Outer Solar System A Program for Exploration, Report of a Study (1969) / Chapter Skim
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PLANETARY ATMOSPHERES
PLANETARY ATMOSPHERES
Pages 25-45
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From page 25... ...
Evidently some other fractionation process has been active in the outermost regions of the solar system as well, since Uranus and Neptune are deficient in hydrogen with respect to the cosmic average and may be deficient in helium as well. In order to identify the causes of these abundance variations, which must obviously be intimately related to the processes involved in the origin and evolution of the solar system, it is essential to have more quantitative data on the abundances of the light and heavy elements in the atmospheres of these planets to complement models giving the bulk composition of their interiors.
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From page 26... ...
There is another aspect of the abundance problem of great importance for understanding the chronology of the solar system -- isotopic abundances, which are virtually unaffected by chemical reactions and are critical tests of astrophysical theories for element formation. A wealth of data related to the abundance of isotopic nuclei in the earth's crust and the meteorites is available, on the basis of which very precise determinations of the time of formation of the elements and of these bodies can be made.
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From page 27... ...
Only H2, NH3, and CH4 have been detected, but, undoubtedly, H20 and H2S are also present in large amounts at lower levels of the atmosphere. In addition, other more complex molecules are doubtless present in trace amounts, which are formed through photolytic processes induced by solar light.
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From page 28... ...
It has already been suggested that some of the coloring matter observed in the Jovian cloud deck could be organic polymers dissolved in the cloud material. Laboratory experiments using mixtures of methane and ammonia subjected to electrical discharge have produced colored substances, thereby lending support to this interpretation.
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From page 29... ...
In the outer planets, methane is expected to be photochemically modified by absorption of ultraviolet radiation. Present knowledge suggests that the detailed chemical processes lead to formation of complicated hydrocarbons, an exciting prospect which may have significant biological overtones.
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From page 30... ...
We therefore recommend that consideration be given to the development of small and relatively simple unshielded entry probes capable of being carried on all outerplanet missions. THERMAL STRUCTURE AND DYNAMICS Studies of motions in the atmosphere of Jupiter have recently begun to achieve a quantitative status and are leading to new ideas about the behavior of rapidly rotating atmospheres and the interaction of cloud and planetary motions.
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From page 31... ...
will be remedied if high-resolution visual pictures required can be acquired during the contemplated missions to the major planets. Thermal Mapping and Vertical Temperature Structure In nonhomogeneous convective atmospheres, such as those of Jupiter and Saturn, upward mass motions occur which transport heat from hotter to cooler regions.
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From page 32... ...
Certain results indicate that at times little or no temperature contrast exists between visual features, such as the Great Red Spot and its surroundings, while at other times the brightness-temperature variations can be pronounced. The Great Red Spot has appeared cooler on two widely separate occasions, once by ~2°.
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From page 33... ...
Recent reassessment of the heat balance on Jupiter has indicated a more complex state of affairs in which thermal radiation mainly influences boundary conditions: The distribution of heat through the lower atmosphere will be dominated by fluid motions on a variety of scales, from that of the general circulation down to small-scale convection. Flybys and orbiters can be used, not only for thermal imaging, but for radio occultations and infrared and microwave thermal soundings.
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From page 34... ...
These measurements add justification to what appears to be the most important single mission concept for outer-planet atmospheric studies. VISUAL IMAGING Studying features of the visible surface of dense cloud over a broad range of horizontal scales requires photography of the planet beginning (in the case of a flyby)
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From page 35... ...
Measurement of the planetary phase function, which is an effect produced by the clouds, gives data useful for other purposes but contributes little to our understanding of the chemistry and physics of the clouds. Photography of the cloud structures near the terminator may help in understanding the scale of motions in the atmosphere near the cloud level.
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From page 36... ...
Since the principal cloud layers on both Jupiter and Saturn are believed to be due to condensation of trace gases from the atmosphere, it appears that mass spectrometric analysis of the atmosphere combined with temperature and cloud-density measurements would permit determination of the cloud composition. At present, it appears that a mass spectrometer of dynamic range l0^:l would be capable of effecting a very detailed elemental and isotopic analyses for the elements H, He, 0, C, N, Ne, S, Ar, and Cl.
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From page 37... ...
A more appealing possibility appears to lie in sending the second such entry probe to Uranus or Neptune. Uranus and Neptune Is an entry probe mission to Uranus or Neptune justifiable in fact?
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From page 38... ...
Almost every piece of information essential to the solution of these problems must be derived from the results of entry probe missions. We therefore strongly recommend Jupiter and Uranus entry probe missions at the earliest possible date.
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From page 39... ...
in addition to several 24-in. planetary patrol telescopes.
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From page 40... ...
The 24-in. NASA planetary patrol telescopes, operating at f/75, should provide good material for analysis.
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From page 41... ...
These weak bands are probably formed deeper in the clouds than the strong bands and should therefore reflect latitudinal variations of the deeper cloud levels. Another aspect of our knowledge of the major planets which should be improved before advanced space probes are sent is their energy budget.
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From page 42... ...
Jupiter occultations of these satellites occur frequently, and in each case the detailed manner of decay or rise of the radio signal can be studied in much the same way as during spacecraft occultation. From this, deductions can be made concerning the density and structure of the ionosphere of Jupiter, as well as the structure of the upper atmosphere and some indication of its chemical composition.
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From page 43... ...
Either type of radar echo detection from Jupiter would give new information concerning the atmosphere: the level of a surface of discontinuity, the amount of change in the value of the dielectric constant, the attenuation above this discontinuity, and the dependence of the attenuation on frequency all would be valuable for creating a model of the atmosphere. In the case of the detection of incoherent scatter from clouds, their height and particle size as well as windspeeds and overlying atmospheric attenuation can be deduced from two-frequency radar observations.
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From page 44... ...
Bistatic radar would also be an important method of studying the surfaces of the major satellites of the giant planets and the particles in the rings of Saturn. In consideration of the above discussion, we recommend that NASA support radar astronomy to the extent necessary to obtain direct measurements of the Jovian atmosphere and with the plan that the same ground-based facilities would be used with spacecraft for bistatic radar investigations.
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From page 45... ...
Ultraviolet spectroscopy from earth-orbiting telescopes can be expected to yield much valuable information on the atmospheres of the major planets. The University of Wisconsin OAO-A2 has already made a substantial number of planetary observations.
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