Mantle Of Venus Research Articles

Propagation of nonlinear ion-acoustic fluctuations in the mantle of Venus

Motivated by the observations of ion-acoustic fluctuations with the Parker Solar Probe (PSP) and earlier by the Pioneer Venus Orbiter (PVO) in the Venusian magnetosheath, we investigate the nature of ion-acoustic solitary and double-layer (DL) structures in the mantle. We employed a hydrodynamic description along with reductive perturbation theory to derive the nonlinear Zakharov—Kuznetsov equation that elucidates the dynamics of three-dimensional ion-acoustic wave packets. Using the spacecraft measurements of the plasma configuration at Venus, we carried out a parametric analysis of these structures, including the influence of the magnetic field strength and the relative densities and temperatures, considering two cases: quasi-parallel and oblique propagation. Moreover, we determined the structural characteristics of these waves, where oblique (quasi-parallel) solitary waves have a potential of 0.4 V (0.4 V) and a maximum electric field amplitude Em ~ 0.024 mV m−1 (8 m V m−1) across spatial and temporal widths of ~40–80 km (~140–200 m) and 0.4 s (1.6 ms). These waves produce low-frequency electrostatic activity in the frequency range of 1.6–10 Hz (630–3160 Hz). Quasi-parallel DLs have potential drops of (6.5–13) V and Em ~ (0.16–0.35) mV m−1 with a width and duration of (100–120) m and ~1 ms, and a frequency range of ~630–3980 Hz. These outcomes can explain the detected electrostatic fluctuations above the ionosphere via PVO in the frequency channels of 730 Hz and 5.4 kHz. Furthermore, the DL features estimated in this work are in line with the recent PSP measurements of the DLs propagating in the magnetosheath of Venus.

Viscosity contrasts in the Venus mantle from tidal deformations

Viscosity contrasts in the Venus mantle from tidal deformations

Felsic Tesserae on Venus Permitted by Lithospheric Deformation Models

AbstractVenus tessera terrain contains the oldest rocks preserved on the surface of the planet. The composition of the tesserae is unknown, but recent observations of near‐infrared emissivity from orbit suggest that they are silica rich. This contrasts with previous interpretation that silica‐rich rheologies are too weak to produce tessera folds. We examine the range of compositions consistent with the observed folds in the tessera. We map folds in Tellus Regio tessera using the highest resolution Magellan radar images (∼100 m) and stereo topography (∼1 km). We use fold spacings as input into a buckling analysis that simulates power law rheology using contemporary dry‐mineral flow laws and modeled igneous rock compositions, consistent with a predicted dry lithosphere for Venus in the recent era. The folds have dominant wavelengths of 6–19 km with amplitudes of 500–1,000 m, consistent with low strains (1%–12%). We confirm that observed folds require heat flows of ≥40 K/km and strain rates faster than 10−16s−1, higher than today. These low strain folds are unlikely to have significantly modified the preexisting tessera surface, thus older impact craters must have been removed by an earlier geologic process. We demonstrate that the folds can form in dry crust with a SiO2 content ranging from 48% to 72%. Dry felsic rheologies could explain low emissivity signatures of tesserae that are attributed to a low Fe2+. If felsic, tessera materials may have formed in a prior era via partial melting of hydrous materials derived from recycling of surface waters into the Venus mantle.

Constraints of the topography, gravity and volcanism on Venusian mantle dynamics and generation of plate tectonics

Venus's mantle convection model was studied in a three-dimensional spherical shell domain with depth- and temperature-dependent viscosity. Numerical results show that key observations of Venus including the number of major “hotspot” volcanic systems, spectral patterns of the surface topography and geoid at long- and intermediate-wavelengths can be explained in models that have a spinel-to-post-spinel endothermic phase change of −3.5MPa/K Clapeyron slope and averaged mantle viscosity of 2×1021Pas (i.e., convective Rayleigh number of 1.8×107). Our models with the endothermic phase change show relatively weak time-dependence, suggesting that the phase change may not be the primary cause for “catastrophic” resurfacing on Venus. Our calculations also show that Venus cannot have a weak asthenosphere that is similar to that on the Earth, in order to match the observations, thus supporting a key role of asthenosphere in producing plate tectonics.

Simulating the thermochemical magmatic and tectonic evolution of Venus's mantle and lithosphere: Two‐dimensional models

Numerical convection models of the thermochemical evolution of Venus are compared to present‐day topography and geoid and recent resurfacing history. The models include melting, magmatism, decaying heat‐producing elements, core cooling, realistic temperature‐dependent viscosity and either stagnant lid or episodic lithospheric overturn. In stagnant lid convection the dominant mode of heat loss is magmatic heat pipe, which requires massive magmatism and produces very thick crust, inconsistent with observations. Partitioning of heat‐producing elements into the crust helps but does not help enough. Episodic lid overturn interspersed by periods of quiescence effectively loses Venus's heat while giving lower rates of volcanism and a thinner crust. Calculations predict 5–8 overturn events over Venus's history, each lasting ∼150 Myr, initiating in one place and then spreading globally. During quiescent periods convection keeps the lithosphere thin. Magmatism keeps the mantle temperature ∼constant over Venus's history. Crustal recycling occurs by entrainment in stagnant lid convection, and by lid overturn in episodic mode. Venus‐like amplitudes of topography and geoid can be produced in either stagnant or episodic modes, with a viscosity profile that is Earth‐like but shifted to higher values. The basalt density inversion below the olivine‐perovskite transition causes compositional stratification around 730 km; breakdown of this layering increases episodicity but far less than episodic lid overturn. The classical stagnant lid mode with interior temperature ∼rheological temperature scale lower than TCMBis not reached because mantle temperature is controlled by magmatism while the core cools slowly from a superheated start. Core heat flow decreases with time, possibly shutting off the dynamo, particularly in episodic cases.

Geoid and topography of Venus in various thermal convection models

Important though indirect information about the internal structure of Venus is provided by its topography and geoid. In the last decades this information has been used to constrain the Venus mantle viscosity structure and its dynamic regime. Recently, the geodynamic inversion of the Venus’ geoid and topography resulted in a group of best fitting viscosity profiles. We use these viscosity models here as an input to our mantle convection code. We carry out simulations of the Venus’ mantle evolution in a 3D spherical shell with depth dependent viscosity and check whether the character of the dynamic topography and the geoid represented by their power spectra fits the observed quantities. We compare the results with several other models obtained for different viscosity stratifications (constant, constant with highly viscous lithosphere, linear increase of viscosity). Further, we estimate the effect of other factors such as internal heating and varying Rayleigh number. We use a 2D spherical axisymmetric convection code to study the effect of lateral viscosity variations. In these 2D models we monitor the topography and the geoid developing above the axisymmetric plume and compare them with the observed elevations of Venus’ geoid and topography in several Regia. Though none of the models fits observed data perfectly, we can generally conclude, that the best fit between the observed and predicted quantities is reached for viscosity profiles with 200 km thick lithosphere followed by a gradual increase of viscosity with depth and with the upper mantle viscosity of 2 × 10 21 Pa s. For all viscosity profiles the predicted geoid and topography spectra match the observed ones only up to the degree 40, thus indicating other than dynamic origin of these quantities for higher degrees.

Radiation by energetic electrons accelerated by wave-particle interaction: a plausible mechanism for x-ray emission from the Venus mantle

Abstract. In this paper it is argued that recently observed x-ray emission from non-magnetic planets (Dennerl et al., 2002) can be explained as a combination of bremsstrahlung and line K-shell radiation produced by the interaction of energetic electrons with the neutral atmosphere. Numerical simulations show that the modified two stream instability can generate energetic 100 eV electrons that are observed and these electrons can produce x-ray emission.

A conceptual model for the relationship between coronae and large‐scale mantle dynamics on Venus

Among the most enigmatic features on Venus are the more than 500 quasi‐circular structures collectively referred to as coronae. Associated volcanism and tectonism suggest that coronae result from small, transient mantle plumes, although only about 20% have clearly associated positive gravity anomalies indicative of current activity. Coronae are concentrated within a region defined by the dynamically active highland areas of Beta, Atla, and Themis Regiones (BAT). Coronae avoid the lowest or highest topography and geoid. Subsets of the corona population indicate that the spatial distribution of coronae has evolved from a previously less concentrated global distribution to the current BAT concentration. Recent laboratory experiments on thermal convection at high Rayleigh number, using fluids with temperature‐dependent viscosity, suggest a straightforward interpretation of these relationships. Small‐scale upwellings from a hot lower thermal boundary layer tend to be excluded by longer‐wavelength downwelling structures from the cold upper boundary layer. The relative paucity of coronae in the broad lowland plains regions is consistent with the inference of mantle downwellings beneath the plains. Coronae are largely excluded from compensated highlands, perhaps because the thickened crust is not easily disrupted in these regions. Laboratory experiments indicate that small, transient, boundary layer instabilities can coexist with, and be focused toward, large‐scale upwellings, possibly explaining the spatial concentration of coronae in the BAT region. Thus coronae provide important insights into interacting scales of convective flow in Venus's mantle and into the evolution of Venus's mantle dynamics. Direct tests of our model are possible through geological mapping.

Numerical models of magmatism in convecting mantle with temperature‐dependent viscosity and their implications for Venus and Earth

Numerical models are presented for magmatism in a convecting mantle that contains internal heat source. Mantle convection is modeled by a convection of binary eutectic material with Newtonian temperature‐dependent rheology driven by thermal, compositional, and melt buoyancy as well as the buoyancy from the “660‐km” phase transitions. Mantle magmatism is modeled by a gravitationally induced permeable flow of magma generated by pressure release melting through matrix. Magma mostly has the eutectic composition, that is, basaltic composition. The numerical models suggest that magmatism and mantle convection strongly influence each other to form a coupled system when the internal heating is sufficiently strong. Episodic magmatism takes place to chemically differentiate the mantle; basaltic materials occupy deeper part of the lower mantle, while magma residue occupies the uppermost mantle. Compositional buoyancy of the differentiated materials makes mantle convection sluggish except when magmatic activities take place. The style of magmatism and mantle convection depends on the viscosity contrast across the cold thermal boundary layer (TBL) along the top surface boundary. When the viscosity contrast is large, the fluid in the coldest part of top TBL becomes a stagnant lid and a conspicuous lateral heterogeneity arises in the upper mantle. Magmatism is induced by hot uprising diapirs originating in the top of the lower mantle. The diapirs induce convective circulation within the upper mantle when the lower mantle is rather cold but induce flushing event, that is, a massive flow across the 660‐km phase boundary and the resulting vigorous magmatism, when the lower mantle becomes sufficiently hot owing to the internal heating. When the viscosity contrast is moderate, the fluid in the coldest part of top TBL behaves as a moving lid. Both the moving lid and hot uprising diapirs induce magmatic activity; the magmatic activity due to a moving lid resembles ridge volcanism. The mantle of Venus and that of the early Earth are suggested to have been on the regime modeled here, and the magmatic activity by flushing event is compared to the magmatism of volcanic plain formation on Venus.

Wave activity in the dayside mantle of Venus, Mars, and Titan

Abstract In this paper we analyse the properties of the waves excited in the interaction layer between the shocked solar wind and planetary plasma on the dayside of non-magnetic bodies. We compare the predictions of existing models for Venus, Mars, and Titan. It is concluded that the currently available measurements at Venus and Mars are not adequate to distinguish between the models, but at Titan the difference between the predictions is significant.

On the dayside region between the shocked solar wind and the ionosphere of Mars

The objective of this paper is to explore the structure and properties of the region between the shocked solar wind and the planetary plasma on the dayside of Mars. This study is based on the observations obtained by the plasma instruments carried on board the Phobos 2 spacecraft. These instruments have independently identified several plasma boundaries in the dayside magnetosphere of Mars, the boundaries are located close to each other but exhibit different plasma features. This investigation leads us to conclude that a dayside region exists on Mars with the following characteristics: (1) the bulk of the shocked solar wind protons are deflected, (2) the total magnetic field increases, (3) a plasma depletion region is formed, (4) both shocked solar wind and planetary plasma are present, (5) accelerated electrons and heavy ions are also present, (6) intensive wave activity is seen in the 5 to 150 Hz frequency interval, and (7) a current layer and associated magnetic shears can be identified. We name this region, located between the ionosphere and the shocked solar wind, the dayside boundary layer or dayside mantle. Its key features are similar to those of the Venus mantle; the variety of names given to the Martian plasma boundaries merely reflects its complicated structure. We conclude that the two boundary layers at Venus and Mars are very similar and that the physical processes (i.e., the interaction of the shocked solar wind with the planetary plasma) at work within these boundary layers are probably similar.

A coupled thermal‐mechanical model for corona formation on Venus

We develop a model of the combined thermal and mechanical evolution of diapirs in the mantle of Venus. The diapir is treated as an oblate spheroid rising through a viscous fluid and ultimately impinging on a rigid overlaying “lid”. Drag forces imposed by the lid cause the diapir to spread and flatten as it rises. We parameterize the heat loss from the rising diapir using a Nusselt number formulation and treat the resulting loss in buoyancy in our flow calculation. The model predicts the evolution timescale and degree of flattening of a diapir as it rises, as well as the stresses exerted on the underside of the lithosphere. In order to explore diapir behavior further, we check our analytical model against the predictions of the finite element code MANTLE. From the combined results of these models and observations of Venusian coronae, we are able to make a number of inferences about Venusian diapirism and mantle properties. We find that the diapirs responsible for formation of Venusian coronae have an initial size distribution extending from about 30 to 100 km in radius. Typical evolution timescales for these diapirs are 30–50 Myr. If Venus' global average resurfacing age is 300–500 Myr, then our results are consistent with a rate of diapirism that has been roughly constant over this period, and with an effective mantle viscosity of 1021 Pa s.

Energetic electrons in the dayside mantle of Venus

The purpose of the current study is to review the available and relevant experimental data in order to examine whether there is any evidence of accelerated electrons above the dayside ionosphere of Venus, where the shocked solar wind encounters the planetary plasma, that is in the dayside mantle. Such energetic electrons were seen around Mars in the corresponding region, and theoretical studies also predict their existence at Venus. Direct measurements, however, are not available, because the retarding potential analyzer (ORPA) onboard the Pioneer Venus Orbiter had an upper energy limit of 60 eV. Therefore we were forced to look for indirect evidences. We have reexamined the ORPA data and the magnetic field measurements in the dayside mantle. We have concluded, using these indirect indicators, that this energetic electron population is very likely to be present in the dayside mantle of Venus.

The interaction of the shocked solar wind and the planetary ions at Mars

In this study we investigate that region of the dayside magnetosphere of Mars where we may expect that the shocked solar wind comes into direct contact with the planetary ions, and where energy and momentum transfer from the solar wind to planetary plasma may take place. It is our intention to analyse whether the dominant microphysical processes are similar to those observed in the equivalent regions of the magnetosphere of Venus, that is in the dayside mantle of Venus. In this study we present results obtained along the first elliptic orbit of the Phobos 2 spacecraft. Our first conclusion is that around Mars it is the magnetic barrier region that corresponds to the dayside mantle of Venus. In this region at Mars wave excitations are observed, dominantly in the 5 to 50 Hz region; accelerated heavy ions and electrons were also detected. The major conclusion of this study is that despite several observed differences in the way that Venus and Mars interact with the solar wind, the dominant plasma features in the magnetic barrier region of Mars and the dayside mantle of Venus are very similar, indicating that probably the underlying physics is also similar.

Low energy (E

Low energy (<400 eV) ion fluxes measured by the hyperbolic retarding potential analyser (HARP), during the first two elliptic orbits of the Phobos 2 spacecraft, are presented. Combining these results with the overlapping higher energy observations of the TAUS instrument, carried on the same spacecraft, we show the presence of flowing and shocked/thermalized solar wind ions inside the magnetosheath and antisunward flowing heavy ion population below the upper boundary of the magnetic barrier. Our results are not consistent with the presence of a dense, stagnated planetary ion population there, in the region explored by Phobos-2. Our results do not indicate the existence of additional sharp plasma boundaries, such as the proposed ion composition (mass loading) boundary. The observed characteristics of the outer regions of the Venus mantle and the combined HARP and TAUS measurements close to and inside of the magnetic barrier at Mars do not indicate the presence of significant differences.

On the interaction between the shocked solar wind and the planetary ions on the dayside of Venus

Around Venus the planetary ionosphere is directly exposed to the shocked solar wind. The interaction takes place in a broad region surrounding the dayside ionosphere, called the mantle, where the shocked solar wind plasma and the plasma of planetary origin have equally important roles. In this paper both the experimentally determined characteristics and the microphysics of the mantle are discussed in detail. It is shown that as a result of the interaction between the two plasma populations, a modified two‐stream instability develops, and waves are excited with a frequency of a few times the lower hybrid frequency. The polarization of the waves is almost perpendicular to the magnetic field. The stabilization of the higher‐frequency part of the wave spectrum is the result of transverse wave convection in the particular sheet‐like geometry of the mantle. The interaction of these waves with planetary ions and electrons is described within the framework of a nonlinear model in which the saturation of the modified two‐stream instability is due to induced scattering of the waves on cold planetary ions. The effective collision frequency between the shocked solar wind protons and planetary ions is also calculated; it is shown how this leads to ion pick up and heating. Other macroscopically observable effects of these processes are electron acceleration along the magnetic field and ionospheric heating. The experimental data collected in the dayside mantle of Venus by the instruments carried onboard the Pioneer Venus Orbiter are compared to our model. It is believed that the observations support the scenario presented.

The effect of the transformation of basalt to eclogite on the internal dynamics of Venus

Differentiation that occurs in zones of partial melting, forms less dense depleted mantle and basaltic crust. While the basaltic crust thickens, it transforms into denser eclogite. Typical values of Earth parameters are assumed to characterize Venus mantle properties except for the gravity acceleration equal to 8.8 m s −2, the surface temperature equal to 700 K and an unmoving surface. Such values provide a thermal Rayleigh number equal to 3 × 10 5. It can be applied to the formation of the Earth's surface during its primitive evolution and to the evolution of Venus considered as a one plate planet. In a 2D rectangular ☐, a steady pattern of thermal convection is achieved before the compositional effects are taken into account. Rocks are modeled with 130,000 particles characterized by their type (peridotite, basalt, eclogite) and their density. This model provides chemical buoyancy related to the distribution of these heterogeneities and the magmatism and its origin. A first study takes into account the chemical buoyancy created by mantle heterogeneities and basalt. In a second study, chemical buoyancy induced by dense eclogite is included. The comparison of the results of these two models allows us to quantify separately the dynamics induced by mantle heterogeneities and that induced by denser eclogite. The less dense depleted mantle accumulates in a surface-layer and tends to make the downwelling zones disappear, favoring more elongate cells. When the depleted mantle reaches the base of the upper mantle, compositional buoyancy enhances the development of new upwellings and new partial melting zones. The amount of magma becomes strongly time dependent and new magma centers are created. The competition between thermal and chemical buoyancy, in addition to the stabilization of the upper depleted layer decreases the efficiency of heat and mass transfer. Once the basalt/eclogite transformation is taken into account, gravitational instabilities form, yielding a recycling of crust into the mantle and lamination of the depleted lithospheric layer. These instabilities carry basaltic crust in the domain of stability of eclogite and limit the crustal thickness to a value lower than 40 km. The formation of compositional upwelling becomes less efficient while thermal downwellings are enhanced by the buoyancy of eclogite. This moderation of chemical perturbations induced by mantle heterogeneities favor both heat transfer and mass transfer. Therefore, both the cooling of the planet and the recycling of eclogite to the partial melting zones are more rapid. The buoyancy of eclogite enhances the recycling of crust in partial melting zones and may provide some “instantaneous” volcanic events at high rates which result from the melting of recycled eclogite. Finally, chemical and thermal convection stabilizes larger cells than a purely thermal convection would. While the mean aspect ratio of the cells is close to 1.3, in a purely thermal convection, it tends to 4 when chemical convection of mantle is included and it tends to 2 when chemical convection of both mantle and eclogite are taken into account.

Comparative hypsometric analysis of Earth and Venus

The comparison between Earth and Venus global hypsometric distributions is reinvestigated on the basis of the high‐resolution data provided by the recent Magellan global topographic coverage of Venus' surface, and the detailed ETOPO‐5 topographic database for the Earth's surface. The study of the cumulative hypsometric curve for both planets reveals that there exists a domain of elevation for which the relationship between the elevation and the square root of cumulative area percentage is linear. This domain covers about 80% of the mapped venusian surface and nearly all the terrestrial oceanic surface (55% of the earth's surface) and corresponds to an elevation range of about 2000 m for both planets. Proposing the upper threshold elevation of this domain as a common geophysical reference level, it is shown that the terrestrial oceanic peak and the venusian peak coincide very closely. In addition, the terrestrial and venusian hypsometric distributions for elevations lower than this reference level are quite similar, in their amplitude, modal distribution, and pronounced symmetry. The correspondence of the venusian peak with the terrestrial oceanic peak is a striking feature, but it does not necessarily imply the occurrence of plate tectonics on Venus. The present observation is consistent with the existence of a similar thermal isostasy acting at the planetary scale. It places an upper bound at 90±10 km for the modal thermal lithosphcric thickness of Venus, if the Venus mantle thermal conditions are earth‐like. This estimate is reduced if hotter conditions occur.

Generation of waves in the Venus mantle by the ion acoustic beam instability

The ion acoustic beam instability is suggested as a mechanism to produce wave turbulence observed in the Venus mantle at frequencies 100 Hz and 730 Hz. The plasma is assumed to consist of a stationary, cold O+ ion plasma and a flowing, shocked solar wind plasma. The O+ ions appear as a beam relative to the flowing ionosheath plasma which provides the free energy to drive the instability. The plasma is driven unstable by inverse electron Landau damping of an ion acoustic wave associated with the cold, ionospheric O+ ions. The instability can directly generate the observed 100 Hz waves in the Venus mantle, as well as the observed 730 Hz waves through the Doppler shift of the frequency caused by the satellite motion.

An unusual volcano on Venus

Materials that issued from an unusual Venusian volcano produced (1) a complex domical structure about 100 km across with thick, broad flow lobes up to 41 km wide, (2) an extensive sheet of thick flows, and (3) radar‐bright surfaces that extend to 360–400 km from the volcano. Altimetry indicates that the relief of the domical structure is about 0.5–1.1 km. The lobes and flows have prominent regularly spaced ridges ∼ 686–820 m apart. Thick flows with large ridge separations and broad lobes are rare on Venus. We suggest that the viscosities these flows were larger than those of most lava flows on Venus. Comparisons of the dimensions of the volcano's lobes with lava flows on Earth suggest that the Venusian lavas may have large silica contents. Radar‐bright surfaces around the volcano may represent the result of an explosive eruption or very thin deposits of low‐viscosity lavas. Thus we suggest that the radar‐bright surfaces and lavas of the volcano were derived from a magma that differentiated within the crust or mantle of Venus. The differentiation produced (1) a gas‐rich low‐viscosity phase, (2) high‐viscosity lavas, and (3) a residual primary magma.