Noble gases on Venus

There are limited probe data available for the isotopic composition of the Venus atmosphere, and of course none available for noble gases in the Venus interior. Wieler (2002, this volume, p. 41 his Table 8 and references therein) provides the most recent compilation and assessment of the available data. 

Nonradiogenic noble gases. For Ne, both of the two °Ne/ Ne ratios derived from different data bases (11.8 0.6 and 12.15 0.40 see Wieler s (2002) Table 8) require some fractionation relative to the solar value, although not quite to the same extent as that of the terrestrial atmosphere. The constraint on the Ne/ Ne ratio of 0.067 cannot be used to further limit the source of Ne. The °Ar/ Ar value of 5.45 0.10 measured by Venera spacecraft instruments is nominally somewhat above the terrestrial ratio but is essentially indistinguishable from it within error. 

Radiogenic nuclides. The measured ratio is 1.11 0.02, substantially less 

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The surface of Venus is hidden under an unbroken layer of clouds 45-60 km above it. Recently, the planet has been subjected to a complete cartography by radar satellites. Its atmosphere contains 96% CO2 by volume, the remainder consisting of N2, SO2, sulphur particles, H2SO4 droplets, various reaction products and a trace of water vapour. The water is probably subject to photolytic decomposition. Noble gases are more abundant than on Earth 36Ar by a factor of 500, neon by a factor of 2,700, and D (deuterium) by a factor of 400. 

Noble gases are most abundant in planetary atmospheres, although even there they are only minor components. They have been measured in the gas envelopes of Venus, Earth (of course), Mars, and Jupiter. We will consider their utility in understanding planetary differentiation and atmospheric evolution shortly, but first we will focus on their rather miniscule abundances in meteorites and other extraterrestrial materials. 

The observed noble-gas abundances and isotopic ratios on Venus are summarized in Tables 3 and 4. The helium mixing ratio is a model-dependent extrapolation of the value measured in Venus upper atmosphere, where diffusive separation of gases occurs. The main differences between Venus and Earth are that Venus is apparently richer in He, Ar, and Kr than the Earth, and the low " Ar/ Ar ratio of — 1.1 on Venus, which is —270 times smaller than on Earth. The low " Ar/ Ar ratio may reflect more efficient solar-wind implantation of Ar in solid grains accreted by Venus and/or efficient early outgassing that then stopped due to the lack of plate tectonics. Wieler (2002) discusses the noble-gas data. Volkov and Frenkel (1993) and Kaula (1999) describe implications of the " Ar/ Ar ratio for outgassing of Venus. 

Table 4 summarizes the data on the isotopic composition of Venus atmosphere. Aside from the noble gases, the most important difference between Venus and Earth is the high D/H ratio, which is —150 times greater than the D/H ratio of 1.558 X 10 in standard mean ocean water (SMOW). The high D/H ratio strongly suggests. 

Venus is also rich in nonradiogenic noble gases, with the absolute abundance of on Venus exceeding that on Earth by a factor >70. This is clearly due to the amount of noble gases initially supplied and retained by the planet, and is discussed further in Chapter 4.12. 

While considerations of the origin of planetary noble gases have been predominantly focused on those presently found in the atmosphere, noble gases still within the Earth provide further constraints about volatile trapping during planet formation. A wide range of noble-gas information for the Earth s mantle has been obtained from mantle-derived materials, and indicates that there are separate reservoirs within the Earth that have distinctive characteristics that were established early in Earth history. These must be included in comprehensive models of Earth volatile history. Also, data are now available for the atmospheres of both Venus and Mars, as well as from the interior of Mars, so that the evolution of Earth volatiles can be considered within the context of terrestrial-planet formation across the solar system. 

On Venus, the noble gases do not appear to have greatly evolved from solar characteristics. The heavy rare-gas elemental abundances are similar to solar values, although this similarity does not extend to neon, since the e/ Ar ratio is low. Nonetheless, the je/ Ne ratio is closer to the solar value. Venus is also gas rich, with the absolute abundance of argon on Venus exceeding... 

Table 1 summarizes the techniques used to measure noble gases. By far the most important is mass spectrometry. Mass spectrometers in space are used, e.g., for solar wind and solar energetic particle measurements or atmospheric analyses on Venus, Moon, Mars and Jupiter, while mass spectrometers in the laboratory allow us to analyze extraterrestrial samples available on Earth, i.e., lunar samples, meteorites, interplanetary dust or solar corpuscular radiation trapped by foils exposed in space. Of course. 

Noble gases in the Venusian atmosphere have been analyzed by mass spectrometry and gas chromatography on board the Pioneer Venus and several Venera spacecraft (Hoffman et al. 1980a Donahue and Pollack 1983 Istomin et al. 1983 Moroz 1983 Donahue and Russell 1997). The data and their sources are summarized in Table 8. In the early 1980s, greatly different Kr and Xe abundances were reported by the Pioneer Venus and the Venera teams (see Appendix in Donahue and Pollack 1983). This discrepancy was settled, when Venera 13 and 14 mass spectrometer data were found to be in agreement with the Pioneer Venus results (Donahue 1986). The early Venera results appear to have been compromised by contamination with calibration gas (Istomin et al. 

Clathrated gases therefore appear unlikely to be the source of atmospheric noble gases, at least for Venus, and one must appeal to physical adsorption on ice. 

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