Search

Abstract: Venus' tectonic evolution is not well understood. Thousands of kilometers of possible subduction sites on Venus have been identified along networks of rift zone trenches called chasmata. Rift zones are strong candidates for tectonic recycling due to pre‐existing weaknesses in the lithosphere. Recently, peel‐back delamination (PBD) was proposed as a mechanism of regional‐scale lithospheric recycling initiated at Venusian rift zones (Adams et al., 2022, https://doi.org/10.1029/2022je007460). PBD occurs when the lithospheric mantle becomes sufficiently thick and negatively buoyant to decouple and peel away from the overlying crust remaining at the surface. Both positively and negatively buoyant lithosphere were shown to undergo buoyancy‐driven PBD, though delamination is inhibited by increasing positive plate buoyancy. In this study, we use 2D numerical models to verify that delamination can be initiated in thinner, more positively buoyant lithosphere than in models with no plume‐rift interactions. Our results show that plume‐induced PBD in positively buoyant plates is facilitated by the excess negative buoyancy in the lithospheric mantle and increasing plume buoyancy force, and it is inhibited by increasing crustal buoyancy and decreasing rift width. We propose an age‐progressive framework for delamination at rift zones, where young, thin plates require a larger plume buoyancy force to be destabilized than thicker, yet still positively buoyant plates. We use lithospheric thickness constraints to predict PBD may be most likely to initiate near the Dali‐Diana Chasmata system.

Constraining the thermal and compositional state of the mantle is crucial for deciphering the formation and evolution of Mars. Mineral physics predicts that Mars' deep mantle is demarcated by a seismic discontinuity arising from the pressure-induced phase transformation of the mineral olivine to its higher-pressure polymorphs, making the depth of this boundary sensitive to both mantle temperature and composition. Here, we report on the seismic detection of a midmantle discontinuity using the data collected by NASA's InSight Mission to Mars that matches the expected depth and sharpness of the postolivine transition. In five teleseismic events, we observed triplicated P and S waves and constrained the depth of this discontinuity to be 1,006 [Formula: see text] 40 km by modeling the triplicated waveforms. From this depth range, we infer a mantle potential temperature of 1,605 [Formula: see text] 100 K, a result consistent with a crust that is 10 to 15 times more enriched in heat-producing elements than the underlying mantle. Our waveform fits to the data indicate a broad gradient across the boundary, implying that the Martian mantle is more enriched in iron compared to Earth. Through modeling of thermochemical evolution of Mars, we observe that only two out of the five proposed composition models are compatible with the observed boundary depth. Our geodynamic simulations suggest that the Martian mantle was relatively cold 4.5 Gyr ago (1,720 to 1,860 K) and are consistent with a present-day surface heat flow of 21 to 24 mW/m2.

We currently have a limited understanding of the tectonic framework that governs Venus. Schubert and Sandwell (1995) identified over 10,000 km of possible subduction sites at both coronae and chasmata rift zones. Previous numerical and experimental studies have shown the viability of regional-scale lithospheric recycling via plume-lithosphere interactions at coronae, yet little work has been done to study the possibility of resurfacing initiated at Venusian rift zones. We created 2D numerical models to test if and how regional-scale resurfacing could be initiated at a lateral lithospheric discontinuity. We observed several instances of peel-back delamination - a form of lithospheric recycling in which the dense lithospheric mantle decouples and peels away from the weak, initially 30 km-thick crust, leaving behind a hot, thinned layer of crust at the surface. Delamination initiation is driven by the negative buoyancy of the lithospheric mantle and is resisted by the coupling of the plate across the Moho, the significant positive buoyancy of the crust arising from a range of crustal densities, and the viscous strength of the plate. Initial plate bending promotes yielding and weakening in the crust, which is crucial to allow decoupling of the crust and lithospheric mantle. When there is sufficient excess negative buoyancy in the lithospheric mantle, both positively and negatively buoyant plates may undergo delamination. Following a delamination event, the emplacement of hot, buoyant asthenosphere beneath the crust may have consequences for regional-scale volcanism and local tectonic deformation on Venus within the context of the regional equilibrium resurfacing hypothesis.

Analyses of seismic data from the InSight mission have provided the first in situ constraints on the thickness of the crust of Mars. These crustal thickness constraints are currently limited to beneath the lander that is located in the northern lowlands, and we use gravity and topography data to construct global crustal thickness models that satisfy the seismic data. These models consider a range of possible mantle and core density profiles, a range of crustal densities, a low-density surface layer, and the possibility that the crustal density of the northern lowlands is greater than that of the southern highlands. Using the preferred InSight three-layer seismic model of the crust, the average crustal thickness of the planet is found to lie between 30 and 72 km. Depending on the choice of the upper mantle density, the maximum permissible density of the northern lowlands and southern highlands crust is constrained to be between 2,850 and 3,100 kg m−3. These crustal densities are lower than typical Martian basaltic materials and are consistent with a crust that is on average more felsic than the materials found at the surface. We argue that a substantial portion of the crust of Mars is a primary crust that formed during the initial differentiation of the planet. Various hypotheses for the origin of the observed intracrustal seisimic layers are assessed, with our preferred interpretation including thick volcanic deposits, ejecta from the Utopia basin, porosity closure, and differentiation products of a Borealis impact melt sheet.

SEIS, the seismometer of the InSight mission, which landed on Mars on 26 November 2018, is monitoring the seismic activity of the planet. The goal of the Mars Structure Service (MSS) is to provide, as a mission product, the first average 1-D velocity model of Mars from the recorded InSight data. Prior to the mission, methodologies have been developed and tested to allow the location of the seismic events and estimation of the radial structure, using surface waves and body waves arrival times, and receiver functions. The paper describes these validation tests and compares the performance of the different algorithms to constrain the velocity model below the InSight station and estimate the 1-D average model over the great circle path between source and receiver. These tests were performed in the frame of a blind test, during which synthetic data were inverted. In order to propagate the data uncertainties on the output model distribution, Bayesian inversion techniques are mainly used. The limitations and strengths of the methods are assessed. The results show the potential of the MSS approach to retrieve the structure of the crust and underlying mantle. However, at this time, large quakes with clear surface waves have not yet been recorded by SEIS, which makes the estimation of the 1-D average seismic velocity model challenging. Additional locatable events, especially at large epicentral distances, and development of new techniques to fully investigate the data, will ultimately provide more constraints on the crust and mantle of Mars.

On Venus, radar observations of the surface have highlighted two categories of craters: bright‐floored, interpreted as pristine, and dark‐floored, interpreted as being partially filled by lava. While volcanic resurfacing occurs within and outside craters in the plains, it seems mainly concentrated within the interior of dark‐floored craters in the crustal plateaus, suggesting that the magma is negatively buoyant there. Indeed, crater unloading may facilitate vertical ascent of a negatively buoyant magma by decompressing the underlying crust. However, the crater topography also generates a shear stress which would tend to horizontalize the vertical propagation of a dyke. We use numerical simulations of magma ascent in an axisymmetric crater stress field to demonstrate that, depending on the crust thickness and the magma‐crust density contrast, a negatively buoyant magma can indeed erupt only in the crater interior while remaining stored in the crust elsewhere. In particular, we identify four different behaviors depending on if and where a magma‐filled crack ascending below a crater reaches the surface. We draw a regime diagram as a function of two characteristic dimensionless numbers. For eruption to occur only in the crater interior requires a crust thinner than 45 km and a limited range of magma‐crust density contrasts, between 40 and 280 kg m−3 for crust thicknesses between 20 and 45 km, the permissible range decreasing for increasing crustal thicknesses. These results suggest that the crustal plateaus may not be particularly thick and could be slightly differentiated, but probably not very felsic. Plain Language Summary: The Magellan mission revealed two categories of impact craters at the surface of Venus: the pristine bright‐floored and the dark‐floored craters, which are interpreted as craters partially filled by smooth lava after their formation. In the crustal plateaus of Venus, the magma reaches the surface mainly within craters, suggesting that it is denser than the crust. Because of the crater negative topography, the underlying crust is decompressed relative to its surroundings, which, in turn, facilitates magma ascent below the crater despite its negative buoyancy. We first gather surface observations on a set of craters located in the crustal plateaus of Venus to construct a characteristic fresh crater topography. We then use a model of magma ascent below a crater in the crust of Venus to constrain the magma and crust densities as well as the initial magma storage depth that allow for magma eruption within the crater interior only. We show that magma reaches the surface only in the interior of the crater if the crust is slightly less dense than the magma and if it is not too thick (≤45 km in thickness). Key Points: We identify four different behaviors for magma‐filled crack propagation below cratersWe draw a behavior diagram as a function of two dimensionless numbers characterizing dyke propagation below a craterMagma infilling of dark‐floored craters in Venus' plateaus requires a crustal thickness ≤45 km and a small crust‐magma density contrast [ABSTRACT FROM AUTHOR]

An innovative approach to pattern recognition in Synthetic Aperture Radar (SAR) images is described.

An investigation of the relationship between the topographic and gravitational features of four large volcanic swells on planet Venus reveals the presence of hotspots in various stages of evolution. The four swells are Atla, Beta, Bell and Western Eistla Regiones. The investigation is performed by calculating the admittance spectra from topographic and gravitational data by using spherical harmonic and local inversion models.

The InSight Mission began acquiring the first seismic data on Mars in early 2019 and has detected hundreds of events. The largest events recorded to date originate at Cerberus Fossae, a young volcanic region characterized by high volume, low viscosity lava flows. A handful of Low Frequency (LF) quakes that share key attributes of Long Period quakes recorded on Earth's volcanoes are also traced to Cerberus Fossae. This study explores whether a traditional volcanic source model that simulates the generation of tremor as pressurized fluid makes its way through a channel at depth, can explain these atypical LF events. We consider a wide range of physical parameters including fluid viscosity, the ratio of driving pressure to lithostatic pressure, aspect ratio of the channel, and the equilibrium channel opening. We find that the model can produce the observed seismic signature, with a combination of low‐viscosity magma and high volume flux of ∼104 − 105 m3/s that are within an order‐of‐magnitude agreement with Cerberus Fossae lava flow properties deduced from analysis of lava flow dimensions. It is impossible, however, at this stage to conclude whether or not this is a likely explanation for Mars, as the model results in fluxes that are extreme for Earth yet are just within bounds of what has been inferred for Cerberus Fossae. We therefore conclude that we cannot rule out active magma flow as the mechanism responsible for the atypical LF events that likely originate from Cerberus Fossae. Plain Language Summary: A number of Marsquakes are located at a region of Mars that hosted geologically recent volcanic eruptions. A subset of these events resemble seismic events recorded at volcanoes on Earth. We set out to study whether these events can be explained by fluid flow at depth, using a model of fluid flow through a channel. We find that low viscosities and high flow rates that are within an order‐of‐magnitude agreement with flow properties deduced from modeling of Cerberus Fossae lava flows are required to match the observed events in question. It is impossible at this stage of the InSight mission, however, to conclude whether or not this is a likely explanation for Mars. Key Points: Low Frequency (LF) events that share attributes of volcanic quakes are traced to a young volcanic region on MarsLF events are modeled as deep volcanic quakes caused by pressure‐driven flow across a channel at Cerberus FossaeLF attributes are matched by fluids less viscous and of higher fluxes than terrestrial flood basalts [ABSTRACT FROM AUTHOR]

Copyright of Reflets de la Physique is the property of EDP Sciences and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Heat is primarily generated in planetary interiors by the decay of long‐lived heat producing elements (HPE). Planetary heat flow estimates can thus provide critical insights into the thermal state of a planet and the bulk distribution of the HPE. The lack of appreciable lithospheric deflection in the north polar region of Mars by the weight of the polar ice cap is suggestive of low heat flow. Here we model the deflection of the Martian lithosphere and show that the present‐day mantle heat flow cannot exceed 7 mW m−2 in the north polar region of Mars. Our mantle heat flow estimate is notably lower than the heat flow expected from a chondritic mantle suggesting the Martian mantle to be depleted in HPE. If our result is globally representative, lower levels of heat generation in the planet's mantle may have inhibited widespread late‐stage volcanism on Mars. Plain Language Summary: The geophysical evolution of a planet is governed by the rate of heat production within the planet and the mechanism and efficiency through which that heat is lost. After the accretion and differentiation of a planet, the major source of internal heat generation is through the decay of long‐lived radiogenic elements such as uranium, thorium, and potassium. We constrain the present‐day thermal structure in the north polar region of Mars by modeling observed geophysical flexure beneath the north polar cap. Our results suggest that most of the heat producing elements on Mars may be sequestered in the crust and that the mantle is depleted in these elements. If globally representative, our results have important implications for the thermal and geological evolution of Mars. Key Points: We constrain the Martian mantle heat flow by modeling the lithospheric deflection from the weight of the north polar cap on MarsWe find that the mantle heat flow cannot exceed 7 mW m−2 on the north pole of MarsThe estimated low mantle heat flow suggests a strong fractionation of heat producing elements into the Martian crust [ABSTRACT FROM AUTHOR]

Abstract: Signatures of lithospheric flexure were previously identified at a dozen or more large coronae on Venus. Thin plate models fit to topographic profiles return elastic parameters, allowing derivation of mechanical thickness and surface heat flows given an assumed yield strength envelope. However, the low resolution of altimetry data from the NASA Magellan mission has hindered studying the vast majority of coronae, particularly those less than a few hundred kilometers in diameter. Here we search for flexural signatures around 99 coronae over ∼20% of the surface in Magellan altimetry data and stereo‐derived topography that was recently assembled from synthetic aperture radar images. We derive elastic thicknesses of ∼2 to 30 km (mostly ∼5 to 15 km) with Cartesian and axisymmetric models at 19 coronae. We discuss the implications of low values that were also noted in earlier gravity studies. Most mechanical thicknesses are estimated as <19 km, corresponding to thermal gradients >24 K km−1. Implied surface heat flows >95 mW m−2—twice the global average in many thermal evolution models—imply that coronae are major contributors to the total heat budget or Venus is cooling faster than expected. Binomial statistics show that “Type 2” coronae with incomplete fracture annuli are significantly less likely to host flexural signatures than “Type 1” coronae with largely complete annuli. Stress calculations predict extensional faulting where nearly all profiles intersect concentric fractures. We failed to identify systematic variations in flexural parameters based on type, geologic setting, or morphologic class. Obtaining quality, high‐resolution topography from a planetwide survey is vital to verifying our conclusions. [ABSTRACT FROM AUTHOR]