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.     

Assemblages of geologic/morphologic units in the northern hemisphere of Venus

The geologic/morphologic map of the northern mid-to-high latitudes of Venus prepared by a Soviet science team on the basis of Venera 15/16 mission radar image coverage is analyzed and used to define six discrete assemblages of geologic/morphologic units that have well-defined geographic distributions. These assemblages have distinctive and differing geological and tectonic expressions and include: Plains Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, and a high concentration of small volcanic domes; Plains-Corona Assemblage - which is dominated by lowland smooth plains and lowland rolling plains interpreted to be of volcanic origin, at least ten coronae structures concentrated in the northern half of the region, and at least five large volcanoes, generally concentrated in the southern and western half of the region; Plains-Ridge Belt Assemblage - which is dominated by lowland smooth plains and lesser amounts of lowland rolling plains, major occurrences of ridge belts in a distinctive fan-shaped pattern, and very minor and patchy occurrences of tessera; Plains-Corona-Tessera Assemblage - which is dominated by approximately equal amounts of lowland smooth plains and lowland rolling plains, at least five coronae concentrated in the northern part of the region, a small number of large volcanoes, also in the northern part of the region, and numerous small patches of tesserae scattered throughout, and the highest abundance of small volcanic domes observed in the northern hemisphere; Tessera-Ridge Belt Assemblage — which is dominated by a few large areas (Fortuna, Laima, Tellus) and several smaller areas (Dekla, Meni) of tesserae, ridge belts generally arrayed in an angular and often orthogonal pattern different from the fan-shaped pattern of the Plains-Ridge Belt Assemblage, lowland rolling plains and lesser amounts of lowland smooth plains, and an upland rise (Bell Regio); Tessera-Mountain Belt Assemblage - which is centered on the two volcanoes Colette and Sacajawea in Lakshmi Planum, and characterized by the peripheral mountain belt/tessera pairs, with the tessera on the outboard side: Danu/Clotho (S), Akna/Atropos (W), Freyja/ltzpapalotl (N), and Maxwell/Fortuna (E).The distribution and characteristics of assemblages demonstrate that vertical and horizontal tectonic forces are operating on the crust and lithosphere of Venus in different ways in specific localized areas. Alternative models are outlined for the origin of each assemblage and the relationship between assemblages, and important unresolved questions are identified. A key to the further understanding of these assemblages is the origin of ridge belts and tessera terrain.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Geology of the Venus equatorial region from Pioneer Venus radar imaging

Pioneer Venus radar data has provided images (resolution 20- to 40-km) of approximately 50% of the total surface of Venus in a band between 45 ° N to 15 ° S. These data are used to map the broad radar characteristics of the equatorial region on the basis of radar brightness and texture. Seven radar units are defined and are used to assess the geologic character of the equatorial region. These units fall into two distinct classes, those that are radar-bright (35% of the equatorial region) which correspond to highlands and zones of intense tectonic deformation, and radar-dark units, corresponding primarily to plains (65% of the equatorial region). The correspondence between features in the 15 ° region of overlap between the Pioneer Venus and Venera 15/16 images is examined and used to extend units mapped in the northern high latitudes into the equatorial region. On the basis of the distribution of the radar units, properties of RMS slope, reflectivity, the scattering behavior of the surface, and topographic signature, seven physiographic units are mapped in the equatorial region and are identified by increasing complexity as plains (undivided), dark halo plains, upland rises, upland plateaus, interhighland tectonic zones, tectonically segmented linear highlands, and tectonic junctions. The physiographic units are distributed in a nearly continuous interconnecting zone of volcanic rises and tectonic features that extends for nearly 360 ° around the equator of the planet. The distribution of large circular structures interpreted as coronae is also examined and it is concluded that the abundances of the largest structures, diameters greater than 500 km, is less than in the northern high latitudes with a notable absence of smaller coronae. The absence of small coronae may be due to the resolution limit of the Pioneer Venus data since analyses of higher resolution Arecibo and Goldstone imagery suggests that a number of corona-like features not identified in the PV data are present.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Geologic interpretation of the near-infrared images of the surface taken by the Venus Monitoring Camera,Venus Express

We analyze night-time near-infrared (NIR) thermal emission images of the Venus surface obtained with the 1-μm channel of the Venus Monitoring Camera onboard Venus Express. Comparison with the results of the Magellan radar survey and the model NIR images of the Beta-Phoebe region show that the night-time VMC images provide reliable information on spatial variations of the NIR surface emission. In this paper we consider if tessera terrain has the different NIR emissivity (and thus mineralogic composition) in comparison to the surrounding basaltic plains. This is done through the study of an area SW of Beta Regio where there is a massif of tessera terrain, Chimon-mana Tessera, surrounded by supposedly basaltic plains. Our analysis showed that 1-μm emissivity of tessera surface material is by 15–35% lower than that of relatively fresh supposedly basaltic lavas of plains and volcanic edifices. This is consistent with hypothesis that the tessera material is not basaltic, maybe felsic, that is in agreement with the results of analyses of VEX VIRTIS and Galileo NIMS data. If the felsic nature of venusian tesserae will be confirmed in further studies this may have important implications on geochemical environments in early history of Venus. We have found that the surface materials of plains in the study area are very variegated in their 1-μm emissivity, which probably reflects variability of degree of their chemical weathering. We have also found a possible decrease of the calculated emissivity at the top of Tuulikki Mons volcano which, if real, may be due to different (more felsic?) composition of volcanic products on the volcano summit.     

Sequential deformation of plains at the margins of Alpha Regio,Venus: Implications for tessera formation

Abstract— The boundaries between the highly deformed tessera terrain and adjacent volcanic plains are primarily those of embayment, where the tessera are stratigraphically older than the plains. Previous studies show that <3% of these boundaries display evidence of tectonic tilting after the emplacement of the plains. One of these unusual boundaries is the western margin of Alpha Regio tessera, a zone ～ 100 km in width that separates the plains from the interior structures of Alpha. This zone is characterized by margin parallel, fine‐scale (1–5 km) fractures, graben, and ridges that truncate and postdate the broad‐scale (10–30 km) ridges and troughs of the interior of Alpha. The western margin is embayed by several volcanic plains units that are progressively tilted and deformed by graben with closer proximity to Alpha Regio. The earliest deformation of the plains consists of northeast‐trending graben ～1 km in width that are similar in morphology and spacing to graben that deform intratessera plains and plains at the eastern boundary of Alpha. Northwest‐trending graben then formed over an interval marked by the emplacement of two additional plains units; their similarity to northwest‐trending structures emanating from Eve corona and the Lada Terra rift suggests a possible genetic relationship. The tilting of the plains adjacent to western Alpha implies relative vertical movement of the margin, either uplift of tessera or downwarping of plains subsequent to the formation and relaxation of the interior of Alpha Regio. Subsidence of plains at this locale is supported by the presence of a basin to the west of Alpha surrounded by a fracture belt contiguous with western Alpha. Thus, the fractures and deformation at the western boundary of Alpha may be related to the formation of a basin to the west of Alpha with some influence from the northernmost extension of the Lada Terra rift. Such a basin is not present at a section along the eastern boundary of Alpha Regio, where the origin of tilted plains remains equivocal. We conclude that the deformation along the western margin of Alpha Regio is not directly related to the process of tessera formation but is an example of tessera modification and is consistent with the stratigraphic position of tessera as the oldest unit observed on Venus.     

Small domes on Venus: Characteristics and origin

Approximately 22,000 small domes have been identified on the 25% of the surface of Venus imaged by Venera 15/16. The word dome is used to imply a broad, lens-shaped, positive topographic feature. The domes: (1) are generally circular in planimetric outline; (2) range in diameter from the effective limit of Venera resolution (2 km) to 20 km; (3) show flank slopes generally 10 ° and possibly 5 °; and (4) occur in association with mottled plains units. Associated features include summit pits, radar bright surfaces, and basal topographic platforms. There are two significant areas of major dome concentrations approximately 180 ° in longitude apart: (1) the largest concentration occurs in the Akkruva Colles area of Niobe Planitia, centered at approximately 45 ° N/120 ° E, just north of the flanks of the Thetis Regio rise; and (2) another concentration occurs in northwestern Guinevere Planitia, centered at approximately 35 ° N/300 ° E, on the north flank of the Beta Regio rise. In addition to these major areas of concentrations, domes occur in smaller concenrations throughout the imaged area of Venus, in association with coronae, arachnoids, intermediate sized hills interpreted to be volcanic constructs, large volcanic centers and calderas. The characteristics and geologic associations of small domes are consistent with an interpretation of their origin as volcanic, and on the basis of their low slopes, individual characteristics, and geologic associations they are interpreted to represent dominantly effusive low shield volcanoes. The large number of small domes implies a large number of multiple centralized eruptions, each one of which represents a discrete, relatively small, volume of material available to build an edifice over a finite time period. Calculated modal volume is 0.73 km³ for individual edifices. Based on the number identified by Venera, the total number of small domes estimated for the entire planet 4.4 × 10⁶ and total edifice volume over the entire planet represents a minimum volume equivalent to a layer approximately 7 m thick over the planet and representing 0.03% of the estimated crustal volume of Venus. In absolute number, size range, and distribution they appear to be similar to terrestrial oceanic seamounts. The global abundance and distribution, size frequency distribution, minimum size, and changes in these characteristics with latitude for the domes will be particularly important in understanding the way in which the domes form and their relationship to global models of tectonism and heat flow on Venus. Increased spatial resolution and coverage from Magellan data will enable a more thorough assessment of these features and associated questions, particularly where radar incidence angles are 15 °.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Diffuse scattering of radar on the surface of Venus: Origin and implications for the distribution of soils

Previous analysis of PV altimeter data has shown that ~25% of the surface of Venus is characterized by low values of reflectivity, interpreted as being due to the presence of porous materials such as soils. However, examination of a corrected reflectivity data set in combination with PV altimeter data suggests that no more than 5% of the surface of Venus is covered by soils more than several to tens of cm in depth. Most regions of apparent low reflectivity are instead interpreted to be due to the presence of small (5–50 cm) roughness elements on the surface that cause diffuse scattering at the 17 cm PV wavelength. Regions of low apparent reflectivity are of interest because of a correlation with tessera, a complex tectonic unit mapped from Venera 15/16 SAR data. Regions of tessera are characterized by a complex system of intersecting ridges and valleys thought to be of tectonic origin. Examination of possible models for the form of diffuse scatterers in the tessera suggests that they are rock fragments and originate from a mass-wasting process that is linked to the rugged nature of the terrain. Further, these diffuse scatterers are associated with other tectonic landforms, suggesting that they originate as part of tectonic deformation of the surface. Viewed from a geologic standpoint, the PV data sets are important tools for understanding tectonic, volcanic, and degradational processes on Venus, as well as for future interpretation of data from the Magellan mission.     

The frequency-area distribution of volcanic units on Venus: Implications for planetary resurfacing

The areas of volcanic units on Venus have been measured on the 1:5000000 geological maps published by NASA/USGS. These data were used to obtain a frequency-area distribution. The cumulative frequency-area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit (30 × 10⁶ km²) to the smallest (20 km²). The probability distribution function has been calculated. The medium and large volcanic units correlate well with a power-law (fractal) relation for the dependence of frequency on area with a slope of −1.83. There are fewer small units than the expected values provided by the power-law relation. Our measurements cover 21.02% of the planetary surface, 3.59% of the study area was found to be tessera terrain and is excluded from this study of volcanism. The measurements were restricted to areas where geological maps have been published. The analysis was performed on two independent areas of the planet, with a complete coverage of published maps. In both areas the largest volcanic unit covers a significant portion of the surface (58.75% and 63.64%, respectively). For the total measured volcanic units (excluding tessera), these two largest units (that could correspond to the same unit or not) cover the 61.18% and they are stratigraphically superimposed on older volcanic units which cover 3.37% of the area. The remaining area (35.45%) is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit(s). These results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of Venus and different criteria for geological mapping. Despite this fact, the presence of these very large units is incompatible with the equilibrium resurfacing models, because their generation at different ages would destroy the crater randomness. Our frequency-area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit(s) followed by a decay of volcanism. Our data for the frequency-area distribution of volcanic units provide new support for catastrophic resurfacing models. It is difficult to make our observations compatible with equilibrium, steady-state resurfacing models.     

Tectonics of Venus: A review

The article presents a new tectonic scheme of Venus and gives the following interpretation of the planet's main structural units: (1) plains — areas of flood volcanism over stretched crust; (2) dome-like uplifts — areas of uplifting and volcanic activity above the mantle hot-spots; (3) coronae —former dome-like uplifts, partially subsided and diffused by gravity; (4) ridge belts — fold zones; (5) tesserae — fragments of ductile compression and shortening of crust; (6) supercoronae — coronae formed in the course of further evolution and relaxation of Beta-type uplifts. Ishtar Terra is considered to be a fragment of an ancient tessera paleocontinent, on the edge of which the Lakshmi supercorona is superimposed. Aphrodite Terra is considered as a belt of mantle hot-spot structures (dome-like uplifts, coronae, supercoronae, volcanoes, rifts).Three types of planetary belts have been distinguished on Venus: uplifted 'weakened' belts with an abundance of mantle hot-spot structures; a northern fan of ridge belts; and belts of low basalt plains. The center of the planetary system of uplifted weakened belts is situated in Atla Regio.The present tectonic structure of Venus is inferred to have formed during two stages of evolution characterized by different tectonic regimes. Stage I is a regime of soft ductile plates (formation of tessera uplifts and volcanic plains). Stage II is a formation of 'weakened' uplifted planetary belts, various tectonic regimes of mantle hot-spots, and plains-forming volcanism.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Tectonic and kinematic study of a strike-slip zone along the southern margin of Central Ovda Regio, Venus: Geodynamical implications for crustal plateaux formation and evolution

The tectonic system of the southern margin of Central Ovda Regio, a crustal plateau which straddles Venus equator, has been interpreted as a dextral strike-slip array, on the basis of evidence clearly identifiable, as are Riedel fracture patterns of different scales, en échelon folds and brittle strike-slip faults. This transcurrent regime developed two main shear belts (Inner and Outer, on respectively thicker and thinner crust), whose minimum dextral displacement has been estimated in 30-50 km. Since the up or downwelling models for plateau formation cannot easily explain tectonic shears of this magnitude along their margins, an alternative hypothesis has been built, which stands on the proposed collisional belt which could form Ovda northern border (King et al., 1998, Lunar Planet. Sci. Conf. 29, Abstract 1209; Tuckwell and Ghail, 2002, Lunar Planet. Sci. Conf. 33, Abstract 1566). Within this framework, the shear would represent a transcollisional transcurrent zone, similar to the strike-slip zones produced in the foreland of the Himalayas-Tibet collision front. Eastern Ovda would be an independent area of thickened crust, pushed to the SSE by the northern collision, with the deformation concentrated at its margins, and experiencing a shear strain on its southern margin. None of the data, however, either supports nor helps to discard theoretical subduction events as a cause of the collision. On the contrary, image relationships could be interpreted as evidence that the main shear deformation took place during the last global resurfacing event on the planet.     

Morphology of the Tessera Terrain on Venus: Implications for the Composition of Tessera Material

The main goal of this paper is to estimate the possible composition of the tessera material on the basis of an interpretation of the morphology of the tessera precursor terrain. The results of detailed photogeologic analysis of tessera are presented. For the study, 56 randomly chosen areas that characterize the surface of large and small tessera massifs were selected. Each area represents a portion of the F-MAP photomosaics acquired at a 75 m/px resolution. The results of this study show that the tessera precursor terrain appears everywhere as plains. In its morphology, these plains are similar to the plains outside the tessera massifs. An overview of all possible mechanisms of the formation of plains on Venus and comparison of these mechanisms with the data of the chemical measurements on the surface of Venus suggests that the Venusian plains were formed as a result of the emplacement of low-viscous basaltic lava. This rather well-known conclusion is made here for the first time in order to estimate the possible composition of the tessera material. Thus, it is likely that the composition of the tessera precursor plains is similar to the composition of the basaltic plains on Venus. The products of posttessera volcanism in the form of morphologically smooth plains commonly occur within the tessera terrains. Morphologically, these plains are similar to the regional Venusian plains, which strongly suggests a basaltic composition of such plains. There are only two volcanic flows within the whole tessera terrain on Venus whose morphology permits one to interpret them as a manifestation of nonbasaltic, more siliceous volcanism. This means that the material of the regional tessera-bearing highlands very rarely responded to the thermal influence from below by siliceous volcanism. If some hypothetical granitelike material makes up the main portion of the tessera highlands, this material remains hidden. Therefore, the hypothesis of the granitelike bulk composition of the tessera highlands has little support from observations. At the current stage of the study of Venus, a model in which tessera highlands are composed predominantly of basalt with a possible, but insignificant component of more siliceous material is thought to be correct.     

Global stratigraphy of Venus: analysis of a random

The age relations between 36 impact craters with dark paraboloids and other geologic units and structures at these localities have been studied through photogeologic analysis of Magellan SAR images of the surface of Venus. Geologic settings in all 36 sites, about 1000 × 1000 km each, could be characterized using only 10 different terrain units and six types of structures. These units and structures form a major stratigraphic and geologic sequence (from oldest to youngest): 1) tessera terrain; 2) densely fractured terrains associated with coronae and in the form of remnants among plains; 3) fractured and ridged plains and ridge belts; 4) plains with wrinkle ridges; 5) ridges associated with coronae annulae and ridges of arachnoid annulae which are contemporary with wrinkle ridges of the ridged plains; 6) smooth and lobate plains; 7) fractures of coronae annulae, and fractures not related to coronae annulae, which disrupt ridged and smooth plains; 8) rift-associated fractures; 9) craters with associated dark paraboloids, which represent the youngest 10% of the Venus impact crater population (Campbellet al., 1992), and are on top of all volcanic and tectonic units except the youngest episodes of rift-associated fracturing and volcanism; surficial streaks and patches are approximately contemporary with dark-paraboloid craters.Mapping of such units and structures in 36 randomly distributed large regions (each 10⁶ km²) shows evidence for a distinctive regional and global stratigraphic and geologic sequence. On the basis of this sequence we have developed a model that illustrates several major themes in the history of Venus. Most of the history of Venus (that of its first 80% or so) is not preserved in the surface geomorphological record. The major deformation associated with tessera formation in the period sometime between 0.5–1.0 b.y. ago (Ivanov and Basilevsky, 1993) is the earliest event detected. In the terminal stages of tessera formation, extensive parallel linear graben swarms representing a change in the style of deformation from shortening to extension were formed on the tessera and on some volcanic plains that were emplaced just after (and perhaps also during the latter stages of the major compressional phase of tessera emplacement. Our stratigraphic analyses suggest that following tessera formation, extensive volcanic flooding resurfaced at least 85% of the planet in the form of the presently-ridged and fractured plains. Several lines of evidence favor a high flux in the post-tessera period but we have no independent evidence for the absolute duration of ridged plains emplacement. During this time, the net state of stress in the lithosphere apparently changed from extensional to compressional, first in the form of extensive ridge belt development, followed by the formation of extensive wrinkle ridges on the flow units. Subsequently, there occurred local emplacement of smooth and lobate plains units which are presently essentially undeformed. The major events in the latest 10% of the presently preserved history of Venus (less than 50 m.y. ago) are continued rifting and some associated volcanism, and the redistribution of eolian material largely derived from impact crater deposits.Detailed geologic mapping and stratigraphic synthesis are necessary to test this sequence and to address many of the outstanding problems raised by this analysis. For example, we are uncertain whether this stratigraphic sequence corresponds to geologic events which were generally synchronous in all the sites and all around the planet, or whether the sequence is simply a typical sequence of events which occurred in different places at different times. In addition, it is currently unknown whether the present state represents a normal consequence of the general thermal evolution of Venus (and is thus representative of the level of geological activity predicted for the future), or if Venus, has been characterized by a sequence of periodic global changes in the composition and thermal state of its crust and upper mantle (in which case, Venus could in the future return to levels of deformation and resurfacing typical of the period of tessera formation).     

Gravity studies of Aphrodite Terra on Venus

Aphrodite Terra is the largest highland area on Venus of the size of Africa. It is traversed by the Aphrodite-Beta belt of troughs with a length of 21 000 km. There are two other large belts of troughs on Venus: Themis-Atla, 14 000 km long, and Beta-Phoebe, 8000 km long. In this paper, four gravity profiles across Aphrodite Terra are studied and compared with the morphology.Western Aphrodite and Niobe Planitia to the north seem to be in isostatic equilibrium under the assumption of Airy compensation with a mean crustal thickness of T = 50 km. The graben area in the middle part of Aphrodite Terra shows negative isostatic gravity anomalies indicating deficit masses. The adjacent Atla Regio to the east is regionally Airy compensated with T = 50 km, and the mountains Nokomis, Maat and Ozza Montes are locally undercompensated, i.e. they are associated with surplus masses in the depth. Ulfrun Regio, a hilly terrain just east of Atla Regio is Airy compensated with T = 30 km. These results give a mean crustal thickness around 50 km for Aphrodite Terra. The isostatic disturbed zones in the middle of Aphrodite (grabens) and Atla Regio as well as the undercompensated Beta Regio have been associated with recent volcanism from the observation of the concentrations of electrical discharges in these areas. Atla and Beta Regiones are both located at intersections of the systems of troughs described above.Contribution No. 308, Institut für Geophysik der Universität Kiel, F.R.G.     

Morphologic and gravimetric investigations of Bell and Eisila Regiones on Venus

Bell Regio is a highland fragment south of Ishtar Terra, extending 1300 km in N-S direction and 900 km in E-W direction. South of this region Eisila Regio is located with an E-W extension of 8000 km and a width of 2000 km. Bell Regio consists of two large massifs: a northern massif with maximum altitudes of 2.5 to 3.0 km above the 6051 km datum and with a semi-corona (other coronae on Venus are associated with volcanic-tectonic processes) and a southern massif with a maximum of 4 to 4.5 km above the datum. The possible shield volcano Tepev Mons of 250 km in diameter is superimposed on the southern massif. It shows a radar dark crater of 40 km diameter on its eastern flank, a crater-like feature of 15 km diameter on the top and a radar bright area extending from the dark crater across the summit. South of Tepev Mons are several volcanic structures with summit depressions. The crest of Bell Regio exhibits a N-S extending fossa system. The whole fresh appearing plain-like area has been classified as rather young compared to other units. Gravity data show a maximum of 33 mGal at Bell Regio and 35 mGal at eastern Eisila Regio. The basins north and south of the highland fragments are associated with gravity lows.Density models have been calculated along the gravity profile Rev. 163 of Pioneer Venus Orbiter across Bell and Eisila Regiones assuming Airy isostatic compensation of the topography and considering several boundary conditions (e.g. mean crustal thickness T<- 100 km). There are two groups of density models in the case of Airy compensation. In the first group global total compensation is assumed along the profile and regional partial compensation for Bell and Eisila Regiones. This solution gives a range of possible models with 10 km <- T <- 100 km and a partial compensation for Bell and Eisila Regiones between 12% and 55%. Thus these two highland fragments show subsurface surplus masses.The second group of models considers for the whole profile total compensation with a global T <- 100 km and a regional very large depth of compensation for Bell and Eisila Regiones, i.e. T > 100 km.The highland of Beta Regio has, like Bell Regio, a N-S rifting system, volcanic structures, a fresh appearing plain-like surface and either deep-seating compensating masses or near surface surplus masses. Bell can be considered as little sister of Beta. The geological and geophysical results imply a volcanic-tectonic uplift over a hot spot. The conditions of Atla Regio in eastern Aphrodite Terra are similar. Thus the existence of volcanic-tectonic uplifts support the important role of hot spot volcanism on Venus.Contribution Nr. 343, Institut für Geophysik der Universität Kiel, F.R.G.     

Beta Regio, Venus: Evidence for uplift, rifting, and volcanism due to a mantle plume

Geophysical data have led to the interpretation that Beta Regio, a 2000×25000 km wide topographic rise with associated rifting and volcanism, formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume. We have tested this hypothesis through detailed geologic mapping of the V-17 quadrangle, which includes a significant part of the Beta Regio rise, and reconnaissance mapping of the remaining parts of this region. Our analysis documents signatures of an early stage of uplift in the formation of the Agrona Linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging. We see evidence that the Theia rift-associated volcanism occurred during the first part of post-regional-plains time and cannot exclude that it continued into later time. We also see evidence that Devana Chasma rifting was active during the first and the second parts of post-regional-plains time. These data are consistent with uplift, rifting and volcanism associated with a mantle diapir. Geophysical modeling shows that diapiric upwelling may continue at the present time. Together these data suggest that the duration of mantle diapir activity was as long as several hundred million years. The regional plains north of Beta rise and the area east and west of it were little affected by the Beta-forming plume, but the broader area (at least 4000 km across), whose center-northern part includes Beta Regio, could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain.     

Characteristic features in Venus’ nightside cloud-top temperature obtained by Akatsuki/LIR

Mid-infrared images of almost the entire Venus nightside hemisphere obtained by the Longwave Infrared Camera (LIR) onboard Akatsuki on December 9 and 10, 2010 reveal that the brightness temperature of the cloud-top ranges from 237 K in the cold polar collars to 243 K in the equatorial region, significantly higher than the values obtained by Venera 15. Other characteristic features of the temperature distributions observed are zonal belt structures seen in the middle and low latitudes and patchy temperature structures or quasi-periodic streaks extending in a north–south direction in the northern middle latitudes and southern low latitudes.     

Geology of the southern Ishtar Terra/Guinevere and Sedna Planitae region on Venus

Recent high resolution, high incidence angle Arecibo radar images of southern Ishtar Terra and flanking plains of Guinevere and Sedna on Venus reveal details of topographic features resolved by Pioneer Venus. The high incidence angles of Arecibo images favor the detection of surface roughness-related features, and complement recently obtained low incidence angle Venera 15/16 images in which changes in surface topographic slope are well portrayed. Four provinces have been defined on the basis of radar characteristics in Arecibo images and topography. Volcanism and tectonism are the dominant processes in the mapped area, which has an average age of about 0.5–1.0 billion years (Ivanov et al., 1986). These processes vary in relative significance in the mapped provinces and it is likely that geologic activity has occurred simultaneously in all four provinces. On the basis of stratigraphic evidence, however, a general sequence is proposed which represents the major activity in each area. The low predominantly volcanic plains of Guinevere and Sedna Planitiae are the relatively oldest terrain. A major region of complex tectonic deformation, the Southern Ishtar Transition Zone, postdates much of the low plains and delineates the steep-sloped flanks of Ishtar Terra. Lakshmi Planum is characterized by a distinctive volcanic style (large low edifices, calderas, flanking plains) and at least in part postdates the Southern Ishtar Transition Zone. Relatively recent plains-style volcanism occurs locally in Sedna Planitia and embays the Southern Ishtar Transition Zone. Compressional deformation appears to dominate the mountains of the Ishtar plateau, but the nature of the tectonic deformation in the Southern Ishtar Transition Zone is very complex and likely represents a combination of extension, compression and strikeslip deformation. Arecibo data reveal additional coronae in the lowlands, suggesting that corona formation is an even more widespread process than indicated by the Venera data.     

Radiating graben-fissure systems in the Ulfrun Regio area, Venus

Radiating graben-fissure systems are common on Venus. Most are thought to be underlain by mafic dykes, fed by centrally-located magmatic centres. From previous work it has been shown that these magmatic plumbing systems can extend out up to 2000 km or more and that interaction between neighbouring systems can provide insight into the relative chronology of their magmatic centres. Systematic mapping of graben-fissure systems has potential as a tool for regional magmatic chronology and correlation on Venus.This methodology is applied to the Ulfrun Regio area (200-240°E, 0-25°N) where we mapped 47,000 graben and fissures. From these, 66 radiating systems comprised of 13,000 individual graben and fissures, and having radii of up to 2000 km have been identified, and are interpreted to be underlain by dyke swarms focussed on magmatic centres. Cross-cutting relationships among these systems and with the Hecate Chasma rift zone have been examined to provide a relative chronology for the magmatic centres. Two trends emerged: (a) an apparent younging from the southwest to northeast of the study area and (b) a cluster of older ages in the southwest, linked to the Atla Regio mantle plume.     

Seasonal redistribution of water in the surficial Martian regolith: Results from the <Emphasis Type="Italic">Mars Odyssey</Emphasis> high-energy neutron detector (HEND)

The seasonal variation of neutron emissions from Mars in different spectral intervals measured by the HEND neutron detector for the entire Martian year are analyzed. Based on these data, the spatial variations of the neutron emissions from the planet are globally mapped as a function of season, and the dynamics of seasonal variation of neutron fluxes with different energies is analyzed in detail. No differences were found between seasonal regimes of neutron fluxes in different energy ranges in the southern hemisphere of Mars, while the regime of fast neutrons (with higher energies) during the northern winter strongly differs from that during the southern winter. In winter (L _s = 270°–330°), the fast neutron fluxes are noticeably reduced in the northern hemisphere (along with the consecutive thickening of the seasonal cap of solid carbon dioxide). This provides evidence of a temporary increase in the water content in the effective layer of neutron generation. According to the obtained estimates, the observed reduction of the flux of fast neutrons in the effective layer corresponds to an increase in the water abundance of up to 5% in the seasonal polar cap (70°–90°N), about 3% at mid-latitudes, and from 1.5 to 2% at low latitudes. The freezing out of atmospheric water at the planetary surface (at middle and high latitudes) and the hydration of salt minerals composing the Martian soil are considered as the main processes responsible for the temporary increase in the water content in the soil and upper layer of the seasonal polar cap. The meridional atmospheric transport of water vapor from the summer southern to the winter northern hemisphere within the Hadley circulation cell is a basic process that delivers water to the subsurface soil layer and ensures the observed scale of the seasonal increase in water abundance. In the summer northern hemisphere, the similar Hadley circulation cell transports mainly dry air masses to the winter southern hemisphere. The point is that the water vapor becomes saturated at lower heights during aphelion, and the bulk of the atmospheric water mass is captured in the near-equatorial cloudy belt and, thus, is only weakly transferred to the southern hemisphere. This phenomenon, known as the Clancy effect, was suggested by Clancy et al. (1996) as a basic mechanism for the explanation of the interhemispheric asymmetry of water storage in permanent polar caps. The asymmetry of seasonal meridional circulation of the Martian atmosphere seems to be another factor determining the asymmetry of the seasonal water redistribution in the “atmosphere-regolith-seasonal polar caps” system, found in the peculiarities of the seasonal regime of the neutron emission of Mars.     

Divergent plate boundary characteristics and crustal spreading in Aphrodite Terra,Venus: A test of some predictions

It has been proposed that divergence and crustal spreading occur in Western Aphrodite Terra and some adjacent equatorial regions of Venus at rates in the range of a few centimeters per year. If equatorial spreading is common and widespread, then a consequence of this should be: (1) a young average age of the surface of the planet, (2) a trend in age from older terrain in the polar regions to younger terrain toward the equator, and (3) a latitudinal distribution of extensional features in equatorial regions and compressional deformation features in middle to high latitudes. These predictions are tested using published results from Arecibo, VENERA 15/16, and Pioneer Venus data, and it is found that: (1) the northern mid-to-high latitudes are characterized by a young average age, (2) there is a trend in the total number of craters per unit area from high values in the north polar regions to low values toward the equator, and (3) there is evidence for a latitudinal distribution of tectonic features of different types, with extensional features common in equatorial regions and compressional deformation features common in the northern middle to high latitudes. Further tests of these and other predictions can be made using data from the upcoming Magellan mission.