A lower limit of atmospheric pressure on early Mars inferred from nitrogen and argon isotopic compositions
A lower limit of atmospheric pressure on early Mars inferred from nitrogen and argon isotopic compositions
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根据氮和氩同位素组成推断早期火星大气压力下限
DOI：
10.1016/j.icarus.2017.08.020
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发表时间：
2018
期刊：
影响因子：
3.2
通讯作者：
Usui Tomohiro
中科院分区：
文献类型：
--
作者：
Kurokawa Hiroyuki;Kurosawa Kosuke;Usui Tomohiro
我们使用氮和稀有气体的元素和同位素组成来研究火星大气的损失和补充的历史。计算大气层的演变时考虑到各种过程：小行星和彗星的撞击侵蚀和补充、太阳辐射和风引起的大气逃逸、火山脱气和行星际尘埃颗粒的气体沉积。我们的模型再现了火星大气中N和稀有气体（除N2）的元素和同位素组成，从探测任务和火星陨石分析中推断。其他过程，如电离诱导的分馏，这是不包括在我们的模型中，可能会作出很大的贡献，在生产目前的同位素组成。由于猛烈轰击期间的强烈撞击极大地影响了大气质量，大气压力随机演变。虽然稠密的大气层保留了原始的同位素组成，但早期火星上稀薄的大气层受到随机撞击事件和逃逸引发的分馏的严重影响。在大气压力降低后分馏的开始被解释为在较低的大气压力下同位素分馏的时间尺度较短。我们的数值计算结果与较少分馏的N（15 N/14 N）和Ar（38 Ar/36 Ar）同位素组成的火星陨石艾伦山84001中记录的古代大气的比较提供了一个下限的大气压在4 Ga保存原始同位素组成。我们得出结论，在4 Ga时，大气压力高于约0.5 bar。
We examine the history of the loss and replenishment of the Martian atmosphere using elemental and isotopic compositions of nitrogen and noble gases. The evolution of the atmosphere is calculated by taking into consideration various processes: impact erosion and replenishment by asteroids and comets, atmospheric escape induced by solar radiation and wind, volcanic degassing, and gas deposition by interplanetary dust particles. Our model reproduces the elemental and isotopic compositions of N and noble gases (except for Xe) in the Martian atmosphere, as inferred from exploration missions and analyses of Martian meteorites. Other processes such as ionization-induced fractionation, which are not included in our model, are likely to make a large contribution in producing the current Xe isotope composition. Since intense impacts during the heavy bombardment period greatly affect the atmospheric mass, the atmospheric pressure evolves stochastically. Whereas a dense atmosphere preserves primitive isotopic compositions, a thin atmosphere on early Mars is severely influenced by stochastic impact events and following escape-induced fractionation. The onset of fractionation following the decrease in atmospheric pressure is explained by shorter timescales of isotopic fractionation under a lower atmospheric pressure. The comparison of our numerical results with the less fractionated N (15N/14N) and Ar (38Ar/36Ar) isotope compositions of the ancient atmosphere recorded in the Martian meteorite Allan Hills 84001 provides a lower limit of the atmospheric pressure in 4 Ga to preserve the primitive isotopic compositions. We conclude that the atmospheric pressure was higher than approximately 0.5 bar at 4 Ga.