Jupiter’s atmosphere

For many years, meteorites have provided the only means to determine the abundance of 3He in protosolar material. The values obtained by mass spectroscopy techniques in the so-called planetary component of gas-rich meteorites have been critically examined by Geiss (1993) and Galli et al. (1995). The latter recommend the value 3He/4He= (1.5 0.1) x 10-4. The meteoritic value has been confirmed by in situ measurement of the He isotopic ratio in the atmosphere of Jupiter by the Galileo Probe Mass Spectrometer. The isotopic ratio obtained in this way, 3He/4He= (1.66 0.04) x 10 4 (Mahaffy et al. 1998), is slightly larger than, but consistent with, the ratio measured in meteorites, reflecting possible fractionation in the protosolar gas in favor of the the heavier isotope, or differential depletion in Jupiter s atmosphere. 

Fig. 1. Evolution of 3He/H in the solar neighborhood, computed without extra-mixing (upper curve) and with extra-mixing in 90% or 100% of stars M < 2.5 M (lower curves). The two arrows indicate the present epoch (assuming a Galactic age of 13.7 Gyr) and the time of formation of the solar system 4.55 Gyr ago. Symbols and errorbars show the 3He/H value measured in meteorites (empty squares) Jupiter s atmosphere (errorbar) the local ionized ISM (filled triangle) the local neutral ISM (filled circle) the sample of simple Hll regions (empty circles). Data points have been slightly displaced for clarity. The He isotopic ratios has been converted into abundances relative to hydrogen assuming a universal ratio He/H= 0.1. See text for references.

In contrast to the terrestrial planets, the giant planets are massive enough to have captured and retained nebular gases directly. However, concentrations of argon, krypton, and xenon measured in Jupiter s atmosphere by the Galileo spacecraft are 2.5 times solar, which may imply that its atmosphere preferentially lost hydrogen and helium over the age of the solar system. 

Fig. VIII—1-4. Proposed temperature profile of Jupiter s Atmosphere. The tropo-pause is chosen as height reference since there is no evidence of a solid surface. The temperature at the tropopause is 9S.5°K and the number density is 2 x I01 cm 3. Contrary to the case of the upper atmosphere of earth, there appears to be no boundary between stratosphere and mesosphere. The observed cloud deck is believed to be solid ammonia. (M) signifies the number of molecules per cm3. From Hunten (490b), reprinted by permission of the American Meteorological Society. 

Jupiter and Uranus are outer planets composed mainly of gases. Jupiter s atmosphere contains reddish-brown clouds of ammonia. Uranus has an atmosphere made up mainly of hydrogen and helium with clouds of water vapor. This combination looks greenish to an outside observer. In addition, Mars has an atmosphere that is 95% carbon dioxide, and Venus has a permanent cloud cover of sulfur dioxide that appears pale yellow to an observer. Mercury has no permanent atmosphere. Saturn has 1 km thick dust and ice rings that orbit the planet. The eight planets in our solar system are diverse, each having different chemical compositions within and surrounding the planets. Out Earth is by far the friendliest planet for human existence. 

From the isotopic decomposition ofnormal He one finds thatthe mass-3 isotope, 3He, is quite rare relative to 4He. It is 0.0142% of all helium in solar gases at the time the Sun was forming, as recorded in planetary gases trapped within gas-rich meteorites that formed in the primitive solar disk. It is somewhat larger, 0.0166% in the helium in Jupiter s atmosphere, which could be a better measure of initial 3He/4He. But3He is about 100 times more rare in the Earth s atmosphere (relative to 4He) because the history ofradioactive decay of uranium in the Earth (see the Rutherford anecdote above) has enriched our atmosphere in daughter 4He. 

Figure 2.6 Carbon- and nitrogen-isotopic compositions of presolar SiC grains. Predictions from stellar models are shown for comparison. Solar metallicity AGB star models Nollett et al. (2003), Type II SN Rauscher et al. (2002), novae Jose et al. (2004). For data sources see Lodders Amari (2005) Zinner (2007). Note that for the solar 14N/15N ratio the value inferred for Jupiter s atmosphere is shown.

Von Zahn U., Hunten D. M., and LehmacherG. (1998) Helium in Jupiter s atmosphere results from the Galileo probe helium interferometer experiment. J. Geophys. Res. 103, 22815-22830. 

Jupiter s atmosphere consists mainly of hydrogen (90 percent) and helium (9 percent). How does this mixture of gases contrast with the composition of Earth s atmosphere Why does the composition differ ... 

Mahaffy PR, Donahue TM, Atreya SK, Owen TC, Niemann HB (1998) Galileo probe measurements of D/H and He-3/He-4 in Jupiter s atmosphere. Space Sci Rev 84 251-263 Marti K, Kim JS, Thakur AN, McCoy TJ, Keil K (1995) Signatures of the martian atmosphere in glass of the Zagami meteorite. Science 267 1981-1984... 

Figure 3. Isotopic composition of various Xe components, normalized to Xe and the terrestrial atmospheric composition. Solar wind Xe (SW), U-Xe, and Xe in Jupiter s atmosphere are discussed in the Sun and Jupiter sections (Tables 5 and 7), the meteoritic component Xe-Q (Busemann et al. 2000) is discussed in the chapter by Ott (2002). Xe in Jupiter s atmosphere is probably heavier than terrestrial atmospherie Xe, and might be eonsistent with either of the other eompositions shown. Note that the absolute ordinate position of the Jupiter pattern depends heavily on the ehoiee of the normalisation isotope, as visualized by the error bar on the normalizing isotope Xe (cf. Mahaffy et al. 2000).

The accurate Galileo probe interferometer data has allowed confirmation of a He depletion in Jupiter s atmosphere. Actually, the Jovian He abundance is, within error, the same as the value in the solar convective zone, but this is accidental, because He settling also occurs in the Sun (Sun section). On the other hand, the He abundance in Jupiter is lower than the lower limit of the protosolar value by about 6 sigma. So, whereas the implication from the Voyager data of a considerable He depletion in Jupiter s atmosphere relative to the solar convective zone does not hold any more, the Galileo data nevertheless clearly indicate that He has segregated, or still does so, in the atmosphere and interior of Jupiter (vs. Zahn et al. 1998). 

The Galileo probe mass spectrometer also showed that the abundances of the four heavier noble gases in Jupiter s atmosphere relative to H are distinctly non-solar (Table 7). Ne is depleted by about an order of magnitude, whereas Ar, Kr, and Xe all are enriched by a factor of 2.5. The striking underabundance of Ne is viewed as supporting evidence for He segregation in Jupiter s interior (Niemann et al. 1998 von Zahn et al. 

The Ne/ Ne ratio in Jupiter has a relatively large uncertainty, but the reported value is within 6% identical to values deduced for the solar wind. As the difference between the solar Ne composition and that of atmospheric or meteoritic Ne is considerably larger than in the case of Ar (e.g., Ott 2002), the Ne data from the Galileo probe therefore allow us to conclude what was not possible for Ar Ne in Jupiter has a solar-like isotopic composition that is distinct from primordial Ne in meteorites or Ne in the Earth s atmosphere. However, the uncertainties of the e/ Ne ratio are too large to decide whether or not the Ne depletion in Jupiter s atmosphere by an order of magnitude resulted in any isotopic fractionation. 

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