Prepared for the National Aeronautics and Space Administration Geologic Map of the Mylitta Fluctus Quadrangle (V–61), Venus By Mikhail A. Ivanov and James W. Head, III Pamphlet to accompany Scientifiic Investigations Map 2920 75° 75° V–1 V–6 V–3 50° 50° V–7 V–2 V–17 V–10 V–18 V–9 V–19 V–8 25° 25° V–29 V–22 V–30 V–21 V–31 V–20 270° 300° 330° 0° 30° 60° 90° 0° 0° V–43 V–32 V–42 V–33 V–41 V–34 –25° –25° V–55 V–44 V–54 V–45 V–53 V–46 V–61 V–56 –50° –50° V–60 V–57 V–62 2006 –75° –75° U.S. Department of the Interior U.S. Geological Survey INTRODUCTION nic ash, can absorb the incident energy and produce a lower observed echo. On Venus, there also exists a rapid increase in THE MAGELLAN MISSION reflectivity at a certain critical elevation, above which high- The Magellan spacecraft orbited Venus from August 0, dielectric minerals or coatings are thought to be present. This 990, until it plunged into the Venusian atmosphere on October leads to very bright SAR echoes from virtually all areas above 2, 994. Magellan Mission objectives included: () improv- that critical elevation. ing knowledge of the geological processes, surface properties, The measurements of passive thermal emission from and geologic history of Venus by analysis of surface radar Venus, though of much lower spatial resolution than the SAR characteristics, topography, and morphology, and (2) improv- data, are more sensitive to changes in the dielectric constant of ing the knowledge of the geophysics of Venus by analysis of the surface than to roughness. As such, they can be used to aug- Venusian gravity. ment studies of the surface and to discriminate between rough- The Magellan spacecraft carried a 2.6-cm radar system ness and reflectivity effects. Observations of the near-nadir to map the surface of Venus. The transmitter and receiver sys- backscatter power, collected using a separate smaller antenna tems were used to collect three data sets: () synthetic aper- on the spacecraft, were modeled using the Hagfors expression ture radar (SAR) images of the surface, (2) passive microwave for echoes from gently undulating surfaces to yield estimates thermal emission observations, and (3) measurements of the of planetary radius, Fresnel reflectivity, and root-mean-square backscattered power at small angles of incidence, which were (rms) slope. The topographic data produced by this technique processed to yield altimetric data. Radar imaging, altimetric, have horizontal footprint sizes of about 0 km near periapsis, and radiometric mapping of the Venusian surface was done in and a vertical resolution on the order of 00 m. The Fresnel mission cycles , 2, and 3 from September 990 until Septem- reflectivity data provide a comparison to the emissivity maps, ber 992. Ninety-eight percent of the surface was mapped with and the rms slope parameter is an indicator of the surface tilts, radar resolution on the order of 20 meters. The SAR observa- which contribute to the quasi-specular scattering component. tions were projected to a 75-m nominal horizontal resolution, and these full-resolution data compose the image base used in MYLITTA FLUCTUS QUADRANGLE geologic mapping. The primary polarization mode was hori- zontal-transmit, horizontal-receive (HH), but additional data The Mylitta Fluctus quadrangle (V–6, fig. ) is in the for selected areas were collected for the vertical polarization southern hemisphere of Venus and extends from lat 50° to 75° sense. Incidence angles varied between about 20° and 45°. S. and from long 300° to 360° E. The northern third of the High resolution Doppler tracking of the spacecraft took quadrangle covers the southern part of Lavinia Planitia, which place from September 992 through October 994 (mission represents a class of equidimensional lowlands on Venus (Bind- cycles 4, 5, 6). Approximately 950 orbits of high-resolution schadler and others, 992b) and is as deep as about .5–2 km gravity observations were obtained between September 992 below mean planetary radius. The southeastern part of the map and May 993 while Magellan was in an elliptical orbit with area shows the western part of the upland of Lada Terra, the a periapsis near 75 km and an apoapsis near 8,000 km. An summit of which is about 3 km above mean planetary radius additional ,500 orbits were obtained following orbit-circular- (fig. 2). ization in mid-993. These data exist as a 75° by 75° harmonic Before the Magellan mission, the geology of the Mylitta field. Fluctus quadrangle was known on the basis of topographic data acquired by the Pioneer-Venus altimeter (Masursky and others, 980; Pettengill and others, 980) and radar images received MAGELLAN RADAR DATA by the Arecibo telescope (Campbell and others, 99). Arecibo Radar backscatter power is determined by () the mor- radar images showed that the area of Mylitta Fluctus quadran- phology of the surface at a broad range of scales and (2) the gle was populated by several coronae and coronalike features intrinsic reflectivity, or dielectric constant, of the material. and riftlike fracture zones parallel to the northern margin of the Topography at scales of several meters and larger can produce upland of Lada Terra (Campbell and others, 99; Senske and quasi-specular echoes and the strength of the return is greatest others, 99). Arecibo data further revealed that the western when the local surface is perpendicular to the incident beam. and southern parts of the map area were characterized by com- This type of scattering is most important at very small angles plex patterns of features such as deformational belts and vol- of incidence because natural surfaces generally have few large canic plains. Several regions along the margins of Lada Terra tilted facets at high angles. The exception is in areas of steep upland were thought to be the sources of extensive outpourings slopes, such as ridges or rift zones, where favorably tilted ter- of digitate lava flows down the regional slopes of Lada. rain can produce very bright signatures in the radar image. For Early Magellan results showed that the interior of Lavinia most other areas, diffuse echoes from roughness at scales com- Planitia was populated with complex patterns of deformational parable to the radar wavelength are responsible for variations features in the form of belts of ridges and grooves (Squyres in the SAR return. In either case, the echo strength is also mod- and others, 992; Solomon and others, 992). The most prom- ulated by the reflectivity of the surface material. The density inent belts of ridges within the map area are Molpadia and of the upper few wavelengths of the surface can have a signifi- Penardun Lineas at the southern edge of Lavinia Planitia and cant effect. Low-density layers, such as crater ejecta or volca- Morrigan Linea west of Lada Terra. The upland of Lada Terra hosts an assemblage of numerous coronae including one of and roughness data (root mean square, rms, slope). The back- the largest corona on Venus, Quetzalpetlatl (Stofan and others, ground for our unit definition and characterization is described 992), interconnected by belts of fractures and graben (Baer in Tanaka (994), Basilevsky and Head (995a, b), Basilevsky and others, 994; Magee and Head, 995). One of these belts, and others (997), and Ivanov and Head (998). Kalaipahoa Linea, runs from east to west through the middle of the Mylitta Fluctus quadrangle connecting Kamui-Huci Magellan SAR AND RELATED Data Corona in the west-central part and Jord Corona and Tarbell Patera in the eastern part of the map area. Among the most The synthetic aperture radar (SAR) instrument flown on spectacular features of the quadrangle are large complexes of the Magellan spacecraft (2.6 cm, S-band) provided the image lava flows, Mylitta Fluctus (Roberts and others, 992) ema- data used in this mapping and interpretation. SAR images are nating from Tarbell Patera and Cavillaca Fluctus and Juturna a record of the echo (radar energy returned to the antenna), Fluctus originating at Boala Corona, which is inside larger which is influenced by surface composition, slope, and wave- Quetzalpetlatl Corona. Magellan gravity data (Konopliv and length-scale surface roughness. Viewing and illumination Sjogren, 994; Konopliv and others, 999) show that the east- geometry also influence the appearance of surface features in ern half of the quadrangle, which is dominated by the western SAR images. Guidelines for geologic mapping using Magel- part of Lada Terra, is characterized by high values of both ver- lan SAR images and detailed background to aid in their inter- tical gravity acceleration (as much as 20 mGal) and height of pretation can be found in Elachi (987), Saunders and others geoid (as much as 40 m). Lowlands that surround the upland (992), Ford and Pettengill (992), Tyler and others (992), are characterized by a –30 mGal gravity anomaly and a –0 and Tanaka (994). In the area of the Atalanta Planitia quad- m geoid anomaly northwest and southwest of the Lada Terra rangle, incidence angles are such, 6.3–2.3 for Cycle and upland. 9.7–25. for Cycle 2 (Ford and others, 993), that backscat- The key geological theme of the quadrangle is that the map ter is dominated by variations in surface roughness at wave- area portrays a transition zone from uplands to lowlands. The length scales. Rough surfaces appear relatively bright, whereas characteristics and configuration of Lavinia Planitia north of smooth surfaces appear relatively dark. Variations also occur the map area have been cited as evidence for the region being depending on the orientation of features relative to the incident the site of large-scale mantle downwelling (Bindschadler and radiation (illumination direction), with features normal to the others, 992b).