Absorption Bands in the Infra-Red Spectrum of Venus

Abstract

In 1922 St. John and Nicholson investigated the spectrum of Venus by photographing with a plane-grating spectrograph regions near λ5900, the α-band of oxygen near λ6300, and the Β-band of oxygen at λ6867. The radial velocity of the planet and the dispersion employed were sufficient to separate planetary lines from those of terrestrial origin. No trace was found of lines due to oxygen or to water vapor in the spectrum of the planet. Recent progress at the Research Laboratory of the Eastman Kodak Company in sensitizing photographic plates to the infra-red has made it possible to extend this investigation to the region of longer wavelengths where the Α-band of oxygen at λ7594 and the group of strong water-vapor lines in the interval λ8150-λ8300 afford excellent material for a sensitive test of the presence of molecules of these gases in the atmosphere of Venus. For this purpose we have used a plane-grating auto-collimating spectro-graph, nine feet in focal length, at the coudé focus of the 100-inch reflector. The photographs, which cover the region λ7400-λ8800, were taken in the first order of the grating and have a linear scale of about 5.6 A.U. to the millimeter. The exposure times were from two to four hours. Our observations were made in April and May when the planet was near greatest eastern elongation. The radial velocity of Venus relative to the Earth for the average date of observation was about-14 kilometers per second, which at λ8000 corresponds to a displacement of 0.37 A.U. Since this represents a shift of 0.066 millimeters on our spectrograms, lines due to the planet would be separated clearly from terrestrial lines. The quantitative study of the spectrograms has not as yet been completed , but it is clear from inspection and microphotometer trac-ings that no lines of measurable intensity due either to oxygen