緯度相關覆蓋層
大部分火星表面都披覆了一層厚厚的富冰覆蓋層,該覆蓋層是過去多次從天空飄落的冰核塵埃所組成[1] [2] [3]。在部分地區可看到覆蓋層中的一些分層[4]。
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前一幅法厄同區覆蓋層分層圖像的放大,可看到4到5層分層。
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HiWish計劃下高解像度成像科學設備顯示的埃里達尼亞區覆蓋層。
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HiWish計劃下高解像度成像科學設備顯示的埃里達尼亞區披覆及未披覆氣候變化時從天空落下的覆蓋層地表近景。
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HiWish計劃下高解像度成像科學設備拍攝的刻布壬尼亞區覆蓋層近景,覆蓋層可能由過去氣候條件下從天空降落的冰及塵埃組成。
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HiWish計劃下高解像度成像科學設備顯示的希臘區含分層的平坦覆蓋層。
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HiWish計劃下高解像度成像科學設備顯示的伊斯墨諾斯湖區覆蓋層近景,箭頭指示隕石坑邊緣突出的覆蓋層厚度。
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HiWish計劃下高解像度成像科學設備顯示了伊斯墨諾斯湖區覆蓋層厚度的近景。
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HiWish計劃下高解像度成像科學設備顯示的覆蓋層近景圖。
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HiWish計劃下高解像度成像科學設備顯示的覆蓋層近景圖。
從天空降落的冰核塵埃,很好地證明了這層覆蓋層富含水冰。許多表面常見的多邊形形狀也表明土壤中富含冰。2001火星奧德賽號發現了高含量的氫(可能來自水)[5][6] [7] [8] [9]。從軌道上進行的熱輻射測量表明了冰的存在[10] [11];鳳凰號火星探測器降落在一片多邊形區域中,它發現了水冰,並進行了直接觀測[12][13],事實上,它的着陸火箭暴露了純冰。理論預測在幾厘米厚的土壤下會發現冰。該覆蓋層被稱為「緯度相關覆蓋層」,因為它的出現與緯度有關。正是這層覆蓋層後來的破裂,才形成了多邊形地面。這種富含水冰地面的破裂是根據物理作用所預測的[14][15] [16] [17][18] [19][20]。另一種表面被稱為「腦紋地形」,因為它看起來像人腦的表面。當兩種區域同時出現時,腦紋地形高度較多邊形地面更低。
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HiWish計劃下高解像度成像科學設備拍攝的線狀谷底沉積,顯示了下一幅圖像來源的背景照片。
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HiWish計劃下高解像度成像科學設備顯示的細胞開放型和細胞封閉型腦紋地形。
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HiWish計劃下高解像度成像科學設備顯示,腦紋地形形成於較厚的地層,箭頭指示較厚的單元分裂成小細胞。
儘管相鄰下層的腦紋地形參差不齊,但從頂層開始,多邊形層相當平整。據信,含多邊形的覆蓋層深度需達10-20米,才能形成平整表面。在所有的冰消失之前,覆蓋層會持續很長一段時間,因為頂部會形成一層保護性的滯留沉積物[21] [22] [23]。覆蓋層中含有冰和塵埃。當一定數量的冰升華後,塵埃停留在頂部,形成滯留沉積層[24] [25] [26] [27]。 根據多邊形地面的總面積計算,估計覆蓋層中鎖住的總水量約有10米深,這一體積相當於在整個星球覆蓋了一層2.5米深的水。但相比之下,地球北極和南極冰蓋融化的水則可覆蓋整個星球30米深[28]。
覆蓋層形成於火星氣候與現在不同的時期[29] [30] [31],火星自轉軸的傾斜或傾角變化很大[32] [33] [34],而地球的傾斜變化則很小,因為我們相當大的月球穩定了地球。火星只有兩顆非常小衛星,它們沒有足夠的引力來穩定火星的傾斜。當火星傾斜度超過40度(今天是25度)左右時,冰就會沉積在某些緯度帶上,而這些緯度帶現今存在着大量的覆蓋層 [35] [36]。
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