Potential Mechanisms of Atmospheric Loss on MarsAtmospheres form and evolve during planetesimal accretion and over the lifetime of aplanet. Though the mechanisms underlying atmospheric evolution are not yet definitivelyunderstood, there is a burgeoning body of scientific literature dedicated to reconciling currentobservational evidence with the mechanisms thought to be significant in this evolutionaryprocess. In the case of Mars, the disagreements about the evolution of the atmosphere arepronounced. The composition of the primordial atmosphere, its surface pressure, the strengthand effect of the early solar flux, and the magnitude of the effect of mechanisms active inaccreting or eroding atmosphere have not yet been definitively answered, but are still beingwidely debated. The one commonality among the majority of the theories of Martianatmospheric evolution is that the Martian atmosphere was once much denser and this denseatmosphere, which created a much warmer, wetter climate on Mars, was lost. This papercritically examines these different theories of atmospheric evolution and the evidence used tojustify them.The present day Martian Atmosphere is composed primarily of carbon dioxide with traceamounts of water, carbon monoxide and nitrogen (McElroy 443) the surface pressure liessomewhere 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 unknownquantity of carbon dioxide and water trapped in the polar caps, which are composed of carbondioxide 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 regolithis 30 bars water and 20 bars of carbon dioxide respectively. (915) The atmospheric isotope ratiosas reported by a number of sources are organized in table 1.Table 1:D/H 15N/14N129Xe/132Xe40Ar/36ArHunten61.62Anders2.5 10Though the values reported differ, all invariably show significant isotopic enrichment, the valuesare relative to Earth’s isotope ratios.In order for a model of atmospheric evolution to hold up under scrutiny it must predictconditions which are consistent with our observational evidence. This includes accounting forthe isotopic enrichment, the present day surface pressure and composition as well as the surfacefeatures and evidence of past Martian atmospheric change.Understanding the initial conditions of the Martian atmosphere is essential to being ableto accurately model the effect that different mechanisms would have over time. The impact ofmany of the mechanisms proposed to have eroded the Martian atmosphere is heavily dependentupon 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 outflowchannels in the heavily-cratered terrain, eroded crater features, and layered deposits, all point tosubstantial 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.” (Hunten915) Therefore in order for liquid water to exist on the surface of Mars the atmosphericconditions would have to be altered drastically from those observed today. In a primarily carbon dioxide, water based atmosphere, like that of the present Martianatmosphere, 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 enoughgreenhouse to maintain a surface temperature at which liquid water could exist for a substantialperiod of time range from 1 bar-5 bar. (Vlassopoulos 79) Most authorities use 1 bar of surfacepressure 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 atmospherecould ever create a large enough greenhouse effect for liquid water to exist arguing that thecondensation 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 thegreenhouse effect. (Galimov 473). This argument though fails to take into account the fact thatin addition to decreasing the solar flux incident on the lower levels of the atmosphere and thesurface the clouds would effectively prevent electromagnetic radiation in the infrared given offby the planet from escaping. (Crisp 21) Thus even though the clouds were decreasing the solarflux, the trapping of the infrared heat radiated by the planet would be sufficient to negate thisdecrease and the clouds would not effectively change the greenhouse effect generated by thecarbon dioxide atmosphere. (21)Additional evidence for the presence of a thicker primordial atmosphere is found in theisotopic 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 theplanet’s history. (See Table 1) These mass selective atmospheric loss processes could haveeroded a substantial fraction of the atmosphere and the isotopic fraction serves as “a means toisolate and quantify loss by sputtering”. (Hutchins 14934) How much atmospheric loss theseisotopic ratios indicate is not entirely clear since though the process is mass selective the heavierisotopes would have also been lost to some extent over time and the atmosphere could bereplenished by different mechanisms. Among these replenishing mechanisms would beatmospheric accretion by small impactors, outgassing through volcanic processes and release ofelements 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 therewas in fact a methane-based primordial Martian atmosphere. There are several pieces ofobservational evidence that support this. One such piece of evidence is the fact that “theredoxpotential of the mantle has not remained unchanged through geological time and the mantleevolved from an initially reduced to present oxidized state.”(Galimov 473) Also no carbonatespectral lines have been detected on the