Site Section Links
VENUS'S INTERNAL HEAT: AN UPDATE
C. Leroy Ellenberger
The discussion of the thermal imbalance that may exist on Venus [KRONOS VI :2 (1981), pp. 23-24] was based on the paper by Tomasko et al. [J. Geophys. Res., Vol. 85, No. A13, pp. 8187-99, Dec. 30, 1980] which had been recommended by F. W. Taylor, one of the co-authors. That paper avoided giving credence to the presence of a significant internal heat flux on Venus by emphasizing the overlap between the ranges of the flux components. The adequacy of the greenhouse effect as the sole explanation for Venus's high surface temperature was thereby implied, while they concluded that "it appears that the very high surface temperature of Venus is due almost entirely to the greenhouse effect" [p. 8197]. While Tomasko et al. were not incorrect in their conservatism, other authors in the same issue were more forthright about the inadequacy of greenhouse models while proposing alternatives in addition to internal heat.
The abstract for the paper by Suomi et al ventures: "Net flux radiometer measurements in the atmosphere of Venus question the effectiveness of the greenhouse mechanism in maintaining the high surface temperature" [J. Geophys. Res., op. cit, pp. 8200-18]. Suomi had earlier concluded that "the greenhouse effect alone is unable to account for the high surface temperatures" [Science News, Vol. 116, Nov. 3, 1979, p. 308]. Suomi et al. state: "The energy budget dilemma in the deep atmosphere actually occurs below the measurements. The large magnitude of the infrared fluxes in comparison with the solar fluxes at 13 km implies that either the atmosphere below 13 km or the surface (or both) are losing energy. Replacement of the energy lost below 13 km would require a non radiative mechanism to transport heat downward from the clouds or upward from the planet's interior" [p. 8208]. However, they go on to consider a thermal oscillation on the scale of decades in which "Both entropy and energy balance of the atmosphere could oscillate under the influence of feedback processes which cause slow oscillations in the global cloud properties or composition" [p. 8209]. Suomi et al. conclude: "The measured net fluxes imply that the deep atmosphere is cooling radiatively. This surprising result could mean that the lower atmosphere is actually cooling at the present time or that a yet unknown mechanism is acting to maintain thermal balance. The observed cooling is not found to be inconsistent with the assumption that the atmosphere is in a steady state, although an increasing atmospheric entropy [i.e., cooling] appears to be a distinct possibility" [pp. 8216-17].
Ingersoll and Pechmann state in their abstract: "If all observations are correct and the probe sites are typical of the planet, the second law of thermodynamics requires that the bulk of the lower atmosphere heating must come from a source other than direct sunlight or a thermally driven atmospheric circulation. Neither the so-called greenhouse models nor the mechanical heating models are consistent with this interpretation of the observations" [J. Geophys. Res., op. cit., pp. 8219-22]. They conclude: "In summary, we cannot claim to understand the high surface temperatures of Venus on the basis of the Pioneer Venus observations. If the variability of the observed infrared net flux profiles is real and if the night and north profiles are not anomalously high due to local conditions, then a transfer of 23 W m-2 from the upper to the lower atmosphere is implied. This transfer cannot be due to any steady state, thermally driven process in the atmosphere. We conclude either that some of the net flux data are in error or that they are from anomalous regions or else that there is another source of energy for the deep atmosphere. Examples include 23 W m-2 internal heat source, a secular cooling of the atmosphere, or a chemically energetic rain carrying solar energy downward" [p. 8222]. With respect to the validity of a 23 W m-2 flux, they observed in an earlier discussion: "An imbalance at the top of the atmosphere of 18 W m-2 is almost ruled out by orbiter data . . ., however, and is 250 times the earth's internal heat flux" [p.8221].
Taylor redressed New Scientist's exaggeration in the 19 Nov. 1980 news item with his letter in the 29 Jan. 1981 issue, p. 301. He admonished that "the enormously complex questions of the global energy balance of an entire planet . . . are not settled so quickly.... The problem is the familiar one of needing to determine the small difference of two large numbers; when it is realized that millions of separate measurements are needed to determine each component, the work can be better appreciated." The overall problem is complicated further because only the northern hemisphere has been scrutinized by the orbiter and surface conditions reported by the three landers indicate a wider range than expected.
If the measurements are accurate and representative, there is no question that more than the greenhouse effect is required to explain the high surface temperature. However, the investigators have proposed processes in addition to internal heat to explain the observed thermal imbalance. The evaluations are handicapped by inconsistencies between lander and orbiter data, and the questionable representativeness of the lander sites. Clearly, the resolution of these issues is not close at hand; despite the continuing claims for the success of the greenhouse effect, the words of Suomi et al. and Ingersoll and Pechmann, quoted above, should be kept in mind.
C. Leroy Ellenberger