Planetary Astronomy; an Appraisal of Ground-Based Opportunities (1968) / Chapter Skim
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Atmospheres: Planets and Comets
Atmospheres: Planets and Comets
Pages 26-38
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From page 26... ...
With each of these techniques, the basic procedures are the same: to observe the given object, to interpret the observations with the help of laboratory calibrations and physical theory, and to develop comprehensive atmospheric models. Substantial progress in all three areas is required if a thorough understanding of planetary atmospheres is to be achieved.
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From page 27... ...
The inner or terrestrial planets appear to have secondary atmospheres derived from outgassing. The outer or Jovian planets have reducing atmospheres containing large amounts of hydrogen, which strongly suggest an origin similar to that of the solar system itself.
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From page 28... ...
atmospheric composition. The validity of this assumption can be tested in part by examining the atmospheres of the outer planets for signs of prebiological organic molecules, since these planets presently exhibit one type of atmosphere postulated for the primordial Earth.
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From page 29... ...
JUPITER: Recent ultraviolet spectra from rockets revealed unidentified absorption at 2600 A and below 2100 A that may be caused by large organic molecules. A preliminary spectrum of the 8.5-13.5-/t region contains several puzzling features that require verification and explanation.
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From page 30... ...
Since neither of these gases has ground-state absorption lines in the readily accessible spectral region, their presence and abundances must either be deduced indirectly or determined from Earth-orbital observations. With regard to minor constituents, isotopic forms of gases already identified, such as 18CH4 and 1H2H, should be sought and abundances estimated, and the possible presence of organic molecules and water vapor in the atmosphere of Jupiter further investigated.
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From page 31... ...
The resulting temperatures may correspond to different levels in a planetary atmosphere or may be indicative of surface or subsurface conditions, depending on the strength of the atmospheric opacity and its wavelength dependence. Radio observations allow investigation of the lower atmospheres of cloud-covered planets and should contribute to studies of the atmospheres of Venus and the major planets.
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From page 32... ...
If the observed spectrum is produced by absorption in a CO2-N2 atmosphere, very high pressures must exist; if by absorption by water vapor, a great deal more water vapor must be present than seems consistent with observations; if by absorption by liquid water in the clouds, again too much water vapor is implied to be consistent; if by absorption by dust, large quantities of suspended material are required; and if by scattering, large particle sizes are needed. In the case of Jupiter, thermal radiation dominates the radio spectrum for wavelengths shorter than about 3 cm.
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From page 33... ...
Group velocity delay, extinction, and the Doppler phase shift of these echoes can all be used to gain information on the atmospheric scale height and dielectric properties. The observed disk brightness temperature of Saturn increases from about 100°K at short millimeter wavelengths to about 300°K at 20-on wavelengths, a spectrum very similar to that of Jupiter's thermal radiation.
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From page 34... ...
Temperatures derived from analyses of molecular bands are difficult to interpret because of the nonequilibrium excitation mechanisms. GAS DYNAMICS Gas motions in comet tails are relatively easy to observe, but, as mentioned above, the relationship of these motions to the interplanetary medium through which the comets pass is not clear and constitutes an important area of research.
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From page 35... ...
The opacity caused by such particles may contribute significantly to the planetary heat balance. Finally, planetary meteorology may be strongly affected by the presence of condensates able to absorb and liberate substantial amounts of latent heat during phase changes.
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From page 36... ...
Atmospheric models have been proposed that take into account the effect of condensation on the temperature gradient, but they require elaboration and observational verification. On Saturn, the visible cloud surface may consist of ammonia crystals at very low temperatures; on the other hand, these clouds may be condensed methane or a mixture of the ammonia crystals and the methane.
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From page 37... ...
Such a mechanism is known to operate on Earth in arid regions, and it appears reasonable that winds capable of initiating the enormous Martian dust storms would have sufficient force to cause erosion. However, the details of the soil-moving process at very low pressures remain uncertain owing to the large extrapolation that is required to go from theories based on observations in terrestrial deserts at atmospheric pressure to expected
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From page 38... ...
There is no direct evidence on the wind forces that would be required to dislodge dust and carry it into the atmosphere nor on the effects of such dust on surface terrain under conditions assumed to exist in the lower atmosphere. Some insight into these questions can again be gained by laboratory experiments on dust-raising and -transport mechanisms, in this case for high pressures and temperatures.
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