IC.XRCS 33, 554--557 (1978) Venus: The 17- to 38-Micron Spectrum R. A. REED, 1 W. J. FORREST, AND J. R. HOUCK Center for Radiophysics and Space Research, Cornell University, Ithaca, New York 14858 AND J. B. POLLACK N A S A Ames Research Center, Moffett Field, California 00000 Received March 18, 1977; revised July 27, 1977 The Venus emission spectrum was measured from the NASA Lear Jet on five nights in June 1975. A cooled grating spectrometer with a resolution of h/Ak --~ 25 over the spectral interval 17 to 38 t~m was used. The main features in the observed spectrum are consistent with the theoretical emission spectrum of a haze of aqueous sulfuric acid droplets suspended in a CO2 atmosphere. INTRODUCTION The thermal emission spectrum of a planet contains information on the chemical composition and thermal structure of the planetary atmosphere. In the case of Venus, the peak surface temperature of ~750°K is observed only in the radio region; the 200- 250°K temperatures observed in the ther- mal infrared originate from a much higher level in the Venus atmosphere. It has been proposed (Sill, 1972; Young, 1973, 1974; Pollack et al., 1974) that a haze cloud of sulfuric acid droplets exists in the primarily CO2 atmosphere of Venus at the levels observed in the thermal infrared. The present 17-38-~m spectrum of Venus will allow a further test of this hypothesis. OBSERVATIONS The spectrum of Venus has been obtained from an altitude of 14 km with the 30-cm i Present address: Grumman Aircraft, Bethpage, N. Y. 11714. telescope of the NASA Lear Jet. The in- strument employed was a cooled grating spectrometer utilizing both the first- and second-order spectra from the grating to achieve a resolution of 25 over the wave- length range 17-38 ~m (Houck et al., 1975 ; Schaack, 1975). The beam size of 3.6 arc- rain included the whole disk of Venus. The instrumental response was calibrated using the emission of Mars in conjunction with the model of Wright (1976), which at this time gave brightness temperatures of 256 and 248°K at wavelengths of 20 and 34 ~m, respectively (Wright, personal communica- tion, 1976). The observed spectrum, shown in Fig. I, is the average of five nights of observation in June 1975. Data points have been taken at a density of three per resolu- tion element so that the statistical errors may be estimated from the scatter of adjacent points. The spectrum is charac- terized by an overall continuum level, with noticeable absorption shortward of 20 t~m and longward of 30t~m, compared to a 245°K blackbody. This spectral shape has 554 0019-1035/78/0333-0554502.00/0 Copyright ~ 1978 by Academic Press, Inc. All rights of reproduction in any form reserved.