A region‐by‐region condensed description of almost all of the area that was radar‐photographed by Veneras 15 and 16 is presented. Using some generalizations, the diversity of terrain was reduced to a discrete set from which a geological‐morphological map was constructed. The predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types: ridge‐and‐band, patchy rolling plain, dome‐and‐butte plain, smooth plain, and high smooth plain. Stratigraphically, the ridge‐and‐band plains are the oldest and the smooth plains are the youngest. The stratigraphic position of the other types is yet to be determined. Large sections of the plains show similarities to the mare‐type basaltic plains of the moon, Mercury, and Mars. Other types of terrain are combinations of ridges and grooves in various patterns: linear parallel, orthogonal, diagonal or chevron‐like, and chaotic. In some places the ridge‐and‐groove terrain is Stratigraphically below the plain material, but in other places it appears to be plain material that has been subsequently deformed. Near the eastern and western boundaries of Ishtar Terra large (several hundred kilometers in diameter) ring‐like features can be seen that are named coronae or ovoids. Evidence of tectonic deformation and the presence of flow‐like patterns support their designation as volcano‐tectonic features. Beta Regio seems to be an uplifted plain showing evidence of rifting and volcanism. All types of terrain are sparesely peppered with craters of obvious impact morphology. Their average density gives the plain an age range of 0.5 to 1×109 years. The fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking, at least for the last 0.5 to 1×109 years. No evidence for water‐erosion‐sedimentation processes has been found. The tectonic activity of Venus has no equivalent on the moon, Mercury, or even Mars, and can be compared only with that of the Earth. Intensive horizontal deformation, previously known only on Earth, occurs on Venus, but in a characteristic Venusian style.

Analysis of the radar images obtained by Veneras 15 and 16 leads to the conclusion that the ridge‐and‐groove structures on the surface of Venus are the result of tectonic deformation. Although the mechanism of such deformation cannot yet be unequivocally deduced, several styles of deformation can be described. Areal deformation occurs where horizontal stresses have operated over large areas. Shear deformation appears in bands showing differential longitudinal deformation. Transversal stresses operating over long and relatively narrow areas have produced belt deformation. Circular deformation is related to a specific locus of the stresses. The absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment. The origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity‐induced spreading of surface material over upwellings. A general conclusion is that in the surveyed area of Venus neither terrestrial plate tectonics nor lunar‐highland‐type terrain exists.

Additional studies of Venusian impact craters have been made based on an analysis of Venera 15/16 radar imagery and altimetry. The crater population on Venus has been subdivided into groups representing different morphological classes and types. The craters display the size‐dependent variations in morphology which are well known from other planets. Assuming a crater production rate based on estimates by Hartmann et al. (1981), their areal density indicates an age for the total population of approximately 1 b.y. Alternative estimates based on the assumption that the recent cratering rates on the earth and Venus were similar would give a significantly lower age. The areal and size frequency distribution of a number of circular features of unclear origin have also been analyzed, and it is proposed that some of these features may be highly degraded and modified impact craters corresponding to a population approximately 3 b.y. in age. A suggested model of the effect of the atmosphere on the crater diameter‐frequency curve indicates that cometary impacts played only a minor role in forming the contemporary Venus environment. A comparative study of craters in the area where the Arecibo earth based radar and Venera 15/16 images overlap indicates that the radar dark inner zones of the craters in the high incidence angle Arecibo images correspond to the flat bottoms of the craters seen in the Venera images.

The interaction of both the particle and photon component of the solar wind with the lunar surface material is expected to produce diverse chemical reactions. Experimental evidence for proton-induced OH formation was obtained by bombarding a glass, chemically similar in composition to common silicate minerals, with high-energy protons. The concentration of OH, before and after irradiation, was determined by infrared absorption measurements. The OH formation rate was greatest at the start of the bombardment and decreased with increasing dose. The maximum proton to OH conversion rate, at the start of the irradiation, is at least 5 or 10% and may be as high as 100%. Using this result, together with estimates of the lunar age and recent solar proton flux data, we were able to make very rough calculations of the minimum proton-induced OH content in the lunar surface. If mixing or churning is not important, the upper centimeter could contain 4 X 10 •6 OH per cm 8. When protons below 40 Mev and the higher conversion rate are included in the computation, the estimated OH concentrations could increase by a factor of 10 or more. If surface mixing or churning has occurred, they should be divided by an average churning depth.

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