




How Mars lost its atmosphere.
Zahnle K; Workshop on the Evolution of the Martian Atmosphere (1992 Jun 29-Jul 1 : Kona, HI).
Pap Present Workshop Evol Martian Atmos. 1992; 29-30.
NASA Ames Research Center, Moffett Field, CA 94035, USA.
Mars is a small planet with a thin atmosphere. That Mars is small has been known for centuries, and we have gotten used to it; but its thin atmosphere has been called "one of the great disappointments of the space age", and we have not gotten used to this. The apparent dearth of volatiles is not confined to one or two key elements, but may well apply to all plausible atmospheric constituents. It is surest for the noble gases, for which alternative reservoirs larger than the atmosphere are unlikely. It is less certain for water or CO2, which can be hidden in surface and subsurface reservoirs. There is a widespread suspicion that Mars's thin atmosphere is in some way attributable to the planet's size. Three more or less complementary hypotheses have been suggested. The current favorite is that Mars, being small, cooled relatively quickly, and long ago ceased to recycle volatiles effectively. Weathering reactions of water, CO2, and rock would then have progressively and irreversibly consumed the atmosphere. This modern incarnation of Lowell's "dying planet" applies to CO2 and water in particular; it is more problematic for nitrogen, and it is very hard to see how it could apply to the noble gases. Another possibility is that the atmosphere was never degassed or outgassed in the first place. Although inefficient outgassing of modern Mars is a reasonable hypothesis, and fully consistent with the great antiquity of its few volcanos and the low absolute abundance of 40Ar compared to Earth, inefficient outgassing of ancient Mars is less appealing, and is contradicted by the relatively high abundance of 129Xe in the martian atmosphere. I prefer escape. Hydrodynamic escape (vigorous thermal escape) and impact erosion (expulsion of atmosphere by impacts) are two processes that should have been operative early. Although in principle hydrodynamic escape could have shrunk Mars's atmosphere a hundredfold while leaving the composition of the remnant atmosphere nearly unaltered, very high escape fluxes are required. The implicated escape mechanism must have been efficient, nearly non-fractionating, and vastly more potent for Mars than for Earth or Venus. Impact erosion is an appealing candidate. Noble gases are the obvious first test. Noble gases are the most volatile elements and so are the most likely to have been affected by impact erosion and the easiest to address quantitatively. Xenon in particular imposes three constraints on how Mars lost its atmosphere: (i) the very low abundance of nonradiogenic Xe compared to Earth, Venus, and likely meteoritic sources; (ii) its nonradiogenic isotopes distinct from likely meteoritic sources; and (iii) the relatively high absolute abundance of radiogenic 129Xe, daughter of the extinct radionuclide 129I (half-life 17 Myr). In impact erosion the first two become constraints on the composition, mass distribution, and orbital elements of the impactors. The third requires that Mars lost its nonradiogenic Xe early, probably before it was 100 Myr old. Impact erosion can explain Mars by any of three stories. (i) Mars is unlikely. In a sort of planetary brinkmanship, impact erosion almost removed the entire atmosphere but was arrested just in time. (ii) Martian noble gases are cometary and cometary Xe is as isotopically mass fractionated as martian and terrestrial Xe. This is most easily accomplished if a relatively thick geochemically controlled CO2 atmosphere protected trace atmophiles against escape. (iii) Mars was indeed stripped of its early atmospheres but a small remnant was safely stored in the regolith, later released as a byproduct of water mobilization.
The Invention of Lying has received mixed reviews











