Donald M. Hunten

The Escape of Light Gases from Planetary Atmospheres

Abstract The Jeans-Spitzer treatment of atmospheric escape by evaporation must be supplemented, for a mixture of gases, by a discussion of the mutual diffusion and mixing of the components. A recent description for the atmosphere of Titan is here applied to other objects. A range of conditions is found (corresponding to easy escape) in which the escape flux is determined by diffusion, and a simple expression is given for the flux. The state of hydrodynamic blowoff of the light gas is included, and is found to be orderly, not “chaotic” normally, the heavier gases are not carried along. For the present Earth, the flux of hydrogen is obtained within a factor of 2 from the H2O mixing ratio in the stratosphere. The Spitzer description applies to helium: the loss rate is determined by the exospheric temperature, not by diffusion. However, diffusion-limited flow could prevail during occasional periods at 2000K; if most of the helium loss is at such times, equal time constants for He4 and He3 would follow automat...

Theory of Planetary Atmospheres: An Introduction to Their Physics and Chemistry

Theoretical models of planetary atmospheres are characterized in an introductory text intended for graduate physics students and practicing scientists. Chapters are devoted to the vertical structure of an atmosphere; atmospheric hydrodynamics; the chemistry and dynamics of the earth stratosphere; planetary astronomy; ionospheres; airglows, auroras, and aeronomy; and the stability of planetary atmospheres. Extensive graphs, diagrams, and tables of numerical data are provided.

Vertical Transport in Atmospheres

Two decades ago, a revolutionary paper appeared, explaining the presence of O2 at heights above 100 km through the rapidity of molecular diffusion (Nicolet and Mange, 1954). Subsequent papers and reviews discussed this idea further (Mange, 1955, 1957; Nicolet, 1960). A second revolution took place some ten years later, with the formulation of a steady-state model incorporating eddy diffusion (Colegrove et al., 1965, 1966). These papers gave the first quantitative description of the upward transport of O, between 90 and 150 km. The aeronomy to today is permeated with eddy diffusion, which for many problems is a first-order effect.

The Upper Atmosphere of Jupiter

Abstract The aeronomy of Jupiters atmosphere at pressures <25 mb is surveyed. Attention is drawn to those areas most in need of further work, including both planetary and laboratory studies. The heat balance is still uncertain at nearly all heights. Potential stratospheric heat sources are under consideration, and their presence is suggested by the probable existence of emission features of methane in the infrared and ammonia at radio wavelengths. The thermal emissivity at the mesopause is highly uncertain, and this uncertainty propagates throughout the thermosphere to the exosphere. The photochemistry of methane and ammonia is beginning to receive serious attention, but many difficulties remain. There is still a strong possibility that local photochemical equilibrium cannot exist, and that exchange with the lower atmosphere is essential for a steady state. The photochemistry of H2 is much more satisfactory; the model can be checked against the observed Lyman-α albedo, and an effective eddy diffusion coe...

N2+ emission in the twilight☆

Abstract About a years observations of the N 2 + band (3914 A) at Kitt Peak (latitude 32°) are reported. Morning intensities are the same throughout the year, but there is a strong winter maximum in the evening. It is suggested that the additional ionization is produced by photoelectrons from the magnetic conjugate point. Heights are estimated by the zenith-horizon method, which gives 235 km for the constant component and 350 km during the evening enhancement. The intensity variation through twilight is therefore entirely due to changes of the N 2 + concentration; each ion scatters light at a constant rate. The rotational distribution resembles that for a temperature of 1600°K, much higher than the temperature of the atmosphere. It is suggested that part of the ions may be produced by charge transfer from metastable O + ( 2 D ). N 2 + concentrations resulting from photoionization are calculated; they give a fair account of the observed horizon intensities, but not the zenith. Non-local electrons from higher in the atmosphere are suggested as a possible extra source; alternatively, the zenith measurements may be perturbed by scattered horizon light. The band intensity in the nightglow cannot be measured; the upper limit is 1 R.

Soft electrons as a possible heat source for Jupiter's thermosphere

Abstract The 850 K exospheric temperature inferred for Jupiter from the radio-occultation experiments on Pioneers 10 and 11 is shown to imply a heat input of 0.25–0.5 erg cm−2s−1. One possible source of this energy is precipitation of electrons from a warm plasma (temperature corresponding to energies of the order of 30–500 eV). A mechanism is suggested wherein the presence of this plasma can be accounted for by centrifugal acceleration and adiabatic compression of ionospheric electrons and protons. Present ideas of the source strength of ionospheric plasma, however, give heating rates that are too small by 1–2 orders of magnitude, although inferences from direct plasma measurements suggest that the required plasma is indeed present.

9. The venus ionosphere and solar wind interaction

The current state of knowledge of the chemistry, dynamics and energetics of the upper atmosphere and ionosphere of Venus is reviewed together with the nature of the solar wind-Venus interaction. Because of the weak, though perhaps not negligible, intrinsic magnetic field of Venus, the mutual effects between these regions are probably strong and unique in the solar system. The ability of the Pioneer Venus Bus and Orbiter experiments to provide the required data to answer the questions outstanding is discussed in detail.

TITAN'S ATMOSPHERE AND SURFACE

This review is based on the report of the 1973 Titan Atmosphere Workshop ( The Atmosphere of Titan , NASA SP-340, 1974). The interior is presumably dominated by a melted NH3 - H2O solution, and the presence of CH4 is suggested by its abundance in the atmosphere. There is also evidence for H2, although a very high escape rate is implied. Nitrogen, from NH3 photolysis, could help in retarding this escape. Several lines of evidence suggest the presence of clouds and haze; both frozen CH4 and organic polymers are plausible. The thermal-emission spectrum shows peaks at 8 and 12 μ m, presumably due to CH4 and C2H6 in a warm stratosphere, but no minimum at 17 μm due to a pressure-induced H2 greenhouse. A decline in brightness temperature at longer wavelengths suggests a weak greenhouse effect from pressure-induced absorption in CH4. A surface temperature near 125°K is suggested by the weight of the evidence, but a value as low as 90°K is not excluded.

Spectrographic Detection of Topographic Features on Mars

Observations of the martian carbon dioxide band at 1.05 microns made with a three-channel multislit spectrophotometer indicate gross height variations in the vicinity of Syrtis Major and surrounding desert regions. Syrtis Major appears to be very high with essentially no detectable carbon dioxide above it. The data appear to confirm local trends and, in magnitude at least, the large variations of height found in earlier radar observations. A one-to-one correlation of height with albedo is not evident in the results. Elevated areas are found in both desert and dark regions. In several regions dark areas are associated with relatively steep slopes.

ON EJECTION OF NEUTRAL HYDROGEN FROM THE SUN AND THE TERRESTRIAL CONSEQUENCES

Abstract A critical examination is made of a suggestion by Akasofu that large amounts of neutral hydrogen emitted from the Sun may be responsible for the main phase of terrestrial magnetic storms. This suggestion encounters such overwhelming objections from our knowledge of solar, interplanetary, and terrestrial physics that it appears to be without sound foundation.