{"title":"Isotopic fractionation of methane on Mars via diffusive separation in the subsurface","authors":"John E. Moores, Haley M. Sapers","doi":"10.1016/j.pss.2024.105971","DOIUrl":null,"url":null,"abstract":"<div><p>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 <sup>13</sup>CH<sub>4</sub> compared to reservoir concentrations by approximately −31‰. Over time, both the reservoir and the emitted gas will evolve to become isotopically enriched in <sup>13</sup>CH<sub>4</sub> compared to a standard of constant isotopic ratio. This necessitates temporal measurements of emitted methane to understand the <strong><em>δ</em></strong><sup>13</sup>C of the reservoir and depth of the release, preferably with hourly or better frequency. Finally, a seasonal cycle in <strong><em>δ</em></strong><sup>13</sup>C 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.</p></div>","PeriodicalId":20054,"journal":{"name":"Planetary and Space Science","volume":"251 ","pages":"Article 105971"},"PeriodicalIF":1.8000,"publicationDate":"2024-09-13","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://www.sciencedirect.com/science/article/pii/S0032063324001351/pdfft?md5=8e924cd9a4592418f7b9e727ff8b9bcf&pid=1-s2.0-S0032063324001351-main.pdf","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Planetary and Space Science","FirstCategoryId":"101","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0032063324001351","RegionNum":4,"RegionCategory":"物理与天体物理","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q3","JCRName":"ASTRONOMY & ASTROPHYSICS","Score":null,"Total":0}
引用次数: 0
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.
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