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底座形撞擊坑

維基百科,自由的百科全書

底座形隕石坑(pedestal crater)是行星地質學中一種隕石坑類型,它的撞擊噴射物散佈於周圍地形之上,從而形成一座凸起的平台(如底座)。當撞擊坑噴出的物質形成一層抗侵蝕層,並使鄰近區的侵蝕速度比其他區域慢時,就形成一座底座形隕石坑。一些底座被精確測量出高於周圍區域數百米,這也意味着撞擊坑中有數百米的材料被侵蝕掉,結果是隕坑及其噴出物覆蓋層都明顯高出周邊環境。底座型隕石坑是在水手號任務期間被首次觀測到的[1][2][3][4]

描述

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通過進一步的研究,研究人員將相關隕石坑分為三種不同的類別[5],並對它們的形成有了新的見解。過度噴射坑(Excess ejecta craters)[6]和棲留坑(perched craters)[7][8][9]都比底座形坑大[10][11]。所有這三種隕坑都有相似的碗狀坑形和高於周邊地表的環坑平台。過度噴射坑和棲留坑顯示有噴發物堆積,但底座形撞擊坑通常不顯示。三者都位於相同的區域,並且高出周邊地形的高度似乎都有相同,平均高出近50米[12]。過度噴射坑和棲留坑之間的主要區別為棲留坑的碗坑很淺,有時幾乎填滿了物質。底座形隕坑一般則靠近絕壁(懸崖)高原中央。

現在認為,所有這三種類型的隕石坑都是撞擊冰層後造成的。較大的過度噴射坑和棲留坑完全擊透了冰層,也抵達了較淺的岩石層。部分岩石層堆積在撞擊坑邊緣周圍,形成一圈粗糙的噴發沉積物,噴出物保護了它們下方的區域不受侵蝕,使得這些隕坑高出周圍地表之上。較小的「底座形隕坑」則通過不同的作用過程形成了一層保護層。模擬顯示,對冰層巨大撞擊將產生強烈的熱浪,足以融化部分積冰,由此產生的水可溶解礦物質,並產生抗侵蝕的覆蓋層[13]

對這些不同隕石坑形成過程的新認識,使幫助科學家們了解了火星上的富冰物質如何在亞馬遜紀時期多次沉積在兩半球的中緯度區的。例如[14],在那段時期,火星自轉軸傾角(傾斜)經歷了許多大的變化[15][16],這些變化導致了氣候的變遷。由於目前的傾角,火星兩極都擁有一層厚厚的積冰。有時,兩極面向太陽,導致極地冰層中的冰向中緯度轉移,正是這段時間才形成了富冰層[12]。  

圖集

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另請參閱

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參考文獻

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  1. ^ Pedestal Crater Development. JPL – NASA. 2015-07-01 [2017-08-10]. (原始內容存檔於2021-08-01). 
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  9. ^ Meresse, S.; Costard, F.; Mangold, N.; Baratoux, D.; et al. Martian perched craters and large ejecta volume: Evidence for episodes of deflation in the northern lowlands. Meteorit. Planet. Sci. 2006, 41 (10): 1647–1658 [2013-03-03]. Bibcode:2006M&PS...41.1647M. doi:10.1111/j.1945-5100.2006.tb00442.x可免費查閱. (原始內容存檔於2017-10-04). 
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