AI 中文总结
研究火星大气中氧同位素分馏，通过开发含CO₂光解和O₃形成相关氧同位素分馏的一维光化学模型，发现化学分馏过程使逃逸物种重氧同位素贫化，增强氧向太空逃逸相关分馏，MMX任务或可探测。
AI 中文摘要
火星大气中挥发性元素重同位素的富集表明火星通过向太空逃逸失去了大部分大气。ExoMars痕量气体轨道器（TGO）最近的大气测量表明，氧同位素组成的垂直分布受涉及同位素分馏的化学反应影响，但其定量影响尚未完全评估。本研究开发了一个一维光化学模型，纳入与CO₂光解和O₃形成相关的氧同位素分馏，以研究氧同位素组成的垂直分布。计算表明，相对于CO₂，CO中的重氧同位素贫化，主要是由于CO₂光解期间的同位素分馏。模型与TGO测量的氧和碳同位素组成垂直分布吻合良好。O₃在¹⁸O和¹⁷O中强烈富集，而原子氧中重氧同位素高度贫化，以补偿其在O₃中的富集。这些化学分馏过程会使从高层大气逃逸的物种中的重氧同位素贫化，从而增强与氧向太空逃逸相关的同位素分馏。火星卫星探测（MMX）任务可能探测到这种逃逸氧的分馏同位素组成。
英文摘要
The enrichment of heavy isotopes of volatile elements in the Martian atmosphere indicates that Mars lost a large portion of its atmosphere through escape to space. Recent atmospheric measurements by ExoMars Trace Gas Orbiter (TGO) have suggested that the vertical profiles of oxygen isotopic compositions are influenced by chemical reactions involving isotopic fractionation. However, their quantitative impacts have not yet been fully evaluated. In this study, we develop a 1D photochemical model that incorporates oxygen isotopic fractionation associated with CO$_2$ photolysis and O$_3$ formation to investigate the vertical profiles of oxygen isotopic compositions. Our calculations show that CO is depleted in heavy oxygen isotopes relative to CO$_2$, reaching $δ^{18}$O $\sim -25$ per mil and $δ^{17}$O $\sim -15$ per mil, primarily due to isotopic fractionation during CO$_2$ photolysis. The vertical profiles of oxygen and carbon isotopic compositions are in good agreement between our model and the TGO measurements. O$_3$ is strongly enriched in $^{18}$O and $^{17}$O, reaching $δ^{18}$O $\sim 100$ per mil and $δ^{17}$O $\sim 50$ per mil as a consequence of the isotopic fractionation during its formation, whereas atomic oxygen is highly depleted in the heavy oxygen isotopes with $δ^{18}$O $\lesssim -100$ per mil and $δ^{17}$O $\lesssim -50$ per mil so as to compensate for their enrichment in O$_3$. These chemical fractionation processes can deplete the heavy oxygen isotopes in species that escape from the upper atmosphere, and thereby enhance the isotopic fractionation associated with oxygen escape to space. Such fractionated isotopic compositions of escaping oxygen may be detectable by the Martian Moons eXploration (MMX) mission.
CommentsAccepted for publication in The Planetary Science Journal
