The origin of inner planet atmospheres.

Abstract

In a series of laboratory experiments, A. Bar-Nun and colleagues (Bar-Nunet al. 1985, 1988; Owenet al. 1991) have succeeded in analysing the amounts of gases trapped in amorphous ice forming at low temperatures (16-190 K). They found that a temperature-dependent fractionation of noble gases occurs. The relative abundances of argon, krypton and xenon trapped by ice formed at 50 K fit an extrapolated mixing line that passes through points for the abundances of these same gases in the atmospheres of Mars and Earth on a three-isotope plot. The noble gases in the Shergottites, one of the three types of meteorites originating on Mars, also fall on this line (figure 1). A study of the xenon isotopes in the Shergottites and Nakhlites (a second type of Martian meteorite) suggest that these meteorites contain gases from the Martian interior and atmosphere, and the Shergottites may also include a component from the impactor that expelled them from the planet (Owen & Bar-Nun 1993). Taken together, these data provide a good indication that icy planetesimals (comets) that formed in the Uranus-Neptune region (T ~ 50 K) played a major role in delivering the noble gases to Mars and the Earth. Venus could have obtained its noble gases from the impact(s) of one or more comets formed at the lower temperatures (T ~ 30 K) prevailing in the Kuiper Belt (Owenet al. 1992; Owen Bar-Nun 1993). Given that ice is the carrier, one naturally expects that comets could have brought inallthe volatiles. This idea is buttressed by a study of nitrogen. The same laboratory experiments referenced above demonstrate that CO is trapped twenty times more efficiently than N2in ice forming at 50 K. Since N2is expected to be the dominant carrier of nitrogen in the solar nebula, whereas CO is likely to contain no more that 15-30% of the total carbon, this would lead to a severe depletion of nitrogen in icy planetesimals. In evaluating C/N in Halley’s comet, the atmospheres of Venus and Mars, and the volatile inventory of the Earth, one sees exactly this effect. In all four inventories C/N = 20 ± 10, whereas the solar value is 3.2. Thus we again discover support for the cometary delivery hypothesis (Owen & Bar-Nun 1994).