Mikhail A. Ivanov

Tessera terrain on Venus: A survey of the global distribution, characteristics, and relation to surrounding units from Magellan data

The tessera terrain on Venus, comprised of areas of high radar backscatter, complex deformation patterns relative to other units, and topography standing higher than surrounding plains, covers ∼35.33 × 106 km2, about 8% of the surface of Venus, and is nonrandomly distributed, being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30°S. Individual tessera occurrences range in area from the lower limits of our measurements (about 200 km2) up to the largest tessera, Ovda, with an area of about 8.6 × 106 km2, or about 2% of the surface area of Venus. The size-frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes, reflecting the great abundance of small tessera fragments. Modes of occurrence include (1) large clusters (e.g., Aphrodite Terra and Ishtar Terra); (2) arc-like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it; (3) areas where tesserae are rare or absent which occur both as low-lying plains (e.g., Guinevere Planitia), and as elevated regions (e.g., Atla Regio). Tessera terrain has a bimodal elevation-frequency distribution, with the main peak at about 0–1 km and an additional peak at about 3 km above mean planetary radius. In terms of number of occurrences, however, tesserae do not display a correlation with elevation at the global scale, since small tessera patches commonly occupy low-lying regions. Although tessera exhibit a range of gravity signatures, many occurrences are interpreted to represent relatively shallow (crustal) levels of compensation. Tessera boundaries include Type I (sinuous/embayed, dominated by adjacent lava plains embaying tessera massifs; 73% of total tesserae boundaries) and Type II (linear/tectonic). Only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation. Occurrence of broad slopes of post-tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places. Several lines of evidence suggest the possibility that a widespread tessera-like basement, comprising at least 55% of the surface of Venus, is buried under a cover of lava plains a few hundred meters to as much as 2–4 km thick. A wide variety of deformational structures and patterns is observed within the tessera including those representing extension, compression, shear, and transpression; in some cases the apparently complex patterns can be resolved into single-event kinematic interpretations involving noncoaxial deformation (e.g., Itzpapalotl), while in other cases, polyphase deformation is more likely (e.g., central Ovda and Thetis). Where relations can be determined stratigraphically, earliest deformation within the tessera is primarily related to crustal shortening and compression (Phase I), followed by pervasive extensional deformation commonly oriented normal to the strike of Phase I features, generally along the same principal stress direction (Phase II). Evidence also exists for the contemporaneous formation of these distinctive deformation patterns. Lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with Phase II extensional deformation and with regional tilting. Tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed, an observation consistent with many other mapping studies. We see no evidence for transitional stages between tessera and volcanic rises and/or lowlands, that might represent a long-term sequence of upwelling or downwelling followed by crustal deformation and tessera formation. No impact craters deformed by Phase I deformation have yet been observed on tessera, suggesting that Phase I tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before ∼300–500 m.y. ago; however, the starting time, and thus duration of tessera formation, is unknown. On the basis of the very small number of on-tessera craters deformed by Phase II extensional deformation, this period probably did not last more than several tens of millions of years after the cessation of Phase I. Little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years, during which time the vast volcanic plains were emplaced, although tilting of early plains along some tessera margins is observed. Building on the global synthesis presented here, future analyses of individual tessera occurrences will provide the detailed descriptions, kinematic interpretations, and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation.

Style and sequence of extensional structures in tessera terrain, Venus

Recent studies have focused on the question of the stratigraphic sequence and thus the stages of tessera formation, specifically, if tessera are formed by contractional deformation followed by extensional deformation or vice versa. A major question centers on the interpretation of specific lineaments within tesserae as graben (bounded by faults ∼60°) or, alternatively, open tension fractures (dipping ∼90°). We document and assess the origin of extensional structures in tesserae at several locations on Venus, noting the morphology, continuity along strike, parallelism of walls, stratigraphic position and interaction with other structures, and variability due to radar viewing geometry. In each study area, our analyses demonstrate that (1) the extensional structures have variable widths, interior subparallel lineaments, and ramp terminations; (2) ridges and lineaments are continuous across the troughs, where the floors of many of these structures contain the lowered sections of preexisting structures; and (3) intratessera plains are seen to embay ridges and an impact crater is superposed on a ridge and in both cases these features are subsequently deformed by the extensional structures. We conclude that the morphology of these extensional structures is consistent with an origin as graben, not open tension fractures, and that these graben postdate the ridges in each study area. Both the graben and the ridges of the sizes found in our survey can be formed when the brittle crust is of the order of 1 to 10 km thick. To further test the tension fracture model, we examine the conditions of a Venus that could produce tension fractures of the dimension (∼1 km width) of extensional structures found in tessera terrain and find that thermal gradients of a minimum of 400 to 1500 K km−1 (heat flows of 800 to 3000 mW m−2) are required for a range of diabase rheologies and strain rates thought typical of Venus during tessera formation. Such a thermal structure would favor partial melting at depths <1 km. Dike propagation from this region of shallow melting within the tensile stress field would produce vast quantities of volcanism, mitigating against the preservation of the closely associated tension fractures; this volcanism is not observed. Both the amplitude and sign of changes in surface temperature induced by atmospheric warming due to massive outpourings of lava are not consistent with the hotspot model. On the basis of our analysis of tesserae, we conclude that the ridges formed first in response to large-scale contraction of the crust and that the graben formed contemporaneously and largely following this phase as the thickened crust relaxed in a manner to what is predicted and observed for plateau regions on Earth such as Tibet and the Altiplano.

Syrtis Major and Isidis Basin contact: Morphological and topographic characteristics of Syrtis Major lava flows and material of the Vastitas Borealis Formation

Received 18 October 2002; revised 10 March 2003; accepted 14 April 2003; published 28 June 2003. [1] The floor of Isidis Basin is covered by materials of the Vastitas Borealis Formation (VBF) that appear to be emplaced essentially as a single unit. Along its western boundary, Isidis Basin is in contact with volcanic flows from Syrtis Major Planum. The contact between the Isidis unit and volcanic flows from Syrtis Major is sharp to gradational and in places is characterized by a high (� 500 m) scarp or by a network of faults that separate pieces of lava plains off the main plateau of Syrtis. Clusters of knobs and mesas, sometimes arranged in flow-like features, are also typical features of the transition zone. Several important characteristics of the transition from Syrtis Major to Isidis Basin are documented. (1) The small-scale surface texture seen in MOC images appears to be the same for both the Syrtis lava plateau and the knobs and mesas that characterize the transition. (2) There is strong evidence for the breakup of the coherent surface of Syrtis Major where it is in contact with materials in Isidis Basin. (3) The plateau breakup (the knobby terrain) occurs basinward after the major break of slope of Syrtis Major where it enters the Isidis Basin. (4) There is no evidence for plateau breakup anywhere up on the slopes of Syrtis Major Planum. (5) The lavas of Syrtis remain morphologically intact where they are in contact with other units, such as the Noachian cratered terrain or where lava flows are stacked within Syrtis Major itself. These characteristic features of the transition zone from Syrtis to Isidis are readily explained if the zone of plateau breakup consists of relatively young lava flows that have been superimposed onto the surface of a volatile-rich substratum, such as the interior unit of Isidis Basin (the Vastitas Borealis Formation). Thus simple superposition of volcanic materials on top of volatile-bearing sediments can explain the key geological and topographic aspects of the transition zone from Syrtis Major to Isidis Basin. On the basis of our findings, we outline the following scenario for the evolution of this region. In the Early Hesperian, volcanic plains are emplaced in Syrtis Major (the lower part of the Syrtis Major Formation), and wrinkle ridges deform their surfaces soon thereafter. Concurrently, volcanic plains are emplaced on the floor of the Isidis Basin, and wrinkle ridges deform their surfaces soon thereafter. The apparent simultaneity of these units may mean that Syrtis Major was the source of many of the flows in the Isidis Basin. In the early part of the Upper Hesperian, subsequent to the formation of most of the wrinkle ridges, the Vastitas Borealis Formation was emplaced in the Isidis Basin and elsewhere in the northern lowlands. Following the emplacement of the Vastitas Borealis Formation, the upper part of the Syrtis Major Formation was emplaced, erupting from the eastern margins of Syrtis Major Planum and flowing down into the westernmost part of the Isidis Basin on top of the recently emplaced Vastitas Borealis Formation. Modification of the superposed lavas by degradation and evolution of the VBF formed the scarps and unusual morphology of the marginal areas. We found no compelling evidence for massive or sudden erosion from Syrtis Major to produce the plains currently on the surface of the floor of the Isidis Basin (the Vastitas Borealis

Geology of Venus: Mapping of a global geotraverse at 30°N latitude

We mapped a swath of terrain on Venus that extends circumferentially completely around the globe at 30°N latitude representing about 11% of the surface of the planet in order to address the following questions: Do presently defined stratigraphic units prove useful for broad application to regional and global mapping? Do units described in one area occur in other regions of the planet and how laterally continuous are they? If similar sequences of units are found in different parts of the planet, do they correlate temporally (e.g., time-correlative), or do they represent similar sequences formed at different times (e.g., time-transgressive or independently locally repetitive)? We found that similar sequences of units are seen in different parts of the global geotraverse and that key stratigraphic units could be traced across almost the entire circumference of Venus at this latitude, providing evidence that these units are not only global in extent, but also laterally contiguous. This gives confidence that the stratigraphic sequence can be extended to other parts of Venus for further testing and refinement. On the basis of this mapping, we find that several areas, including tessera massifs and Beta Regio, show a concentration of older units and appear to have been topographically high over much of the visible history of Venus. Tessera terrain is not uniformly distributed in some areas where these older units are concentrated. Ridge belts and fracture belts are not randomly distributed throughout the area, suggesting that they represent regional patterns of deformation. The altitude distribution of the stratigraphic units and structures shows that the regional plains tend to be the lowest, with older units (tessera and earlier plains) at intermediate to high elevations, consistent with regional plains emplacement following the formation of the most significant regional topography. Sixty corona and corona-like features were found in the geotraverse, and the vast majority show evidence of being initiated in the earliest posttessera time (early Guineverian) in an extensional tectonic environment; few show evidence of postregional plains volcanism, and no positive evidence was found for the initiation of coronae in the postregional plains period of the history of Venus. Regional mapping shows that shield plains represent a unit occurring primarily prior to the emplacement of regional plains with wrinkle ridges (early part of the middle Guineverian), perhaps overlapping somewhat with its lower member. Evidence is found for changes in styles of volcanism with time, and tectonic activity changed in time in terms of pervasiveness, style, and areal distribution. On the basis of mapping of this globally continuous swath representing about 11% of the planet, we find that our analysis supports the presence of changes in the style and relative importance of different types of geological activity as a function of time, rather than the “nondirectional” history proposed by Guest and Stofan [1999].

Duration of tessera deformation on Venus

The density and distribution of impact craters superposed on the highly deformed tessera terrain on Venus permit analysis of the amount and duration of deformation prior to the emplacement of the stratigraphically younger global volcanic plains. Eighty percent of tesserae craters are undeformed. No existing craters exhibit evidence of contractional deformation, suggesting that the early compressional stage of tessera deformation ended abruptly. The small number of craters fractured by late-stage tessera extension constrains the duration of this phase to less than 20% of the average crater retention age of the tesserae, or approximately 30–60 Ma. These results suggest a geologically rapid decline in the magnitude of surface strain rates associated with the transition from the terminal stages of tessera compressional deformation to the eruption of the global volcanic plains.

Stratigraphic and geographic distribution of steep‐sided domes on Venus: Preliminary results from regional geological mapping and implications for their origin

Analysis of the stratigraphic position of 44 steep-sided domes in a global band of C1-MIDRs at 30°N latitude shows that in a region representing over 11% of the surface of Venus, the vast majority of steep-sided domes are closely associated stratigraphically with shield plains (Psh), a unit characterized by a very high abundance of small shields of apparent basaltic extrusive origin. Previously reported heterogeneities in areal and altitude distribution are now more plausibly interpreted in terms of formation in association with this unit and burial by subsequent regional plains units. Measurement of abundances of steep-sided domes presently exposed in shield plains and extrapolation to areas below younger units suggest that over a thousand of these features may have formed originally. Interpretation of the mode of formation of the steep-sided domes in the context of these new data favors hypotheses for their origin which call on remelting of basaltic crust to produce more silica-rich magmas. Occurrences of some steep-sided domes in association with summits of large shields (e.g., Tepev Mons and Sapas Mons) indicate that other mechanisms of dome formation (e.g., evolution of magma reservoirs) have also operated at other times in the history of Venus.

Topographic and morphologic characteristics of Reull Vallis, Mars: Implications for the history of the Reull Vallis fluvial system

[1] The eastern rim region of Hellas basin is characterized by the four prominent and quite extensively researched (cf. Crown et al., 2005, and references therein) outflow channels. In this work we focus on the Reull Vallis. On the basis of observations from available data sets, we present a hypothesis for the evolution of Reull Vallis and its complimentary fluvial system. We suggest that this system consists of parts that were formed during several phases rather than being a single continuous channel. Our results show that the fluvial system of Reull Vallis consists of two main parts and likely had independent formation phases and different sources of water. Our results also show that the upper portion of the Reull Vallis was formed by outflow from beneath Hesperia Planum (as proposed already in earlier works), but the suggested segments 1 and 2 (Mest and Crown, 2001) of the Vallis are not directly linked. There seems to have been an on-surface source for the formation of the segment 2 in the form of a topographic depression that was filled before the subsequent draining and formation of segment 2. Our interpretation of the evolution and formation implies a complex history for the Reull Vallis system.

Volcanism on Venus

Abstract Venus is almost as large as Earth and has both thermal and compositional potential for long-lasting volcanic activity, in contrast to the smaller terrestrial planets. Extrusive volcanic materials make up about 80% of the surface of the planet and its volcanic landforms range from small (several kilometers in diameter) to large (tens- to a few hundreds of kilometers across) volcanic constructs to volcanic plains thousands of kilometers in extent. The CO2 atmosphere of Venus is dense and its pressure at the surface is nearly 100 times that of Earth (or equivalent to pressures at about 1 km depth in the sea). The high pressure inhibits gas exsolution, magma disruption, and pyroclastic volcanism. The surface temperatures are high enough (~770 K) to potentially increase the time of cooling of magmatic bodies in near subsurface and lava flows on the surface. In contrast to the terrestrial plate tectonics, a hot spot style of volcanism, which is characteristic of the intraplate areas on Earth, prevails on Venus. In this style, heat is released by advection in isolated volcanoes distributed over the surface; the majority of the heat is currently lost by conduction through the lithosphere. Two major classes of volcanic features have been identified on Venus. (1) Features in which effusive characteristics are most important are volcanic plains, edifices, steep-sided domes, and channels. Among these landforms, plains are the most important and make up ~76% of the surface of Venus. (2) Coronae, arachnoids, and novae represent magmatic features that are dominated by tectonic structures with relatively minor effusive characteristics. Strongly deformed (densely lineated and ridged plains), mildly deformed (shield and regional plains), and non-deformed plains (smooth and lobate plains) compose three major types of volcanic plains on Venus. Regional plains represent the most extensive unit that makes up about 40% of the surface of the planet. Volcanic constructs and channels usually associate with a specific type of volcanic plains: small volcanoes and most of the steep-sided domes belong to shield plains, lava channels characterize regional and lobate plains, large shield volcanoes make a significant portion of lobate plains. The main volcanic plains on Venus display consistent relationships of relative ages among each other at the global scale: shield plains postdate strongly deformed units and predate regional plains; lobate plains overlay structures of regional plains. These stratigraphic relationships consistent are consistent at the global scale and allow division of the visible portion of the geologic history of Venus into three regimes of resurfacing during which specific types of endogenous activity dominated. (1) The majority of the tectonized terrains (e.g., densely lineated and ridged plains) define the first, tectonically dominated, regime. During this time, large regions of thickened crust (tesserae) were formed; a limited contraction and possible underthrusting along specific zones resulted in formation of ridge and mountain belts. The later phases of the ancient tectonic regime were manifested by the mutual development of groove belts and many coronae. All tectonized terrains of the first regime represent local- to regional topographic highs in the background topography. (2) During the second, volcanically dominated regime, the vast plains such as shield plains and regional plains were emplaced preferentially in regional lows. The density of craters on regional plains suggests that the first two regimes (tectonic and volcanic) operated during about the first one-third of the observable history. (3) Contemporaneous lobate plains and rift zones define the third, network rifting-volcanism regime. This regime dominated the last two-thirds of the observable geologic history and is linked to the later stages of evolution of the dome-shaped rises.

Crystal growth of GdVO4 by the micropulling down method

Undoped and neodymium-doped (1 at %) gadolinium vanadate (GdVO4) single crystals 5 mm in diameter and up to 25 mm in length, uniform in cross section, have been grown by the micropulling down method. The chemical and phase compositions of the crystals have been determined, and their absorption spectra were measured.

Steepness of Slopes at the Luna-Glob Landing Sites: Estimating by the Shaded Area Percentage in the LROC NAC Images

The paper presents estimates of the occurrence probability of slopes, whose steep surfaces could be dangerous for the landing of the Luna-Glob descent probe (Luna-25) given the baseline of the span between the landing pads (~3.5 m), for five potential landing ellipses. As a rule, digital terrain models built from stereo pairs of high-resolution images (here, the images taken by the Narrow Angle Camera onboard the Lunar Reconnaissance Orbiter (LROC NAC)) are used in such cases. However, the planned landing sites are at high latitudes (67°–74° S), which makes it impossible to build digital terrain models, since the difference in the observation angle of the overlapping images is insufficient at these latitudes. Because of this, to estimate the steepness of slopes, we considered the interrelation between the shaded area percentage in the image and the Sun angle over horizon at the moment of imaging. For five proposed landing ellipses, the LROC NAC images (175 images in total) with a resolution from 0.4 to 1.2 m/pixel were analyzed. From the results of the measurements in each of the ellipses, the dependence of the shaded area percentage on the solar angle were built, which was converted to the occurrence probability of slopes. For this, the data on the Apollo 16 landing region ware used, which is covered by both the LROC NAC images and the digital terrain model with high resolution. As a result, the occurrence probability of slopes with different steepness has been estimated on the baseline of 3.5 m for five landing ellipses according to the steepness categories of <7°, 7°–10°, 10°–15°, 15°–20°, and >20°.