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

Abstract

Delay-Doppler positions of three radar features are given for observations made during the 1964, 1967, and 1969 inferior conjunctions of Venus. The latitude and longitude of each feature were determined from these data by the method of weighted least squares. The results indicate that the rotation period of Venus may be slightly smaller than the earth-synchronous period of 243.16 days. Some preliminary measurements of the polarized and cross-polarized scattering properties of the average surface and the anomalous features are given. These data indicate that the bulk of the surface of Venus is significantly smoother than the moon and that at least two anomalous features give larger ratios of cross-polarized to polarized power than any features known on the moon. number of the 1967 observations and some of the 1969 observations have been processed. The combination of the data from the three inferior conjunctions indicates that Venus may have a slightly smaller rotation period than the earth-synchronous rotation period of 243.16 days. Also, a slight shift in the apparent latitude of the features indicates that the assumed direction of the spin axis is in error by approximately 2 ø of arc. Such an error is consist-Copyright (•) 1970 by the American Geophysical Union. ent with the formal errors specified for the various reductions of the spin axis reported so far. Since 1964, several spin axis reductions using radar data from one or more inferior conjunctions of Venus have been performed. Four of the more recent reductions are given in Table 1. The first and last entries in the table are based on feature data covering two inferior conjunctions. These data strongly suggest the earth-synchronous rotation period. The fairly small error estimates assigned to the values of, and I• in the case of the second entry in the table were later relaxed in a reevaluation of the data, the results of which appear as the third entry. Recent radar maps published by Rogers et al. [1968] and Ingalls et al. [1968] adopted the spin axis given in the second entry but using the earthsynchronous period. For the purpose of direct comparison, the same spin axis and rotation period were used here.After establishing the spin axis direction and the rotation period, it is necessary to fix the longitude system to the planet. In the past, several systems have been used, but these can be approximately reduced to two systems. The first and most commonly used places the -40 ø meridian on the subearth point at the time of inferior conjunction. This definition places the feature, [Goldstein, 1965] on the prime meridian. Both the 1964 and 1967 inferior conjunctions have been used as the reference epochs, and in view of the earth-synchronous rotation characteristic, both epochs establish the same coordinate system within a few degrees. A second 435 436 RAYMOND F. JURGENS 70-cm RADAR STUDIES OF VENUS 439TABLE 3. List of delay-Doppler frequency measurements for feature Hertz Time, UT Date hms Delay, msec Frequency, Hz

Planetary radar astronomy

“…Ground-based images and altimetry led initially to the discovery of elevation variations (e.g., Campbell, Dyce, et aL, 1972) and reflectivity features (e.g., Goldstein, 1965;Jurgens, 1970) and then to tentative geologic identification of those features (e.g., Campbell, Head, et aL, 1983Burns and Campbell, 1985). Eventually, images with resolution as fine as that in Fig.…”

The surfaces of solar‐system bodies

“…There is a small dark area below the center of the disk and a dark area with a central peak near feature a Rogers and Ingalls, 1970]. The bulk of the surface of Venus is significantly smoother than the surface of the moon [Jurgens, 1970]. It has a polarized radar (12.5 cm) reflectivity of 0.0067 ± 0.005 and an average dielectric constant of 3.75 ± 0.3 [Carpenter, 1966].…”

Venus Express bistatic radar: High‐elevation anomalous reflectivity

“…Early Earth‐based radar images of Venus revealed three very bright areas: “Alpha” and “Beta” near the equator [ Goldstein , 1965; Rogers and Ingalls , 1970] and “Maxwell” at 70°N latitude [ Jurgens , 1970]. On the basis of then‐current understanding, investigators hypothesized that these were regions of very rough surface texture, where highly tilted surface elements preferentially backscattered radar signals toward Earth.…”