Site Section Links
KRONOS Vol VII, No. 3
VENUS: VOLCANISM AND HOT-FUDGE
In three recent issues of KRONOS, the possibility of a thermal imbalance on Venus resulting from a significant internal source was discussed (7, 8, 9). Because the imbalance was not statistically definite, it was hoped that additional data analysis would ascertain the true state of affairs by the time of the International Conference on the Venus Environment last November.
THERMAL IMBALANCE : While the uncertainty over the thermal imbalance has not been resolved, a consensus is emerging that at least three "regions on Venus have been, and perhaps still are, centers of volcanic activity" (3, p. 135). Naturally, widespread, on-going volcanism could produce the imbalance. However, investigators are not voicing this possibility just yet.
Initially, a 16% thermal imbalance was indicated by measurements showing that on average the atmosphere absorbed 132 W/m2 and radiated 153 W/m2 (9, p. 24). While the numbers have changed, an imbalance, though reduced, is still indicated. Adjusted values reported by F. W. Taylor indicate about 145 W/m2 absorbed on average with 158 W/m2 radiated, for a 9% excess. Noting this excess, Taylor suggests that the planet's albedo may be lower than the currently accepted value of 0.80. In other words, the imbalance is not real, but results from having over-estimated the reflectivity of the cloud tops (13).
That our understanding of Venus's thermal state is incomplete is indicated by the fact that calculations for the greenhouse effect allow 200 W/m2 on average to escape while only 158 W/m2 was observed by the Pioneer Venus Orbiter (PVO). This suggests that a fairly important source of opacity in the clouds has not been identified (13). It also suggests that it is too soon to foreclose our options concerning the thermal state in general.
VOLCANISM : Giving serious consideration to the possibility of on-going volcanism on Venus has not come easily to NASA scientists. David Morrison's attitude in 1978, perhaps, is illustrative. When this was suggested to him in the context of apprising him of George R. Talbott's cooling model, then coming in KRONOS IV:2 (Letter, CLE to DM, Aug. 22,1978), he replied: "The presence of volcanic land forms on Venus, as suggested by radar imagery, does not imply currently active volcanism on that planet anymore than on Mars or the Moon" (Letter, DM to CLE, Sept. 20,1978). This reply, tempered by faith in the green house effect, ignored the significance both of Talbott's model and of the temperature difference between Earth, Moon and Venus.
Today, the case for volcanism is supported by five lines of reasoning (3, 11). a) In the apparent absence of plate tectonics, volcanism would be a likely way for heat from radioactive decay to reach the surface. b) The shapes and overall sizes of the suspected areas suggest shield volcanoes, probably supported by expanding hot spots in the mantle. c) The suspected areas coincide with strong gravity anomalies consistent with the existence of up-welling hot spots. d) The radar brightness of the areas suggests that fresh, rough lava flows cover them. e) The lightning observed on Venus(2) clusters over two of the suspected areas which Fred Scarf of TRW, Inc. suspects is produced, not in the sparse clouds over the sites, but "directly above the surface, within the turbulent dust and ash rising from volcanic vents" (3, p. 137).
J. Kelly Beatty ventured that "the combined weight of all these lines of evidence makes the idea of volcanism on Venus an attractive one". Roger Phillips of the Lunar and Planetary Institute and Harold Masursky of the U. S. Geological Survey go even further: Since Venus apparently cannot release its internal heat by "seafloor" spreading, "they think that small volcanoes may dot the extensive 'rolling uplands' that dominate most of the globe. In that case, Beta Regio, Aphrodite, and Maxwell Montes would not be the only eruptive centers on Venus, just the largest ones", according to Beatty (3, p. 137).
TOPOGRAPHY AND TECTONICS : Nothing about the results from Pioneer Venus has been more ludicrous than the strained and even false attempts to see Venus as another Earth with mountains, giant valleys, continents and "ocean" basins. Yet, as a comparison of relief maps shows, Venus is much smoother than Earth (3, 10). That Venus does not resemble Earth is also clear from the distribution of topography: Venus's is unimodal; Earth's, bimodal. On Venus, 60% of the surface lies within 500 meters of the average radius; only about 5% is classified as "continental". In contrast, 35% of Earth is continental; 65% is oceanic with an average depth of about 5 km below sea level (3, p. 135; 12, p. 64).
Some scientists have strained to find evidence for plate tectonics on Venus (1, 4, 6, 10). However, mid-ocean ridges are absent and subduction trenches apparently so. As Beatty observed: "The case against subduction zones is shaky, since these features would appear just at the resolution limit of the PVO radar mapper. In fact . . . the presence of plate tectonics on Earth does not automatically follow from examining our planet at PVO resolution" (3, p. 134). This is probably irrelevant, however, because as Phillips and others have calculated, "Venus' surface and shallow interior are too hot to allow subduction anyway. They argue that high temperatures make the crust too buoyant and dry to power plate tectonic activity" (3, pp. 134-5).
The essence of the tectonic issue has been captured succinctly by Robert Muir Wood:
This suggestion finds support in the estimate that over geologic time the heat at the surface could eradicate 300 km diameter craters simply by plastic deformation (3, p. 135).
This confirms what Ralph E. Juergens reported in his critique of Morrison (KRONOS IV:2), namely that "craters on Venus with strongly stunted rims not at all in keeping with their great diameters seem to suggest that such formations are indeed deformed by subsidence" (p. 78). Juergens also suggested the plastic nature of Venus's surface with "Anomalous features built of rubble or excavated in rubble may indeed be slowly disappearing as we observe them, or they may already have achieved precarious equilibrium isostatically by impressing inverted images of themselves into pliant near surface materials" (p. 76).
RARE GASES AND WATER : The rare gases krypton, neon, argon, and xenon continue to vex investigators. Neon and xenon have yet to be accommodated by theories of solar system formation. According to George Wetherill of the Carnegie Institution of Washington, "There seems to be no simple explanation of the rare gases" (11).
Re-evaluation of data on the ratio of deuterium to hydrogen, originally thought to be useless, has indicated that Venus's atmosphere once contained at least 1.5% water. Some researchers have suggested that Venus could have outgassed as much as 300 bars of water. In this case, a greenhouse effect would have raised the temperature to 1,500°K, melting the crust. As the water dissociated, hydrogen escaped and oxygen reacted with the molten surface and carbon monoxide. This would explain the observed, but hitherto enigmatic, depletion of water and oxygen in the atmosphere (5). When the partial pressure of water dropped to about 15 bars, cooling would have begun (3, pp. 137-8).
In conclusion, consistent with Velikovsky's history of Venus, evidence is mounting for the presence of widespread, on-going volcanism. The validity of the indicated thermal imbalance is still unclear. The depletion of water and oxygen in the atmosphere is being explained, but on a timetable of millions, rather than thousands, of years.
C. Leroy Ellenberger
REFERENCES1. Anonymous, "Venus not a comet!", New Scientist (26 June 1980), p. 410.
2. Anonymous, "Venus, Lightning and volcanoes", Science News (Dec.5, 1981), p. 362.
3. J. Kelly Beatty, "Venus: The Mystery Continues", Sky and Telescope (February, 1982), pp. 134-8.
4. Eric Burgess, "Beneath the veils of Venus", New Scientist (28 August 1980), pp. 661-4.
5. J. Eberhart, "Venus: The Waters of Yesteryear", Science News (Dec.12,1981), pp.372-3.
6. C. Leroy Ellenberger, "Heretics . . .", KRONOS VI:4 (Summer 1981), pp. 79-80, 84.
7. C. Leroy Ellenberger, "Venus's Internal Heat: An Update", KRONOS VI:3 (Spring 1981), pp. 96-7.
8. C. Leroy Ellenberger, "Venus's Greenhouse: Premature Sufficiency?", KRONOS VI:4 (Summer 1981), p.94.
9. Lewis M. Greenberg and C. Leroy Ellenberger, "Venus's Internal Heat", KRONOS VI:2 (Winter 1981), pp. 18-24.
10. James W. Head, et al., "Topography of Venus and Earth: A Test for the Presence of Plate Tectonics", American Scientist (Nov.-Dec. 1981), pp. 614-23.
11. Richard A. Kerr, "Venus Highlights", Science 215 (15 January 1982), pp. 278-9.
12. Gordon H. Pettengill, et al., "The Surface of Venus", Scientific American (August 1980), pp. 54-65.
13. F. W. Taylor, "Structure and Energetics of the Atmosphere of Venus", Summary report submitted by request to Nature, November 1981. Publication pending.
14. Robert Muir Wood, Review of Earthlike Planets, New Scientist (5 Nov. 1981), p. 391.