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底座形撞击坑

维基百科,自由的百科全书

底座形陨石坑(pedestal crater)是行星地质学中一种陨石坑类型,它的撞击喷射物散布于周围地形之上,从而形成一座凸起的平台(如底座)。当撞击坑喷出的物质形成一层抗侵蚀层,并使邻近区的侵蚀速度比其他区域慢时,就形成一座底座形陨石坑。一些底座被精确测量出高于周围区域数百米,这也意味着撞击坑中有数百米的材料被侵蚀掉,结果是陨坑及其喷出物覆盖层都明显高出周边环境。底座型陨石坑是在水手号任务期间被首次观测到的[1][2][3][4]

描述

通过进一步的研究,研究人员将相关陨石坑分为三种不同的类别[5],并对它们的形成有了新的见解。过度喷射坑(Excess ejecta craters)[6]和栖留坑(perched craters)[7][8][9]都比底座形坑大[10][11]。所有这三种陨坑都有相似的碗状坑形和高于周边地表的环坑平台。过度喷射坑和栖留坑显示有喷发物堆积,但底座形撞击坑通常不显示。三者都位于相同的区域,并且高出周边地形的高度似乎都有相同,平均高出近50米[12]。过度喷射坑和栖留坑之间的主要区别为栖留坑的碗坑很浅,有时几乎填满了物质。底座形陨坑一般则靠近绝壁(悬崖)高原中央。

现在认为,所有这三种类型的陨石坑都是撞击冰层后造成的。较大的过度喷射坑和栖留坑完全击透了冰层,也抵达了较浅的岩石层。部分岩石层堆积在撞击坑边缘周围,形成一圈粗糙的喷发沉积物,喷出物保护了它们下方的区域不受侵蚀,使得这些陨坑高出周围地表之上。较小的“底座形陨坑”则通过不同的作用过程形成了一层保护层。模拟显示,对冰层巨大撞击将产生强烈的热浪,足以融化部分积冰,由此产生的水可溶解矿物质,并产生抗侵蚀的覆盖层[13]

对这些不同陨石坑形成过程的新认识,使帮助科学家们了解了火星上的富冰物质如何在亚马逊纪时期多次沉积在两半球的中纬度区的。例如[14],在那段时期,火星自转轴倾角(倾斜)经历了许多大的变化[15][16],这些变化导致了气候的变迁。由于目前的倾角,火星两极都拥有一层厚厚的积冰。有时,两极面向太阳,导致极地冰层中的冰向中纬度转移,正是这段时间才形成了富冰层[12]。  

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另请参阅

参考文献

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