PS4.1

Our neighbouring planet Venus holds key insights into terrestrial planet evolution. At present, there is no mosaic of mobile tectonic plates on the planet, yet Venus’ surface is scarred with many tectonic and volcanic structures. Surface deformation seems to be related to regional-scale tectonic deformation and/or mantle upwellings, but it remains questionable exactly when, and how, Venus is resurfacing. “Coronae” are ~circular crown-like structures with traces of tectonic and volcanic activity. They are commonly proposed to be surface manifestations of mantle plume upwellings and/or magmatism, and may therefore provide fundamental insights to Venus’ interior dynamics through time. The exact processes underlying their development and the reasons for their diverse morphologies have been widely debated in the past, with several key outcomes for the Venus scientific community. 

In this presentation, I focus on our recent 3D numerical studyof plume-induced corona formation ^[1] and discuss what insights this study gives on the thermal evolution of Venus, as well as its present-day geological activity. The modelled corona morphologies are strongly on the lithospheric structure and the underlying dynamic processes at play. By a detailed comparison the modeling results with observed corona features (data from NASA’s Magellan mission), widespread plume activity on Venus was identified.  Moreover, I present prompting new results on the gravitational signatures of these modelled corona structures, and discuss whether we can distinguish between different stages of corona evolution in the gravity field. These outcomes may be important for future radio science experiments aboard ESA’s EnVision orbiter.

Finally, I’ll touch upon several key directions for future research on these enigmatic coronae structures, which are relevant in light of the upcoming ‘Decade(s) of Venus Science’. While I mainly formulate these key questions form a geodynamical point-of-view, I invite scientists from all disciplines of the Geo- and Planetary sciences to join the discussion on how these unique coronae can provide key information on the evolution of the interior and surface of Earth’s twin planet.


[1] Gülcher, A.J.P., Gerya, T.V., Montési, L.G.J., and Munch, J., (2020). Corona structures driven by plume–lithosphere interactions and evidence for ongoing plume activity on Venus. Nature Geoscience, 13, 547–554. 

How to cite: Gülcher, A., Gerya, T., and Montési, L.: Corona structures as a window into volcano-tectonic activity on Venus: key insights and ways forward, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7680, https://doi.org/10.5194/egusphere-egu22-7680, 2022.

Rifting and rises are prominent landscape features in the roughly triangular area characterized by the presence of three major rises (Atla, Beta and Themis) and two corona-dominated long chasmata (Hecate and Parga). The coronae population associated with these chasmata represents 35% of all Venusian coronae and 56% of coronae associated with fracture zones (Smrekar et al., 2010). We focused on the spatial analysis of the coronae population associated with Parga chasma for identifying the depth of the main thermal anomaly that fed (and maybe still feeds) them.

We explore a formation mechanism for coronae based on the Rayleigh–Taylor (R-T) gravitational instability (Tackley and Stevenson, 1991) of the lithosphere that may occur when a layer of dense fluid overlies a layer of less dense fluid. The R-T gravitational instability theory can be used to draw a relationship between the spacing of volcanic structures and edifices at the surface and the depth of the source of instability beneath the volcanic fields (i.e. the lithosphere-asthenosphere boundary depth where partial melting is initiated and starts the vertical upwelling of material). We performed the analyses both on the entire population and on two sub-groups obtained from automatic clustering based on point spacing analysis. Overall, the results obtained from the analysis of the entire population can be considered a global average while the information extracted from the analyses of the two clusters are to be interpreted as end members. Hence, the lithosphere-asthenosphere boundary depth results to be located at 117 ± 10 km underneath Parga.

Additionally, we ran geodynamical models using a variable thermal conductivity and expansivity, and reference viscosities between 1e20 and 1e22 Pa s. These models use an extrusive to intrusive magmatism ratio of 0.1, a typical terrestrial value (Crisp et al., 1984). The intrusive melt is assumed to stall at the base of the crust (~20 km depth; James et al., 2013), since the latter represents a density barrier. According to these models,  a mantle reference viscosity of 1e20 Pa s is best compatible with the geologically inferred lithosphere thickness as well as a thin mechanical thickness as suggested by elastic thickness estimates (e.g., O’Rourke & Smrekar 2018).

As future missions will return higher resolution imagery and topographical information, we suggest the area of Parga chasma as a region of high interest for future data acquisitions. In fact, more detailed data can allow the observation of stratigraphic relationships between rises, rifts, coronae, and volcanoes in order to reconstruct the event sequences. By means of R-T analysis and similar techniques, we would thus be able to refine current analyses and perform more detailed estimates from smaller volcanic features and obtain more precise information about magma reservoir distribution in the subsurface.

How to cite: De Toffoli, B., Mazzarini, F., Plesa, A.-C., Vaujour, T., Breuer, D., and Hauber, E.: Revealing Venus Interior from Coronae Analysis, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-954, https://doi.org/10.5194/egusphere-egu22-954, 2022.

Our team has discovered first impact craters under the thick Venusian atmosphere with radar images during the Venera 15/16 mission. Later Magellan radar images of a better quality allowed us to count all impact craters and to find amazing features, splotches, resulted most probably from “airbursts” - total explosive disruption in flight of small celestial bodies. Splotches could have a central feature (possibly caused by terminal impacts of fragments), or could be diffusive patches of increased (bright) or decreased (dark) areas of changed radar reflectivity. The main explanation so far is that atmospheric shock waves, generated by airbursts, somehow change the surface radar reflectivity, e.g. creating smoother (radar dark) or more rough (radar bright) zones due to reflection of shocks. Size of splotches vary from ~10 km to ~200 km, being comparable with the characteristic atmosphere thickness. The exact mechanisms of air shock wave interaction with the surface is still under debates, but promises to help us better understand the presence of dust/sand/pebbles/boulders at the surface of Venus as well as to estimate mechanical properties of surface rocks. We start a small project to support the issue. The project includes the numerical modeling of atmospheric shock waves on Venus due to cratering impacts and due to airbursts. Our modeling is compared with results published in 1990s-2000s. Airbursts are modeled as a hot spheric volume gas explosion 10 to 40 km above the surface in the Venusian stratified atmosphere. In addition to trivial parameters like maximum pressure, dynamic pressure and the wind speed behind the shock front, necessary for the following analysis of a possible “aeolian” motion of surface’s fines, we try to formulate a general picture of shock wave propagation in the atmosphere after an airburst. We find that the large-scale hot gas bubble from the source zone creates a n x 10 km plume (a kind of a classical “mushroom”), which effectively expands laterally at high altitudes, pushing forward an enhanced shock wave. This wave is looking like a gradual conversion of the main shock wave from a hemispheric one to a conic front, returning back to surface. The other trivial (but not discussed quantitatively) phenomenon is the seismic wave, created by an air shock, but finally overrun the atmospheric shock front. It means that the surface air shock front at large distances arrive after the seismic wave shakes the surface. We plan to investigates all these phenomena and compare models with observations. An interesting possibility seems to be satellite observation of rare meteoroid entry to the Venusian atmosphere, as it now available for terrestrial bolides.

How to cite: Ivanov, B.: Impact features on Venus: Modeling craters and splotches, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6178, https://doi.org/10.5194/egusphere-egu22-6178, 2022.

The idea of a habitable Venus epoch has gained traction in recent years via 1-D and 3-D General Circulation Modeling (GCM) efforts [1,2,3]. Although recent work has supported an alternative permanent hot and dry scenario [4].  However, the habitable scenario presents us with a conundrum - how does a terrestrial planet transform from temperate to hot-house? For decades it was proposed that the gradual brightening of the sun was the probable cause [5]. Yet 4 billion years ago Venus was receiving nearly 1.4 times the insolation that Earth receives today, and many studies have put Earth at the inner boundary of the habitable zone today [6].  The newer 3-D GCM efforts have demonstrated that if Venus had an early habitable period, that the cloud-albedo feedback responsible for maintaining temperate surface conditions [7] could still be in operation today. From this perspective increasing insolation through time cannot be an answer to the transition from habitable to hot-house. We propose that the 'Great Climate Transition' (GCT) was trigged by simultaneous large igneous provinces (LIPs) akin to those like the Siberian Traps responsible for the End Permian [8].  We have taken the most up to date LIP database for Earth [9] and characterized their distribution through time as random or nearly random. Next we initiate a large suite of Monte Carlo simulations based on this record and generate the likelihood for simultaneous, or environmentally overlapping events in this hypothetical setup. We find the probability of such events to be quite high, a probable cause for Venus' GCT, and a possible harbinger of things to come for Earth.

[1] Grinspoon, D.H. & Bullock, M.A. (2007) AGU https://doi.org/10.1029/176GM12
[2] Way, M. J. et al. (2016) GRL 43, 8376–8383
[3] Way, M. J. and Del Genio, A D.  (2020) JGR Planets 125, e2019JE006276
[4] Turbet et al. (2021) Nature,598,276 https://doi.org/10.1038/s41586-021-03873-w
[5] Kasting J. F., Pollack J. B. and Ackerman T. P. (1983) Icarus, 57, 335-355
[6] Kopparapu, R.K. et al. (2013) ApJ 765, 131
[7] Yang, J. et al. (2014) ApJ 707, L2
[8] Wignall, P. (2001) Earth Science Reviews, 53 (1-2), 1-33
[9] Ernst R. E. et al. (2021) AGU Geophys. Mon. 255, pp. 3-26

How to cite: Way, M., Ernst, R., and Scargle, J.: Heat-death by volcano - how Venus went rogue?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4357, https://doi.org/10.5194/egusphere-egu22-4357, 2022.

The habitability of a terrestrial planet depends on its surface conditions, which can vary greatly during its evolution. Volatile exchanges between the interior, the surface and the atmosphere - the cycles of volatile species or their absence - are largely responsible for these variations and the resulting complex feedback mechanisms between the processes involved. The differences between Earth and Venus climate conditions highlight how similar processes and base characteristics can lead to divergent states after billions of years of evolution. While Venus exhibits hostile conditions and its atmosphere appears mostly desiccated, some hypothetical evolution scenarios suggest it was not always so, and that it could have sustained liquid water oceans for an undefined period of time during its past history. We investigate what mechanisms are likely to be responsible for this type of catastrophic change on Venus and possibly on terrestrial planets, using coupled numerical evolution simulations of planetary evolution, involving mantle dynamics, volcanism, atmospheric greenhouse, escape mechanisms, meteoritic impacts and surface solid-gas exchanges. Increasing solar luminosity (the faint young sun paradox) only marginally affects surface temperature changes. Atmospheric escape could only hide the results of a runaway greenhouse phase by removing water rather than cause the observed climate change. Moreover, it is shown, especially in light of recent measurements interpretation, to be unlikely to be responsible for massive water loss. Large impacts, capable of releasing large amounts of volatiles in the atmosphere, are infrequent and unlikely to occur during late evolution. The smaller impactors do not have enough mass to affect the mantle or atmosphere substantially.  The cause of catastrophic transitions and the means to dessicate the atmosphere of Venus post-runaway greenhouse may be internal. We investigate volcanic gas release based on mantle composition and mantle dynamics over time, as well as oxidation mechanisms of fresh material that can trap volatiles into the surface. Solid surface oxidation is inefficient and appears to be roughly as efficient (within 0.1-1 order of magnitude) as recent atmospheric escape, when considering O removal during the last few billion years. Ashes oxidation could be more efficient but requires explosive volcanism that is not widespread on Venus, given the few traces detected from surface observation. We compare its effects to that on Earth. However, large variations in atmospheric composition and vertical structure resulting from runaway greenhouse could affect all the mechanisms involved in the evolution of terrestrial planets and, under some circumstances lead to a late molten surface phase. Surface exchanges and atmospheric loss would therefore be affected in turn.

How to cite: Gillmann, C. and Golabek, G.: The role of surface volatile exchanges in evolving climate conditions on terrestrial planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3336, https://doi.org/10.5194/egusphere-egu22-3336, 2022.