Planetary characteristics from radar observations

Abstract

Meaningful radar experiments have been achieved with the planets Venus, Mercury, and Mars since the first successful radar detection of Venus in early 1961. Venus in particular has been carefully studied radarmetrically in the wavelength range from 3.6 cm to 7 meters, with several spectacular results yielding both fundamental information concerning the planet as well as corollary data for synthesizing a complete physical picture of Venus from all of the astronomical measuring techniques. It has been discovered that the Venusian axial rotation is retrograde with a period of about 250 days, the atmosphere is essentially transparent at wavelengths longer than 10 cm and that radar echos are an order of magnitude weaker at 3.6 cm than at the longer wavelengths suggesting a considerable atmospheric effect in the short centimeter region.Observations of Mercury and Mars are far less complete owing to the planets' relatively great distances from the Earth which effect the radar detectability through an inverse R fourth law. However, it appears that an accurate rotational period of Mercury of 59 _+ 5 days has been established as opposed to the long accepted synchronous period of 88 days. Furthermore, significant variations in the reflectivity of Mars with the Martian-centered coordinates of the subearth point have been observed, some of which are strongly correlated with visual markings on the planet's surface.Finally, through straining the available radar facilities to their limits a vast catalogue of range and doppler velocity data for Venus and Mercury has been assembled for the study of the motions of the Earth and planets and for the computation of greatly improved values of the fundamental solar system constants, the most notable of which is the astronomical unit.The purpose of this paper is to review the interpretation of the radar observations from the standpoint of the above and to outline the direction of current radar astronomical research. The questions of celestial mechanics, however, will not be discussed. Fundamental ConceptsWe will take the basic photometric approach as developed by MUHLEMAN et al. (1965) as opposed to that of conventional radar engineering since it appears to allow a smoother integration with other astronomical measurements. The planets may be studied in terms of how the measurements differ from the radar response of an ideal

Planetary radar astronomy

“…Estimated values of n usually fall between unity (geometric scattering, which describes the optical appearance of the full Moon) and 2 (Lambert scattering). Another expression that encompasses quasispecular and diffuse scattering components is a function derived heuristically by Muhleman (1966),…”

Some Preliminary Results of the 70‐cm Radar Studies of Venus

“…Some systematic errors in the measurements are believed to be caused by the varying amount of resolution with which the feature is viewed as it moves through the delay-Doppler map. Also, as Muhleman [1966] pointed out, the apparent location of the features may shift as the aspect angle changes. It is expected that this effect is not large enough to be seen with the present resolution capability and is less likely to be a problem with the cross-polarized observations, which are less dependent on the angle of incidence.…”

Interferometric Investigations of The Atmosphere of Venus

“…Many such models can be found that match the data quite well, but most involve an atmosphere that is optically thin in the cm region, e.g., Ho et al [1966]. However, the analysis of the radar reflectivity spectrum by Muhleman [1966] and Evans et al [1966] strongly indicates that the opacity is greater than unity at wavelengths shorter than about 4 cm. The importance o.f this relatively high opacity in terms of microwave interferometer observations for a general Venus atmosphere has been studied by Muhleman [1969] (hereafter called paper I) and that for an adiabatic CO• atmosphere by Berge and Greisen [1969] (hereafter called paper II).…”