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Geologic Investigations Series I-2808 Prepared for the National Aeronautics and Space Administration Geologic Map of the Ovda Regio Quadrangle (V–35), Venus By Leslie F. Bleamaster, III, and Vicki L. Hansen Pamphlet to accompany Geologic Investigations Series I–2808 75° 75° V–1 V–2 V–4 50° 50° V–3 V–8 V–13 V–9 V–12 V–10 V–11 25° 25° V–20 V–25 V–21 V–24 V–22 V–23 0° 30° 60° 90° 120° 150° 180° 0° 0° V–34 V–35 V–33 V–36 V–32 V–37 –25° –25° V–46 V–47 V–45 V–48 V–44 V–49 V–57 –50° V–56 V–58 –50° V–62 2005 –75° –75° U.S. Department of the Interior U.S. Geological Survey 0 THE MAGELLAN MISSION comparable to the radar wavelength are responsible for variations in the SAR return. In either case, the echo The Magellan spacecraft orbited Venus from August strength is also modulated by the reflectivity of the sur- 10, 1990, until it plunged into the Venusian atmosphere face material. The density of the upper few wavelengths on October 12, 1994. Magellan had the objectives of (1) of the surface can have a significant effect. Low-density improving knowledge of the geologic processes, surface layers, such as crater ejecta or volcanic ash, can absorb properties, and geologic history of Venus by analysis of the incident energy and produce a lower observed echo. surface radar characteristics, topography, and morphol- On Venus, a rapid increase in reflectivity exists at a cer- ogy and (2) improving knowledge of the geophysics of tain critical elevation, above which high-dielectric miner- Venus by analysis of Venusian gravity. als or coatings are thermodynamically stable. This effect The Magellan spacecraft carried a 12.6-cm radar leads to very bright SAR echoes from virtually all areas system to map the surface of Venus. The transmitter above that critical elevation. and receiver systems were used to collect three datasets: The measurements of passive thermal emission from synthetic aperture radar (SAR) images of the surface, Venus, though of much lower spatial resolution than the passive microwave thermal emission observations, and SAR data, are more sensitive to changes in the dielec- measurements of the backscattered power at small angles tric constant of the surface than to roughness. As such, of incidence, which were processed to yield altimet- they can be used to augment studies of the surface and to ric data. Radar imaging and altimetric and radiometric discrimnate between roughness and reflectivity effects. mapping of the Venusian surface were done in mission Observations of the near-nadir backscatter power, col- cycles 1, 2, and 3, from September 1990 until September lected using a separate smaller antenna on the spacecraft, 1992. Ninety-eight percent of the surface was mapped were modeled using the Hagfors expression for echoes with radar resolution of approximately 120 m. The SAR from gently undulating surfaces to yield estimates of observations were projected to a 75-m nominal horizontal planetary radius, Fresnel reflectivity, and root-mean- resolution; these full-resolution data compose the image square (rms) slope. The topography data produced by this base used in geologic mapping. The primary polarization technique have horizontal footprint sizes of about 10 km mode was horizontal-transmit, horizontal-receive (HH), near periapsis and a vertical resolution of approximately but additional data for selected areas were collected for 100 m. The Fresnel reflectivity data provide a compari- the vertical polarization sense. Incidence angles varied son to the emissivity maps, and rms slope parameter is from about 20° to 45°. an indicator of the surface tilts, which contribute to the High-resolution Doppler tracking of the sapcecraft quasi-specular scattering component. was done from September 1992 through October 1994 (mission cycles 4, 5, 6). High-resolution gravity observa- tions from about 950 orbits were obtained between Sep- tember 1992 and May 1993, while Magellan was in an OVDA REGIO QUADRANGLE elliptical orbit with a periapsis near 175 km and apoapsis near 8,000 km. Observations from an additional 1,500 INTRODUCTION orbits were obtained following orbit-circularization in The Ovda Regio quadrangle (V–35), at lat 0° N. to mid-1993. These data exist as a 75° by 75° harmonic 25° S. and long 90° to 120° E., encompasses approxi- field. mately 8.4 M km2 of complex geology with intricate vol- cano-tectonic relationships. The quadrangle was mapped at 1:5,000,000 scale under the Venus Geologic Mapping MAGELLAN RADAR DATA program. The northern part of V–35 includes south-cen- Radar backscatter power is determined by the mor- tral Aphrodite Terra, a high-standing, equatorial, plateau phology of the surface at a broad range of scales and by region that contains eastern Ovda Regio and western the intrinsic reflectivity, or dielectric constant, of the mate- Thetis Regio; the southern region, which is lower in topog- rial. Topography at scales of several meters and larger raphy, hosts Tahmina Planitia, Gauri Mons, Boszorkany can produce quasi-specular echoes, with the strength of Dorsa, the extensive flows of Inari Corona, and numerous the return greatest when the local surface is perpendicu- other unnamed deformation centers and corona flows. lar to the incident beam. This type of scattering is most The Kuanja, Ralk-umgu, and Vir-ava Chasmata system, important at very small angles of incidence, because nat- a highly deformed east-trending zone, defines a geomor- ural surfaces generally have few large tilted faces at high phic boundary between the high-standing crustal plateaus angles. The exception is in areas of steep slopes, such of the north and the southern low-lying plains (fig. 1, map as ridges or rift zones, where favorably tilted terrain can sheet). Cross-cutting and embayment relations constrain produce very bright signatures in the radar image. For the relative spatial and temporal relations of map units most other areas, diffuse echoes from roughness at scales and tectonic structures within three geographic regions. 1 Within each region, individual tessera units, large corona observe on Earth: igneous, metamorphic, and sedimentary. flows, local volcano-tectonic complexes, chasmata defor- The absence of surface water and the paucity of eolian mation, and impact craters describe a coherent, or logi- erosion on Venus (for example, Kaula, 1990) dismiss the cally consistent, geologic history. idea that many surface rocks are of sedimentary origin. Detailed mapping was conducted using cycles 1, 2, Furthermore, the apparent absence of widespread erosion and 3 Magellan synthetic aperture radar (SAR) images suggests surface rocks are not likely exposed crustal met- at C-MIDR and F-MIDR scale and Magellan altimetry, amorphic or intrusive igneous rocks. Thus, surface rocks gravity, emissivity, and slope data where available and most likely originated as extrusive igneous rocks, that is, applicable (fig. 2, map sheet). FMAPs in both printed volcanic flows. Exceptions to these flows include impact- photographic and CD-ROM format provided the high- related materials, eolian deposits, and atmospheric chemi- est image resolution to view complicated volcanic and cal precipitates; although these materials are present on tectonic contacts. Stereo images, generated using cycle 1 Venus they are areally minor. Although one might argue and cycle 3 SAR stereo pairs (Plaut, 1993), and synthetic that sedimentary or metamorphic rocks exist on the sur- stereo images (fig. 3, map sheet), generated using topog- face of Venus, we have come across no convincing evi- raphy and cycle 1 SAR (Kirk and others, 1992), proved dence for widespread distribution of either of these two useful in resolving the interactions of flows, primary and general rock types and we will consider them no further secondary structures, and topography. Incidence angles in because no evidence supports their presence. V–35 provide a moderate range in viewing angle: cycle 1, Venus geologic units (or “material units”; for exam- 35.5°–45.2°; cycle 2, 24.9°–25.1°; cycle 3 stereo, 18.7°– ple, volcanic flows, eolian deposits, crater deposits) are 24.5° (for more details see Ford and others, 1993). typically differentiated in Magellan data by patterns in SAR, emissivity, or rms slope data that reflect primary MAPPING TECHNIQUES features such as lobate flows, mottling, or homogene- ity. The first order task in mapping material units is to Mapping began with the compilation of Magellan C- determine their spatial distribution and to examine con- MIDR framelet scale images using Adobe Illustrator1 6.0 tact relations between adjacent units (Wilhelms, 1990). and 8.0 and progressed to the use of a synthetic stereo Several problems must be kept in mind. Available data (Kirk and others, 1992) cycle 2 base map with FMAP might inhibit unique distinction between different mate- framelet inlays. Use of NIH Image software, with a set of rial units or may result in division of a single unit into macros developed by Duncan Young at Southern Meth- two apparently different units (Hansen, 2000). For exam- odist University, Dallas, Tex., proved very useful for ple, spatially separate lava flows may show similar radar, integrating multiple data sets and for allowing interac- emissivity, or rms slope characteristics and, hence, one tive adjustment of image stretch for detailed mapping, a might conclude (incorrectly) that these units are time cor- technique that is particularly critical for mapping subtle relative. Alternatively, a single (or composite) volcanic material unit and structural facies boundaries. Through- flow unit emplaced within a single eruptive event may out the mapping, material units were treated separately have both pahoehoe and aa flow facies and show radically from tectonic (secondary) structures, and we delineated different radar and rms slope signatures, and therefore the spatial distribution of each (Hansen, 2000) in order they might be interpreted (incorrectly) as temporally dis- to avoid assumption that tectonic structures form at the tinct geologic units.
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