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On comparative planetary studies
mAaUS 3, 83-84 (1964)
LETTER TO THE EDITORS
On Comparative Planetary Studies
esses on Venus and E a r t h .leads to the conclusion that chemical processes and plastic flow in the subcrustal regions of both planets should have approximately the same time scale. However, the surface temperature of Venus is believed to be of the order of 700°K (Sagan, 1962). Therefore, the crustal processes on that planet should operate on a contracted time scale as compared to analogous processes in Earth's crust. This conclusion led the writer (Mueller, 1963) to suggest that topogr'lphic relief on Venus should be less than on Earth. Indeed, both the distance and time scales of isostatic adjustment should be contracted as a consequence of the weaker crust. However, tile thermal gradients of Venus and Earth should converge at some subcrustal depth if our expected parallelism holds. If for the m o m e n t we assume that planetary crusts are derived by chemical differentiation as a response to tile gravitational field and tile temperature gradient in subcrustal regions, then Venus and Earth should have similar crusts both with respect to volume and chemical constitution. However, we must allow for slight differences due to the different temperatures and tile very probable interaction of the lithosphere and atmosphere of Venus (Mueller, 1963). If on the other hamt Earth's crust came from Moon, we should expect little or no crust on Venus (assuming this planet never had a moon). Checking Alfven's hypothesis thus reduces to determining the volume and chemistry of the Venus crust. I t is interesting that the same data may also provide a test of the older hypothesis that Moon was in some way derived from Earth. In this case Venus' crust might be expected to be proportionally larger in volmne than Earth's crust. Of course, this conclusion is valid only if the Moo~ material was lost after differentiation. Other potentialities for comparative studies present themselves. One of these concerns the distribution of the inferred erust'd material on Venus (if we adopt the differentiation hypothesis)
The increasing pace of planetary investigations is providing us with a broader basis for the clarification of certain long-standing geophysical problems. An example of such a problem is the origin of Earth's crust. The scope of this problem was recently enlarged by Alfv~n (1963) when he suggested that the crust may have been derived from Moon. However, it is difficult to conceive of good test for Alfv~n's hypothesis as long "is we confine ourselves to the E a r t h - M o o n system. The purpose of this letter is to discuss the advantage of the comparative approach to planetary studies with emphasis on the Venus-Earth pair. This approach then appears to lead to tests of several geophysical hypotheses including that of Alfv~n. I t has been suggested by the writer (Mueller, 1963) that the similarities in the densities and radii of Venus and Earth imply a certain similarity in their internal constitution and parallelism in their evolution. I t is, of course, assumed that this evolution is not greatly affected by external factors such as the rate of rotation. The magnetic field of Venus appears to be of low intensity by terrestrial standards (Smith et al., 1963), and the rate of rotation is apparently nearly a captured one (Goldstein and Carpenter, 1963). It is natural to assume that these two facts are related. But we shall disregard such implications and assume that coupling between these factors and the processes of interest here is slight. Now it would be expected that the processes of chemical reaction, particle diffusion, and plastic movements would all involve analogous kinetic parameters such as an activation energy. The consequences of this have indeed been discussed by Ramberg (1948) for the problem of chemical differentiation in the gravitational field and by Staeey (1963) for plastic m o v e m e n t in Earth's mantle. The rates of these phenmnena may be highly temperature dependent by a factor of e x p ( - - A E / R T ) , where AE is the energy of activation for the kinetic process. The expected parallelism for deep-seated proc83
84
LETTER TO THE EDITORS
in terms of the exl)ected contracted crustal time scale. Perhaps t h a t planet has only poorly developed "continents" or even a uniformly distributed crust. Also the character of the Venus atmosphere and its very probable interaction with the lithosphere suggest ~t study which might lead to a greater understanding of the early (('rreslri:tl atmosphere. t~EFERENCES ALFV~, H. (1963). The early history of ~he Moon and the Earth. Icarus 1, 357-363. {~OLBSTEIN, R. M., AND CARPENTER, R. L. (1963). Rotation of Venus: Period estimated from radar measurements. Science 139, 910--911. MUEbLER, R. F. (1963). Chemistry and petrology of Venus: Preliminary deductions. Science 141, 1046-1047.
RAMBERG, I-L (1948). Radial diffusion and ehemic'fl stability in the gravitational fieht, d. Geology 56, 448-458. S.~C,AN, C. (1962). Structure of the lower atmosphere of Venus. Icarus 1, 151-159. SMITH, E. J., LEVERETW, 1)., COLEMAN, P. J., AND SONETT, C. P. (1963). Mariner I I : Preliminary reports on measurements of Venus, magnetic field. Science 139, 909-910. S'rAcEY, F. D. (1963). Tile theory of creep in rocks :md the problem of convection in the E a r t h ' s manth', lc.rw~ 1, 304-313. ROBERT F, ~IUELLER
Department o] Geophysical Sciences University of Chicago Chicago, Jllinois Rcccivcd December 22, 1963
LETTER TO THE EDITORS
On Comparative Planetary Studies
esses on Venus and E a r t h .leads to the conclusion that chemical processes and plastic flow in the subcrustal regions of both planets should have approximately the same time scale. However, the surface temperature of Venus is believed to be of the order of 700°K (Sagan, 1962). Therefore, the crustal processes on that planet should operate on a contracted time scale as compared to analogous processes in Earth's crust. This conclusion led the writer (Mueller, 1963) to suggest that topogr'lphic relief on Venus should be less than on Earth. Indeed, both the distance and time scales of isostatic adjustment should be contracted as a consequence of the weaker crust. However, tile thermal gradients of Venus and Earth should converge at some subcrustal depth if our expected parallelism holds. If for the m o m e n t we assume that planetary crusts are derived by chemical differentiation as a response to tile gravitational field and tile temperature gradient in subcrustal regions, then Venus and Earth should have similar crusts both with respect to volume and chemical constitution. However, we must allow for slight differences due to the different temperatures and tile very probable interaction of the lithosphere and atmosphere of Venus (Mueller, 1963). If on the other hamt Earth's crust came from Moon, we should expect little or no crust on Venus (assuming this planet never had a moon). Checking Alfven's hypothesis thus reduces to determining the volume and chemistry of the Venus crust. I t is interesting that the same data may also provide a test of the older hypothesis that Moon was in some way derived from Earth. In this case Venus' crust might be expected to be proportionally larger in volmne than Earth's crust. Of course, this conclusion is valid only if the Moo~ material was lost after differentiation. Other potentialities for comparative studies present themselves. One of these concerns the distribution of the inferred erust'd material on Venus (if we adopt the differentiation hypothesis)
The increasing pace of planetary investigations is providing us with a broader basis for the clarification of certain long-standing geophysical problems. An example of such a problem is the origin of Earth's crust. The scope of this problem was recently enlarged by Alfv~n (1963) when he suggested that the crust may have been derived from Moon. However, it is difficult to conceive of good test for Alfv~n's hypothesis as long "is we confine ourselves to the E a r t h - M o o n system. The purpose of this letter is to discuss the advantage of the comparative approach to planetary studies with emphasis on the Venus-Earth pair. This approach then appears to lead to tests of several geophysical hypotheses including that of Alfv~n. I t has been suggested by the writer (Mueller, 1963) that the similarities in the densities and radii of Venus and Earth imply a certain similarity in their internal constitution and parallelism in their evolution. I t is, of course, assumed that this evolution is not greatly affected by external factors such as the rate of rotation. The magnetic field of Venus appears to be of low intensity by terrestrial standards (Smith et al., 1963), and the rate of rotation is apparently nearly a captured one (Goldstein and Carpenter, 1963). It is natural to assume that these two facts are related. But we shall disregard such implications and assume that coupling between these factors and the processes of interest here is slight. Now it would be expected that the processes of chemical reaction, particle diffusion, and plastic movements would all involve analogous kinetic parameters such as an activation energy. The consequences of this have indeed been discussed by Ramberg (1948) for the problem of chemical differentiation in the gravitational field and by Staeey (1963) for plastic m o v e m e n t in Earth's mantle. The rates of these phenmnena may be highly temperature dependent by a factor of e x p ( - - A E / R T ) , where AE is the energy of activation for the kinetic process. The expected parallelism for deep-seated proc83
84
LETTER TO THE EDITORS
in terms of the exl)ected contracted crustal time scale. Perhaps t h a t planet has only poorly developed "continents" or even a uniformly distributed crust. Also the character of the Venus atmosphere and its very probable interaction with the lithosphere suggest ~t study which might lead to a greater understanding of the early (('rreslri:tl atmosphere. t~EFERENCES ALFV~, H. (1963). The early history of ~he Moon and the Earth. Icarus 1, 357-363. {~OLBSTEIN, R. M., AND CARPENTER, R. L. (1963). Rotation of Venus: Period estimated from radar measurements. Science 139, 910--911. MUEbLER, R. F. (1963). Chemistry and petrology of Venus: Preliminary deductions. Science 141, 1046-1047.
RAMBERG, I-L (1948). Radial diffusion and ehemic'fl stability in the gravitational fieht, d. Geology 56, 448-458. S.~C,AN, C. (1962). Structure of the lower atmosphere of Venus. Icarus 1, 151-159. SMITH, E. J., LEVERETW, 1)., COLEMAN, P. J., AND SONETT, C. P. (1963). Mariner I I : Preliminary reports on measurements of Venus, magnetic field. Science 139, 909-910. S'rAcEY, F. D. (1963). Tile theory of creep in rocks :md the problem of convection in the E a r t h ' s manth', lc.rw~ 1, 304-313. ROBERT F, ~IUELLER
Department o] Geophysical Sciences University of Chicago Chicago, Jllinois Rcccivcd December 22, 1963