Isotopic fractionation of methane on Mars via diffusive separation in the subsurface

IF 1.8 4区 物理与天体物理 Q3 ASTRONOMY & ASTROPHYSICS
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Abstract

Many processes have been identified in the Martian subsurface which may produce or release methane that eventually can be emitted into the atmosphere. Given the wide range of isotopic values for source carbon reported on Mars and the importance of atmospheric methane isotopologues as a tracer for subsurface processes, it is critical to quantify the level of isotopic fractionation that can occur during subsurface transport. On Earth, isotopic fractionation occurs when methane transport is dominated by Knudsen diffusion through small pores. However, unlike the Earth, on Mars the low atmospheric pressure and commensurate longer mean free path suggest that most subsurface transport of methane occurs in the Knudsen regime, amplifying this effect. Here, we report on simulations of diffusion through the martian subsurface and report on the level of fractionation that would be expected under two end-member scenarios. For Interplanetary Dust Particles (IDPs) incorporated in near-surface sediments in which methane is released quickly upon generation, atmospheric emissions of methane are expected to be representative of the reservoir isotopic ratio. However, for deeper sources in which methane accumulates as trapped gas, subsurface transport will result in depletions of 13CH4 compared to reservoir concentrations by approximately −31‰. Over time, both the reservoir and the emitted gas will evolve to become isotopically enriched in 13CH4 compared to a standard of constant isotopic ratio. This necessitates temporal measurements of emitted methane to understand the δ13C of the reservoir and depth of the release, preferably with hourly or better frequency. Finally, a seasonal cycle in δ13C with an amplitude of 5.3‰ is expected with adsorption acting to create small temporary reservoirs that are filled and emptied over the year by the subsurface thermal wave. This effect may provide a way to probe near-surface thermophysical properties.

通过地表下的扩散分离实现火星上甲烷的同位素分馏
火星地表下有许多过程可能会产生或释放甲烷,这些甲烷最终会排放到大气中。鉴于火星上报告的源碳同位素值范围很广,以及大气甲烷同位素作为地下过程示踪剂的重要性,量化地下迁移过程中可能发生的同位素分馏水平至关重要。在地球上,当甲烷传输主要是通过小孔隙进行克努森扩散时,就会发生同位素分馏。然而,与地球不同的是,火星上的低气压和相应的较长的平均自由路径表明,甲烷的大部分次表层迁移是在努森机制下发生的,从而放大了这种效应。在此,我们报告了通过火星地下扩散的模拟情况,并报告了在两种末端成员情况下的预期分馏水平。对于纳入近地表沉积物的行星际尘埃粒子(IDPs),甲烷在生成后会迅速释放,大气中的甲烷排放预计将代表储层的同位素比值。然而,对于甲烷作为滞留气体积聚的较深来源,地下传输将导致 13CH4 与储层浓度相比减少约 -31‰。随着时间的推移,与恒定同位素比的标准相比,储层和排放气体的 13CH4 都将发生同位素富集。这就需要对排放的甲烷进行时间测量,以了解储层和排放深度的 δ13C,测量频率最好为每小时或更高。最后,δ13C 的季节性周期振幅预计为 5.3‰,吸附作用会产生小型临时储层,这些储层在一年中会被地下热浪填满和排空。这种效应可为探测近地表热物理特性提供一种方法。
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来源期刊
Planetary and Space Science
Planetary and Space Science 地学天文-天文与天体物理
CiteScore
5.40
自引率
4.20%
发文量
126
审稿时长
15 weeks
期刊介绍: Planetary and Space Science publishes original articles as well as short communications (letters). Ground-based and space-borne instrumentation and laboratory simulation of solar system processes are included. The following fields of planetary and solar system research are covered: • Celestial mechanics, including dynamical evolution of the solar system, gravitational captures and resonances, relativistic effects, tracking and dynamics • Cosmochemistry and origin, including all aspects of the formation and initial physical and chemical evolution of the solar system • Terrestrial planets and satellites, including the physics of the interiors, geology and morphology of the surfaces, tectonics, mineralogy and dating • Outer planets and satellites, including formation and evolution, remote sensing at all wavelengths and in situ measurements • Planetary atmospheres, including formation and evolution, circulation and meteorology, boundary layers, remote sensing and laboratory simulation • Planetary magnetospheres and ionospheres, including origin of magnetic fields, magnetospheric plasma and radiation belts, and their interaction with the sun, the solar wind and satellites • Small bodies, dust and rings, including asteroids, comets and zodiacal light and their interaction with the solar radiation and the solar wind • Exobiology, including origin of life, detection of planetary ecosystems and pre-biological phenomena in the solar system and laboratory simulations • Extrasolar systems, including the detection and/or the detectability of exoplanets and planetary systems, their formation and evolution, the physical and chemical properties of the exoplanets • History of planetary and space research
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