纬度相关覆盖层

大部分火星表面都披覆了一层厚厚的富冰覆盖层，该覆盖层是过去多次从天空飘落的冰核尘埃所组成Hecht, M.  2002.  Metastability of water on Mars. Icarus 156, 373–386 Mustard, J., et al.  2001. Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice. Nature 412 (6845), 411–414.  Pollack, J., D. Colburn, F. Flaser, R. Kahn, C. Carson, and D. Pidek.   1979.  Properties and effects of dust suspended in the martian atmosphere.  J. Geophys. Res.  84, 2929-2945. 。在部分地区可看到覆盖层中的一些分层。

从天空降落的冰核尘埃，很好地证明了这层覆盖层富含水冰。许多表面常见的多边形形状也表明土壤中富含冰。2001火星奥德赛号发现了高含量的氢（可能来自水）Boynton, W., and 24 colleagues.  2002.  Distribution of hydrogen in the nearsurface
of Mars: Evidence for sub-surface ice deposits. Science 297, 81–85Kuzmin, R, et al.  2004. Regions of potential existence of free water (ice) in the near-surface martian ground: Results from the Mars Odyssey High-Energy Neutron Detector
(HEND). Solar System Research 38 (1), 1–11. Mitrofanov, I. et al. 
2007a. Burial depth of water ice in Mars permafrost subsurface. In: LPSC 38,
Abstract #3108. Houston, TX. Mitrofanov, I., and 11 colleagues.   2007b. Water ice permafrost on Mars: Layering
structure and subsurface distribution according to HEND/Odyssey and MOLA/
MGS data. Geophys. Res. Lett. 34 (18). doi:10.1029/2007GL030030. Mangold, N., et al.  2004.  Spatial
relationships between patterned ground and ground ice detected by the
neutron spectrometer on Mars. J. Geophys. Res. 109 (E8). doi:10.1029/
2004JE002235.。从轨道上进行的热辐射测量表明了冰的存在Feldman, W., and 12 colleagues.  2002.  Global distribution of neutrons from Mars:Results from Mars Odyssey. Science 297, 75–78. Feldman, W., et al.  2008.North to south asymmetries in the water-equivalent hydrogen distribution at high latitudes on Mars. J. Geophys. Res. 113. doi:10.1029/2007JE003020.；凤凰号火星探测器降落在一片多边形区域中，它发现了水冰，并进行了直接观测Bright Chunks at Phoenix Lander's Mars Site Must Have Been Ice  – Official NASA press release (19.06.2008)，事实上，它的着陆火箭暴露了纯冰。理论预测在几厘米厚的土壤下会发现冰。该覆盖层被称为“纬度相关覆盖层”，因为它的出现与纬度有关。正是这层覆盖层后来的破裂，才形成了多边形地面。这种富含水冰地面的破裂是根据物理作用所预测的Mutch, T.A., and 24 colleagues, 1976. The surface of Mars: The view from the Viking2 lander. Science 194 (4271), 1277–1283.Mutch, T., et al. 1977. The geology of the Viking Lander 2 site. J. Geophys. Res. 82, 4452–4467. 
Levy, J., et al.  2009. Thermal contraction crack polygons on Mars: Classification, distribution, and climate implications from HiRISE observations. J. Geophys. Res. 114. doi:10.1029/2008JE003273. Washburn, A.  1973.  Periglacial Processes and Environments. St. Martin’s Press,New York, pp. 1–2, 100–147.Mellon, M.  1997. Small-scale polygonal features on Mars: Seasonal thermal contractioncracks in permafrost. J. Geophys. Res. 102, 25,617-625,628. Mangold, N. 2005.  High latitude patterned grounds on Mars: Classification, distribution
and climatic control. Icarus 174, 336–359.Marchant, D., J. Head.  2007.  Antarctic dry valleys: Microclimate zonation, variable geomorphic processes, and implications for assessing climate change on Mars. Icarus 192, 187–222。另一种表面被称为“脑纹地形”，因为它看起来像人脑的表面。当两种区域同时出现时，脑纹地形高度较多边形地面更低。

尽管相邻下层的脑纹地形参差不齐，但从顶层开始，多边形层相当平整。据信，含多边形的覆盖层深度需达10-20米，才能形成平整表面。在所有的冰消失之前，覆盖层会持续很长一段时间，因为顶部会形成一层保护性的滞留沉积物Marchant, D., et al.  2002.  Formation of patterned ground
and sublimation till over Miocene glacier ice in Beacon valley, southern Victoria
land, Antarctica. Geol. Soc. Am. Bull. 114, 718–730.  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.。覆盖层中含有冰和尘埃。当一定数量的冰升华后，尘埃停留在顶部，形成滞留沉积层 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.   Schorghofer, N., O. Aharonson.  2005.  Stability and exchange of subsurface ice on
Mars. J. Geophys. Res. 110 (E05). doi:10.1029/2004JE002350. Schorghofer, N., 2007. Dynamics of ice ages on Mars. Nature 449 (7159), 192–194 Head, J., J. Mustard, M. Kreslavsky, R. Milliken, D. Marchant.  2003. 
Recent ice ages on Mars. Nature 426 (6968), 797–802.。
根据多边形地面的总面积计算，估计覆盖层中锁住的总水量约有10米深，这一体积相当于在整个星球覆盖了一层2.5米深的水。但相比之下，地球北极和南极冰盖融化的水则可覆盖整个星球30米深Levy, J. et al.  2010.  Thermal contraction crack polygons on Mars:  A synthesis from HiRISE, Phoenix, and terrestrial analog studies.  Icarus: 206, 229-252.。

覆盖层形成于火星气候与现在不同的时期Mustard, J., et al.  2001. Evidence for recent climate change on Mars from the identification of youthful near-surface ground ice. Nature 412 (6845), 411–414. Kreslavsky, M.A., Head, J.W., 2002. High-latitude Recent Surface Mantle on Mars:
New Results from MOLA and MOC. European Geophysical Society XXVII, Nice. Head, J.W., Mustard, J.F., Kreslavsky, M.A., Milliken, R.E., Marchant, D.R., 2003.Recent ice ages on Mars. Nature 426 (6968), 797–802.，火星自转轴的倾斜或倾角变化很大 name= 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.    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. ，而地球的倾斜变化则很小，因为我们相当大的月球稳定了地球。火星只有两颗非常小卫星，它们没有足够的引力来稳定火星的倾斜。当火星倾斜度超过40度（今天是25度）左右时，冰就会沉积在某些纬度带上，而这些纬度带现今存在着大量的覆盖层 Kreslavsky, M., J. Head, J.  2002.  Mars: Nature and evolution of young,
latitude-dependent water-ice-rich mantle. Geophys. Res. Lett. 29, doi:10.1029/
2002GL015392. Kreslavsky, M., J. Head.  2006.  Modification of impact craters in the northern
plains of Mars: Implications for the Amazonian climate history. Meteorit. Planet.
Sci. 41, 1633–1646.。


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