REFLECTION PROPERTIES OF VENUS AT 3.8 cm

Abstract

During the fall of 1967, the M.I.T. Lincoln Laboratory Haystack radar was employed to study the scattering properties of the planet Venus at 3.8-cm wavelength. An increase in the transmitter power to 300kW CW and a reduction in the system noise temperature to 60°K provided a considerable improvement in the radar performance compared with that available for earlier measurements. The frequency power spectra of the echoes were determined by digital Fourier analysis of the received signals. The total power in each spectrum was computed to yield the value for the radar cross section of Venus. These average 1.75 percent of the projected area of the disk, in good agreement with a reanalysis of results obtained in 1966. The signal spectra have been averaged and employed to derive the angular scattering dependence P(<p) for waves reflected from a region near the center of the disk (0 < <p < 60°), where atmospheric refraction effects seem likely to be small. Comparison of this law with a law derived from similar measurements at 12.5-cm wavelength performed at the Jet Propulsion Laboratory (J p L) shows limb darkening, which is attributed to atmospheric absorption of the shorter wavelength signals. From the amount of limb darkening, the difference in the two-way atmospheric absorption at the two wavelengths is estimated to be 5 ± 1 db. The radar cross section observed at 3.8 cm is lower than that at 12.5cm by 8 ± 3db. Thus it is concluded that the oneway absorption of 3.8-cm microwaves by the atmosphere of Venus is at least 2.5db and possibly more. This is significantly greater than can be accounted for by the recent U.S. and Soviet probe experiments, which imply an atmosphere of CO^ with surface pressure of 20 atmospheres. Either the pressure is considerably greater than this or other gases that are more effective microwave absorbers are present. The scattering behavior of the disk center, i.e., in a region where atmospheric refraction and absorption have not significantly altered the shape of the scattering law, has been compared with that at other wavelengths. It is concluded that Venus, like the moon, appears to be rougher at centimeter wavelengths than meter or decimeter wavelengths. However, for scales important in these measurements, i.e., over horizontal distances of the order of a few times the wavelength, the rms slopes found for Venus are of the order of 1/N/2 times those found for the moon. Delay-Doppler mapping experiments are reported which yielded a resolution of ~100 x 100 km. These show the existence of bright features and the locations of these are tabulated. The report closes with a discussion of the nature of these features. It is concluded that many, if not all, are rougher and denser than their environs and, in view of the general smoothness found elsewhere, are possibly young features.

Radar Mapping of Venus With Interf Erometric Resolution of the Range‐Doppler Ambiguity

“…For example, differences in intrinsic reflectivity caused by changes in dielectric constant should serve to alter the brightness of the regions relative to their environs, regardless of the angle of incidence o.f the radar, whereas rougher regions would appear relatively bright for large angles of incidence and dark for small angles of incidence. A detailed study by Ingalls et al [1968] of the features observed in the CW spectra suggest that differences in local roughness are undoubtedly the major cause of anomalously bright features. However, computation of the specific cross section of region , from the map yields value about twice that of the mean planet, which suggests that, if this region is almost perfectly rough, it must also either have a higher intrinsic reflectivity or be sufficiently elevated above the mean planet so that the atmospheric absorption measured by Ingalls and Evans [1969] is reduced by abo.ut 3 db of a total of 5 db.…”