PS3.3

Introduction: Hydrogen isotopic compositions (D/H or 𝛿D) in chondrites are a powerful tool for deciphering the source of water delivered to terrestrial planets (1). CM-type carbonaceous chondrites contain up to ~10wt.% H₂O, retained as OH in phyllosilicates. The D/H ratio of phyllosilicates (a direct proxy for water) in chondrites cannot be determined directly using whole rock measurements, because their matrices also accreted D-rich organics which are mixed with D-poor phyllosilicates at the sub-micrometer scale. To address this issue, water D/H has been estimated by in-situ measurements of both D/H and C/H in hydrated chondrites, which define a mixing line in a D/H vs. C/H plot. The intercept gives the isotopic composition of the phyllosilicate alone (1). However, SIMS measurements of water D/H using this method can be compromised by (i) contamination and (ii) limited dispersion of the phyllosilicates/organics ratio measured with a large primary beam.

Methods: We addressed both issues using the Wash U NanoSIMS50 which allows us to obtain coordinated isotopic and elemental data with high-spatial resolution. H⁻,D⁻ with ¹²C⁻,¹²C¹⁴N⁻,¹²C¹⁵N⁻,²⁸Si⁻ are collected using magnetic-field peak-jumping in “Combined Analysis” mode. Centering of the secondary ions beam in Cy and P2/P3 planes of the secondary column changes between the low and high masses, resulting in misaligned ion images. So, we used AutoHotkey scripts to send a different Cy voltage for every B-field set up through the virtual keyboard of the NanoSIMS. To separate phyllosilicate-rich from organic-rich pixels, we assume that D/H is not simply a linear function of C/H, but in general D/H is approximated by a function using all measured species: . The true phyllosilicate composition [C,N,Si,H] is estimated from the data and is then used to estimate the water D/H composition from the linear regression model. NanoSIMS isotopic analyses were carried out in a matrix area of the CM Maribo and our analytical conditions were the same as outlined in (2).

Results: First, we calculated a 𝛿D value of −178±46‰ (2σ) for the phyllosilicates in Maribo using the D/H vs. C/H correlation from the resized pixels. This value is higher than previous measurements using SIMS [𝛿D ≈ −420 to −270‰, (2, 3)], demonstrating that D/H ratio of phyllosilicate cannot be simply determined using the D/H vs. C/H line in this matrix area. Second, we calculated the 𝛿D value of the phyllosilicates in Maribo using all the measured species and the linear regression model described above. We found that the phyllosilicate D/H is best correlated for dominant contributions of N, Si and H (b=0.14, c=0.58 and d=−0.86) and minor contributions of C (a=0.06). We calculated a 𝛿D value of −286+/-60‰. This value is consistent with those previously determined by SIMS, demonstrating that our method can be used to precisely determine the water D/H on very small areas.

(1) Alexander C.M.O’D. et al. (2012) Science, 337, 721–723.

(2) Vacher L.G. and Ogliore R.C. (2022) 53rd LPSC, 2653.

(3) van Kooten E.M.M.E. et al. (2018) GCA, 237, 79–102.

(4) Piani L. et al. (2021) EPSL, 567, 117008.

How to cite: Vacher, L. G. and Ogliore, R. C.: Determination of Water D/H in Hydrated Chondrites using NanoSIMS Imaging, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6584, https://doi.org/10.5194/egusphere-egu22-6584, 2022.

The application of the short-lived radiogenic ¹⁸²Hf/¹⁸²W-system (t_1/2 = 8.9 Ma [1]) is a good approach to study early differentiation processes or potential involvement of long-term isolated and/or core-influenced mantle domains as components for ocean island basalts (OIB) [2,3].

Several examples of OIB worldwide (e.g., Hawaii, Samoa and Iceland) exhibit a negative He-W correlation [2], possibly connected to the incorporation of primordial material characterized by high ³He/⁴He ratios and negative µ¹⁸²W (¹⁸²W/¹⁸⁴W deviation of a sample from laboratory standards in parts per million). Anomalous W isotope compositions in combination with elevated ³He/⁴He ratios have previously been connected to seismically anomalous structures in the lowermost mantle, so-called “(mega) ultra-low velocity zones” [3]. Recently, such a structure was discovered beneath the Marquesas Archipelago [4]. This volcanic island chain is located in the South Pacific, in proximity of the Marquesas Fraction Zone. Its formation process is not yet fully understood. Based on high ³He/⁴He ratios in combination with other geochemical characteristics, such as Sr, Nd and Pb isotopes, a deep-lying mantle source has been suggested [5].

In this study, we have analysed seven samples from two islands of the Marquesas Archipelago, which exhibit ³He/⁴He ratios up to 14.4 Ra [5]. µ¹⁸²W ranges from -3.6 ±3.1 to 4.7 ±8.5. Hence, despite elevated ³He/⁴He in some of the samples, none of them display resolved negative ¹⁸²W anomalies and thus, no negative He-W correlation is observed. Interpretations for the decoupling of He-W systematics in samples from the Marquesas Archipelago will be discussed.

References:

[1] Vockenhuber et al., 2004, Phys. Rev. Lett.

[2] Mundl et al., 2017, Science

[3] Mundl-Petermeier et al., 2020, Geochim. Cosmochim. Acta

[4] Kim et al., 2020, Science

[5] Castillo et al., 2007, Chem. Geol.

How to cite: Herret, M.-T., Mundl-Petermeier, A., Castillo, P., and Kim, D.: Tungsten isotope implications for the source of ocean island basalts from the Marquesas Archipelago, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4890, https://doi.org/10.5194/egusphere-egu22-4890, 2022.

The Earth is cooling down and its surface heat flux is the highest among all the terrestrial planet of the Solar System. The total heat loss (Q) is due to the energy released by the secular cooling of our planet (C) and of the radiogenic heat (H) produced by the radioactive decays of the radioelements contained therein. Can geoneutrino disentangle these two contributions?

Since while decaying, the uranium, thorium and potassium radioisotopes contained in the Earth release geoneutrinos in a well-fixed ratio, we can attempt to answer affirmatively to this question. Indeed, geoneutrinos are able to pass through most matter without interacting, so they can bring to surface useful information about the Earth’ deep interior. Concretely, measuring the geoneutrino flux at surface hence translates in estimating H and in turn constraining C once that Q is known.

The only two experiments which collected data in the last 15 years are KamLAND (Japan) and Borexino (Italy). By combining theoretical models and experimental flux with a sophisticated analysis, we inferred valuable insights on mantle radioactivity and of contribution of H to the Earth’s energy budget. We estimated a total radiogenic heat accounting for H = 20.8^+7.3_-7.9 TW and, by subtracting this value from the total heat power of the Earth, we derived a secular cooling C = 26 ± 8 TW. The obtained results are discussed and framed in the puzzle of the diverse classes of formulated Bulk Silicate Earth models, analyzing their implications on planetary heat budget and composition.

The effectiveness in investigating deep earth radioactivity demonstrated by geoneutrino studies confer them a prestigious role in the comprehension of geodynamical processes of our planet.

How to cite: Strati, V., Bellini, G., Inoue, K., Mantovani, F., Serafini, A., and Watanabe, H.: Studying the Earth’s heat budget with geoneutrinos, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-490, https://doi.org/10.5194/egusphere-egu22-490, 2022.

The position of the critical point determines the top of the liquid-vapor coexistence dome, and is a physical parameter of fundamental importance in the study of high-energy shocks, including those associated with large planetary impacts. For most major planetary materials, like oxides and silicates, the estimated position of the critical point is below 1 g/cm³ at temperatures above 5000 K. Here we compute the position of the critical point of one of the most ubiquitous materials: MgO. For this, we perform first-principles molecular dynamics simulations. We find the critical density to be in the 0.4 - 0.6 g/cm³ range and the critical temperature in the 6500 - 7000 K range. We investigate in detail the behavior of MgO in the subcritical and supercritical regimes and provide insight into the structure and chemical speciation. We see a change in Mg-O speciation towards lower degrees of coordination as the temperature is increased from 4000 K to 10000 K. This change in speciation is less pronounced at higher densities. We observe the liquid-gas separation in nucleating nano-bubbles at densities below the liquid spinodal. The majority of the chemical species forming the incipient gas-phase consist of isolated Mg and O atoms and some MgO and O₂ molecules. We find that the ionization state of the atoms in the liquid phase is close to the nominal charge, but it almost vanishes close to the liquid-gas boundary and in the gas phase, which is consequently largely atomic.

This research was supported by the European Research Council under EU Horizon 2020 research and innovation program (grant agreement 681818–IMPACT to RC). This research was performed by access to supercomputing facilities via eDARI stl2816 grants, PRACE RA4947 grant, Uninet2 NN9697K grant.

How to cite: Bögels, T. and Caracas, R.: The critical point and the supercritical regime of MgO, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5002, https://doi.org/10.5194/egusphere-egu22-5002, 2022.

The Moon deforms in response to tidal forcing exerted by the Earth, the Sun and, to a lesser extent, by other planetary bodies. Their observations from ground-based and space-borne instruments, as well as Lunar surface missions, provide one of the most significant constraints that can be employed to unravel the deep interior (Williams et al. [2014], Williams and Boggs [2015]). The tidal forcing generates periodic variations of the harmonic degree-2 shape and gravity that depend on the internal composition and structure of the Moon. These changes in shape and gravity of the Moon are described by three geodetic parameters, called Tidal Love numbers (TLNs). Because of their low degree, these TLNs are sensitive to the structure of the deep interior (e.g., Khan et al. [2004]). Apart from the geodetic constraints, the Moon and Mars (e.g. Zweifel et al. [2021]) are the only other bodies besides the Earth for which seismic data are available. Seismic studies using the Apollo Passive Seismic Experiment (PSE) constrain the seismic wave velocity distribution and therefore give a glimpse of the Moon’s interior structure (Garcia et al. [2011], Weber et al. [2011]). However, at greater depth, seismic data do not provide sufficient resolution on the velocity profile, leaving the near-centre Moon structure uncertain. Other studies based upon geophysical constraints (Khan et al. [2004], Harada et al. [2014, 2016], Matsumoto et al. [2015]) and the re-analysis of the Apollo seismic data suggested the existence of an attenuated region called low viscosity zone (LVZ) originated from a melting layer at the core-mantle boundary (Khan and Mosegaard [2001], Weber et al. [2011], Harada et al. [2014], Rambaux et al. [2014]).

Based on geodetic observations and seismic studies, we perform Monte Carlo simulations for combinations of thicknesses, densities and viscosities for two classes of Moon’s models, one including an undifferentiated core and one including an inner and outer core, with both classes assuming an LVZ at the core-mantle boundary. By comparing predicted and observed tidal deformation parameters we find that the existence of an inner core cannot be ruled out. Furthermore, by deducing temperature profiles for the LVZ and the mantle following Earth assumptions, we obtain stringent constraints on the radius, viscosity, and density of the LVZ. We also infer the first estimation for the outer core viscosity, for our two possible scenarios.

How to cite: Briaud, A., Fienga, A., Melini, D., Rambaux, N., Mémin, A., Spada, G., Saliby, C., Hussmann, H., and Stark, A.: Constraints on the Moon’s deep interior from tidal deformation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7919, https://doi.org/10.5194/egusphere-egu22-7919, 2022.

A star and its planets are born from the same molecular cloud, so they share the same origin of the essential building blocks: elements. The compositional deviations between stars and (particularly rocky) planets are associated with the gas-dust fractionation process in the protoplanetary disk and subsequent formation processes of the planets. During these processes, a key differentiator between forming a gas giant (e.g. Jupiter) and a rocky planet (e.g. Earth) is devolatilisation – i.e. depletion of volatiles (e.g. H, C, and O) resulting in completely different bulk compositions between the two types of planets, with former being dominated by gases/ices and the latter by rocks. This devolatilisation mechanism has been empirically observed in both the Solar System and other planetary systems (e.g. in polluted white dwarf atmospheres), but has yet to be explored and implemented in the prevalent planet-formation models.

I will explore both the nebular and post-nebular devolatilization processes based on the first principals starting from the stellar nebulae to rocky planetary bodies. These processes will then be coupled with a state-of-the-art planet formation model. Such a coupled/hybrid devolatilisation-dynamics model will enable a detailed and accurate estimation of the volatile (subject to devolatilisation) and refractory (resistant to devolatilisation) contents of a small (rocky) planet, as well as the physical properties (e.g. mass, radius, and orbit) of the planet. These unprecedentedly detailed predictions of planetary elemental composition will provide crucial constraints, together with mass, radius and orbital properties, for further modelling of planetary interiors, surfaces, and atmospheres. Together, these will lead to a new level of predictive statistical understanding of the detailed properties of small (rocky) planets in our solar neighbourhood.

How to cite: Wang, H.: Devolatilisation during planet formation: A hybrid model of chemistry and dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10146, https://doi.org/10.5194/egusphere-egu22-10146, 2022.

In the early solar system radiogenic heating by ²⁶Al and collisions are the two prominent ways expected to modify the internal composition of icy planetesimals, building blocks of comets, by removing highly volatile compounds like CO, CO₂ and NH₃. However, observations indicate that even large comets like Hale-Bopp (R ≈ 35 km) can be rich in these highly volatile compounds [1].
Here we constrain under which conditions icy planetesimals experiencing both internal heating and collisions can retain pristine interiors [2]. For this purpose, we employ both the state-of-the-art finite difference marker-in-cell code I2ELVIS [3] to model the thermal evolution in 2D infinite cylinder geometry and a 3D SPH code [4] to study the interior heating caused by collisions among icy planetesimals. For simplicity we assume circular porous icy planetesimals with a low density (≈ 470 kg/m³) based on measurements for comet 67P/Churyumov-Gerasimenko [5].
For the parameter study of the thermal history we vary (i) icy planetesimal radii, (ii) formation time and the (iii) the silicate/ice ratio. For the latter we keep the mean density fixed and change the porosity of the icy planetesimal. For the impact models we use porous, low-strength objects and vary (i) target and (ii) projectile radii, (iii) impact velocity as well as (iv) impact angle. Potential losses of volatile compounds from their interiors are calculated based on their critical temperatures taken from literature [6]. Our combined results indicate that only small or late-formed icy planetesimals remain mostly pristine, while early formed objects can even reach temperatures high enough to melt the water ice.

REFERENCES
[1] Morbidelli & Nesvorný, In: The Trans-Neptunian Solar System. 25–59 (2019). [2] Golabek & Jutzi, Icarus 363, 114437 (2021). [3] Gerya & Yuen, Phys. Earth Planet. Int. 163, 83-105 (2007). [4] Jutzi, Planet. Space Sci. 107, 3–9 (2015). [5] Sierks et al., Science 347, 1044 (2015). [6] Davidsson et al., Astron. Astrophys. 592, A63 (2016).

How to cite: Golabek, G. and Jutzi, M.: Modification of icy planetesimal interiors by early thermal evolution and collisions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1433, https://doi.org/10.5194/egusphere-egu22-1433, 2022.

During the differentiation of terrestrial planets, the metal phase from the impactor core segregates from the silicate phase of the magma ocean. This buoyant mass forms a turbulent thermal and settles toward the proto-core. During this descent, thermal and chemical exchange occurs at the boundary between the metallic and silicate phases. Based on laboratory fluid dynamic experiments mimicking the settling of the metallic thermal turbulent, we develop a Lagrangian approach of the mixing from the experimental velocity field. We are able to track the evolution of the material elongated as lamellae by the turbulent stirring. We have characterised the elongation rate, the aggregation of lamellae, and the probability density function of the elongation and concentration, which are not accessible from direct measurements in the experiments. We have also investigated the effect of the Reynolds number and density ratio on these quantities. These results will allow us to develop a new predictive model of the mixing and chemical transfer in thermal turbulent to better understand the equilibrium between metals and silicates during the accretion of terrestrial planets.

How to cite: Huguet, L. and Deguen, R.: Lagrangian approach of the mixing in a turbulent thermal, and implications for metal-silicate equilibrium during Earth's formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3776, https://doi.org/10.5194/egusphere-egu22-3776, 2022.

The Martian dichotomy features a ~25 km difference in crustal thickness and ~5 km contrast in topography between the southern highlands and northern lowlands [1]. Among various origin hypothesis, a southern impact [2,3] creates a magma pond which, upon cooling, induces crustal thickening and thereby forms the crustal dichotomy within 10s of million years.

Our previous study [4], which utilizes a head-on parametrized impact in 2D geometry, shows that an impact-induced magma pond in the southern hemisphere is able to not only create a thickened crust in the south, but also a satisfying volcanic history with localized melt production in the equatorial region at geologically recent time.  Depleted material, formed from crystallization of the magma pond, spreads and underplates the thicker and colder Northern lithosphere undisturbed by the impact, reinforcing the lesser extent of volcanism in the northern hemisphere. Our resultant mantle structure is consistent with existing simulation efforts that focus on the post-dichotomy formation evolution history [5], and in addition gives the context of how such thermochemical structure is developed.

In order to include a more realistic impact scenario, we use smoothed particle hydrodynamics (SPH) simulations [6] to model the first 24-36 hours of a giant impact between proto-Mars and its impactor. The SPH result is then transferred to the mantle convection code StagYY [7], as an initial thermal condition, to simulate the long-term evolution of the crust and mantle for the subsequent 4.5 billion years. We systematically vary the impactor size, impact velocity and pre-impact Martian mantle temperature. Our preliminary results show that a 45-degree impact does not form a Martian dichotomy-like crustal structure, while a 15-degree impact is a better match.  With a realistic impact, the mechanisms reported in our parametrized impact study still hold.

References:

[1] Watters, T., McGovern, P., & Irwin III, R. (2007). Hemispheres Apart: The Crustal Dichotomy on Mars. Annual Review Of Earth And Planetary Sciences, 35(1), 621-652.

[2] Reese, C., Orth, C., & Solomatov, V. (2011). Impact megadomes and the origin of the martian crustal dichotomy. Icarus, 213(2), 433-442.

[3] Golabek, G., Keller, T., Gerya, T., Zhu, G., Tackley, P., & Connolly, J. (2011). Origin of the martian dichotomy and Tharsis from a giant impact causing massive magmatism. Icarus, 215(1), 346-357.

[4] Cheng, K.W., Tackley, P.J., Rozel, A.B., Golabek, G.J. (2021). Martian Dichotomy: Impact-induced Crustal Production in Mantle Convection Models, Abstract [DI35B-0023] presented at 2021 Fall Meeting, AGU, New Orleans, LA, 13-17 Dec.

[5] Plesa, A., Padovan, S., Tosi, N., Breuer, D., Grott, M., & Wieczorek, M. et al. (2018). The Thermal State and Interior Structure of Mars. Geophysical Research Letters, 45(22), 12,198-12,209.

[6] Emsenhuber, A., Jutzi, M., Benz, W. (2018). SPH calculations of Mars-scale collisions: The role of the equation of state, material rheologies, and numerical effects. Icarus, 301, 247-257

[7] Tackley, P. (2008). Modelling compressible mantle convection with large viscosity contrasts in a three-dimensional spherical shell using the yin-yang grid. Physics Of The Earth And Planetary Interiors, 171(1-4), 7-18.

How to cite: Cheng, K. W., Rozel, A., Ballantyne, H., Jutzi, M., Golabek, G., and Tackley, P.: Forming the Martian dichotomy with realistic impact scenarios, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6591, https://doi.org/10.5194/egusphere-egu22-6591, 2022.

Every planet is singular, with scars and bumps at their surface. One planet, one history. But the physics at play is common to them, connecting planetary bodies together. Tectonics is a common theme of what we can observe on planets of the solar system, and a central question for explanets. More than 20 years of geodynamic modelling has resulted in  identifying a diversity of tectonic regimes for mantle convection, from very active, like heat-pipe (Monnereau and Dubuffet, 2002 among others) and squishy lid (Lourenço et al., 2020) to almost inert, like stagnant lid (Schmeling and Jacoby, 1982). Tectonics is an emergent property deriving from the intimate structure and composition of a planet. It is also a fundamental piece shaping the surface environment. This presentation will attempt to give an overview of tectonic regimes of planets and propose typical evolutional scenari, connecting structural and compositional histories from the depth to the surface.

References

Lourenço, D. L., Rozel, A. B., Ballmer, M. D., & Tackley, P. J. (2020). Plutonic‐squishy lid: A new global tectonic regime generated by intrusive magmatism on earth‐like planets. Geochemistry, Geophysics, Geosystems, 21, e2019GC008756.

Monnereau, M., & Dubuffet, F. (2002). Is Io's mantle really molten?. Icarus, 158, 450-459.

Schmeling, H., & Jacoby, W. R. (1982). On modelling the lithosphere in mantle convection with non-linear rheology. Journal of Geophysics, 50, 89-100.

How to cite: Coltice, N.: An overview of modeled dynamic histories of rocky planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6028, https://doi.org/10.5194/egusphere-egu22-6028, 2022.

      The crystallization of the Basal Magma Ocean (BMO) sets the stage for the long-term evolution of terrestrial planets and may leave behind large-scale thermochemical structures in the lower mantle. Previous work shows that a FeO-enriched molten layer or basal magma ocean (BMO) is stabilized at the core-mantle boundary of large rocky planets such as Earth for a few billion years. The BMO itself is expected to freeze by fractional crystallization (FC) because it cools very slowly. However, the fate of BMO cumulates has not yet been systemically explored.

To explore the fate of the BMO cumulates in the convecting mantle, we explore 2D geodynamic models with a moving-boundary approach. Flow in the mantle is explicitly solved, but the thermal evolution and related crystallization of the successively crystallizing BMO (i.e., below the moving boundary) are fully parameterized. The composition of the crystallizing cumulates is self-consistently calculated in the FeO-MgO-SiO₂ ternary system according to Boukaré et al. (2015). In some cases, we also consider the effects of Al₂O₃ on the cumulate density profile. We then investigate the entrainment and mixing of BMO cumulates by solid-state mantle convection over billions of years as a function of BMO initial composition and volume, BMO crystallization timescales, distribution of internal heat sources, and mantle rheological parameters (Rayleigh Number and activation energy). We vary the initial composition of BMO by manipulating the bulk molar fraction of FeO, MgO, and SiO₂, e.g. considering BMO compositions such as pyrolite, lower-mantle partial melts of pyrolite (after 50% batch crystallization), or Archean Basalt.

For all our model cases, we find that most of the cumulates (first ~90% by mass) are efficiently entrained and mixed through the mantle. However, the final ~9% of the cumulates are too dense to be entrained (either fully or partially) over the age of the Earth, and rather remain at the base of the mantle as a strongly FeO-enriched solid layer. Unless the initial thickness of the BMO is ≤100 km, this strongly enriched and intrinsically dense layer should cover the CMB globally. We highlight that this outcome of BMO fractional crystallization is inconsistent with the geophysical constraints. Our results suggest that the BMO was either very small initially or did not crystallize by end-member FC. An alternative mode of crystallization may be driven by an efficient reaction between a highly-enriched last-stage BMO with the overlying mantle. Such reactive crystallization may be much faster than FC of the BMO, as it is driven by chemical disequilibrium instead of (slow) planetary cooling.

How to cite: Ismail, M. and Ballmer, M.: Fractional Crystallization Of The Basal Magma Ocean: Consequences For Present-day Mantle Structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6651, https://doi.org/10.5194/egusphere-egu22-6651, 2022.

            Ferropericlase is the second most abundant phase of Earth’s lower mantle and is also considered to be one of the main constituents of the mantles of super-Earth exoplanets. Since ferropericlase is more ductile compared to silicates (Girard et al. 2016), it is expected to control the rheological behavior of mantle aggregates which governs solid-state convection of planetary mantles. The mechanical behavior of polycrystalline aggregates is strongly affected by the presence of grain boundaries. Despite previous work on MgO grain boundaries (e.g. Verma & Karki 2010; Hirel et al. 2019), little is yet known about the properties and mobility of ferropericlase grain boundaries at pressure conditions of deep planetary interiors.

            In this study, we carried out atomistic simulations based on the density functional theory to model the structures, energies and spin states of iron of a series of [001] symmetrical tilt grain boundaries in ferropericlase as a function of pressure. Based on these results, we investigated the mechanical behavior of the Σ5 tilt grain boundary by applying simple shear increments to the simulation cell to trigger grain boundary migration. Here, we will present the different mechanisms of grain boundary migration and the evolution of the ideal shear strengths up to a pressure of 400 GPa. Our results show that the mechanical strength of the grain boundaries and the directionality of their motion strongly varies with increasing pressure. Especially at pressure conditions of super-Earth exoplanets, significant grain boundary weakening is observed with increasing depth.  Implications for the deformation of ferropericlase at conditions of Earth’s and super-Earth’s mantles will be finally discussed.

How to cite: Ritterbex, S. and Tsuchiya, T.: Ab initio investigation of the intercrystalline mechanical behavior of ferropericlase at extreme pressures of planetary mantles, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1140, https://doi.org/10.5194/egusphere-egu22-1140, 2022.

Accurate measurements of a planet's mass, radius and age (provided for example by the PLATO mission and follow-up measurements) together with compositional constraints from the stellar spectrum can help us to deduce potential evolutionary pathways that rocky planets can evolve along, and allow us to predict the range of likely atmospheric properties that can then be compared to observations.

However, for the evolution of composition and mass of an atmosphere, a large degeneracy exists due to several planetary and exterior factors and processes, making it very difficult to link the interior (and hence outgassing processes) of a planet to its atmosphere. The community therefore thrives now to identify the key factors that impact an atmosphere, and that may lead to distinguishable traces in planetary, secondary outgassed atmospheres. Such key factors are for example the planetary mass (impacting atmospheric erosion processes) or surface temperature (impacting atmospheric chemistry, weathering and interior-atmosphere interactions).

Here we investigate the signature that a planet evolving into plate tectonics leaves in its atmophere due to its impact on volcanic outgassing fluxes and volatile releases to the atmosphere - leading possibly to distinguishable sets of atmospheric compositions for stagnant-lid planets and plate tectonics planets. These preliminary findings will need to be further investigated with coupled atmosphere-interior models including various feedback mechanisms such as condensation and weathering as well as atmospheric escape to space.

How to cite: Noack, L. and Brachmann, C.: Is planetary resurfacing a key factor for outgassing and gas speciation on rocky planets?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4128, https://doi.org/10.5194/egusphere-egu22-4128, 2022.

Both the Moon¹ and Mars² are known to have significant degree-1 variations in their crustal thicknesses, with the Moon's far side and Mars's southern hemisphere having far thicker crusts than their respective opposing hemispheres. A number of potential mechanisms have been proposed to explain these dichotomies, including large impacts in both cases^3,4, radiant heat from the Earth⁵ (in the case of the Moon), and large-scale volcanism⁶ (in the case of Mars). However, the effectiveness of these mechanisms are limited by the difficulty of sustaining a large hemispheric difference during the tens to hundreds of Ma of crustal formation. Both planets' lithospheres are examples of a fluid-dynamical boundary layer known as a stagnant lid, caused by temperature-dependent viscosity in a convecting system. We consider the effect of pressure on the viscosity of magma oceans and mantles, finding that under certain circumstances a spherically-symmetric stagnant lid is linearly unstable to asymmetric perturbations. The fastest-growing wavenumbers of this instability is degree 1, meaning that a small initial asymmetry may grow into a full-scale hemispherical dichotomy. We then numerically examine the stability of these asymmetric states, finding that they may last for hundreds of Ma. We also compare to the case of Mercury, a similarly-sized planet with no such crustal dichotomy, to determine if our analysis matches observations.

¹ Wieczorek, M.A., Jolliff, B.L., Khan, A., Pritchard, M.E., Weiss, B.P., Williams, J.G., Hood, L.L., Righter, K., Neal, C.R., Shearer, C.K., McCallum, I.S., Tompkins, S., Hawke, B.R., Peterson, C., Gillis, J.J. & Bussey, B. 2006 The Constitution and Structure of the Lunar Interior. Reviews in Mineralogy and Geochemistry 60, 221–364.

² Thiriet, M., Michaut, C., Breuer, D. & Plesa, A.-C. 2018 Hemispheric dichotomy in lithosphere thickness on mars caused by differences in crustal structure and composition. Journal of Geophysical Research: Planets 123 (4), 823–848.
Weiss, Benjamin P. & Tikoo, Sonia M. 2014 The lunar dynamo. Science 346 (6214), 1198

³ Garrick-Bethell, I., Perera, V., Nimmo, F. & Zuber, M.T. 2014 The tidal-rotational shape of the Moon and evidence for polar wander. Nature 512 (7513), 181–184.

⁴ Andrews-Hanna, J.C., Zuber, M.T. & Banerdt, W.B. 2008 The borealis basin and the origin of the martian crustal dichotomy. Nature 453 (7199), 1212–1215.

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How to cite: Watson, C., Neufeld, J., and Michaut, C.: Asymmetric growth of planetary stagnant lids, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4758, https://doi.org/10.5194/egusphere-egu22-4758, 2022.

From the observations on ~1000 recognizable impact craters on Venus’ surface, the average surface age for Venus is comparable to the average surface age for Earth, and is significantly younger than the surface ages of other solar terrestrial planets. To explain Venus’ young surface without plate tectonics, the global tectonics of Venus have often been proposed to be in an episodic-lid regime with catastrophic global overturns. Previous episodic-lid geodynamic models often assume an olivine-diffusion-creep rheology for Venus’ crust, resulting in global overturns followed by stagnant-lid phases with near-zero surface mobilities. However, some tectonic units on Venus’ surfaces show substantial tectonic deformation, such as tesserae and coronae. Recent analyses of satellite images on Venus' surface also suggest possible widespread lithospheric mobilities in the lowland basins. And these observations can hardly be explained by the stagnant-lid phases between overturns in the episodic-lid models.

In this study, we test the influence of (1) a composite, experiment-based crustal rheology (including diffusion creep, dislocation creep, and plasticity), and (2) intrusive magmatism, on Venus’ surface tectonics, using the mantle convection code StagYY in a 2D spherical annulus geometry. Our results show that applying the experiment-based rheology and intrusive magmatism in the model results in (1) both global and regional overturns, (2) high and continuous surface mobilities that indicate substantial surface deformation between global overturns, and (3) a young and thinner crust that is consistent with current estimations.  As for volcanic activities, contrary to olivine-diffusion-creep models, there is no persistent mantle plume in our models when the realistic crustal rheology is applied. The basalt cumulated between the upper and lower mantle affects convective flows in the mantle and mantle upwellings from the core-mantle boundary. Also, there are short-term, randomly located volcanisms within crust between global overturns, which are consistent with recent observations of active magmatism on Venus’ surface and the short-term plumes suggested by coronae formation models. The surface tectonics in our models are dependent on the heat transfer efficiency in the upper mantle. And the tectonic regime is different from both episodic-lid regime and plutonic-squishy-lid regime that are proposed in previous literature, and can provide insights on the tectonic style for Venus and early Earth.

How to cite: Tian, J., Tackley, P., and Rozel, A.: Implications of a realistic crustal rheology and intrusive magmatism on Venusian tectonics: a geodynamic perspective, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12407, https://doi.org/10.5194/egusphere-egu22-12407, 2022.