liquid water on present-day Mars, Robert M. Haberle, Christopher P. McKay, James Schaeffer, Nathalie A. Cabrol, Edmon A. Grin, Aaron P. Zent, and Richard Quinn.〕 This is 103% higher than the pressure at the topographical datum (610 Pa, or 6.1 mbar or 0.09 psi) and above the triple point of water, suggesting that the liquid phase could be present under certain conditions of temperature, pressure, and dissolved salt content.〔(Making a Splash on Mars ), NASA, 29 June 2000〕 It has been theorized that a combination of glacial action and explosive boiling may be responsible for gully features in the crater. Some of the low elevation outflow channels extend into Hellas from the volcanic Hadriacus Mons complex to the northeast, two of which Mars Orbiter Camera images show contain gullies: Dao Vallis and Reull Vallis. These gullies are also low enough for liquid water to be transient around Martian noon, if the temperature would rise above 0 Celsius.〔 para 3 page 2 Martian Gullies Mars#References〕 Hellas Planitia is antipodal to Alba Patera. It and the somewhat smaller Isidis Planitia together are roughly antipodal to the Tharsis Bulge, with its enormous shield volcanoes, while Argyre Planitia is roughly antipodal to Elysium, the other major uplifted region of shield volcanoes on Mars. Whether the shield volcanoes were caused by antipodal impacts like that which produced Hellas, or if it is mere coincidence, is unknown. Wikiterracimmeriaboundaries.jpg|MOLA map showing boundaries of Hellas Planitia and other regions Hellas basin topo.jpg|Geographic context of Hellas Image:Twisted Ground in Hellas.jpg|Apparent viscous flow features on the floor of Hellas, as seen by HiRISE. Image:Twisted Terrain in Hellas Planitia.jpg|Twisted terrain in Hellas Planitia (actually located in Noachis quadrangle). 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「Hellas Planitia」の詳細全文を読む スポンサード リンク
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