We now know from ground-based radio observations and from the remarkably successful direct entry and landing probes of the Soviet Union that the surface temperature of Venus is within a few degrees of 750°K (Marov, 1972). The surface atmospheric pressure is about ninety times that at the surface of the Earth, and is comprised primarily of carbon dioxide. This large abundance of carbon dioxide, plus the smaller quantities of water vapor which have been detected on Venus, are adequate to heat the surface to the observed temperature via the greenhouse effect. The Venera 8 descent module, the first spacecraft to land on the illuminated hemisphere of Venus, found it illuminated at the surface, and the Soviet experimenters concluded that the amount of sunlight reaching the surface and the atmospheric constitution were together adequate to drive the required radiative-convective greenhouse (Marov, et al., 1973). These results were confirmed by the Venera 9 and 10 missions, which obtained clear photographs, in sunlight, of surface rocks. Velikovsky is thus certainly mistaken when he says (page ix) “light does not penetrate the cloud cover,” and is probably mistaken when he says (page ix) the “greenhouse effect could not explain so high a temperature.” These conclusions received important additional support late in 1978 from the U.S. Pioneer Venus mission.
A repeated claim by Velikovsky is that Venus is cooling off with time. As we have seen, he attributes its high temperature to solar heating during a close solar passage. In many publications Velikovsky compares published temperature measurements of Venus, made at different times, and tries to show the desired cooling. An unbiased presentation of the microwave brightness temperatures of Venus-the only nonspacecraft data that apply to the surface temperature of the planet-are exhibited in Figure 1. The error bars represent the uncertainties in the measurement processes as estimated by the radio observers themselves. We see that there is not the faintest hint of a decline in temperature with time (if anything, there is a suggestion of an increase with time, but the error bars are sufficiently large that such a conclusion is also unsupported by the data). Similar results apply to measurements, in the infrared part of the spectrum, of cloud temperatures: they are lower in magnitude and do not decline with time. Moreover, the simplest considerations of the solution of the one-dimensional equation of heat conduction show that in the Velikovskian scenario essentially all the cooling by radiation to space would have occurred long ago. Even if Velikovsky were right about the source of the high Venus surface temperatures, his prediction of a secular temperature decrease would be erroneous.
FIGURE 1. Microwave brightness temperatures of Venus as a function of time (after a compilation by D. Morrison). There is certainly no evidence of a declining surface temperature. The wavelength of observation is denoted by Λ.
The high surface temperature of Venus is another of the so-called proofs of the Velikovsky hypothesis. We find that (1) the temperature in question was never specified; (2) the mechanism proposed for providing this temperature is grossly inadequate; (3) the surface of the planet does not cool off with time as advertised; and (4) the idea of a high surface temperature on Venus was published in the dominant astronomical journal of its time and with an essentially correct argument ten years before the publication of Worlds in Collision.
PROBLEM IX. THE CRATERS AND MOUNTAINS OF VENUS
IN 1973 AN IMPORTANT aspect of the surface of Venus, verified by many later observations, was discovered by Dr. Richard Goldstein and associates, using the Goldstone radar observatory of the Jet Propulsion Laboratory. They found, from radar that penetrates Venus’ clouds and is reflected off its surface, that the planet is mountainous in places and cratered abundantly; perhaps, like parts of the Moon, saturation-cratered-i.e., so packed with craters that one crater overlaps the other. Because successive volcanic eruptions tend to use the same lava tube, saturation cratering is more characteristic of impact than of volcanic cratering mechanisms. This is not a conclusion predicted by Velikovsky, but that is not my point. These craters, like the craters in the lunar maria (plural for Latin mare, “sea”), on Mercury and in the cratered regions of Mars, are produced almost exclusively by the impact of interplanetary debris. Large crater-forming objects are not dissipated as they enter the Venus atmosphere, despite its high density. Now, the colliding objects cannot have arrived at Venus in the past ten thousand years; otherwise, the Earth would be as plentifully cratered. The most likely source of these collisions is the Apollo objects (asteroids whose orbits cross the orbit of the Earth) and small comets we have already discussed (Appendix 1). But for them to produce as many craters as Venus possesses, the cratering process on Venus must have taken billions of years. Alternatively, the cratering may have occurred more rapidly in the very earliest history of the solar system, when interplanetary debris was much more plentiful. But there is no way for it to have happened recently. On the other hand, if Venus was, several thousand years ago, in the deep interior of Jupiter, there is no way it could have accumulated such impacts there. The clear conclusion from the craters of Venus is, therefore, that Venus has for billions of years been an object exposed to interplanetary collisions-in direct contradiction to the fundamental premise of Velikovsky’s hypothesis.
The Venus craters are significantly eroded. Some of the rocks on the surface of the planet, as revealed by the Venera 9 and 10 photography, are quite young; others are severely eroded. I have described elsewhere possible mechanisms for erosion on the Venus surface-including chemical weathering and slow deformation at high temperatures (Sagan, 1976). However, these findings have no bearing whatever on the Velikovskian hypotheses: recent volcanic activity on Venus need no more be attributed to a close passage to the Sun or to Venus’ being in some vague sense a “young” planet than recent volcanic activity on Earth.
In 1967 Velikovsky wrote: “Obviously, if the planet is billions of years old, it could not have preserved its original heat; also, any radioactive process that can produce such heat must be of a very rapid decay [sic], and this again would not square with an age of the planet counted in billions of years.” Unfortunately, Velikovsky has failed to understand two classic and basic geophysical results. Thermal conduction is a much slower process than radiation or convection, and, in the case of the Earth, primordial heat makes a detectable contribution to the geothermal temperature gradient and to the heat flux from the Earth’s interior. The same applies to Venus. Also, the radionuclides responsible for radioactive heating of the Earth’s crust are long-lived isotopes of uranium, thorium and potassium-isotopes with half-lives comparable to the age of the planet. Again, the same applies to Venus.