July 25, 1955 Dear Dr. Velikovsky: It is pleasant to have a note from you and to realize that you are as active as usual. Before answering your questions may I presume to suggest that it would help you greatly to read a few chapters in a sinmple textbook in physics dealing with the production of a spectrum. This would enable you to predict what elements might be expected possibly to occur in the realitvley cool atmospheres of the planets. To recall a few simple facts: a solid body cannot give out a spectrum of lines; if heated sufficiently it will give out a continuous spectrum, but with no lines. Gases, however, when sufficiently heated or energized give out a spectrum consisting of a pattern of bright lines, each element showing its own characteristic pattern. These lines, however, become dark when the light from a continuous specturm of higher temperature, like the body of the sun, for example, shines throught he gases. The planets, which are relatively cool and give out no light of their own, act like mirrors, reflecting the sunlight to the observer. If a planet, like Mercury, for example, has no atmosphere, its spectrum is merely that of sunlight. If a planet does have an atmosphere, hwoever, the situation may be slightly different. Both the atmosphere and the surface of the planet reflect the sunlight so that the solar spectrum is still the dominating feature, but the light which reaches the surface and is reflected to the observer has traversed the atmosphere twice, once from the sun to the surface, and once from the surface to the observer. As we all know some of the best-known elements occur as gases even at low temperatures, hydrogen, oxygen, nitrogen, and others which are less abundant. The elements exist in the form of molecules, because the temperatures of the planets are too low to break them up into atoms, and usually show a strong affinity form combination with other elements. So we have H2O, CO2, N2O, etc. They are also capable of absorbing out of the sunlight a pattern of lines, each characteristic of the molecule concerned. They are called molecular spectra, as distinct from atomic spectra, and provide us with all the spectroscopic evidence we posssess about the presence of certain gases in planetary atmospheres. The lines and bans of these gases usually are found in the red or infrared part of the spectrum. Now to apply these facts and considerations to your questions: The presence of chlorine in Saturn is improbable. It is not an abundant gas, shows great affinity for chemical combinations, and so far as I know has never been identified with certainty even in the sun or stars. Water or water vapor might be present in the atmosphere of Saturn, but would be completely frozen at the temperature, and hence unobservable. Ionized iron and sulphur could not possibly be present in the atmospheres of Jupiter and Venus, because their spectra are atomic and would require very high temperatures for their production. You realize, of course, that these negative results apply only to the atmospheres and not the bodies of the planets. It is probable that the latter contain much the same elements as does the body of earth, but in somewhat different relative proportions. Spectroscopic methods can give us no information regarding this question because no spectrum is formed at these comparatively low temperatures.   Sincerely yours ,   Walter S. Adams