火星探测是太阳系内行星探测的重点之一。探测表明今天的火星寒冷而干燥，缺少液态水，不适宜居住。而水则以固态的形式存在于其表两极极冠中或广泛存在于中低纬度的次表层。大气中极为稀少的水汽也在近期的探测活动中被发现有季节性的变化。但火星地质、地貌的遥感和就位探测数据发现其表面有着大量过去的水文活动的残余痕迹。因此火星大气的演化历史一直是过去几十年的研究热点和重点。对火星的观测和大多数理论研究倾向于支持如下演化历史：1）火星大约在38亿年前的诺亚纪晚期有过富含二氧化碳的大气及相对于今天火星更为温暖湿润的表面环境；2）火星表面环境在诺亚期之后随着火星大气的逃逸发生了剧烈变化；3）大气中的氢、氘、氦等元素在火星形成后最初的4-5亿年就以热逃逸的形式逃走，而碳、氮、氧、氩等在地球上基本无法逃逸的重元素也可以通过多种非热逃逸机制从火星有效地逃逸；4）光化学逃逸机制是35亿年前至今火星大气中氧和碳逃逸的最主要的机制。因此，研究早期火星不同地质时期的光化学逃逸、以及氧和碳在光化学逃逸过程中的分馏效应，对我们开展火星大气和水的逃逸分析、探索火星大气稳定性、理解火星环境（液态水）和宜居性的演化有重要作用。根据上述研究背景，我们提出了以下两大科学问题：1）火星大气的光化学逃逸是如何影响火星大气的演化？本论文从光化学逃逸率、光化学逃逸过程对冕层结构的影响、光化学逃逸过程对火星大气和水耗散的贡献、光化学逃逸过程中C和O的分馏效应四个方面进行论证；2）如何利用有限的C和O的地质历史观测数据、结合火星大气的多种源汇过程，构建同位素比值的演化模型，从而对火星大气的演化过程进行评估？蒙特卡洛方法在测试粒子数目足够充分的条件下，可以较为真实地描述高能粒子形成后的运动状态、运动轨迹以及碰撞发生后的运动状态，从而可以从宏观上定量描述整个大气在演化过程中的总逃逸率以及逃逸率在各演化阶段的差异。为探索上述科学问题，本论文通过构建了一维和三维的蒙特卡洛逃逸模型，开展火星大气的光化学逃逸和演化的模拟研究。论文的主要创新点有：1) 构建了火星大气早期的光化学逃逸模型和冕层结构模型：为探索早期火星大气的演化提供了更合理的研究工具；2) 基于该模型的模拟分析揭示了火星大气中氧和碳的光化学逃逸率对太阳极紫外辐射的响应并非单调递增，并揭示了次生高能粒子对火星大气逃逸不可或缺的贡献；3) 该模型的模拟结果给出了光化学逃逸过程中的氧和碳的同位素分馏系数。

Mars is dominant target to explore in the solar system. The explorations show that Mars is cold, dry, lack of liquid water, and inhabitable today. Only ice water in polar caps, hydrated minerals in surface/subsurface, and rarely seasonal changing water vapor have been found in recent explorations. However, there are lots of geological evidences produce that liquid water was wildly existed on Martian surface. Observations and theoretical studies on Mars are support that: 1) Mars atmosphere was thicker, warmer and wetter than current during the late Noachian period; 2) while changed dramatically with the atmospheric escape after Noachian period; 3) lighter elements (H, He, etc.) are effectively lost through thermal escape in the first 0.4-0.5 bys after Mars formation, while heavier elements (C, N, O, etc.) could escape effectively by non-thermal escape mechanisms; 4) photochemical escape mechanisms are the main escape mechanisms of O and C since 3.5 Ga. Therefore, to study the isotopic fractionation effect of O and C, the evolution history of the atmosphere and water during photochemical escape, are important to understanding the habitability of Mars. Through above background, we propose two scientific questions: 1) How does the photochemical escape of the Martian atmosphere impact the Martian atmospheric evolution? We demonstrate this question by photochemical escape rates, coronal structures, atmospheric and water loss, and the isotopic fractionation effects in the photochemical escape process. 2) How to evaluate the Martian atmospheric evolution (pressure, isotopic ratio, atmospheric age)?In order to explore those scientific questions, this thesis has built a 1D and a 3D Monte-Carlo model to simulate the photochemical escape and evolution of Martian Atmosphere. The main innovative points of this thesis are: 1) The photochemical escape model and coronal structure model provides a reasonable tool to research the Martian atmospheric evolution; 2) the model reveals that the photochemical escape rates of O and C do not increase monotonically with the increasing of the solar EUV radiation, and contributions of secondaries’ escape are indispensable; 3) the simulation results produce the isotopic fractionation factors of O and C in the photochemical escape process, and we evaluate the Martian atmospheric age by C isotope fractionation effect.