Potential Mechanisms of Atmospheric Loss on MarsAtmospheres form and evolve during planetesimal accretion and over the lifetime of a planet. Though the mechanisms underlying atmospheric evolution are not yet definitively understood, there is a burgeoning body of scientific literature dedicated to reconciling current observational evidence with the mechanisms thought to be significant in this evolutionary process. In the case of Mars, the disagreements about the evolution of the atmosphere are pronounced. The composition of the primordial atmosphere, its surface pressure, the strength and effect of the early solar flux, and the magnitude of the effect of mechanisms active in accreting or eroding atmosphere have not yet been definitively answered, but are still being widely debated. The one commonality among the majority of the theories of Martian atmospheric evolution is that the Martian atmosphere was once much denser and this dense atmosphere, which created a much warmer, wetter climate on Mars, was lost. This paper critically examines these different theories of atmospheric evolution and the evidence used to justify them. The present day Martian Atmosphere is composed primarily of carbon dioxide with trace amounts of water, carbon monoxide and nitrogen (McElroy 443) the surface pressure lies somewhere between 5-7 mbars though it varies seasonally, with most sources favoring 7 mbars. (Ahrens 11) The surface temperature is 215 degrees Kelvin. (11) There is also an unknown quantity of carbon dioxide and water trapped in the polar caps, which are composed of carbon dioxide and water, as well as an unknown amount of water and carbonates trapped in the Martianregolith. (Hunten 915) One estimate for the amount of water and carbon dioxide in the regolith is 30 bars water and 20 bars of carbon dioxide respectively. (915) The atmospheric isotope ratios as reported by a number of sources are organized in table 1. Table 1:D/H 15N/14N129Xe/132Xe40Ar/36ArHunten6 1.62Anders2.5 10Though the values reported differ, all invariably show significant isotopic enrichment, the values are relative to Earth’s isotope ratios.In order for a model of atmospheric evolution to hold up under scrutiny it must predict conditions which are consistent with our observational evidence. This includes accounting for the isotopic enrichment, the present day surface pressure and composition as well as the surface features and evidence of past Martian atmospheric change. Understanding the initial conditions of the Martian atmosphere is essential to being able to accurately model the effect that different mechanisms would have over time. The impact of many of the mechanisms proposed to have eroded the Martian atmosphere is heavily dependent upon the conditions under which the mechanism was acting. There is strong evidence for a warmer, wetter Martian climate in the past. Much of this evidenceis provided by the morphology of the Martian surface. “Extensive valley networks and outflow channels in the heavily-cratered terrain, eroded crater features, and layered deposits, all point to substantial physical erosion by liquid water on the surface.” (Hutchins 14933) Liquid water isunstable today on the Martian surface and would “either freeze or flash into vapor.” (Hunten 915) Therefore in order for liquid water to exist on the surface of Mars the atmospheric conditions would have to be altered drastically from those observed today. In a primarily carbon dioxide, water based atmosphere, like that of the present Martian atmosphere, the surface pressure of the atmosphere would have to be increased substantially. Estimates for how thick the atmosphere would have to be in order to have a large enough greenhouse to maintain a surface temperature at which liquid water could exist for a substantial period of time range from 1 bar-5 bar. (Vlassopoulos 79) Most authorities use 1 bar of surface pressure in their models since this would be a sufficient condition for liquid water to exist. Thereare some in the field who contest that a primarily carbon dioxide and water-based atmosphere could ever create a large enough greenhouse effect for liquid water to exist arguing that the condensation of carbon dioxide in the troposphere would generate clouds with a high albedo. These clouds they stipulate would decrease the solar flux reaching the surface thus negating the greenhouse effect. (Galimov 473). This argument though fails to take into account the fact that in addition to decreasing the solar flux incident on the lower levels of the atmosphere and the surface the clouds would effectively prevent electromagnetic radiation in the infrared given off by the planet from escaping. (Crisp 21) Thus even though the clouds were decreasing the solar flux, the trapping of the infrared heat radiated by the planet would be sufficient to negate this decrease and the clouds would not effectively change the greenhouse effect generated by the carbon dioxide atmosphere. (21) Additional evidence for the presence of a thicker primordial atmosphere is found in the isotopic ratios of elements in the present-day atmosphere. All of these isotope ratios are enrichedindicating that some sort of mass selective atmospheric loss mechanism has been effective in the planet’s history. (See Table 1) These mass selective atmospheric loss processes could have eroded a substantial fraction of the atmosphere and the isotopic fraction serves as “a means to isolate and quantify loss by sputtering”. (Hutchins 14934) How much atmospheric loss these isotopic ratios indicate is not entirely clear since though the process is mass selective the heavier isotopes would have also been lost to some extent over time and the atmosphere could be replenished by different mechanisms. Among these replenishing mechanisms would be atmospheric accretion by small impactors, outgassing through volcanic processes and release of elements adsorbed in the regolith or trapped in the polar caps and in other subsurface reservoirs.An alternative theory to the thicker carbon dioxide based atmosphere suggests that there was in fact a methane-based primordial Martian atmosphere. There are several pieces of observational evidence that support this. One such piece of evidence is the fact that “the redoxpotential of the mantle has not remained unchanged through geological time and the mantleevolved from an initially reduced to present oxidized