It is not yet entirely clear whether Mars began as a warm and wet planet that evolved towards the present-day cold and dry body or if it always was cold and dry with just some sporadic episodes of liquid water on its surface. An important clue into this question can be gained by studying the earliest evolution of the Martian atmosphere and whether it was dense and stable to maintain a warm and wet climate or tenuous and susceptible to strong atmospheric escape. In this review we therefore discuss relevant aspects for the evolution and stability of a potential early Martian atmosphere. This contains the EUV flux evolution of the young Sun, the formation timescale and volatile inventory of the planet including volcanic degassing, impact delivery and removal, the loss of the catastrophically outgassed steam atmosphere, atmosphere-surface interactions, as well as thermal and non-thermal escape processes affecting a potential secondary atmosphere at early Mars. While early non-thermal atmospheric escape at Mars before 4 billion years ago is poorly understood, in particular in view of its ancient intrinsic magnetic field, research on thermal escape processes and the stability of a CO₂-dominated atmosphere around Mars against high EUV fluxes indicate that volatile delivery and volcanic degassing cannot counterbalance the strong atmospheric escape. Therefore, a catastrophically outgassed steam atmosphere of several bars of CO₂ and H₂O, or CO and H₂ for reduced conditions, through solidification of the Martian magma ocean could have been lost within just a few million years. Thereafter, Mars likely could not build up a dense secondary atmosphere during its first \(\sim400\) million years but might only have possessed an atmosphere sporadically during events of strong volcanic degassing, potentially also including SO₂. This indicates that before \(\sim4.1\) billion years ago Mars indeed might have been cold and dry with at maximum short and sporadic warmer periods. A denser CO₂- or CO-dominated atmosphere, however, might have built up afterwards but must have been lost later-on due to non-thermal escape processes and sequestration into the ground.

目前尚不清楚火星是从温暖湿润的星球发展到如今的寒冷干燥的身体，还是一直处于寒冷干燥的状态，表面仅散布着一些液态水。通过研究火星大气的最早演变，以及保持温暖湿润的气候是否稠密，稳定，还是脆弱易受大气逃逸的影响，可以找到有关这个问题的重要线索。因此，在这篇综述中，我们讨论了潜在的早期火星大气层的演化和稳定性的相关方面。其中包括年轻太阳的EUV通量演化，行星的形成时间尺度和挥发性清单，包括火山脱气，影响传递和去除，灾难性除气的蒸汽气氛的损失，大气与地表的相互作用，以及影响火星早期潜在次级大气的热和非热逃逸过程。尽管对40亿年前的火星早期非热大气逸散知之甚少，尤其是鉴于其古老的固有磁场，对热逸散过程和CO稳定性的研究，火星周围以₂为主的大气对EUV通量较高的情况表明，挥发物的输送和火山脱气无法抵消强烈的大气逸散。因此，在几百万年之内，由于火星岩浆海洋的凝固，几条CO ₂和H ₂ O或CO和H _2的条形物在减少条件下的灾难性脱气蒸汽气氛可能已经消失。此后，火星可能在最初的\（\ sim400 \）百万年期间无法建立密集的二次大气，但可能仅在强烈的火山脱气事件中偶发地拥有了大气，可能还包括SO ₂。这表示在\（\ sim4.1 \）之前十亿年前，火星确实确实处于寒冷和干燥的状态，最多只有短暂的零星的温暖时期。但是，之后可能会形成更密集的CO _2-或以CO为主的气氛，但由于非热逃逸过程和螯合到地下，因此以后必定会丢失。