Venus Lithosphere Research Articles

Sound velocities of Tremolite at high pressure: Implications for detecting water on Venus

While water may be stored in Venus' interior in the form of hydrous minerals, the velocity structure on Venus has been inferred based on anhydrous conditions. Due to its high decomposition temperature, tremolite could potentially store water on Venus, and thus, it is important to assess if the water-bearing tremolite could potentially have a distinct signal in velocity profiles of the Venusian lithosphere that future missions could measure. In this study, we measured the elastic properties of tremolite polycrystalline samples at high pressure up to 6 GPa using an ultrasonic interference method. Our results show that both VP and VS for tremolite increase linearly with pressure, and the values are much lower than those of nominally anhydrous minerals. The VP/VS ratio of 1.59–1.73 is less than that of most hydrous minerals. Combining the thickness of the crust and mineralogic models in previous studies, we attempt to build a possible wave velocity structure of Venus bearing tremolite deep to 100 km. Our results indicate that under cold temperature gradients, tremolite decomposes at 56 km resulting in a jump of 3.0% and 2.2% in VP and VS respectively, while under hot temperature gradients, tremolite decomposes at 35.2 km resulting in a jump of 3.1% and 2.2% in VP and VS respectively. When seismic velocity profiles for Venus become available through future missions, these theoretical profiles could provide insight into the potential presence of tremolite in the lithosphere of Venus and therefore the global water budget of the planet.

The formation of tonalitic and granodioritic melt from Venusian basalt

The crust of Venus is composed of the low lying volcanic planitiae and the elevated, deformed tesserae. It is thought that the tesserae may be composed of silicic igneous rocks and that it may resemble proto-continental crust. The initial development of terrestrial continental crust is likely due to melting and deformation of primitive mafic crust via mantle-plume upwelling and collisional plate processes. Unlike Earth, the lithosphere of Venus is not divided into plates and therefore evolved continental crust, if present, developed primarily by melting of pre-existing mafic crust. Here, we report the results of high pressure equilibrium partial melting experiments using a parental composition similar to the basalt measured at the Venera 14 landing site in order to determine if silicic melts can be generated. It was found that at pressures of 1.5 GPa and 2.0 GPa and temperatures of 1080 °C, 1090 °C, and 1285 °C that tonalitic and granodioritic melts can be generated. The experimental results indicate that silicic rocks may be able to form in the crust of Venus providing the thermal regime is suitable and that the lower crust is basaltic. The implication is that the older, thicker regions of Venusian crust may be partially composed of silicic igneous rocks.

Plume-induced crustal convection: 3D thermomechanical model and implications for the origin of novae and coronae on Venus

Novae and coronae are large volcanotectonic features on Venus, with a contentious and possibly non-unique origin and enigmatic relationship. Their formation is most commonly explained by flexure and fracturing of the strong rigid Venus lithosphere atop mantle upwelling and/or downwelling. Here we present new 3D high-resolution thermomechanical model of mantle plume/diapir impingement into warm and thin lithosphere with Venus-like surface temperature. Numerical results suggests that nova-like and corona-like structures can result from magma-assisted convection of weak ductile crust, induced by decompression melting of the hot rising mantle plume. During the initial stage, nova forms by stellate fracturing of a topographic rise forming atop the growing crustal convection cell. Few million years later, nova can convert to coronae by inward dipping concentric fracturing of the nova rise margins and subsequent outward thrusting of partially molten crustal rocks over the surface. An outer annulus of concentric normal faults forms in the outer rise region of the downbending brittle upper crust, whereas an inner annulus of concentric thrust faults forms in front of the outward thrusting crustal wedge. A trench-like depression forms between these two annuli. Resembling retreating subduction, the rudimentary concentric upper-crustal slab warms up rapidly and recycles into the convection cell. The convection cell remains active for up to 15 million years, fueled by heat and magma from the plume. Predicted surface topography and fracturing patterns agree with some small to moderate size novae and coronae on Venus.

Fabric development as the key for forming ductile shear zones and enabling plate tectonics

Lithospheric deformation on Earth is localized under both brittle and ductile deformation conditions. As high-temperature ductile rheologies are fundamentally strain-rate hardening, the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature. In this contribution, I develop a localization potential that quantifies the weakening associated with these changes. The localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions. I provide analytical expressions for the localization potential associated with a temperature increase, grain size reduction, an increase in water fugacity, melt content, or the abundance of a weak mineral phase. I show that these processes cannot localize deformation from a mantle convection scale (103 km) to a ductile shear zone scale (1 km). To achieve this, is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers. This process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non-linear rheology. Micas, melt, and fine-grained aggregates – unless dry rheologies are used – have the necessary characteristics. As none of these phases is expected to be present in the dry lithosphere of Venus, this concept can explain why Venus, unlike the Earth, does not display a global network of plate boundaries. The diffuse plate boundary in the Central Indian Ocean may be as yet non-localized because serpentinization has not reached the ductile levels of the lithosphere.

Aerial electromagnetic sounding of the lithosphere of Venus

Electromagnetic (EM) investigation depths are larger on Venus than Earth due to the dearth of water in rocks, in spite of higher temperatures. Whistlers detected by Venus Express proved that lightning is present, so the Schumann resonances ∼10–40 Hz may provide a global source of electromagnetic energy that penetrates ∼10–100 km. Electrical conductivity will be sensitive at these depths to temperature structure and hence thermal lithospheric thickness. Using 1D analytic and 2D numerical models, we demonstrate that the Schumann resonances—transverse EM waves in the ground-ionosphere waveguide—remain sensitive at all altitudes to the properties of the boundaries. This is in marked contrast to other EM methods in which sensitivity to the ground falls off sharply with altitude. We develop a 1D analytical model for aerial EM sounding that treats the electrical properties of the subsurface (thermal gradient, water content, and presence of conductive crust) and ionosphere, and the effects of both random errors and biases that can influence the measurements. We initially consider specified 1D lithospheric thicknesses 100–500 km, but we turn to 2D convection models with Newtonian temperature-dependent viscosity to provide representative vertical and lateral temperature variations. We invert for the conductivity-depth structure and then temperature gradient. For a dry Venus, we find that the error on temperature gradient obtained from any single local measurement is ∼100%—perhaps enough to distinguish “thick” vs. “thin” lithospheres. When averaging over thousands of kilometers, however, the standard deviation of the recovered thermal gradient is within the natural variability of the convection models, <25%. A “wet” interior (hundreds of ppm H 2O) limits EM sounding depths using the Schumann resonances to <20 km, and errors are too large to estimate lithospheric properties. A 30-km conductive crust has little influence on the dry-interior models because the Schumann penetration depths are significantly larger. We conclude that EM sounding of the interior of Venus is feasible from a 55-km high balloon. Lithospheric thickness can be measured if the upper-mantle water content is low. If H 2O at hundreds of ppm is present, the deeper, temperature-sensitive structure is screened, but the “wet” nature of the upper mantle, as well as structure of the upper crust, is revealed.

Tectonic evolution of Bell Regio, Venus: Regional stress, lithospheric flexure, and edifice stresses

In order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of Venus, we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in Bell Regio, using Magellan synthetic aperture radar (SAR) images and altimeter measurements of topography. The major volcanoes of Bell Regio, Tepev Mons and Nyx Mons, exhibit tectonic characteristics that are unique relative to other volcanic edifices on Venus. The most prominent distinction is the lack of large rift zones within the overall highland uplift, which characterize many other highland rises on Venus. Also, previous studies have determined that many large Venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on Earth and Mars. Tepev and Nyx Montes exhibit both the radial tectonic features associated with other Venusian edifices and numerous concentric graben. Nyx Mons implies a more distributed magmatic system by its broad shape, radial chains of pit craters, and expansive flow fields, whereas Tepev Mons is a more centralized volcanic system, with limited associated long flows. We investigate the regional stresses associated with Bell Regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading, modeling both Nyx Mons and Tepev Mons as axisymmetric loads with Gaussian mass distributions on an elastic plate. The relationship between the tectonic features surrounding Tepev Mons and stresses associated with magma chamber inflation are also examined through finite element analysis. Using topography data to model the shape of the volcano, we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in Magellan images. These results imply a shift in volcanic style within Bell Regio from an early phase of broad, low shield formation to later steep‐sided, more centralized edifice development. Such changes are consistent with an increase in the thickness of the lithosphere over time.

How does Venus lose heat?

The tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior. On the Earth, about 70% of the heat flow through the mantle is attributed to the subduction of cold lithosphere. In order to understand the tectonic and volcanic processes on Venus it is necessary to understand how heat is transported through its mantle. In this paper, three alternative end‐member hypotheses are considered. The first is the steady state loss of heat through the mantle to the surface in analogy to the Earth. However, without plate tectonics and subduction on Venus, a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination. The required plume flux would be equivalent to about 80 plumes with the strength of the Hawaiian plume. The required delamination flux implies a 50% delamination of the entire Venus lithosphere every 10 m.y. Neither appears possible, so that it is concluded that Venus cannot transport heat through its mantle to its surface on a steady state basis. The second hypothesis is that there has been a strong upward concentration of the heat‐producing elements into the crust of Venus; the heat generated is then lost by conduction. Surface measurements of the concentrations of the heat‐producing elements place constraints on this model. If everything is favorable this hypothesis might be marginally acceptable, but it is considered to be highly unlikely. The third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence. The near‐random distribution of craters suggests that the last subduction event occurred about 500 Ma. This model implies a thick thermal lithosphere (≈300 km) at the present time, which is consistent with a variety of surface observations. Lava lakes on the Earth are considered as analogies to plate tectonics; they also exhibit episodic subduction events.

Gravity over Coronae and Chasmata on Venus

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m km -1. The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km -1. The ADCs and GTRs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations. Instead, they suggest compensation by thermally induced thickness variations in a moderately thick (≈100 km) lithosphere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting. Weighted linear least squares fits to GTR vs h (long-wavelength topography) data from Heng-o and Fatua Coronae, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Corona, at Artemis Chasma, and other arcuate segments of Parga and Hecate Chasmata occur on the concave sides of the arcs. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Corona, and elsewhere along Parga and Hecate Chasmata.

Lithospheric flexure beneath the Freyja Montes Foredeep, Venus: Constraints on lithospheric thermal gradient and heat flow

Analysis of Venera 15 and 16 radar images and topographic data from the Freyja Montes region on Venus suggest that this mountain belt formed as a result of a sequence of underthrusts of the lithosphere of the North Polar Plains beneath the highlands of Ishtar Terra, The Freyja Montes deformation zone consists, south to north, of a linear orogenic belt, an adjacent plateau, a steep scarp separating the plateau from the North Polar Plains, a linear depression at the base of the scarp, and an outer rise. The topographic profile of the depression and outer rise are remarkably similar to that of a foreland deep and rise formed by the flexure of the underthrusting plate beneath a terrestrial mountain range. We test the lithospheric flexure hypothesis and we estimate the effective thickness Te of the elastic lithosphere of the underthrusting portion of the North Polar Plains by fitting individual topographic profiles to deflection curves for a broken elastic plate. The theoretical curves fit the observed topographic profiles to within measurement error for values of flexural rigidity D in the range (0.8–3) × 1022 N m, equivalent to Te in the range 11–18 km. Under the assumption that the base of the mechanical lithosphere is limited by the creep strength of olivine, the mean lithospheric thermal gradient is 14–23 K/km. The portion of the North Polar Plains to the immediate north of Freyja Montes stands at a regional elevation very near the modal elevation for the planet, so the inferred elastic lithosphere thickness and thermal gradient may be at least approximately representative of a significant fraction of the Venus lithosphere. That the inferred thermal gradient is similar to the value expected for the global mean gradient on the basis of scaling from Earth provides support for the hypothesis that simple conduction dominates lithospheric heat transport on Venus relative to lithospheric recycling and volcanism.

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.

Convection‐driven tectonics on Venus

An analysis is presented for convective stress coupling to an elastic lithosphere as it applies to tectonic deformation on Venus. Theoretical solutions are introduced for the response of a mathematically thick elastic plate overlying a Newtonian viscous medium with an exponential depth dependence of viscosity. A Green's function solution is obtained for the viscous flow driven by a harmonic density distribution at a specified depth. It is shown that during the transient phase of the solution, the field quantities (stress, velocity, displacement) in both media depend upon the rheology of both media, whereas in steady state the solutions become uncoupled. Due to this uncoupling, stresses obtained from a purely viscous solution can be applied as boundary conditions at the base of an isolated elastic plate. I show for such a plate that (1) shear stress imparted at the base of the lower boundary can induce large in‐plane stretching mode forces and (2) these forces can be estimated directly from free‐air gravity anomalies without detailed knowledge of the density distribution in the underlying flow. Free‐air gravity anomalies for Venus imply in‐plane forces of about 2–3×1012 N/m. In‐plane forces of this magnitude are capable of extensive disruption of the Venusian lithosphere. To demonstrate this, an elastic‐plastic analysis is carried out for the deformation of a model Venus lithosphere; the calculation is based on a lithospheric strength envelope and uses gravity and topography as a guide to the magnitude of stresses imparted by convective flow. The results predict that dynamic uplift of Venusian topography must be accompanied by extensive brittle failure and viscous flow in the lithosphere. Geoid anomalies and geoid‐to‐topography ratios for Venusian topographic features are large compared to values for thermal swells in Earth's ocean basins; this suggests that dynamic support of topography must be an important process on Venus. Further, a low‐viscosity zone is probably not present beneath the Venusian lithosphere. A comparative analysis suggests that tectonic deformation of the lithosphere associated with dynamically supported topography on Earth is unimportant.

Constraints on the lithospheric structure of Venus from mechanical models and tectonic surface features

Radar images of the surface of Venus show numerous structures that are interpreted as having formed due to horizontal compression and extension of the lithosphere. Many of these features exhibit characteristic scales (widths or spacings) of deformation, one of 10–20 km and another of 100–300 km. For a range of simple models, we test the hypothesis that these length scales are controlled by dominant wavelengths arising from unstable compression or extension of the Venus lithosphere. Results show that the existence of tectonic features that exhibit both length scales can be explained if, at the time of deformation, the lithosphere consisted of a crust that was relatively strong near the surface and weak at its base and an upper mantle that was stronger than or of nearly comparable strength to the upper crust. The spacings of these features imply crustal thicknesses in the approximate range 5–30 km and a thermal gradient not greater than 25 K km−1. Features that exhibit only the smaller scale of deformation can be explained by either a lithosphere with a thick crust that overlies a weak mantle or a lithosphere with a strong mantle but with small internal strength contrasts. For a broad range of parameters, the models require not only that smaller scale compressional and extensional features have similar spacings, but also that the widths and spacings of larger scale compressional features be greater than those formed in extension. This is consistent with observed differences in the characteristic length scales of larger scale surface features, as well as the similarities in spacings of observed smaller scale features.

Impact craters of Venus: Analysis of Venera 15 and 16 data

From description of impact craters on Venus and crater size‐and‐depth distributions, models are developed that analyze the size‐distribution curve and radar‐bright haloes around fresh craters. The dense Venusian atmosphere has a strong influence on cratering. The estimated age of the surface of the area under study is from 0.5 to 1.0×109 years. The estimated rate of resurfacing during this time interval is about 0.04 to 0.8 cm/106 years. The lithosphere of Venus seems to be sufficiently rigid to sustain craters of 100–140 km diameter and 0.8–2.5 km depth for about 108 years.

Support of long‐wavelength loads on Venus and implications for internal structure

A thin‐shell formulation for computing stress distributions on the surfaces of one‐plate planets is developed. The lithospheric model includes laterally varying density anomalies at two depths, corresponding to undulations on a crust‐mantle boundary and variations in upper mantle density. For a given set of model parameters, values for the vertical displacement, crustal thickness anomaly, and mantle density anomaly required to satisfy the topographic and gravitational boundary conditions are computed, along with the resulting stress field. The computed stresses can then be compared to stress directions inferred from observed tectonic features. This formulation is used to study loads on Venus's lithosphere, and it is found that most highland areas appear to have been formed in a state of isostasy or by uplift, with a significant portion of their support probably derived from dynamic processes in the mantle. A possible exception is Ishtar Terra, whose circumferential mountain belts are most consistent with a radially compressive stress regime generated by flexural loading. Reasonable lithospheric models predict low‐density mantle beneath Atla, Beta, Eisila, Thetis, and Tethus Regiones and thick crust accompanied by higher‐density mantle under Ishtar Terra and Ovda Regio. Thus western Aphrodite Terra appears to be more closely related to Ishtar than to the rest of the equatorial highlands. These results favor a hot spot model of Venus tectonics in which Beta, eastern Aphrodite, Eisila, and Tethus are expected to be relatively young and thermally active, while Ovda and Ishtar should be older and less active.

Venus banded terrain: Tectonic models for band formation and their relationship to lithospheric thermal structure

Recent radar images of the mountains of Ishtar Terra, Venus, at approximately 3‐km radar resolution show a series of linear bands of alternatively greater and lesser backscatter; the bands are generally aligned with the topographic strike of the mountain ranges. We test the hypothesis that these band features are tectonic in origin, the product either of folding during lithospheric compression or of block faulting during lithospheric extension. Tested models for folding include uniform elastic and viscous plates overlying in viscid substrates, layered plates, and a viscous half space in which viscosity decreases exponentially with depth. The uniform plate models can produce folds at a dominant wavelength equal to the characteristic spacing between bands on Venus, about 15 to 20 km, if the elastic or high‐viscosity layer is, at most, a few kilometers in thickness, though the required compressive stresses are several kilobars in magnitude. For the case of an exponentially decreasing viscosity the skin depth plays the role of layer thickness, and similar results hold. Layered elastic or viscous plates, however, can fold at the required wavelength with subkilobar stresses and, therefore, are favored over uniform plate models. Tested extensional models include graben and horst formation in a surface elastic‐brittle layer, imbricate normal faulting, and necking of a surficial layer that is everywhere in a state of extensional failure. The spacing between adjacent extensional features can match the spacing between bands on Venus if the surficial brittle layer is no more than a few kilometers in thickness. Both compressional and extensional models for band formation are therefore consistent with radar images of Venus mountain ranges at the image resolution presently available. Additional evidence, however, including the linearity and continuity of individual bands, the relationship between band trends and topographic contours in some regions near the ends of mountain ranges, and the very different manifestations of lithospheric extension in other highland areas of Venus, leads us to favor the hypothesis that banded terrain formed contemporaneously with the mountain ranges of Ishtar Terra as a result of horizontal compression of the Venus lithosphere. Independent arguments on the thermal structure of the Venus crust and the mechanical behavior of crustal rocks as a function of temperature indicate that the elastic lithosphere of Venus is approximately 1–10 km thick. We therefore suggest that the surficial layer of elastic‐brittle or high‐viscosity behavior required for either the compressional or extensional models may in fact be the elastic lithosphere of the Venus highlands at the time of band formation.

Reply to the critique of Reasenberg &amp; Bills on “Elastic thickness of the Venus lithosphere estimated from topography and gravity”

Reply to the critique of Reasenberg &amp; Bills on “Elastic thickness of the Venus lithosphere estimated from topography and gravity”

Critique of “Elastic thickness of the Venus lithosphere estimated from topography and gravity” by A. Cazenave and K. Dominh

Cazenave and Dominh compare the topography in a portion of the rolling plains province of Venus with a measure of the corresponding gravity perturbations, the residual acceleration of the Pioneer Venus Orbiter. They ignore substantial geometric factors and make unsubstantiated geophysical assumptions to draw conclusions which we consider logically invalid and numerically implausible.

Identifying the coordination of silicon by magic‐angle‐spinning NMR: Thickness of the Venus lithosphere estimated from topography and gravity” by A. Cazenave and K. Dominh

Six‐coordinated Si in stishovite yields a sharp 29Si MASNMR signal at −191.3 ±0.2 ppm from TMS, well‐removed from the characteristic signal associated with tetrahedral coordination in quartz, cristobalite, and other, less crystalline siliceous solids.

Elastic thickness of the Venus lithosphere estimated from topography and gravity

The very close correlation observed on Venus between topography and gravity on a regional scale is clearly related to the isostatic compensation of the topographic loads. Observed gravity anomalies over topographic features of moderate lateral extent, located in the equatorial region (0° to 40° N latitude and 10° W to 60° E longitude) are somewhat larger than those computed with a model of local isostatic compensation (of Airy type). They may rather be explained by a model of compensation including elastic forces within the lithosphere which contribute to support topographic loads through elastic stresses. We have applied the theory of lithospheric flexure : to explain in wavelength and in amplitude the observed gravity anomalies, a flexural rigidity of ∼ 4 × 1028 dyne cm has been estimated for the Venus lithosphere in the rolling plains province. This value is very low compared to the oceanic terrestrial lithosphere (ranging from ∼ 1028 dyne cm near ridge crests to ∼ 2 × 1031 dyne cm near subduction zones). The corresponding thickness of the elastic upper layer able to maintain elastic stresses for long time duration is ∼ 7.5 km. This value may not be representative of the entire Venus lithosphere.

Venus tectonics: Another Earth or another Mars?

The presence of presumably primordial large craters has led to the suggestion that Venus may have a thick lithosphere like that of Mars despite its similarities to Earth in size and density. Howevercrust and upper mantle temperatures on Venus are very likely higher than on Earth so that a dry Venus could have a lithosphere with a thickness similar to that of Earth. If a trace of volatiles is present in the mantlethe lithosphere of Venus could be thinner. Due to the absence of liquid watererosion and deposition will be much slower on Venus than on Earthfavoring retention of primordial cratered surfaces on portions of the crust that have not been destroyed or buried by tectonic and volcanic activity. Geochemical models of solar system origin and petrological considerations suggest that K is about as abundant in Venus as in Earth. The abundance of 40Ar in the atmosphere of Venus lies somewhere between the Earth value and one‐tenth of the Earth value. Because erosional liberation of 40Ar on Venus will be relatively inefficientthis range for 40Ar abundance at least permits an active tectonic historyand if the 40Ar abundance is towards the high end of the rangeit may well require an active tectonic history. Thus we are not constrained to a Mars‐like model of Venus tectonics by craters and possible mantle dryness; an Earth‐like model is equally probable.