Did Mars Possess a Dense Atmosphere During the First $\sim400$ Million Years?

Abstract

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 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 thermal escape. Therefore, a catastrophically

Non-thermal escape of the Martian CO$_2$ atmosphere over time: constrained by Ar isotopes

“…Lammer et al, 2020b, for a discussion on fractionation through different escape processes) at the time when Mars still maintained its intrinsic magnetic field. The lack of any notable fractionation prior to 4.1 Ga, therefore, supports the idea that Mars lost its entire atmosphere early on (Scherf & Lammer, 2021).…”

“…The same atmospheric profiles as in Boesswetter et al (2010) (i.e. assuming no dissociation of the CO 2 molecules) were also used by Terada et al (2009) and Sakata et al (2020) and are therefore not accurate (Tian et al, 2009;Scherf & Lammer, 2021). By using the impact rates of exospheric O + and C + ions we also calculate the corresponding sputter escape rates for O, CO 2 , CO, C, 36 Ar, and 38 Ar.…”

“…While it is yet not entirely clear, how such an intrinsic magnetic field might affect the efficiency of non-thermal escape processes (being it a funnel or a shield, e.g. Gunell et al, 2018;Blackman & Tarduno, 2018;Egan et al, 2019;Scherf & Lammer, 2021), it does not affect thermal escape, which could have been as high as ∼ 10 30 s −1 for carbon at ∼ 4.5 Ga (Tian et al, 2009).…”

“…The evolution and escape of Mars' CO 2 atmosphere in the context of a higher surface partial pressure and a climate that allowed liquid water on the planet's surface, the so-called "warm and wet" early Mars hypothesis, is one of the great riddles in Solar System science (de Pater & Lissauer, 2015;Jakosky & Phillips, 2001;Hurowitz et al, 2017;di Achille & Hynek, 2010;Lammer et al, 2013Lammer et al, , 2018Palumbo et al, 2020;Scherf & Lammer, 2021). Whether early Mars passed through a phase of a warm and wet climate with a much denser atmosphere during the Noachian eon or was predominantly cold and dry with sporadic warm phases is debated.…”

Present-day atomic hydrogen escape on Mars and its variability

“…热逃逸又包含了流体静力学 逃逸(俗称金斯逃逸)和流体动力学逃逸两种极端情形, 前者代表一种随机、缓慢的粒子外逃, 而后者代表一 种有序、高速的粒子外流. 流体动力学逃逸对于当前 火星并不重要, 但在火星演化早期可能起到决定性的 作用 [46][47][48] . 对于非热中性逃逸, 则包含了众多高能中 性粒子释放过程, 如火星电离层中发生的O 2 + 复合分解 可以释放出两个O原子, 是驱动火星O逃逸的最主要通 道 [14,38,[49][50][51][52][53] , 火星高层大气CO 2 , CO和N 2 等成分的光解 是驱动火星C和N逃逸的最主要通道 [16,54,55] , 而溅射 过程(空间环境中的高能粒子沉降至火星大气并与大 气粒子发生碰撞)则是驱动火星大气中较重成分(如 CO 2 , N 2 , Ar等)逃逸的最主要通道 [56][57][58] .…”