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 10to 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. The smaller bodies of the inner solar system (the moon, Mercury, and Mars) are each characterized by a single, globally continuous lithospheric plate dating to the first half of solar system history (1, 2). In contrast, Earth, which is approximately twice the radius of the largest of these small bodies, has many laterally moving lithospheric plates, large portions of which are less than 200 million years old. Conduction dominates the transfer of heat through the lithospheres of the smaller terrestrial planets, while lithospheric plate recycling is the dominant mechanism on Earth (3). Venus is of extreme interest in understanding the thermal and tectonic evolution of the planets because it is approximately the same size and density as Earth and is Earth's closest planetary neighbor. The Pioneer mission to Venus provided a near-global topographic map of the planet with an average horizontal resolution of about 100 km and an average vertical resolution of about 200 m (4, 5). Analysis of the global physiography shows that approximately 8 percent of the surface lies more than 1.5 km above the mean planetary radius. This highland terrain is concentrated in several areas, the three'most distinctive being Ishtar Terra, Aphrodite Terra, and Beta Regio (5). Although the physiography of Venus can be broadly characterized, the Pioneer Venus data are of insufficient resolution to identify specific geologic processes responsible for physiographic provinces and major topographic features. Thus the question of the presence of plate tectonics and associated features on Venus is a subject of intense debate (3, 6-10). Critical to the understanding of the tectonic style of Venus is the origin of the major mountain ranges on Ishtar Terra (Figs. I and 2). These regions are unlike high topography on the moon, Mars, and Mercury, which is predominantly associated with large volcanoes or the rims of large circular impact basins (1). Akna Montes trend north-northeast and are about 1000 km long and up to 300 km wide. Freyja Montes are continuous with Akna, but trend east-west, extending more than 700 km and rising to similar elevations. Maxwell Montes trend approximately northwest in central Fig. 1. Mountains of id m 1 .....! 1 western Ishtar Terra N" data I (mercator projection). Akna Tc:. ̂ Montes ~//. ~ .~Freyja : Ishtar Terra is outntes ones Montes lined by a 1.5-km ele70 ! B// vation contour that A Maxwell -, Montes defines the base of the \ ! highlands province on .. Lakhmi Pl an :.mVenus (5). Dotted regions lie below this altitude. The base of 60 ........ Maxwell Montes is outlined by a 4.5-km1 elevation contour. All 500. 100 km ..........iiiiiiiiiiiiii: other mountains are ::i .:::: ::::: :: outlined by a 4.0-km 300 320 340 0 20 40 contour. Bars labeled A, B, and C correspond to profiles shown in Fig. 2. Elevations are kilometers above mean planetary radius (6051.5 km) (4). 644 Ishtar Terra, are about 1000 km long and up to 500 km wide, and rise more than 11 km above the mean planetary radius. Recently, images of the Ishtar Terra region were obtained with the 12.6-cmwavelength radar system at Arecibo Observatory in Puerto Rico (Fig. 3 and cover). These images, which map the backscatter cross section per unit area (surface reflectivity), have resolutions of 3 km (11), a significant improvement over previous Earth-based images (12). Because the sub-Earth point on Venus is restricted to +9? of the equator, the angle at which the surface is illuminated by the incident radar wave varies from near zero close to the sub-Earth point to a value approximately equal to the latitude at high latitudes. Appropriate scattering laws as a function of incidence angle and methods of data acquisition and display were discussed by Campbell and Burns (12). At the latitudes of Ishtar Terra (55? to 75?N), the slope of the scattering law increases so that both small-scale (wavelength-size) surface roughness and changes in mean surface slope could be important. The average scattering properties of the planet have been removed (12) so that the images show the ratio of the received power to that corresponding to the average scattering behavior of Venus. The images show the major features of Ishtar Terra known from previous Earthbased observations (12, 13), including Lakshmi Planum, a 1200-km region of low backscatter, and a series of regions of very high backscatter dominated by the Maxwell, Akna, and Freyja ranges (Figs. 1 to 3 and cover). More significantly, however, the new higher resolution images reveal that the regions of very high backscatter are characterized by a series of linear bands that are generally parallel to the long axis of the mountain ranges. Akna Montes, extending along the western edge of Lakshmi Planum, rise abruptly from the southwestern edge of Ishtar Terra to an elevation 6 to 7 km above the mean planetary radius within a horizontal distance of less than 100 km. The broad cross-sectional topographic form (Fig. 2A) shows that the mountains are somewhat asymmetric with steeper slopes toward Lakshmi Planum to the east. They extend northeastward in a slightly arcuate band 100 to 300 km in width and turn north-northeastward at about 69?N, continuing in this direction until merging with the generally eastwest trending Freyja Montes. The height and width of Akna vary along the strike of the range. Topography is highest along the southern section, where three