Contains reports on two research projects.United States Air Force (Contract AF19(628)-500)National Aeronautics and Space Administration (Grants NsG-250-62 and NsG-419)Lincoln Laboratory (Purchase Order DDL BB-107

Barrett, Alan H.

Staelin, David H.

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III. RADIO ASTRONOMY*Prof. A. H. Barrett R. K. Breon M. PalfyProf. J. W. Graham N. E. Gaut T. S. SlaterProf. R. P. Rafuse J. I. Glaser J. H. SpoorProf. W. C. Schwab J. W. Kuiper D. H. StaelinR. J. Allen W. B. Lenoir D. H. SteinbrecherJ. M. Moran, Jr.A. MICROWAVE PROPERTIES OF THE ATMOSPHERE AND SURFACE OF VENUSExtensive computations have been made of the microwave spectra that would char-acterize various atmospheric models for the planet Venus. The models considered werethe CO2-N 2 model, the aeolospheric model,2 and three cloud models. A brief summaryof the computations and some of the conclusions are presented here.In the CO 2 -N 2 model the microwave absorption arises from collision-induced dipolemoments, which become important at high pressures. Recent measurements byThaddeus and Ho 3 of the absorption coefficient of CO 2 -N 2 mixtures at high pressuresand temperatures, and estimates of the CO 2 mixing ratio (Kaplan4 and Spinrad 5 ) haveenabled more accurate spectral computations to be made. The CO2-N2 model assumedan atmosphere in adiabatic equilibrium composed of 10 per cent CO 2 and 90 per centSEGMENT WITHE PARALLEL TOTHE SURFACE Fig. III-1. Illustrating how the polar-I SEGMENT WITH ization dependence of thePARALLEL TO surface emissivity can beTHE SURFACE taken into account.NZ , with a constant mixing ratio. The temperature was assumed to be 700° K at thesurface and to decrease 7 ° K/km to an isothermal region at 287 0 K beginning at altitude60 km. The surface of Venus was assumed to be a smooth dielectric sphere withdielectric constant 5. The computations included the effect of surface reflectivity onthe surface emmissivity and on the radiation incident upon the surface. Two brightnesstemperatures have been computed; one, labeled E//, is the average brightness temp-erature for a narrow triangular segment whose surface is everywhere parallel to theelectric vector E, and the other, labeled H//, is the average brightness temperature*This work was supported in part by the National Aeronautics and Space Adminis-tration (Grants NsG-Z50-62 and NsG-419); and in part by Purchase Order DDL BB-107with Lincoln Laboratory, a center for research operated by Massachusetts Institute ofTechnology, with the support of the U. S. Air Force under Contract AF 19(628)-500.QPR No. 73(III. RADIO ASTRONOMY)700-o* * .HH 600 -r 500-UE0-S400- 0 I ..LUzIo3000 P, = 20 ATM.= IOO ATMP = 300 ATM100 I I lii i I I 1 1 1 i I 1 1O I 10 100WAVELENGTH (CM)Fig. III-Z. Theoretical radio spectra forthe CO -N 2 model atmosphere.of a similar segment whose surface is everywhere parallel to the magnetic vector H(Fig. III- 1).The results for the first model are shown in Fig. III-2 for surface pressures of20, 100, and 300 atmospheres. Superimposed upon the figure are dots representingthe observations that have been made. The major conclusion for this model is that ifthe CO2-N 2 absorption is responsible for the decrease in brightness temperature whichoccurs at short wavelengths, surface pressures will be in the range 100-300 atmos-pheres. A second model was considered, which was the same as the first, except thatthe lapse rate was 4. 83 o K/km, and there was no isothermal region. The results indi-cate that even greater surface pressures, 300-1000 atmospheres, are required for thismodel, and even with these high pressures agreement with the experimental points ispoor.The aeolospheric model presumes an atmosphere containing large amounts of dust.This dust can affect the microwave spectrum both by absorption and scattering. Compu-tations for models containing dust with dielectric constants that were not a function offrequency yielded spectra that varied too slowly to match the observations if only absorp-tion was considered, but varied quite sharply when scattering was included. The equationsof radiative transfer were solved for the scattering atmosphere under the assumptionof isotropic Rayleigh scattering. The solution was obtained by an iteration procedureon the IBM 7094 digital computer. Results were obtained for several models. Themodel providing the best agreement with observations assumed dust particle diametersvarying linearly from 0. 6 mm at the ground to 0 at 90 km. The density also variedQPR No. 73(III. RADIO ASTRONOMY)700H/600- " o0UIrS500LJ300// DUST 3.7 8(1-0.0 00 3 j)2 00-- 1 GM/M 3 AT GROUND10 GM/M 3100 1 I 00 G /M I0.1 10 100WAVELENGTH (CM)Fig. 111-3. Theoretical radio spectra for an atmosphereof dust particles with a maximum size of 0. 6mm. Both absorption and scattering are in-cluded in the computation.linearly, from 10 and 100 g/m3 at the ground to 0 at 90 km. The temperature distri-bution and surface model were the same as for the 4. 83 0 K/km CO2 -N 2 model. Theresult is shown in Fig. 111-3. Although the scattering model can explain very rapiddecreases in brightness temperature at short wavelengths, it requires large particlediameters, greater than 0. 5 mm, and large densities, greater than 10 g/mmosphere, to be effec-tive. The required densities are reduced if still larger particles are present. It alsorequires some absorption above the scattering layer to raise the 4-mm temperatureto the observed values. Because of the very strong dependence of the scattering crosssection Qand upon the particle diameter D (Q5 - D ), the break point in the spectrum wouldbe expected to be strongly dependent upon meteorological conditions in the lower atmos-phere.The cloud models considered were: (i) a cloud with absorption proportional to fre-quency; (ii) a cloud with absorption proportional to the square of the frequency; and(iii) a water cloud in equilibrium with the water vapor beneath it. The first cloud wasof uniform density and composed of particles with a complex dielectric constant ofo = 5(1-0. 05j), which is typical of many organic substances. This yields an absorptioncoefficient proportional to frequency. The cloud was 30 km thick with a temperature of415* K at the bottom, and a temperature of 270 ° K at the top. The planetary surfacewas the same as in the previous models. Although this model provided sufficient micro-wave absorption at short wavelengths and at reasonable cloud densities, 0. 1-1 g/m 3it did not provide a sufficiently abrupt change of brightness temperature in the 0. 8-cmQPR No. 73(III. RADIO ASTRONOMY)to 3-cm region. The second model, which is more characteristic of liquid cloud par-ticles composed of freely rotating dipoles restrained by a simple viscous force, hadan absorption coefficient proportional to the square of the frequency. This producedmuch better agreement with the observations, as shown in Fig. 111-4. The 4-mm datacan be better matched if the cloud is the same, but at a higher temperature. The dataat 10 cm could be better matched if the surface emissivity were somewhat greater.This model has the advantage of simplicity in that it requires only a cloud layer ofreasonable densities which absorbs approximately according to the Debye equation forK = 2.78 x 10-6700 --- K = 1.94 x 10-5K = 5.55 x 10 5o 6-..500 E//LUS500-i/400-model with absorption proportional toature of300K at the bottom and 270 0 K at the top. The bottom of the cloud was assumedco 200-WAVELENGTH (CM)Fig. indicate that ifII-4. Theoretical radio spectra for a cloudmodel with absorption proportional tothe square of the frequency.nonresonant absorption.The final model considered was that of a water cloud, 6 km thick, with a temper-ature of 300'°K at the bottom and Z70°*K at the top. The bottom of the cloud was assumedto be in equilibrium with water vapor that had a constant mixing ratio beneath the clouds.The cloud density was 1 g/m3 , and the atmospheric lapse rate was 4. 83 ° K/ki. Theresults indicate that if the surface pressure is as high as Z0 atmospheres, then theatmospheric absorption is too great to be compatible with the 3-cm observations. Lowercloud temperatures, and thus less water vapor beneath the clouds, permit higher sur-face pressures.A more complete discussion of the computations and conclusions has been preparedfor publication.A. H. Barrett, D. H. StaelinQPR No. 73(III. RADIO ASTRONOMY)References1. A. H. Barrett, Astrophys. J. 133, 281 (1961).2. E. J. Opik, J. Geophys. Research 66, 2807 (1961).3. P. Thaddeus (Goddard Institute of Space Studies, New York), private commu-nication, January 1964.4. L. D. Kaplan, A Preliminary Model of the Venus Atmosphere, Technical ReportNo. 32-379, Jet Propulsion Laboratory, Pasadena, California, December 12, 1962.5. H. Spinrad, Icarus 1, 266 (1962).B. K-BAND RADIOMETRYA five-channel radiometer has been constructed and is operating in the 19-25.5 Gcfrequency range. The radiometer is a Dicke-type superheterodyne radiometer withmicrowave channel-dropping filters and five separate mixers. Digital synchronousdetection is used in which the analog signal is converted to pulse frequency so that asynchronously switched up-down counter can detect the signal. The digital output isstored on punched paper tape for processing by the PDP- computer. Preliminary testsindicate that the equipment is stable and has ATrms 0. 5-2 K for a time constant of1 second. Measurements of sky-brightness temperature are now under way. The radiom-eter will be modified soon to permit measurements in the 21-33 Gc band. In thesemeasurements the 28-ft paraboloid at Lincoln Laboratory, M. I. T., will be used; obser-vations of the sun, moon, various other sources, and extensive observations of the planetVenus will be included. It is hoped that the spectral measurements of Venus in theimportant region where the transition in brightness temperature occurs will lead tobetter understanding of the planet's atmosphere.D. H. StaelinQPR No. 73

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