Gravitational capture

It would appear that current observations and modeling cannot rule firmly for or against gravitational capture of massive solar-composition primary atmospheres on Earth and Venus. The extent to which this occurred, creating tenuous or dense, massive atmospheres, depends upon the mass of the protoplanets at the time of nebular gas dispersal. If it did occur, subsequent evolution from primary to present-day noble gas inventories and compositions must have involved loss processes that fractionated both elements and isotopes to generate the presently observed compositions (Fig. 2). There is presently no reason to assume elemental fractionation in the capture process itself, although it may be that this possibility needs more theoretical study. 

Evidence of such a deep Earth abundance pattern coupled with solar isotopic compositions would be a strong indicator that adsorption could have played a role early in the Earth s accretional history (Pepin 1991, 1998). Although there is evidence that solar-like light noble gas isotopic compositions exist, the abundance pattern of trapped noble gases in the Earth cannot be easily constrained, since there is the possibility that subduction of heavy noble gases has since altered the pattern. With a Mars-size terrestrial embryo and the high value of 2 x 10 for the Ne Henry s constant assumed in Pepin s 

As discussed above, the plausibility of this gravitational capture mechanism depends upon whether a sufficient mass of the Earth accretes prior to dispersal of nebular gases, and further work is required to determine if dispersal of the nebula can extend over such time periods. 

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Gravitational capture and dissolution into molten planets 332... 

The atmosphere of Mars has several features that are distinct from that of the Earth and require a somewhat different planetary history. At likely nebular temperatures and pressures at its radial distance. Mars is too small to have condensed a dense early atmosphere from the nebula even in the limiting case of isothermal capture (Hunten, 1979 Pepin, 1991). Therefore, regardless of the plausibility of gravitational capture as a noble-gas source for primary atmospheres on Venus and Earth, some other way is needed to supply Mars. This may include solar-wind implantation or comets. An important feature is that, in contrast to Earth, martian xenon apparently did not evolve from a U-Xe progenitor, but rather from SW-Xe. This requires that accreting SW-Xe-rich materials that account for martian atmospheric xenon are from sources more localized in space or time and so have not dominated the terrestrial-atmospheric xenon budget. There are insufficient data to determineif the martian C/N ratio is like the terrestrial value, but it appears that the initial C/H2O ratio may have been. Further constraints on the sources of the major volatUes are required. 

At likely nebular temperatures and pressures at its radial distance. Mars is too small to have condensed a dense early atmosphere from the nebula even in the limiting case of isothermal capture (Hunten 1979 Pepin 1991). Therefore, regardless of the plausibility of gravitational capture as a noble gas source for primary atmospheres on Venus and... 

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