Atla Regio Research Articles

Bell Regio, Venus: Integration of remote sensing data and terrestrial analogs for geologic analysis

The geology and surface morphology of Bell Regio (18–42°N, 32–58°E) are investigated using a combination of Magellan, Venera, and analogous terrestrial data. The properties of surface units are compared to either direct terrestrial analog measurements or to the behaviors predicted by theoretical models. Five major volcanic sources are identified from geologic mapping (Tepev Mons, Nefertiti corona, a large shield volcano east of Tepev, and two small edifices southeast of Tepev). The volcano Api Mons lies northeast of the main Bell uplift. The oldest volcanic units are associated with an extensive low shield volcano east of Tepev Mons and a small edifice southeast of Tepev. The annular flow apron of Tepev Mons formed next, with volcanism at a second small edifice on the southeast flank of Tepev Mons producing the youngest flow units. Comparisons between Magellan data, terrrestrial radar images, and field topography profiles suggest that only three units resemble terrestrial a'a flows; the remainder are consistent with smoother pahoehoe‐type surfaces. This suggests that most of the flow units were erupted at relatively low volume effusion rates (<100 m3/s) over long periods of time or had very low viscosities. One flow unit has a knobby texture which resembles large tumuli or ridge structures. Much of the tectonic deformation in the area is in the form of fractures circumferential to the edifices and isolated blocks of tessera terrain; there are no rift zones such as those which occur at Beta Regio, Atla Regio, or Western Eistla Regio. Tepev Mons is characterized by very steep slopes (up to 40° along the east flank), a relatively flat summit, and two large (11 and 31 km) calderas. Though covered in places by high‐dielectric material, the Tepev Mons summit area is relatively smooth with the exception of annular rough deposits which ring the two calderas. These calderas are quite shallow (on the order of a few hundred meters at most), with no discernible difference in radius between individual altimeter footprints inside and outside the larger eastern feature. Magellan and Venera data suggest that the eastern caldera is covered by a layer of unconsolidated material. Ejecta from Miriam crater has produced a halo with a dielectric constant of 6–7 surrounding Nefertiti corona, and Potanina crater ejecta is the likely source material for radar‐dark wind streaks south of Tepev Mons. A low‐dielectric (ε = 2–3) triangular region on the south flank of Tepev Mons may be a pyroclastic or crater ejecta deposit which has been spread westward by the wind. This region has undergone a wide variety of volcanic processes, with a major shift in eruptive style from the older low‐relief eastern volcanic center to the steep slopes and large summit calderas characteristic of Tepev Mons and the two smaller edifices.

Age of rifting and associated volcanism in Atla Regio, Venus

An estimation of the age of rifting and associated volcanism in Atla Regio is carried out based on photogeologic analyses of the stratigraphic relationships between rift‐associated features and impact craters which have associated radar‐dark paraboloids. The paraboloids are believed to be thin mantles of debris derived from the fallout of crater ejecta, and their “parent” craters are considered to be among the youngest 10% of Venus' crater population [Campbell et al., 1992]. In the area under study, there are three impact craters with associated dark paraboloids: 1) Von Schuurman, which is clearly superimposed on the Dali Chasma Rift; 2) Sitwell, which is superimposed on Ganis Chasma and may have been followed by subsequent rift‐associated fracturing; and 3) Luxemburg, which is superimposed on lavas from Ozza Mons and dissected by Dali Chasma fractures which, in turn, are covered by lavas from Maat Mons. This means that at least the last stages of tectonic and volcanic rift‐associated activity in Atla Regio are contemporaneous with the formation of impact craters which are young enough to preserve the associated dark paraboloids. If these craters represent the youngest 10% of the Venus crater population, and the average age of the venusian surface is about 500 m.y. [Schaber et al., 1992], this means that rifting and rift‐associated volcanism in Atla Regio were active as recently as during the last ∼50 m.y. In the current debate over models of Venus resurfacing the results of our study favor the equilibrium resurfacing model [Phillips et al., 1992].

Impact craters and Venus resurfacing history

Venusian impact crater size‐frequency distributions, locations, and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism. An atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km, the size‐frequency distribution is close to the atmosphere‐free case. With this result, and assuming that the surface records a crater production population (a catastrophic resurfacing model, CRM), an age of cessation of rapid resurfacing of ∼ 500 Ma is obtained. Crater locations are widely dispersed across Venus and the hypothesis that they are completely spatially random (CSR) cannot be rejected. However, craters that show embayment by plains materials or modification by throughgoing faults (i.e., tectonized) are preferentially found in areas with relatively few craters overall. The primary region where these modified craters are found is the Aphrodite volcanotectonic zone, extending from Ovda Regio on the west to the region east of Atla Regio. These results, together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy, imply that a range of surface ages are recorded by the impact crater population; e.g., the Aphrodite zone is relatively young. An end‐member model (equilibrium resurfacing model, ERM) was developed to quantify resurfacing scenarios. In the ERM, Venus has been resurfacing at an average rate of approximately 1 km2 yr−1. However, the CRM and ERM are idealized end‐member representations of possible resurfacing histories. For both models, the resurfacing rate can be expressed as the product of resurfacing patch area a (normalized by planetary surface area) and the frequency ω of resurfacing events. Numerical simulations of resurfacing showed that there are two solution branches that satisfy the CSR constraint: a < 0.0003 (4° diameter circle ) and a > 0.1 (74° diameter circle). The former range corresponds to resurfacing diameters smaller than the average intercrater distance, whereas the latter is associated with large, infrequent events, resurfacing 10% of the planet every 50 Ma to 100% every 500 Ma. The observed fraction of embayed and tectonized craters further constrains values of a and only values near 0.0003 are admissible. The resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs, in any given geological episode, in only a limited number of regions on the planet.

Regional topographic rises on Venus: Geology of Western Eistla Regio and comparison to Beta Regio and Atla Regio

Regional topographic rises in the equatorial region of Venus (Western Eistla Regio, Beta Regio, and Atla Regio) form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes. The geologically least complex of these regions is Western Eistla Regio, an elongated domical rise that contains the volcanoes Sif and Gula montes. Sif Mons is a large volcano (diameter > 100 km) whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing, widespread outpouring of lava, additional fracturing along its northern flank, and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion. Sif contains a 40‐km‐diameter summit caldera within which are nested smaller calderas and pit structures. Theoretical considerations suggest multiple levels of magma emplacement within the volcano, with the primary reservoir residing in the crust below the edifice. Gula Mons is made up of multiple volcanic source regions and is associated with Guor Linea, a zone of lithospheric stretching, faulting, and rifting. The geologically complex highland of Beta Regio is a tectonic junction with three arms of Devana Chasma intersecting at Theia Mons. Magellan data reveal (1) the details of rifting and volcanism concentrated at Theia Mons, (2) that Rhea Mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question, (3) that tesserae are a major unit, arrayed in a pattern around Theia from east to west starting on the eastern flank of Beta and continuing westward to the northern part of Asteria Regio, (4) that the northern periphery of the highland contains coronae connected by fractures forming chains, and (5) the presence of a deformed crater within Devana Chasma, split and extended in an east‐west direction, indicating that part of the rift has undergone as much as 20–30% extension. Like Beta, Atla Regio is a complex area of converging rifts (five separate arms) that are centered on the volcano Ozza Mons. The Magellan images show that volcanism and rifting have occurred contemporaneously at Ozza, with lava flows both covering and being crosscut by faults. A second volcano, Maat Mons, contributes to the infilling of the rift Dali Chasma and does not appear to be deformed by faulting. Two of the rifts at Atla contain chains of coronae, suggesting that upwelling along linear zones of extension has occurred. Beta, Atla, and Western Eistla are similar in that they are broad regional rises, are associated with major rift zones, have large apparent depths of isostatic compensation, and contain large volcanoes. Their associations with features mapped as coronae and tesserae are quite variable, suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment.

Coldspots and hotspots: Global tectonics and mantle dynamics of Venus

Magellan radar images and altimetry data and Pioneer Venus gravity data of major highlands and lowlands on Venus are examined with the objective of relating these tectonic and volcanic landforms to convection within Venus' mantle. Two roughly circular lowlands, Atalanta and Lavinia planitiae, are bowl‐shaped depressions which contain compressional tectonic features and lack hotspot‐related features such as coronae and shield volcanoes. They are identified as surface expressions of young mantle coldspots or regions of mantle downwelling. Highlands can be divided into two distinct groups: volcanic rises and plateau‐shaped highlands. Volcanic rises are domical, circular to elongate regions characterized by volcanic construction; extensional tectonism; large, positive gravity and geoid anomalies centered on the highlands; and large apparent depths of compensation. Volcanic rises include Beta, Bell, Atla, and Western Eistla regiones and are identified as surface expressions of large mantle hotspots or plumes. Plateau‐shaped highlands are steep‐sided elevated regions that range from circular (Alpha Regio) to elongate or irregular (Ovda Regio, Tellus Regio) in shape. Their surfaces are dominated by complex ridged terrain, rather than the shield volcanoes and flows that dominate volcanic rises. Most plateau‐shaped highlands contain compressional structures, some of which commonly lie along their margins, and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises. Plateau‐shaped highlands, which include those mentioned above and Western Ishtar Terra, Thetis Regio, and Phoebe Regio, are identified as surface expressions of mantle coldspots or regions of downwelling. The highland coldspot features are elevated as a consequence of crustal thickening; crustal thickening is absent or minor at lowland coldspots. The importance of mantle downwelling in the tectonic deformation of Venus suggests that its mantle, like the mantle of Earth, is strongly heated from within. The major Venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet's core.

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. 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.

Lightning on Venus

On the night side of Venus, the plasma wave instrument on the Pioneer-Venus Orbiter frequently detects strong and impulsive low-frequency noise bursts when the local magnetic field is strong and steady and when the field is oriented to point down to the ionosphere. The signals have characteristics of lightning whistlers and we have tried to identify the sources by tracing rays along the B-field from the Orbiter down toward the surface. An extensive data set strongly indicates a clustering of lightning sources near the Beta and Phoebe Regios, with additional significant clustering near the Atla Regio at the eastern edge of Aphrodite Terra. These results suggest that there are localized lightning sources at or near the planetary surface.

Lightning measurements from the Pioneer Venus Orbiter

The plasma wave instrument on the Pioneer Venus Orbiter frequently detects strong and impulsive low‐frequency signals when the spacecraft traverses the nightside ionosphere near periapsis. These particular noise bursts appear only when the local magnetic field is strong and steady and when the field is oriented to point down to the ionosphere thus; the signals have all characteristics of lightning whistlers. We have tried to identify lightning sources between the cloud layers and the planet itself by tracing rays along the B‐field from the Orbiter down toward the surface. An extensive data set, consisting of measurements through Orbit 1185, strongly indicates a clustering of lightning sources near the Beta and Phoebe Regios, with an additional significant cluster near the Atla Regio at the eastern edge of Aphrodite Terra. These results suggest that there are localized lightning sources at or near the planetary surface.