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Vastitas Borealis (Latin, 'northern waste' 〔Charlton T. Lewis, Charles Short, ''A Latin Dictionary'', Oxford. Clarendon Press. 1879. ISBN 0-19-864201-6 ()〕) is the largest lowland region of Mars. It is in the northerly latitudes of the planet and encircles the northern polar region. Vastitas Borealis is often simply referred to as the northern plains or northern lowlands of Mars. The plains lie 4–5 km below the mean radius of the planet. To the north lies Planum Boreum. A small part of Vastitas Borealis lies in the Ismenius Lacus quadrangle. The region was named by Eugene Antoniadi, who noted the distinct albedo feature of the Northern plains in his book ''La Planète Mars'' (1930). The name was officially adopted by the International Astronomical Union in 1973.〔(USGS Planetary Nomenclature ) (click on the feature name for details)〕 The North Polar Basin lies within Vastitas Borealis, while Utopia Planitia, another very large basin, is adjacent to it. Some scientists have speculated the plains were covered by an ocean at some point in Mars' history and putative shorelines have been suggested for its southern edges. Today these mildly sloping plains are marked by ridges, low hills, and sparse cratering. Vastitas Borealis is noticeably smoother than similar topographical areas in the south. In 2005 the European Space Agency's Mars Express spacecraft imaged a substantial quantity of water ice in a crater in the Vastitas Borealis region. The environmental conditions at the locality of this feature are suitable for water ice to remain stable. It was revealed after overlaying frozen carbon dioxide sublimated away at the commencement of the Northern Hemisphere Summer and is believed to be stable throughout the Martian year.〔(【引用サイトリンク】title=Water ice in crater at Martian north pole )〕 A NASA probe named Phoenix landed safely in a region of Vastitas Borealis unofficially named Green Valley on 25 May 2008 (in the early Martian summer). Phoenix landed at 68.218830°N 234.250778°E. The probe, which will remain stationary, collected and analyzed soil samples in an effort to detect water and determine how hospitable the planet might once have been for life to grow. It remained active there until winter conditions became too harsh around five months later.〔(【引用サイトリンク】title=Mars lander aims for touchdown in 'Green Valley' )〕 ==Surface== Unlike some the sites visited by the Viking and Pathfinder landers, nearly all the rocks near the Phoenix landing site on Vastitas Borealis are small. For about as far as the camera can see, the land is flat, but shaped into polygons. The polygons are between 2–3 m in diameter and are bounded by troughs that are 20 to 50 cm deep. These shapes are caused by ice in the soil reacting to major temperature changes.〔Levy, J, J. Head, and D. Marchant. 2009. Thermal contraction crack polygons on Mars: Classification, distribution, and climate implications from HiRISE observations. Journal of Geographical Research: 114. p E01007〕 The top of the soil has a crust. The microscope showed that the soil is composed of flat particles (probably a type of clay) and rounded particles. When the soil is scooped up, it clumps together. Although other landers in other places on Mars have seen many ripples and dunes, no ripples or dunes are visible in the area of Phoenix. Ice is present a few inches below the surface in the middle of the polygons. Along the edge of the polygons the ice is at least 8 inches deep. When the ice is exposed to the Martian atmosphere it slowly disappears.〔(The Dirt on Mars Lander Soil Findings. Andrea Thompson, 2 July 2009 (Space.com) )〕 In the winter there would be accumulations of snow on the surface.〔Whiteway, J. et al. 2009. Mars Water-Ice Clouds and Precipitation. Science: 325. p 68-70〕 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「Vastitas Borealis」の詳細全文を読む スポンサード リンク
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