Deep versus shallow origin of gravity anomalies, topography and volcanism on Earth, Venus and Mars

The relation between gravity anomalies, topography and volcanism can yield important insights about the internal dynamics of planets. From the power spectra of gravity and topography on Earth, Venus and Mars we infer that gravity anomalies have likely predominantly sources below the lithosphere up to about spherical harmonic degree l=30 for Earth, 40 for Venus and 5 for Mars. To interpret the low-degree part of the gravity spectrum in terms of possible sublithospheric density anomalies we derive radial mantle viscosity profiles consistent with mineral physics. For these viscosity profiles we then compute gravity and topography kernels, which indicate how much gravity anomaly and how much topography is caused by a density anomaly at a given depth. With these kernels, we firstly compute an expected gravity-topography ratio. Good agreement with the observed ratio indicates that for Venus, in contrast to Earth and Mars, long-wavelength topography is largely dynamically supported from the sublithospheric mantle. Secondly, we combine an empirical power spectrum of density anomalies inferred from seismic tomography in Earth’s mantle with gravity kernels to model the gravity power spectrum. We find a good match between modeled and observed gravity power spectrum for all three planets, except for 2?l?4 on Venus. Density anomalies in the Venusian mantle for these low degrees thus appear to be very small. We combine gravity kernels and the gravity field to derive radially averaged density anomaly models for the Martian and Venusian mantles. Gravity kernels for l?5 are very small on Venus below ≈800 km depth. Thus our inferences on Venusian mantle density are basically restricted to the upper 800 km. On Mars, gravity anomalies for 2?l?5 may originate from density anomalies anywhere within its mantle. For Mars as for Earth, inferred density anomalies are dominated by l=2 structure, but we cannot infer whether there are features in the lowermost mantle of Mars that correspond to Earth’s Large Low Shear Velocity Provinces (LLSVPs). We find that volcanism on Mars tends to occur primarily in regions above inferred low mantle density, but our model cannot distinguish whether or not there is a Martian analog for the finding that Earth’s Large Igneous Provinces mainly originate above the margins of LLSVPs.     

Gravity studies of Aphrodite Terra on Venus

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

Gravity and Magnetic Investigations in the Haughton Impact Structure,Devon Island,Canada

Abstract— The results of a new gravity survey show that the Haughton impact structure is associated with a 24 km diameter negative Bouguer gravity anomaly with a maximum amplitude of ?12 mgal. A local minimum with a half-width of 2 km and an amplitude of ?4 mgal is located at the center of the structure. A positive magnetic total field anomaly with a half-width of 0.6 km and an amplitude of 700 nT coincides with the local central gravity anomaly. The overall negative gravity anomaly is explained by lowered rock densities due to impact-related fracturing in the crater area. The central gravity and magnetic anomalies are believed to be due to highly shocked and heated sedimentary and crystalline basement rocks forming the unexposed peak of the central uplift in the Haughton impact structure.     

Processes of crustal formation and evolution on Venus: An analysis of topography,hypsometry, and crustal thickness variations

On Venus, present evidence indicates a crust of predominantly basaltic composition and a relatively young average age for the surface (several hundreds of millions of years). Estimates of crustal thickness from several approaches suggest an average crustal thickness of 10–20 km for much of the lowlands and rolling plains and a total volume of crust of about 1 × 10¹⁰ km³, approximately comparable to the present crustal volume of the Earth (1.02 × 10¹⁰ km³). The Earth's oceanic crust is thought to have been recycled at least 10–20 times over Earth history. The near-coincidence in present crustal volumes for the Earth and Venus suggests that either: (1) the presently observed crust of Venus represents the total volume that has accumulated over the history of the planet and that crustal production rates are thus very low, or (2) that crustal production rates are higher and that there is a large volume of missing crust unaccounted for on Venus which may have been lost by processes of crustal recycling.Known processes of crustal formation and thickening (impact-related magma ocean, vertical differentiation, and crustal spreading) are reviewed and are used as a guide to assess regional geologic evidence for the importance of these processes on Venus. Geologic evidence for variations in crustal thickness on Venus (range and frequency distribution of topography, regional slopes, etc.) are outlined. The hypothesis that the topography of Venus could result solely from crustal thickness variations is assessed and tested as an end-member hypothesis. A map of crustal thickness distribution is compiled on the basis of a simple model of Airy isostasy and global Venus topography. An assessment is then made of the significance of crustal thickness variations in explaining the topography of Venus. It is found that the distinctive unimodal hypsometric curve could be explained by: (1) a crust of relatively uniform thickness (most likely 10–20 km thick) comprising over 75% of the surface, (2) local plateaus (tessera) of thickened crust (about 20–30 km) forming less than 15% of the surface, (3) regions of apparent crustal thicknesses of 30–50 km (Beta, Ovda, Thetis, Atla Regiones and Western Ishtar Terra) forming less than 10% of the surface and showing some geologic evidence of crustal thickening processes (these areas can be explained on the basis of geologic observations and gravity data as combinations of thermal effects and crustal thickening), and (4) areas in which Airy isostasy predicts crustal thicknesses in excess of 50 km (the linear orogenic belts of Western Ishtar Terra, less than 1% of the surface).It is concluded that Venus hypsometry can be reasonably explained by a global crust of generally similar thickness with variations in topography being related to (1) crustal thickening processes (orogenic belts and plateau formation) and (2) local variations in the thermal structure (spatially varying thermal expansion in response to spatially varying heat flow). The most likely candidates for the formation and evolution of the crust are vertical differentiation and/or lateral crustal spreading processes. The small average crustal thickness (10–20 km) and the relatively small present crustal volume suggest that if vertical crustal growth processes are the dominant mechanism of crustal growth, than vertical growth has not commonly proceeded to the point where recycling by basal melting or density inversion will occur, and that therefore, rates of crustal production must have been much lower in the past than in recent history. Crustal spreading processes provide a mechanism for crustal formation and evolution that is consistent with observed crustal thicknesses. Crustal spreading processes would be characterized by higher (perhaps more Earth-like) crustal production rates than would characterize vertical differentiation processes, and crust created earlier in the history of Venus and not now observed (missing crust) would be accounted for by loss of crust through recycling processes. Lateral crustal spreading processes for the formation and evolution of the crust of Venus are interpreted to be consistent with many of the observations derived from presently available data. Resurfacing through vertical differentiation processes also clearly occurs, and if it is the major contributor to the total volume of the crust, then very low resurfacing rates are required.Although thermal effects on topography are clearly present and important on both Venus and the Earth, the major difference between the hypsometric curves on Earth (bimodal) and Venus (unimodal) is attributed primarily to the contrast in relative average thickness of the crust between the two terrains on Earth (continental/oceanic; 40/5 km = 35 km, 8:1) and Venus (upland plateaus/lowlands; about 30/15 km = 15 km, 2:1) (35 – 20 km = a difference of 20 km). The Venus unimodal distribution is thus attributed primarily to the large percentage of terrain with relatively uniform crustal thickness, with the skewness toward higher elevations due to the relatively small percentage of crust that is thickened by only about a factor of two. The Earth, in contrast, has a larger percentage of highlands (continents), whose crust is thicker by a factor of eight, on the average, leading to the distinctive bimodal hypsometric curve.Data necessary to firmly establish the dominant type of crustal formation and thickening processes operating and to determine the exact proportion of the topography of Venus that is due to thermal effects versus crustal thickness variations include: (1) global imaging data (to determine the age of the surface, the distribution and age of regions of high heat flux, and evidence for the nature and global distribution of processes of crustal formation and crustal loss), and (2) high resolution global gravity and topography data (to model crustal thickness variations and thermal contributions and to test various hypotheses of crustal growth).'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Rayleigh-Taylor instability as a mechanism for corona formation on Venus

In this study we explore the idea that coronae have formed on Venus as a result of gravitational (Rayleigh-Taylor) instability of the lithosphere. The lithosphere is represented by a system of stratified homogeneous viscous layers (low-density crust over high density mantle, over lower density layer beneath the lithosphere). A small harmonic perturbation imposed on the base of the lithosphere is observed to result in gravitational instability under the constraint of assumed axisymmetry. Topography develops with time under the influence of dynamic stress associated with downwelling or upwelling, and spatially variable crustal thickening or thinning. Topography may therefore be elevated or depressed above a mantle downwelling, but the computed gravity anomaly is always negative above a mantle downwelling in a homogeneous asthenosphere. The ratio of peak gravity to topography anomaly depends primarily on the ratio of crust to lithospheric viscosity. Average observed ratios are well resolved for two groups of coronae (∼40 mgal km⁻¹), consistent with models in which the crust is perhaps 5 times stronger than the lithosphere. Group 3a (rim surrounding elevated central region) coronae are inferred to arise from a central upwelling model, whereas Group 8 (depression) coronae are inferred to arise from central downwelling. Observed average coronae radii are consistent with a lithospheric thickness of only 50 km. An upper low-density crustal layer is 10-20 km thick, as inferred from the amplitude of gravity and topography anomalies.     

Tectonics of the southern escarpment of Ishtar Terra on venus from observations of morphology and gravity

Maxima of calculated topographical line-of-sight (LOS) gravity attractions caused by Ishtar Terra are shifted to the north with respect to the measured LOS free air gravity maxima south of the highland. This implies a tendency to isostatic compensation of central Ishtar and mass surpluses at the continental border and the southern forelands.The following scenario is compatible with the interpretation of the gravity anomalies and morphological features. Relative motions of the lowland Sedna Planitia against continental Ishtar Terra have caused buckling and flat subduction of the lowland lithospheric material. (Deep subduction can be ruled out by thermal reasons). The free air gravity high is modelled by surplus masses of the buckling and of the high density subducting plate. Evidence for this is given by several compressional features like Ut and Vesta Rupes at the southern continental border and ridges at the SW-flanks of Maxwell Montes. It is further supported by several possible volcanic-tectonic depressions located in the southern part of Ishtar. This local interpretation does not necessarily imply the existence of global plate tectonics on Venus like on Earth, but at least limited horizontal movements of the Venusian lithosphere seem to be likely. This result shows that plate recycling must be considered for heat transfer through the lithosphere beside conduction and hot spot volcanism.Contribution No. 273, Institut für Geophysik der Universität Kiel, F.R.G.     

Eastern Aphrodite Terra on Venus: Characteristics,structure, and mode of origin

Eastern Aphrodite Terra and Western Aphrodite form an altimetrically prominent 14,000 km long part of the equatorial highlands on Venus. Several parallel linear discontinuities striking northwest across the general east-west regional strike of the highlands are mapped in the altimetric and radar image data of Eastern Aphrodite and identified on the basis of abrupt termination of rift-like central chasma, offset and segmentation of the center of the highlands, and radar image discontinuities in the lowlands to the north. These characteristics are similar to those of linear discontinuities previously mapped in Western Aphrodite in terms of length, orientation, and influence on the central highlands and adjacent lowlands.Altimetric profiles in directions parallel to the discontinuities are regionally symmetric, more ridge-like in Eastern Aphrodite compared to the plateau-dominated form of topography in Western Aphrodite, and are characterized by alternating paired ridge-and-trough forms near their crests and on their flanks. By mapping the center of symmetry in multiple profiles, the prominent segmentation of the highland is shown to be imparted by an offset of the regional symmetry along the mapped discontinuities. These characteristics are morphologically similar to several of the large-scale characteristics of divergent plate boundaries of Earth, including mid-ocean rise crests and rifts, offset at fracture zones and transform faults, and symmetric thermal boundary layer topography.The altitude of the surface in profiles parallel to the discontinuities decreases as the square root of distance from the symmetry axes and with a slope similar to that predicted for thermal boundary layer topography associated with rates of divergence on Venus of ~ 1 ± 0.5 cm/yr. In order to test the hypothesis that the linear discontinuities are analogous to fracture zones, the predicted altitude of the surface at great distance from the centers of symmetry of the central highland and in directions across the discontinuities was calculated on the basis of a thermal boundary layer topography model with offset of altimetric symmetry at each discontinuity. Similarity of observed Arecibo high-resolution altimetric profiles across the discontinuities with that calculated for thermal boundary layer topography offset by transform faults reveals that in terms of the sense and magnitude of regional steps in altimetry across discontinuities and the altitude of the surface, Eastern Aphrodite is similar to the known characteristics of crustal spreading at divergent boundaries. The plateau-like form of Western Aphrodite and the ridge-like form of Eastern Aphrodite are analogous respectively to the difference between areas of anomalous (Iceland) and normal crustal production along rise crests on Earth. Estimates of volumetric differences in crustal production in the environment of Venus and as it would be influenced by differences in mantle temperature beneath Western and Eastern Aphrodite imply that Eastern Aphrodite represents normal crustal production. On this basis, Western Aphrodite may be characterized by a mantle temperature that is warmer than the mantle beneath Eastern Aphrodite Terra, perhaps in association with deep convective mantle upwelling.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci., Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT. Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

The stress state of Venusian crust and variations of its thickness: Implication for tectonics and geodynamics

The data obtained for the heights of the relief and the external gravitational field of Venus for spherical harmonics with degree and order up to 18 allow one to start theoretical analysis of the crust-mantle boundary (Venusian Moho) and stress state of the planetary interior. We suppose that Venusian convection is confined by floating massive crust. Apparently the convection in the upper mantle of Venus is separated from that one in the lower mantle and its lateral scale must be essentially smaller than on Earth. So, the convection is reflected to a larger degree of the gravitational field of the planet than for Earth. The spherical harmonic expansion of the topography for Venus correlates with corresponding expansion of the non-equilibrium part of the gravitational potential for n = 3–18. At the same time the relief of Venus is significantly compensated. It is reasonable to suppose that the gravity field for these harmonics is due to crustal thickness variations and, probably, to variations of crustal density. Thus, in the proposed scheme the Moho's relief causes the partial isostatic compensation of the topography.All calculations are carried out for the series of realistic models of Venus taking into consideration an asthenosphere. The asthenosphere is modeled either by a weakened (shear modulus is reduced), or by a liquid inviscid layer. We also suppose that the asthenosphere extends from the base of crust to a depth of 418 km, and the density contrast across the Moho boundary is –0.4 g * cm^–3. If the actual density contrast across the Moho is less than the supposed one by some factor, then one must increase the amplitudes of the roots and inverse roots by the same factor. The results for the Moho's relief and stresses in the crust are presented for the case of the mean thickness of the crust of 50 km, which satisfies the probable upper (connected with phase transitions in waterless basalts) and lower (appearing in the framework of our interpretation) limits.On the whole, the crust-mantle boundary on Venus is evidently smooth, and the stress level in the crust is appreciably smaller than the crustal stresses on the Earth. The strong sensitivity of the stresses character to the parameters of the model of external layers of Venus together with geological data allow us to begin a preliminary investigation of the tectonical structure and geodynamics of the planet.'Geology and Tectonics of Venus', special issue edited by Alexander T. Basilevsky (USSR Acad. of Sci. Moscow), James W. Head (Brown University, Providence), Gordon H. Pettengill (MIT, Cambridge, Massachusetts) and R. S. Saunders (J.P.L., Pasadena).     

Strength of Faults on Mars from MOLA Topography

The stresses which must be maintained on faults bounding the rift topography at Tempe Fossae—the “North Tempe Rift” (NTR)—and Valles Marineris (VM) on Mars are estimated, using a simple elastic model and topographic data from the Mars Orbiter Laser Altimeter (MOLA). The absence of rift-flank uplift at the NTR is consistent with an elastic thickness, T_e, of 20 km or greater at the time of rift formation. The maximum resolved shear stresses on bounding faults due to this topography do not therefore exceed 20 MPa, similar to the inferred strength of terrestrial faults. Elastic thickness estimates at VM are mostly around 50 km or greater. Therefore, for canyon widths of ∼400 km, the bounding faults of VM, if present, must be able to withstand stresses of up to approximately 100 MPa. However, if the fault-controlled sections of the canyons do not exceed 150 km in width, as suggested by geomorphological analysis, the fault strength required is only 20 MPa. Although the maximum resolved shear stresses required to support the topography at VM may need to be greater than the stresses which terrestrial faults can support, at least some faults on Mars are no stronger than similar features on Earth. This observation is consistent with the existence of liquid water in the shallow subsurface of Mars at the time the faults were active. On Venus, plate tectonics is probably prevented by the frictional resistance to motion across strong faults. On Mars, it is more likely that the large thickness of the elastic layer of the lithosphere and the possible positive buoyancy of the crust are responsible for the observed lack of plate tectonics.     

Lunar gravity via the Apollo 15 and 16 subsatellites

Dense Doppler tracking coverage of the Apollo 15 and 16 subsatellites over ten and eighteen day periods when periapsis altitudes were 15–50 km has provided detailed gravity mapping of the lunar frontside. Many new gravity features are revealed including one that does not correlate with any visible topographic structure. All unfilled craters sampled are negative anomalies. The mascons consistently produce gravity highs that load the surface with ≈800 kg cm^?2 excess mass. The Orientale region is represented with a solution grid of 177 point masses that clearly show the ringed structure. The eastern limb is also displayed with a solution grid of point masses. The gravity variations over the central portion of the frontface are shown as line-of-sight acceleration contours in milligals.     

Geoid and Gravity in Earth Sciences – An Overview

Precise global geoid and gravity anomaly information serves essentially three different kinds of applications in Earth sciences: gravity and geoid anomalies reflect density anomalies in oceanic and continental lithosphere and the mantle; dynamic ocean topography as derived from the combination of satellite altimetry and a global geoid model can be directly transformed into a global map of ocean surface circulation; any redistribution or exchange of mass in Earth system results in temporal gravity and geoid changes. After completion of the dedicated gravity satellite missions GRACE and GOCE a high standard of global gravity determination, both of the static and of the time varying field will be attained. Thus, it is the right time to investigate the future needs for improvements in the various fields of Earth sciences and to define the right strategy for future gravity field satellite missions.     

A lunar crustal gravity model across the Triesnecker-Hyginus area,Mare Tranquillitatis,and Mare Fecunditatis

LOS Bouguer gravity anomalies have been calculated from a low altitude LOS free air Doppler gravity profile across northern Mare Fecunditatis, southern Mare Tranquillitatis and the Aridaeus Rille. The Hyginus-Triesnecker area has been included in model calculations, though here only free air anomalies are present. A crustal density model has been fitted to the Bouguer anomalies and to the free air anomalies in the case of the Hyginus-Triesnecker area.On a regional scale northern Fecunditatis has Bouguer anomalies up to 80 mgal and lithostatic stresses of 29 bar and thus is nearly in isostatic equilibrium. Tranquillitatis can be divided into three regions of different crustal structure: (1) northern Tranquillitatis with only minor free air gravity anomalies is more or less in isostatic balance, (2) the southeastern region with Bouguer anomalies to –100 mgal and lithostatic stresses of –73 bar has a considerable mass deficit, (3) the southwestern basin is dominated by the local structure Lamont with a Bouguer maximum of 200 mgal and extremely high lithostatic stresses of 285 bar.The Bouguer minimum of –180 mgal of the Aridaeus area has been modelled by two alternative models: (i) a crustal thickening of 33 km and associated lithostatic stresses of –164 bar, and (ii) a crustal thickening of 20 km plus a low density intrusion. The free air maximum of the Hyginus-Triesnecker area has been fitted by a mantle plug connected with stresses of 116 bar.As the old irregular maria could not sustain large mascon stresses, it has been concluded that the local high stresses of Lamont, Aridaeus, and Hyginus-Triesnecker have been evolved after the impacts of the circular maria. Intrusional activities in these areas could have proceeded to fault zones generated by the large impacts.Contribution No. 211, Institut für Geophysik der Universität Kiel, F.R.G.     

Lithospheric structure in the eastern region of Mars’ dichotomy boundary

We examine gravity, topography, and magnetic field data along the well-preserved Martian dichotomy boundary between 105° and 180°E to better understand the origin and modification of the dichotomy boundary. Admittance modeling indicates bottom-loading for the Amenthes region (105–135°E) with crustal and elastic thickness estimates of 15–40 km, and 15–35 km and top-loading for the Aeolis region (145–180°E) with crustal and elastic thickness estimates of 10–20 km and 10–15 km, respectively. There is a general trend from bottom-loading in the west, to top-loading in the east. The bottom-loading signature near Amenthes may reflect its proximity to the Isidis basin or a broad valley southeast of Isidis. Surface volcanic deposits may produce the top-loading seen at Aeolis. Additional processes such as erosion and faulting have clearly affected the dichotomy and may contribute to the loading signature. Low elastic thickness estimates are consistent with loading in the Noachian, when heat flow was high. Significant Bouguer and isostatic gravity anomalies in these areas indicate substantial variations in the crustal density structure. Crater age dating indicates that major surface modification occurred early in the Noachian, and the small elastic thickness estimates also suggest that subsurface modification occurred in the Noachian. Magnetic and gravity anomalies show comparable spatial scales (several hundred kilometers). The similarity in scale and the constant ratio of the amplitudes of the isostatic and Bouguer gravity to the magnetic anomalies along the dichotomy suggest a common origin for the anomalies. Igneous intrusion and/or local thinning or thickening of the crust, possibly with a contribution from hydrothermal alteration, are the most likely mechanisms to create the observed anomalies.     

Effect of topographic bias on geoid and reference ellipsoid of Venus, Mars, and the Moon

Since the continuation of an external gravity field inside topographic masses by a harmonic function results in topographic bias, geoid computation by means of global gravity models (GGMs) in terms of external-type series of spherical harmonics, at locations where the GGMs are evaluated inside the topographic masses, will be biased. Consequently, if the reference ellipsoid is defined based on the geoid, it will also be biased. In this paper, the effects of topographic bias on the geoid and reference ellipsoid of Venus, Mars, and the Moon are studied. Moreover, a thorough error analysis in the geoid and reference ellipsoid computation is presented, and it is shown that the estimated standard deviation (STD) of the geoid potential value, the geoidal heights, and the semimajor and semiminor axes of the reference ellipsoid are independent of the topographic bias. According to the results, the effects of topographic bias on the geoid potential value and the semimajor and semiminor axes of the reference ellipsoid in comparison with their estimated STDs are insignificant for Venus, Mars, and the Moon. Moreover, the effect of topographic bias on the geoidal heights of Venus as compared with the estimated STD of its geoidal heights is insignificant. However, the effects of topographic bias on the geoidal heights of Mars and the Moon can be significant, especially in high mountains such as the Tharsis volcanic region on Mars.     

Topographic comparisons of uplift features on Venus and Earth: Implications for Venus tectonics

Venus and Earth display different hypsography. We use topographic profiles to search for well-understood terrestrial analogs to venusian features. Specifically, by using cross-correlation, we correlate average profiles for terrestrial rifts (slow and fast, “ultra-slow,” incipient and inactive) and also hotspots (oceanic and continental) with those for venusian chasmata and regiones, to draw inferences as to the processes responsible for shaping Venus’ surface. Correlations tend to improve with faster spreading rates; Venus’ correlations rank considerably lower than terrestrial ones, suggesting that if chasmata are analogous to terrestrial spreading centers, then spreading on Venus barely attains ultra-slow rates. Individual features’ normalized average profiles are correlated with profiles of other such features to establish the degree of similarity, which in turn allows for the construction of a covariance matrix. Principal component analysis of this covariance matrix shows that Yellowstone more strongly resembles Atla, Beta and W. Eistla regiones than it does the terrestrial oceanic hotspots, and that venusian chasmata, especially Ganis, most closely resemble the ultra-slow spreading Arctic ridge.     

Transient mantle layering and the episodic behaviour of Venus due to the ‘basalt barrier’ mechanism

Numerical models of mantle convection that include the ‘basalt barrier’ mechanism are explored for Venus. The ‘basalt barrier’ mechanism is due to the positive buoyancy of subducted basaltic crust between the mantle depths of 660 and 750 km. The inclusion of this mechanism in models of Earth’s evolution has been shown to cause episodic mantle layering early in Earth history and we explore whether it can also operate on Venus. The models presented here include a moderately mobile lithosphere, which is not representative of the current state of Venus, but this allows us to exclude the effects of episodic lithosphere mobility and thus to isolate the effect of the basalt barrier. This is a step in a systematic approach to models with a mostly-static lithosphere. We find the basalt barrier does yield episodically layered mantle convection in some Venus models. The likelihood of episodic layering is increased by Venus high surface temperature and by its less mobile or immobile lithosphere. Surprisingly, secondary differences from Earth, including the lower gravity, density and mantle depth also promote episodic layering. The models suggest that mantle layering and overturns may still be likely to occur in Venus. The breakdown of mantle layering and consequent mantle overturns would lead to dramatic episodes of volcanism, formation of large amounts of crust, and tectonic activity on the planet’s surface, as has been inferred to have happened on Venus around 500 Ma ago from surface morphology and cratering. These results thus suggest that a transient layering of the mantle by the ‘basalt barrier’ mechanism and mantle overturns may be part of the explanation for Venus’s recent resurfacing.     

Morphologic and gravimetric investigations of Bell and Eisila Regiones on Venus

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

Numerical assessment of the effects of topography and crustal thickness on martian seismograms using a coupled modal solution-spectral element method

The past 4 decades of Mars exploration have provided much information about the Mars surface, when its interior structure remains relatively poorly constrained. Today available data are compatible with a large range of model parameters. Seismology is able to provide valuable additional data but the number of seismographs will likely be quite limited, specially in the early-stage of future Mars seismic networks. It is thus of importance to be able to correctly isolate effects induced by the crust structure. Mars topography is characterized by spectacular reliefs like the Tharsis bulge or the Hellas basin and by the so-called “Mars dichotomy”: the north hemisphere is made up of low-altitude plains above a relatively thin crust when the south hemisphere is characterized by a thick crust sustaining high reliefs. The aim of this paper is to study the effects induced on seismograms by the topography of the surface and crust-mantle discontinuities. Synthetic seismograms were computed using the coupled spectral element-modal solution method, which reduces the numerical cost by limiting the use of the spectral element method to the regions where lateral variations, like the presence of a topography, are considered. Due to numerical cost, this study is limited to long period and thus focuses on surface waves, mainly on long period Rayleigh waves. We show that reliefs like the Tharsis bulge or the Hellas basin can induce an apparent velocity anomaly up to 0.5% when only the surface topography is introduced. Apparent anomalies can raise up to 1.0% when the surface topography is fully compensated by a mirror-image topography of the crust-mantle discontinuity. Travel-time of surface wave are systematically increased for seismometers in the north hemisphere of Mars and decreased in the south hemisphere. When comparing effects on seismograms by the Earth and Mars topography, we found them to be larger for the Earth. It is due to the fact that we work with a seismic velocity model of Mars with a mean crust thickness of 110 km when the crust thickness has a mean value of 50 km for the Earth. When changing the Mars model for a thinner crust with a mean thickness of 50 km, effects by the topography on Mars seismograms becomes of the same order when not larger than what is observed on the Earth.     

On the possibility of plate tectonics on Venus

Several arguments have been put forward suggesting that Venus has no place tectonics. We examine some of these arguments and suggest that because conditions on the surface of Venus are very different from those on Earth, the arguments should be reconsidered. We show that in the absence of an ocean, the differential hypsographic curve of Earth would probably have only one mode, like that for Venus. We show that the atmosphere of Venus is quite capable of erosion, provided that near-surface velocities are about 1 m · sec^?1 or more, and that therefore the “oceanic” areas on Venus, should they exist, are probably covered with some thickness of sediment. If sedimentation on Venus is at all rapid, it is likely that subduction zones could be filled up and made unrecognizable topographically. Because Venus does not have an ocean, and because its surface temperature is much greater than that on Earth, ridge crests on Venus have a much smaller topographic expression than those on Earth. If significant sedimentation occurs they would be completely unrecognizable topographically.     

Tectonics of Venus: A review

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