Atmosphere xenon

Xenon + water. The solubility data of Potter and Clynne ((5) and of Stephan, Hatfield, Peoples and Pray (10 ) were used to estimate the mole fraction solubility at one atmosphere xenon pressure at the higher temperatures. The two sets of data were combined with the 20 solubilities selected from five papers by Battino (2) in a linear regression. Figure 6 shows the data, and Battino s equation and the three constant equation. The three constants for the tentative equation for use between the temperatures of 350 and 600 K are in Table V. The Stephan et al. solubility value at 574 K was not included in the regression. 

The spatial resolution is 186 pm FWHM, for a 60 mm detector, corresponding to a 8/// = 320, with a 1 atmosphere Xenon-CH4 (90 %, 10 %) filling. This result was obtained at an amplification factor of 6000, such that the signal charge with 8 keV radiation is 0.3 pCoulomb... 

One of the major vexations of noble-gas geo-/cosmochemistry, and even of cosmochemis-try more broadly, is the difficulty in accounting for the isotopic composition of terrestrial atmospheric xenon in terms of plausibly known starting materials and plausible planetary processes. 

Noble gases and nitrogen in martian meteorites reveal several interior components having isotopic compositions different from those of the atmosphere. Xenon, krypton, and probably argon in the mantle components have solar isotopic compositions, rather than those measured in chondrites. However, ratios of these noble gas abundances are strongly fractionated relative to solar abundances. This decoupling of elemental and isotopic fractionation is not understood. The interior ratio in martian meteorites is similar to chondrites. 

The model is consistent with the isotopic evidence that upper mantle xenon does not have a simple direct relationship to atmospheric xenon. The radiogenic xenon presently seen in the atmosphere was degassed from the upper portion of the solid Earth prior to the establishment of the present upper mantle steady state xenon isotope compositions and concentrations. The lower mantle ratios are established early in Earth history by decay of I and Pu decay produces a relatively small fraction of fissiogenic nuclides (Porcelli and Wasserburg, 1995b). The xenon daughters (now in the upper mantle) of the shortlived parents are supplied from the lower mantle. The MORE Xe/ Xe ratio (when corrected for air contamination) has no radiogenic contributions... 

The relatively imprecise measured ratios of the nonradiogenic isotopes in MORE are indistinguishable from those in the atmosphere. However, more precise measurements of mantle-derived xenon in CO2 well gases have been found to have higher " Xe/ Xe ratios (Phinney et al, 1978 Caffee et al, 1999) that can be explained by either (i) a mixture of —10% xenon trapped within the Earth of solar isotopic composition and —90% atmospheric xenon (subducted or added in the crust) or (ii) a mantle xenon component that has not been isotopically fractionated relative to solar xenon to the same extent as air xenon. 

The greatest difficulty in constraining the global xenon budget has been in calculating the abundances of radiogenic xenon in the atmosphere (see Chapter 4.12). The composition for nonradio-genic atmospheric xenon (Section 4.11.2.5) provides ratios of 5 053... 

The source and composition of terrestrial nonradiogenic xenon have been difficult to constrain, because atmospheric xenon does not match any widespread solar system xenon composition. 

Y244 that represents the number of Xe atoms produced per decay of " " Pu, a similar closure age of 96 Ma is obtained. For example, note that if atmospheric xenon loss occurred during a massive Moon-forming impact, then the closure period corresponds to the time after an instantaneous catastrophic loss event. 

Martian atmospheric xenon is likely derived from solar xenon, with addition of radiogenic Xe and plutonium-derived heavy xenon after a closure age of -70 Ma. Xenon-isotope data for the martian atmosphere have been derived entirely from SNC meteorites (Figure 5). Swindle et al. (1986) pointed out that martian atmospheric xenon strongly resembles mass-fractionated Cl-Xe, with addition of radiogenic Xe (comprising 61% of total Xe). This implies that there are no additional contributions to the xenon inventory from degassed fission xenon. Alternatively, Swindle and Jones (1997) demonstrated that SW-Xe could be fractionated to fall below the measured martian atmospheric Xe/ Xe ratios by amounts consistent with the presence of " " Pu-derived... 

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. 

Atmospheric xenon is also isotopically unique among Xe components in the Solar System (with the possible exception of Martian atmospheric Xe, e.g. Swindle 1995) as it is fractionated by 3% per atomic mass unit (a.m.u.). In addition, atmospheric xenon is depleted relative to the chondritic or solar patterns the Kr/Xe ratio of air is 25 times the mean ratio of chondrites (Fig. 2). The depletion of xenon is not consistent with its isotopic fractionation because, in the first case, the heavy element (Xe) is depleted relative to the lightest one (Kr) and, in the second case, the light isotopes (e.g. Xe) are depleted relative to the heavy ones (e.g. Xe). The elemental depletion of Xe could be due to xenon trapping in a terrestrial reservoir, but possibilities exclude ice or sediments, which cannot explain the 25-fold depletion (Podosek et al. 1981 Ber-natowicz et al. 1985). Preferential subduction of Xe trapped in sediments could account qualitatively for it, but is not yet documented. 

Where this uniformitarian model is least likely to apply is early in Earth history. The mere existence of the core, separated from the mantle within 30 Myr of Earth formation (Chapters 2.14 and 2.15), along with evidence in atmospheric xenon for the decay products of short-lived I... 

Gilmour JD, Whitby JA, Turner G (1998) Xenon isotopes in irradiated ALH84001 Evidence for shock-induced trapping of ancient Martian atmosphere. Geochim Cosmochim Acta 62 2555-2571 Gilmour JD, Whitby JA, Turner G (1999) Martian atmospheric xenon contents of Nakhla mineral separates Implications for the origin of elemental mass fractionation. Earth Planet Sci Lett 166 139-147... 

---