尼羅瑟提斯桌山群

尼罗瑟提斯桌山群(Nilosyrtis Mensae)位于火星卡西乌斯区，其中心坐标为北纬36.87°、东经67.9°，它的东西经度分别为74.4°E和 51.1°E；南北纬度分别是 29.61°N和 36.87°N[2] 。尼罗瑟提斯桌山群正对着西面的普罗敦尼勒斯桌山群，二者都坐落在火星分界线上。该桌山群纵横约705公里（438英里），取名自火星古典反照率特征，1973年被国际天文联合会正式接受。

尼罗瑟提斯桌山群的表面被归属为锐蚀地形(Fretted Terrain)。这里有悬崖、台地和宽阔平坦的山谷，其表面地貌被认为是由岩屑覆盖的冰川所造成Greeley, R. and J. Guest.  1987.  Geological map of the eastern equatorial region of Mars, scale 1:15,000,000. U. S. Geol. Ser. Misc. Invest. Map I-802-B, Reston, VirginiaSharp, R.  1973.  Mars Fretted and chaotic terrains.  J. Geophys. Res.:  78.  4073-4083。这些围绕着丘群和桌山的冰川，被称为舌状岩屑坡Plaut, J. et al.  2008. Radar Evidence for Ice in Lobate Debris Aprons in the Mid-Northern Latitudes of Mars. Lunar and Planetary Science XXXIX. 2290.pdfCarr, M. 2006. The Surface of Mars. Cambridge University Press. Squyres, S. 1978. Martian fretted terrain: Flow of erosional debrid. Icarus: 34. 600-613.；当冰川位于山谷中时，又被称为线状谷底沉积Morgan, G. and J. Head III.  2009.  Sinton crater, Mars: Evidence for impact into a plateau icefield and melting to produce valley networks at the Hesperian-Amazonian boundary.  Icarus: 202. 39–59.Morgan, G. et al.  2009.  Lineated valley fill (LVF) and lobate debris aprons (LDA) in the Deuteronilus Mensae northern dichotomy boundary region, Mars: Constraints on the extent, age, and episodicity of Amazonian glacial events.  Icarus: 202. 22–38.Head, J., et al.  2006.  Extensive valley glacier deposits in the northern mid-latitudes of Mars:  Evidence for the late Amazonian obliquity-driven climate change.  Earth Planet. Sci. Lett. 241.  663-671Head, J., et al.  2006. Modification if the dichotomy boundary on Mars by Amazonian mid-latitude regional glaciation.  Geophys. Res Lett.  33。


气候变化导致的富冰特征
几十年来，火星上的许多地貌特征，包括尼罗瑟提斯桌山群，都被认为含有大量的冰。这一想法被火星勘测轨道飞行器上的浅层雷达(SHARAD)所证实，勘测结果表明，舌状岩屑坡（LDA）和线状山谷沉积（LVF）都含有纯净水冰，上面覆盖着一层隔温的薄岩层 Plaut, J. et al.  2008. Radar Evidence for Ice in Lobate Debris Aprons in the Mid-Northern Latitudes of Mars. Lunar and Planetary Science XXXIX.  2290.pdf。在北半球包括尼罗瑟提斯桌山群在内的许多地方都发现了冰Plaut, J., A. Safaeinili,, J. Holt, R. Phillips, J. Head, J., R. Seu, N. Putzig, A. Frigeri.   2009.  Radar evidence for ice in lobate debris aprons in the midnorthern latitudes of Mars.  Geophys. Res. Lett. 36. doi:10.1029/2008GL036379.。

关于冰的起源，最流行的模型是火星自转轴倾角的巨幅变化所引起的气候改变，有时倾斜角甚至超过80度Touma J. and J. Wisdom.  1993.  The Chaotic Obliquity of Mars.  Science 259, 1294-1297.Laskar, J., A. Correia, M. Gastineau, F. Joutel, B. Levrard, and P. Robutel.   2004.   Long term evolution and chaotic diffusion of the insolation quantities of Mars.  Icarus 170, 343-364.，巨大的倾斜变化解释了为何火星上有许多富含冰的特征。

研究表明，当火星的倾角从目前的25度倾斜到45度时，两极的冰就不再稳定Levy, J., J. Head, D. Marchant, D. Kowalewski.  2008.  Identification of sublimation-type thermal contraction crack polygons at the proposed NASA Phoenix landing site: Implications for substrate properties and climate-driven morphological evolution. Geophys. Res. Lett. 35. doi:10.1029/2007GL032813.，此外，在这一高倾角时，固体的二氧化碳(干冰)就会升华，从而增加了大气压力，这种增加的压力会使更多的尘埃保留在大气中，大气中的水分将以雪或冰的形式落在尘埃颗粒上，计算表明这种物质将集中在中纬度地区Levy, J., J. Head, D. Marchant.   2009a. Thermal contraction crack polygons on Mars: Classification, distribution, and climate implications from HiRISE observations.  J. Geophys. Res. 114. doi:10.1029/2008JE003273.Hauber, E., D. Reiss, M. Ulrich, F. Preusker, F. Trauthan, M. Zanetti, H. Hiesinger, R. Jaumann, L. Johansson, A. Johnsson, S. Van Gaselt, M. Olvmo.  2011.  Landscape evolution in Martian mid-latitude regions:  insights from analogous periglacial landforms in Svalbard.  In: Balme, M., A. Bargery, C. Gallagher, S. Guta (eds).   Martian Geomorphology.  Geological Society, London.  Special Publications: 356.  111-131，火星大气环流模式预测，富冰尘埃将会堆积在已发现有富冰特征的同一区域Laskar, J., A. Correia, M. Gastineau, F. Joutel, B. Levrard, and P. Robutel.   2004.   Long term evolution and chaotic diffusion of the insolation quantities of Mars.  Icarus 170,  343-364.。

当火星倾斜角逐惭回归到较低值时，水冰将会升华(直接变成气体)Mellon, M., B.  Jakosky. 1995. The distribution and behavior of Martian ground ice during past and present epochs.  J. Geophys. Res. 100, 11781–11799.Schorghofer, N., 2007. Dynamics of ice ages on Mars. Nature 449, 192–194.并留下一层尘埃，留下的沉积覆盖了底层物质。所以每次自转轴的大幅度摇摆循环，都会使一些富冰层留在下面Madeleine, J., F. Forget, J. Head, B. Levrard, F. Montmessin.   2007.  Exploring the northern mid-latitude glaciation with a general circulation model.  In:  Seventh International Conference on Mars.  Abstract 3096.。注意，光滑的表面覆盖层可能只代表相对较新的物质。

另请参阅
 火星地質
 都特羅尼勒斯桌山群
 普羅敦尼勒斯桌山群 
 火星分界
 伊斯墨诺斯湖区
 冰河

參考資料

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