Galileo probe

A. Balkrishnan, W. Nicolet, S. Sandhu, and J. Dodson, Galileo Probe Thermal Protection Entry Heating Environments and Spallation Experiment Design,... 

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. 

The wind systems on Jupiter and Saturn are strongly westward at the equator, with eastward jets at latitudes above and below the equator. Equatorial wind speeds on Jupiter approach 150 ms in the direction of rotation, and are 3-4 times higher still on Saturn. Measurements by the Galileo probe indicate that the winds persist down to the deepest level, 20 bar, measured by the probe. Uranus shows prograde (in the direction of rotation) winds of 150 m s at midlatitudes, which decline in speed toward the equator. Because of the low contrast of the Uranian clouds at the time of the Voyager flyby, optical tracking... 

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. 

The Galileo probe released into the Jovian atmosphere in 1995 was enormously significant because it provided the first on-site measurements of the planet s physical and chemical characteristics. Essentially all of the features just described have come from Earth-based observations, from data relayed by spacecraft flying tens of thousands of kilometers above the planet, and from theoretical models of the planet. Data relayed by the probe provided a check on the validity of these models and on the previously collected data as well as a new and extensive collection of information about the physical and chemical properties of the planet. 

One of the first discoveries reported by the Galileo probe was the lack of clouds in the Jovian atmosphere. The probe s nephelometer... 

Fortunately or not, the Galileo probe had fallen through a region of the Jovian atmosphere that is not necessarily typical of its overall structure and composition. Had it descended a few thousand kilometers away, it might have found very different conditions, similar to the wet, cloudy, windy atmosphere that astronomers had always expected to find on the planet. 

A number of the Galileo probe s instruments were designed, therefore, to detect the abundance of chemical species present in the atmospheres of both Jupiter and the Sun. One instrument, the Helium Abundance Detector, was designed specifically to measure the amount of helium in the Jovian atmosphere. It found a value for the ratio of helium to hydrogen (He H) of 0.157, which is 0.81 that found in the Sun. Why does Jupiter have so much less helium in its atmosphere than the Sun ... 

This model might also explain the observed depletion of neon in the planet s atmosphere. The Galileo probe s Neutral Mass Spectrometer (GPMS) observed an abundance of neon about one tenth that in the solar atmosphere. Perhaps, researchers hypothesize, neon dissolves in helium droplets as they form in the atmosphere and fall into the planet s interior. This phenomenon is possible because helium and neon, unlike helium and hydrogen, are completely miscible. 

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... 

Jupiter and other giant planets Mass spectrometry (Galileo probe) In-situ interferometry (Galileo probe) Radio occultation infrared spectroscopy (Voyager) He-Ar, Xe elemental and isotopic abundance in outer Jovian atmosphere. Mahaffy et al. 2000. He abundance in outer Jovian atmosphere, v. Zahn et al. 1998. He abundance in giant planet outer atmospheres. Comath et al. 1991 Comath and Gautier 2000. 

Noble gases are intrinsically difficult to detect by spectroscopy. For example, solar photospheric spectra, which form the basis for solar abundance values of most elements, do not contain lines from noble gases (except for He, but this line cannot be used for abundance determinations). Yet, ultraviolet spectroscopy is the only or the major source of information on noble gas abundances in the atmospheres of Mercury and comets. In the Extreme Ultraviolet (EUV), photon energies exceed bond energies of molecules and the first ionization potential of all elements except F, He, and Ne, so that only these elements are visible in this part of the spectrum (Krasnopolsky et al. 1997). Other techniques can be used to determine the abundance of He where this element is a major constituent. Studies of solar oscillations (helioseismology) allow a precise determination of the He abundance in the solar interior, and the interferometer on the Galileo probe yielded a precise value for the refractive index and hence the He abundance in the upper atmosphere of Jupiter (see respective sections of this chapter). 

Figure 6. The He mass fraction Y in the atmospheres of the four giant planets. The Jovian value is from the interferometer on the Galileo probe, while all others (including a less precise earlier Jovian value) are from the Voyager missions (Table...

For He abundance see Table 6. All data from Galileo probe (noble gases Mahaffy et al. 2000 He/ He Mahaffy et al. 1998). 

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 results also prompted a reevaluation of the Saturn He data from Voyager (Conrath and Gautier 2000). The revised He abundance for Saturn is Yg = 0.18-... 

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. 

Jupiter is the only giant planet for which isotopic ratios of noble gases are available, provided by the Galileo probe mass spectrometer (Mahaffy et al. 1998 2000 Table 7). This instrument also yielded quite precise values for D/H, and which are... 

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