D. B. Campbell

Orogenic belts on Venus

Recent radar observations of Venus show evidence for three types of deformational features in the linear mountain belts of Ishtar Terra: linear ridges and troughs oriented parallel to the strike of the mountains and interpreted to be anticlines and synclines; broad low arches similar to mare-ridge-type features seen on the Moon and interpreted to represent low-angle thrusting and buckling; and linear discontinuities that cut across the strike of the ridges and troughs and are interpreted to represent strike-slip movement. On the basis of the localized concentration of these features, their orientations and patterns of distribution, and their association with linear mountainous topography, we conclude that they mark the location of concentrated horizontal compressional deformation and that Akna and Freyja Montes represent erogenic belts on Venus.

Venus: Volcanism and Rift Formation in Beta Regio

A new high-resolution radar image of Beta Regio, a Venus highland area, confirms the presence of a major tectonic rift system and associated volcanic activity. The lack of identifiable impact craters, together with the apparent superposition of the Theia Mons volcanic structure on the rift system, suggest that at least some of the volcanic activity occurred in relatively recent geologic time. The presence of topographically similar highland areas elsewhere on Venus (Aphrodite Terra, Dali Chasma, and Diana Chasma) suggests that rifting and volcanism are significant processes on Venus.

Geology of a rift zone on Venus: Beta Regio and Devana Chasma

Beta Regio is a region of rifting and volcanism on Venus. The nature of Beta, a major topographic rise and rift zone, is herein characterized using Pioneer Venus, Arecibo, and Venera 15/16 data. High-resolution (1-2 km) Arecibo and Venera radar images reveal details of faulting and volcanism, and Pioneer Venus altimetry illustrates the density and location of faults in relation to topography. Faults are distributed throughout Beta but are concentrated in Devana Chasma, where they are spaced 5-20 km apart. The pattern of faulting and distribution and sequence of volcanic activity in Beta can be used to help understand how the rift has evolved, including the origin of the high topography of Beta and the origin and nature of Devana Chasma. On the basis of geologic mapping relations and map patterns, Beta appears to have formed as a result of doming in response to a mantle anomaly. At the same time, Rhea Mons, a major shield volcano, was formed. Formation of Devana Chasma followed, with extensive faulting in the rift trough. Geometry of the trough and fault patterns suggests that some degree of lithospheric stretching has occurred. Later volcanism produced a second major shield volcano, Theia Mons, which is superimposed on the western bounding fault of the rift zone. Both uplift and extension have been involved in forming Devana Chasma and Beta Regio and may be important in the formation of other equatorial high-land regions with systems of chasmata on Venus, such as Aphrodite Terra. The bifurcation of Devana Chasma in the vicinity of Theia Mons, and the extension of the rift system south to Phoebe Regio and west toward Aphrodite Terra, suggest that their origins may be linked.

Radar Characteristics of Viking 1 Landing Sites

Radar observations of Mars at centimeter wavelengths in May, June, and July 1976 provided estimates of surface roughness and reflectivity in three potential landing areas for Viking 1. Surface roughness is characterized by the distribution of surface landing slopes or tilts on lateral scales of the order of 1 to 10 meters; measurements of surface reflectivity are indicators of bulk surface density in the uppermost few centimeters. By these measures, the Viking 1 landing site at 47.5� W, 22.4�N is rougher than the martian average, although it may be near the martian average for elevations accessible to Viking, and is estimated to be near the Mars average in reflectivity. The AINW site at the center of Chryse Planitia, 43.5�W, 23.4�N, may be an area of anomalous radar characteristics, indicative of extreme, small-scale roughness, very low surface density, or a combination of these two characteristics, Low signal-to-noise ratio observations of the original Chryse site at 34�W, 19.5�N indicate that that area is at least twice as rough as the Mars average.

New radar image of venus.

A new radar image of Venus covering the latitude range 46� to 75� and the approximate longitude range 290� to 10� is shown. The resolution is approximately 20 kilometers.

Galilean Satellites of Jupiter: 12.6-Centimeter Radar Observations

Observations of the Galilean satellites with the radar system at the Arecibo Observatory, Puerto Rico, show that their surfaces are highly diffuse scatterers of radio waves of length 12.6 centimeters; spectra of the radar echoes are asymmetric and broad. The geometric radar albedos for the outer three satellites—0.42 � 0.10, 0.20 � 0.05, and 0.09 � 0.02 for Europa, Ganymede, and Callisto, respectively—show about the same relative decreases as do the optical albedos, although the latter presumably bear only on material much nearer the surface. Radii of 1420 � 30, 2640 � 80, and 2360 � 70 kilometers for Europa, Ganymede, and Callisto were determined from the radar data and are in good agreement with the corresponding optically derived values. Io, observed successfully only once, appears to have an albedo comparable to Ganymedes, but no radius was estimated for it.

Radar Interferometric Observations of Venus at 70-Centimeter Wavelength

A radar interferometer was used to map unambiguously the surface reflectivity of Venus in the polarized mode at a wavelength of 70 centimeters. The observed region extended from 260� to 30� in longitude and from -60� to 50� in latitude with a surface resolution of approximately 3� by 3�. The result agrees well in most respects with earlier maps made elsewhere at shorter wavelengths and, in addition, discloses a number of new features.