Crustal plateaus, also called plateau highlands, are prominent geologic features on Venus, with roughly circular planforms and diameters ranging from 1,500 to 2,500 km. They present a steep-sided topography reaching 2-4 km of altitude above the surrounding plains, with the highest elevations generally closer to the margins. The surface of the plateaus is dominated by tessera terrains, that are characterized by complex tectonic fabrics which indicate multiple stages of deformation recording both extensional and contractional events (e.g., Bindschadler,

Venus is a terrestrial planet with dimensions similar to the Earth, but a vastly different geodynamic evolution, with recent studies debating the occurrence and extent of tectonic‐like processes happening on the planet. The precious direct data that we have for Venus is very little, and there are only few numerical modeling studies concerning lithospheric‐scale processes. However, the use of numerical models has proven crucial for our understanding of large‐scale geodynamic processes of the Earth. Therefore, here we adapt 2D thermomechanical numerical models of rifting on Earth to Venus to study how the observed rifting structures on the Venusian surface could have been formed. More specifically, we aim to investigate how rifting evolves under the Venusian surface conditions and the proposed lithospheric structure. Our results show that a strong crustal rheology such as diabase is needed to localize strain and to develop a rift under the high surface temperature and pressure of Venus. The evolution of the rift formation is predominantly controlled by the crustal thickness, with a 25 km‐thick diabase crust required to produce mantle upwelling and melting. The surface topography produced by our models fits well with the topography profiles of the Ganis and Devana Chasmata for different crustal thicknesses. We therefore speculate that the difference in these rift features on Venus could be due to different crustal thicknesses. Based on the estimated heat flux of Venus, our models indicate that a crust with a global average lower than 35 km is the most likely crustal thickness on Venus.

<p align="justify">Venus is a terrestrial planet with dimensions similar to the Earth and, although it is generally assumed that it does not host plate-tectonics, there are indications that Venus might have experienced, or still does experience, some form of tectonics. In fact, there are widespread observations of rifts on Venus called ‘chasma’ (plural ‘chasmata’), from radar-image interpretation of normal-fault-bounded graben structures (Harris & Bédard, 2015).</p> <p align="justify">The rifts on Venus have been likened to continental rifts on Earth such as the East African (e.g., Basilevsky & McGill, 2007) and Atlantic rift system prior to ocean opening (Graff et al., 2018), even if they are commonly wider than their terrestrial equivalent (e.g., Foster & Nimmo, 1996). However, despite being a prominent feature on its surface, little is known about the mechanisms responsible for creating rifts on Venus beyond the assumption that they are extensional features (Magee & Head, 1995).</p> <p align="justify">Since rifting on Earth in both continental and oceanic settings has been extensively studied through modeling, we adapted 2D thermo-mechanical numerical models of rifting on Earth to Venus in order to study how rifting structures observed on the Venusian surface could have been formed. More specifically, we investigated how rifting evolves under the high pressure and temperature conditions of the Venusian surface and the lithospheric structure proposed for Venus.</p> <p align="justify">Our results show that a strong crustal rheology such as diabase is needed to localize strain and to develop a rift under the harsh surface conditions of Venus. The evolution of the rift formation is predominantly controlled by the crustal thickness, with a 25 km-thick diabase crust required to produce mantle upwelling and melting. Lastly, we compared the surface topography produced by our models with the topography profiles of different Venusian chasmata. We observed a good fit between models characterised by different crustal thicknesses and the Ganis and Devana Chasmata, suggesting that differences in rift features on Venus could be due to different crustal thicknesses.</p> <p align="justify"> </p> <p align="justify"><strong>References</strong></p> <p align="justify">Basilevsky, A. T., & McGill, G. E. (2007). Surface evolution of Venus. In Exploring Venus as a terrestrial planet (p. 23-43). American Geophysical Union. doi: 10.1029/176GM04</p> <p align="justify">Foster, A., & Nimmo, F. (1996). Comparisons between the rift systems of East Africa, Earth and Beta Regio, Venus. Earth and Planetary Science Letters, 143 (1), 183-195. doi: 10.1016/0012-821X(96)00146-X</p> <p align="justify">Graff, J., Ernst, R., & Samson, C. (2018). Evidence for triple-junction rifting focussed on local magmatic centres along Parga Chasma, Venus. Icarus, 306 , 122-138. doi: 10.1016/j.icarus.2018.02.010</p> <p align="justify">Harris, L. B., & Bédard, J. H. (2015). Interactions between continent-like ‘drift’, rifting and mantle flow on Venus: gravity interpretations and Earth analogues. In: Volcanism and Tectonism Across the Inner Solar System. Geological Society of London. doi: 10.1144/SP401.9</p> <p align="justify">Magee, K. P., & Head, J. W. (1995). The role of rifting in the generation of melt: Implications for the origin and evolution of the Lada Terra-Lavinia Planitia region of Venus. Journal of Geophysical Research: Planets, 100 (E1), 1527-1552. doi: 10.1029/94JE02334</p>

<p>With the selection of multiple missions to Venus by NASA and ESA planned to launch in the coming decade, we will greatly improve our understanding of Venus as a planet. However, the selected missions cannot tell us anything about the seismicity on Venus, which is a crucial observable to constrain the tectonic activity and geodynamic regime of the planet, and its interior structure.<span> </span></p><p>Here, we provide new, preliminary estimates of Venus’ global annual seismic budget and the expected frequency of venusquakes per year. We obtain this estimate by scaling the seismicity of the Earth recorded in the CMT catalogue. We test different potential scaling factors based on e.g., the difference in mass, radius, potential seismogenic volume, etc. We also sort the earthquakes into their respective tectonic settings, which allows us to exclude irrelevant tectonic settings present on Earth, but most likely not on Venus from our analysis. This enables us to present a range of potential seismic budgets and venusquake frequencies per tectonic setting on Venus. <span> </span></p><p>This then provides a new estimate of the potential amount of seismicity on Venus. However, it is uncertain how valid this simple scaling approach is from Earth to Venus. Indeed, previous attempts of scaling the volcanism of Earth to Venus (Byrne & Krishnamoorthy, 2022; Van Zelst, 2022) resulted in numbers that aligned with independent estimates, but are still unconstrained and hard to verify until the announced missions fly. Therefore, in order to provide a more robust and holistic view of Venus’ anticipated seismicity, estimates using various different, independent methods should ideally be considered.</p><p>To provide exactly that, we set up the ISSI team ‘Seismicity on Venus: Prediction & Detection’. This is an interdisciplinary team of experts in seismology, geology, and geodynamics. Together we aim to assess the seismic activity on Venus from a theoretical and instrumental perspective. In addition to presenting our preliminary seismicity estimates from scaling Earth to Venus, we therefore also use this contribution to briefly introduce the team and its goals and present the preliminary findings from our first, week-long, dedicated in-person meeting aimed at further characterising Venus’ seismicity.<span> </span></p><p><strong>References</strong></p><p>Byrne, Paul K., and Siddharth Krishnamoorthy. "Estimates on the frequency of volcanic eruptions on Venus." Journal of Geophysical Research: Planets 127.1 (2022): e2021JE007040.</p><p>van Zelst, Iris. "Comment on “Estimates on the Frequency of Volcanic Eruptions on Venus” by Byrne & Krishnamoorthy (2022)." Journal of Geophysical Research: Planets (2022): e2022JE007448.</p>

The long‐wavelength gravity and topography of Venus are dominated by mantle convective flows, and are hence sensitive to the planet's viscosity structure and mantle density anomalies. By modeling the dynamic gravity and topography signatures and by making use of a Bayesian inference approach, we investigate the viscosity structure of the Venusian mantle by constraining radial viscosity variations. We performed inversions under a wide range of model assumptions that consistently predicted the existence of a thin low‐viscosity zone in the uppermost mantle. The zone is about 235 km thick and has a viscosity reduction of 5–15 times with respect to the underlying mantle. Drawing a parallel with the Earth, the reduced viscosity could be a result of partial melting as suggested for the origin of the asthenosphere. These results support the interpretation that Venus is a geologically active world predominantly governed by ongoing magmatic processes.

There is a growing consensus that Venus is seismically active, although its level of seismicity could be very different from that of Earth due to the lack of plate tectonics. Here, we estimate upper and lower bounds on the expected annual seismicity of Venus by scaling the seismicity of the Earth. We consider different scaling factors for different tectonic settings and account for the lower seismogenic zone thickness of Venus. We find that 11 - 34 venusquakes >=Mw5 per year are expected for an inactive Venus, where the global seismicity rate is similar to that of continental intraplate seismicity on Earth. For the active Venus scenarios, we assume that the coronae, ridges, and rifts of Venus are currently seismically active. This results in 126 - 391 venusquakes >=Mw5 annually as a realistic lower bound and 465 - 1446 venusquakes >=Mw5 as a maximum upper bound for an active Venus.

<p>The dense atmosphere of Venus and the planet’s young surface, dominated by volcanic features, bear witness to its past and potentially ongoing volcanic activity. While unique among the terrestrial planets of our Solar System, Venus is likely similar to a myriad of extrasolar worlds [1]. Thus, investigating Venus’s interior structure, thermal history, and magmatic processes may guide our understanding of the evolution and present-day state of an entire class of exoplanets.</p> <p>The present-day geodynamic regime of Venus’s mantle is still debated, but models agree that magmatism played a major role in shaping the atmosphere and surface that we observe today [2]. In this contribution we will summarize the evidence for recent and possibly ongoing magmatic activity in the interior of Venus and show how we can combine current and future observations with thermal evolution models to constrain the planet’s present-day interior structure, dynamics, and magmatic activity. </p> <p>We calculate the tidal deformation and moment of inertia in our models to provide estimates on deep interior parameters. While the tidal Love number k<sub>2</sub>, which is sensitive to the size and state of the core, has been determined from Magellan and Pioneer Venus Orbiter tracking data with large uncertainties [3], the phase lag of the deformation, whose value is particularly sensitive to the thermal state of the interior, has not yet been measured. A rough estimate of the core size of 3500 km with large (>500 km) uncertainties comes from the moment of inertia factor that was determined from Earth-based radar observations [4].  </p> <p>Our models address the recent volcanic activity that was suggested by several observations [e.g., 5]. In particular, we focus on investigating the constraints coming from estimates of the elastic lithosphere thickness, which is linked to the thermal state of the lithosphere at the time of the formation of geological features. Gravity and topography analyses indicate small elastic thicknesses for a variety of locations including coronae [6], steep-sided domical volcanoes [7], and crustal plateaus [8]. The young age of many surface features on Venus suggests a warm lithosphere at present-day, potentially linked to partial melting in the interior. Moreover, a recent study found that the inferred heat flux at 75 locations on Venus associated with recent volcanic and tectonic activity is similar to the values measured on Earth in areas of active extension [9].  </p> <p>Future measurements of the NASA VERITAS and ESA EnVision missions aim to constrain present-day volcanic and tectonic activity as well as the thickness of major layers (crust, mantle, and core) in the interior of Venus. These measurements will provide unprecedented information to address the interior structure and thermal history of our neighbor, who can teach us about the diversity of evolutionary paths that rocky planets around other stars might have followed.</p> <p>[1] Kane et al., 2019. [2] Rolf et al., 2022. [3] Konopliv and Yodder, 1996. [4] Margot et al., 2021. [5] Smrekar et al., 2010. [6] O’Rourke & Smrekar, 2018. [7] Borrelli et al., 2021. [8] Maia and Wieczorek, 2022. [9] Smrekar et al., 2022. </p>

<p>One of the most informative ways of studying the interior structure and geodynamics of terrestrial planets is the joint investigation of gravity and topography data. In the case of Venus, this is in fact one of the only sources of information about the planet's interior, along with the lack of an active dynamo (e.g., Phillips and Russell, 1987), estimations of the moment of inertia (Margot et al., 2021) and tidal love numbers (Konopliv and Yoder, 1996). The firsts global gravity-topography analyses of Venus revealed unique characteristics. For long wavelengths, the planet presents correlations which are considerably higher than for Earth (Sjogren et al., 1980). Moreover, the apparent depth of compensation is large, globally deeper than 100 km (e.g., Kiefer et al., 1986). By analyzing the wavelength-dependent ratio between gravity and topography, the so-called spectral admittance, Kiefer et al. 1986 concluded that Venus' long-wavelength topography is mostly supported dynamically, i.e., by convective flows in the mantle.</p> <p>Although today we know that some Venusian highlands, such as the crustal plateaus (e.g., Simons et al., 1997, Maia and Wieczorek, 2022) and Ishtar Terra (e.g., Kucinskas et al., 1996), are more consistent with being in a state of isostasy with significant variations on crustal thickness, it is broadly accepted that dynamic compensation is important on a global scale, notably at long wavelengths. Hence, one can attempt to predict Venus' gravity field and surface displacement using a geophysical model of mantle convection and how this couples to the surface. One of the most used instantaneous dynamic loading models was developed by Hager and Clayton (1989), where mantle flows, triggered by density anomalies, depend on radial viscosity variations.  On Venus, this model has been adopted to estimate mantle mass anomalies maps and to investigate the planet's mantle viscosity profile (e.g., Herrick and Phillips, 1992; Pauer et al. 2006; James et al. 2013).  Herrick and Phillips (1992) conclude that Venus mantle is consistent with a constant viscosity profile while Pauer et al. (2006) suggests that Venus could be more similar to Earth, where the mantle viscosity increases with depth and with a possible low-viscosity zone in the upper mantle.</p> <p>These studies investigate the global topography and gravity signals to make their predictions. However, there are major highlands on Venus, such as Ovda, Thetis and Ishtar Terra, that are inconsistent with dynamic support. Using the Hager and Clayton (1989) dynamic model we do a new mantle viscosity investigation that excludes those areas where the gravity and topography signals are best modeled by a combination of Airy isostasy and lithospheric flexure. Our study is performed in the spectral domain and a multitaper spatio-spectral localization approach (Wieczorek and Simons, 2007) is adopted to suppress the signals of Ishtar Terra and Western Aphrodite Terra. The figure below shows a comparison between Venus global spectral admittance and correlation and the tapered estimations. There is a clear increase in admittance and correlation when the localization tapers are applied, specially for long wavelengths where dynamic loading is expected to dominate. The difference is largely attributed to the high elevations associated with the highlands that are near a state of isostasy, and hence have low associated gravity signals.</p> <p><img src="" alt="" width="519" height="297" /></p> <p>Regarding the dynamic modeling, we consider a viscosity profile with four layers, where the viscosity and depth of each layer are randomly sampled following a log-uniform and a uniform distribution, respectively. After computing the predicted topography and gravity field, we multiply the data by several different orthogonal localization windows and calculate the predicted admittance.  The misfits between observations and predictions are computed in order determine the accepted models and constrain the mantle viscosity structure.</p> <p>The range of models that properly fit the observed admittance can be roughly divided into two groups, illustrated in figure below. The first is overall characterized by a layer of relative low viscosity beneath the lithosphere with maximum thickness of about 400 km, followed by an increase of viscosity with depth down to the core-mantle boundary. The second set of models does not present a clear viscosity boundary between the lithosphere and the underlying mantle and, as for the first group, the viscosity tends to increase with depth. In this scenario, however, a large number of models present a basal low viscosity zone in the mantle with thicknesses ranging from about 300 to 1000 km. In the next steps of our study, we intend to pinpoint what is the most likely viscosity structure of Venus’ mantle, interpreting the results in the framework of a Bayesian analysis to assess the likelihood of each of these scenarios and considering the physical implications of the different models.</p> <p><img src="" alt="" width="494" height="422" /></p> <p>Hager, B. and Clayton, R. (1989) <em>Mantle Convection, 657-763</em>; Herrick, R. and Phillips, R. (1992) <em>JGR, 97</em>; James, P. et al. (2013) <em>JGR-Planets, 118</em>; Kiefer et al. (1986) <em>GRL, 13</em>; Konopliv and Yoder (1996) <em>GRL, 23</em>; Kucinskas et al. (1996) <em>JGR, 101</em>; Maia and Wieczorek (2022) <em>JGR-Planets, 127</em>; Margot et al. (2021) <em>Nat Astron</em>; Pauer et al. (2006) <em>JGR, 111</em>; Phillips and Russel (1987); Simons et al. (1997) <em>JGI, 131</em>; Sjogren et al. (1980) <em>JGR, 85</em>; Wieczorek and Simons (2007) <em>J Fourier Anal Appl, 13</em></p>