Alice A. Hine

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.

Asteroid 1986 DA: Radar Evidence for a Metallic Composition

Echoes from the near-Earth object 1986 DA show it to be significantly more reflective than other radar-detected asteroids. This result supports the hypothesis that 1986 DA is a piece of NiFe metal derived from the interior of a much larger object that melted, differentiated, cooled, and subsequently was disrupted in a catastrophic collision. This 2-kilometer asteroid, which appears smooth at centimeter to meter scales but extremely irregular at 10- to 100-meter scales, might be (or have been a part of the parent body of some iron meteorites.

Reports Venus: Identification of Banded Terrain in the Mountains of Ishtar Terra

High resolution images obtained with the Arecibo radar system at a wavelength of 12.6 centimeters reveal numerous parallel 10- to 20-kilometer-wide bands of high radar backscatter situated on and oriented parallel to the major mountain ranges of the Ishtar Terra region of Venus. Geometric and morphological characteristics suggest that the bands represent Earth-like tectonic deformational features, such as folds and faults.

Asteroid radar astrometry

Measurements of time delay and Doppler frequency are reported for asteroid-radar echoes obtained at Arecibo and Goldstone during 1980-1990. Radar astrometry is presented for 23 near-earth asteroids and three mainbelt asteroids. These measurements, which are orthogonal to optical, angular-position measurements, and typically have a fractional precision between 10 to the -5th and 10 to the -8th, permit significant improvement in estimates of orbits and hence in the accuracy of prediction ephemerides. Estimates are also reported of radar cross-section and circular polarization ratio for all asteroids observed astrometrically during 1980-1990.

Styles of volcanism on venus: new arecibo high resolution radar data.

Arecibo high-resolution (1.5 to 2 km) radar data of Venus for the area extending from Beta Regio to western Eisila Regio provide strong evidence that the mountains in Beta and Eisila Regiones and plains in and adjacent to Guinevere Planitia are of volcanic origin. Recognized styles of volcanism include large volcanic edifices on the Beta and Eisila rises related to regional structural trends, plains with multiple source vents and a mottled appearance due to the ponding of volcanic flows, and plains with bright features surrounded by extensive quasi-circular radar-dark halos. The high density of volcanic vents in the plains suggests that heat loss by abundant and widely distributed plains volcanism may be more significant than previously recognized. The low density of impact craters greater than 15 km in diameter in this region compared to the average density for the higher northern latitudes suggests that the plains have a younger age.

Radar imaging of the lunar poles.

We have used a radio telescope at Arecibo Observatory, Puerto Rico, to map features of the lunar poles — some as small as 300 metres across — by collecting long-wavelength radar images that can penetrate several metres of lunar dust. We find that areas of the crater floors at the poles that are in permanent shadow from the Sun, which are potential cold traps for water or other volatiles, do not give rise to strong radar echoes like those associated with thick ice deposits in the polar craters on Mercury. Any lunar ice present within regions visible to the Arecibo radar must therefore be in the form of distributed grains or thin layers.

Venus Southern Hemisphere: Geologic Character and Age of Terrains in the Themis-Alpha-Lada Region

Arecibo high-resolution radar images of the southern hemisphere of Venus extending to 78�S show that the surface of the Themis-Alpha-Lada region is characterized by linear deformation zones with volcanoes and corona-like features and by regional volcanic deposits (primarily plains, small shields, and large edifices). Large-scale areal deformation is limited to the tessera of Alpha Regio. Lada Terra, in the southern high latitudes, contains several large coronae, in contrast to Ishtar Terra in the northern high latitudes. The density of craters of possible impact origin is somewhat lower than that observed in the Venera 15 and 16 coverage; these data extend to 43 percent of the areas of the surface of Venus with ages of less than about 1 billion years.

Radar images of asteroid 1627 Ivar

Radar echoes from the near-earth asteroid 1627 Ivar, whose orbit crosses the earths, reveal it to be about twice as long as it is wide, with a maximum dimension no less than 7 km and probably within 20 percent of 12 km. The surface is fairly smooth at centimeter-to-meter scales but appears irregular and nonconvex at kilometer scales.

Geology and tectonics of the Themis Regio-Lavinia Planitia-Alpha Regio-Lada Terra area, Venus - Results from Arecibo image data

New radar images obtained from the Arecibo Observatory (resolution 1.5–4.0 km) for portions of the southern hemisphere of Venus show that: the upland of Phoebe Regio contains the southern extension of Devana Chasma, a rift zone extending 4200 km south from Theia Mons and interpreted as a zone of extension; Alpha Regio, the only large region of tessera within the imaged area, is similar to tessera mapped elsewhere on the planet and covers a smaller percentage of the surface than that observed in the northern high latitudes; the upland made of Ushas, Innini and Hathor Montes consists of three distinct volcanic constructs; Themis Regio is mapped as an ovoid chain of radar-bright arcuate single and double ring structures, edifices and bright lineaments. This area is interpreted as a region of mantle upwelling and on the basis of apparent split and separated features, a zone of localized faulting and extension. Linear zones of deformation in Lavinia Planitia are characterized by lineament belts that are often locally elevated, are similar to ridge belts mapped in the northern high latitudes and are interpreted to be characterized mainly by compression; radar-bright lava complexes within Lavinia Planitia are unique to this part of the planet and are interpreted to represent areas of eruption of high volumes of extremely fluid lava; the upland of Lada Terra is bound to the north by a linear deformation zone interpreted as extensional, is characterized by large ovoids and coronae, is interpreted to be associated with an area of mantle upwelling, and is in contrast to the northern high latitude highland of Ishtar Terra. Regions of plains in the southern hemisphere cover about 78%; of the mapped area and are interpreted to be volcanic in origin. Located within the area imaged (10–78 ‡ S) are 52 craters interpreted to be of impact origin ranging from 8 to 157 km in diameter. On the basis of an overall crater density of 0.94 craters/106 km2, it is determined that the age of this part of the Venus surface is similar to the 0.3 to 1.0 billion year age calculated for the equatorial region and northern high latitudes. The geologic characteristics of the portion of the Venus southern hemisphere imaged by Arecibo are generally similar to those mapped elsewhere on the planet. This part of the planet is characterized by widespread volcanic plains, large volcanic edifices, and zones of linear belt deformation. The southern hemisphere of Venus differs from northern high latitudes in that tessera makes up only a small percentage of the surface area and the ovoid chain in Themis Regio is unique to this part of the planet. On the basis of the analysis presented here, the southern hemisphere of Venus is interpreted to be characterized by regions of mantle upwelling on a variety of scales (ovoids, region made up of Ushas, Innini and Hathor Montes), upwelling and extension (Themis Regio) and localized compression (lineament belts in Lavinia Planitia).

Geology and tectonics of Beta Regio, Guinevere Planitia, Sedna Planitia, and Western Eistla Regio, Venus: Results from Arecibo image data

New radar images (resolution 1.5–2.0 km) obtained from the Arecibo Observatory are used to assess the geology of a portion of the equatorial region of Venus (1‡ S to 45‡ N and from 270‡ eastward to 30‡). Nine geologic units are mapped on the basis of their radar characteristics and their distribution and correspondences with topography are examined. Plains are the most abundant unit, make up 80%; of the area imaged, and are divided into bright, dark, and mottled. Mottled plains contain abundant lava flows and domes suggesting that volcanism forming plains is a significant process in the equatorial region of Venus. Tesserae are found primarily on Beta Regio and its eastern flank and are interpreted to be locally stratigraphically older units, predating episodes of faulting and plains formation. Isolated regions of tesserae concentrated to the north of Western Eistla Regio are interpreted to predate the formation of plains in this area. The volcanoes Sif Mons, Gula Mons, Sappho, Theia Mons, and Rhea Mons, are found exclusively in highland regions and their deposits are interpreted as contributing only a small percentage to the overall volume of the regional topography. The northern 15‡ of the image data overlap with Venera 15/16 images making it possible to examine the characteristics of geologic units mapped under various illumination directions and incidence angles. Surface panoramas and geochemical data obtained from Venera landers provide ground truth for map units, evidence that plains are made up of basaltic lava flows, and that linear deformation zones contain abundant blocks and cobbles. On the basis of spatial and temporal relationships between geologic units, the highlands of Beta Regio and Western Eistla Regio are interpreted to have formed in association with areas of mantle upwelling which uplift plains, cause rifting, and in the case of Beta Regio, disrupt a large region of tessera. Zones of linear belt deformation in Beta Regio and Western Eistla Regio are interpreted to be extensional and indicate that at least limited extension has occurred in both regions. The images reveal for the first time that southern Devana Chasma is a region of overlapping rift valleys separated by a distance of 600 km. Linear deformation zones in Guinevere Planitia, separating Beta Regio and Eistla Regio, converge at a region of ovoids forming a discontinuous zone of disruption and completes an equatorial encompassing network of highlands and tectonic features. The similarity between ovoids and coronae suggests a mechanism of formation associated with hotspots or mantle plumes. Analysis of the distribution and density of impact craters suggests a surface age for this part of the planet similar to or slightly less than that determined for the northern high latitudes from Venera 15/16 data (0.3 to 1.5 by) and comparable to that calculated for the southern hemisphere.