Decline of giant impacts on Mars by 4.48 billion years ago and an early opportunity for habitability
Decline of giant impacts on Mars by 4.48 billion years ago and an early opportunity for habitability
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DOI：
10.1038/s41561-019-0380-0
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发表时间：
2019-07-01
影响因子：
18.3
通讯作者：
Davis, C.
中科院分区：
文献类型：
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作者：
Moser, D. E.;Arcuri, G. A.;Davis, C.
内行星的重陨石轰击减弱的时间是有争议的。它的时间决定了地壳条件的开始始终低于微生物生存的热和冲击压力限制，因此限制了允许行星宜居的条件的发生。在这里，我们确定这一时间为火星通过检查已知的最古老的火星矿物，4.476-4.429 Gyr-old锆石和斜锆石颗粒陨石来自南部高地的变质历史。我们使用电子显微镜和原子探针断层扫描显示，这些颗粒都没有暴露在78 GPa的生命限制冲击压力。97%的晶粒表现出弱至无冲击变质特征，没有冲击诱导熔融的热叠印。相比之下，大约80%的来自地球和月球轰击地壳的研究颗粒显示出这种特征。因此，造成火星半球一分为二的巨大撞击必须发生在4.48 Gyr之前，没有后来的灾难性轰击。考虑到热可居住性模型，我们得出结论，火星地壳的部分达到可居住的压力和温度的4.2 Gyr前，火星的“湿”期的开始，大约0.5 Gyr早于地球上最早的已知生命记录。4.2 Gyr前的早期自然发生，现在对两颗行星都是成立的。
The timing of the wane in heavy meteorite bombardment of the inner planets is debated. Its timing determines the onset of crustal conditions consistently below the thermal and shock pressure limits for microbiota survival, and so bounds the occurrence of conditions that allow planets to be habitable. Here we determine this timing for Mars by examining the metamorphic histories of the oldest known Martian minerals, 4.476-4.429-Gyr-old zircon and baddeleyite grains in meteorites derived from the southern highlands. We use electron microscopy and atom probe tomography to show that none of these grains were exposed to the life-limiting shock pressure of 78 GPa. 97% of the grains exhibit weak-to-no shock metamorphic features and no thermal overprints from shock-induced melting. By contrast, about 80% of the studied grains from bombarded crust on Earth and the Moon show such features. The giant impact proposed to have created Mars' hemispheric dichotomy must, therefore, have taken place more than 4.48 Gyr ago, with no later cataclysmic bombardments. Considering thermal habitability models, we conclude that portions of Mars' crust reached habitable pressures and temperatures by 4.2 Gyr ago, the onset of the Martian 'wet' period, about 0.5 Gyr earlier than the earliest known record of life on Earth. Early abiogenesis by 4.2 Gyr ago, is now tenable for both planets.