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PS3.3

EDI
Dynamic, structural, and compositional evolution of Earth and rocky planets from their accretion to present

The present state of Earth and other rocky planets are an expression of dynamical and chemical processes occurring throughout their history. In particular, giant impacts, core formation and magma-ocean crystallisation and other processes occurring in the early solar system set the stage for the long-term evolution of terrestrial planets. These early processes can happen simultaneously or in recurring stages, and are ultimately followed by progressive crustal growth, long-term mantle mixing/differentiation, core-mantle interaction, as well as inner-core crystallization. The rock-record, through geochemistry and magnetism, is used to interrogate changes in the tectono-thermal regime of Earth’s interior through time, while seismic imaging and gravity data, for instance, provide a snapshot of processes occurring in the contemporary mantle, crust and core. These classes of observations may be linked through geodynamic models, whose accuracy is underpinned by the physical properties (e.g., viscosity and density) of its constituent phases (minerals, melts and fluids). Information on the fundamental thermodynamic and physical behaviour of phases is subject to constant advance via experimental and ab-initio techniques.

This session aims to provide a holistic view of the formation, dynamics, structure and composition of Earth and the evolution of terrestrial bodies by bringing together studies from geophysics, geodynamics, mineral physics, geochemistry, and petrology. This session welcomes contributions focused on data analysis, modeling and experimental work that address the formation and evolution of terrestrial planets and moons in the Solar System, and around other stars.

Co-organized by GD4/GMPV4
Convener: Paolo Sossi | Co-conveners: Simone Pilia, Ingo StotzECSECS, Lena Noack, Stephen J. Mojzsis
Presentations
| Mon, 23 May, 08:30–11:50 (CEST), 13:20–14:50 (CEST)
 
Room 1.85/86

Mon, 23 May, 08:30–10:00

Chairpersons: Paolo Sossi, Stephen J. Mojzsis

08:30–08:35
Compositional evolution of rocky bodies

08:35–08:45
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EGU22-6584
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ECS
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solicited
Lionel G. Vacher and Ryan C. Ogliore

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,Dwith 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.

08:52–08:59
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EGU22-3089
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ECS
Stefano Iannini Lelarge et al.

Planetary differentiation in small bodies is believed to be ruled by several partial end-states that were dominated by low degrees of partial melting and melt segregation, before arriving at the formation of rocky planets. Having a better understanding of non-equilibrium melting processes in undifferentiated chondritic materials is critical to characterize planetary differentiation processes and the formation of rocky planets and differentiated asteroids. In this context, partial melting experiments of natural chondrites can provide unique insights into the petrological evolution associated with early planetary differentiation of planetesimals. For this study, we performed partial melting experiments using fragments from the ordinary chondrite DAV01001. Experiments were performed in a piston-cylinder at 1 GPa pressure, at temperatures from 1100 to 1300 °C and for 24 hours run duration. Reducing conditions were imposed by the use of graphite capsules. The experimental products were analysed using electron microprobe and synchrotron radiation computed microtomography (SR-µCT).

DAV01001 is an equilibrated L6 ordinary chondrite that has still visible relic chondrules and contains olivine (Fo75), low-Ca pyroxene (En77Fs21Wo2), high-Ca pyroxene (En47Fs8Wo45), albitic plagioclase (An13Ab81Or6), metal, troilite, chromite, and apatite. Upon heating, metal and troilite disappear at 1100 °C forming two immiscible phases, one made of pure metal with variable amounts of Ni, the other made of a metal-sulphide liquid of variable composition. Chromite starts melting at 1100 °C and disappears at 1300 °C. Silicatic melt forms already at 1100 °C as a result of the melting of plagioclase. With increasing temperature, the pyroxene and olivine begin to melt and shift the composition of the liquid towards trachy-andesitic (1200 °C) and basaltic trachy-andesitic to andesitic (1300 °C) compositions. Melting of olivine and pyroxene is accompanied by the crystallisation of both phases. The newly-formed olivine has a composition varying from Fo80 to Fo59, becoming progressively enriched in Fe and Ca and depleted in Ni at increasing temperature. The newly-formed pyroxene has a variable Ca content, and is enriched in Al and Cr and depleted in Fe and Mn. The new-grown olivine and pyroxene crystals have a strong affinity with chondritic/primitive achondrites compositions, in contrast to the melts that have a good affinity to a bulk HED composition. Overall, the combination of melting and crystallisation fixes the amount of silicatic liquid to a rather constant value of 10% vol.

SR-µCT was used to create 3D reconstructions of the experimental samples, in order to evaluate the efficiency of metal segregation at increasing degrees of partial melting. At increasing temperature, no change in the object density (number of 3D particles divided by the sample volume) is observed but only a progressive increase of the roundness and sphericity of the particles. This suggests that, even in presence of an interconnected liquid silicate phase (~10% vol), the coalescence of the metal phases does not occur spontaneously and other forces such as rotational spin or deformation are needed to segregate metal under these conditions.

How to cite: Iannini Lelarge, S., Masotta, M., Folco, L., Mancini, L., and Pittarello, L.: Non-equilibrium melting of partially differentiated asteroids: insights from partial melting experiments on L6 chondrite DAV01001, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3089, https://doi.org/10.5194/egusphere-egu22-3089, 2022.

08:59–09:06
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EGU22-1746
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ECS
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Highlight
Weronika Ofierska et al.

According to the canonical model, the Moon was formed in the aftermath of a giant impact, when the proto-Earth was struck by a Mars-size impactor leading to a debris disk from which the Moon accreted. This event is thought to have been sufficiently energetic to cause wholesale melting of the Moon. Solidification of the resulting Lunar Magma Ocean (LMO) involves plagioclase flotation and formation of an anorthositic crust that blankets the residual LMO. This crust may form directly through plagioclase flotation or involve more complex reprocessing mechanisms. Extensive fractional crystallization of the LMO likely led to formation of a residual KREEP component in the crust, enriched in K, REE, P and other incompatible elements relative to the bulk Moon, whose signature has been recognized in several lunar samples (e.g.  feldspathic basalt).

The experimentally-constrained liquid lines of descent of a range of plausible LMO compositions bear strong resemblances to one another, crystallizing in the sequence olivine -> opx -> cpx + plagioclase -> quartz + Fe-Ti oxide. Crystallisation of olivine ± orthopyroxene prevails, depending on the composition, between 61-77 PCS (percent solidified), followed by the concomitant appearance of plagioclase + cpx at 1230±30 oC. Crystallisation of plagioclase marks the point at which the crystallisation sequences diverge owing to differences in bulk composition (e.g. refractory element content), which in turn influence phase saturation. Existing experiments on liquid lines of descent lack resolution, in particular at the point of quartz and Fe-Ti oxide saturation. Moreover, these experiments rarely proceed to the extent required to produce a KREEP component. In this work, we aim to more precisely determine the phase relations during crystallisation of the uppermost LMO, and assess possible mechanisms of formation of the KREEP component.

An isobaric series (8 - 5kbar) of six experiments on the bulk silicate Moon composition of O’Neill (1991) yields a crystallization sequence beginning at 1250 oC with olivine ± opx ± Cr-sp (69 PCS), followed by plagioclase and clinopyroxene at 1200 oC (77 PCS). Our mineral and melt major and trace-element abundances constrain the terminal stages of LMO crystallisation. Melt compositions remain near 45 wt% SiO2 during the final crystallization stage while FeO increases from 12 wt% (bulk) to 20 wt% at plagioclase saturation. The Al2O3 and CaO budget is controlled by plagioclase crystallization (but not cpx) as the An# is as high as 97. We report mineral/melt partitioning coefficients for La, Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Y, Zr, Th and U for plagioclase, pigeonite and high-Ca clinopyroxene and use the lattice strain model to evaluate these, also in the context of literature data. These partition coefficients are therefore the most suitable for understanding the origin of the KREEP component.  

Preliminary results suggest KREEP forms only after 99 PCS due to the evolved melt and the relatively rapid cooling rate of the surface magma ocean once crystal fraction is high. The last stage of eutectic crystallisation should lead to gabbroic rocks as the final crystallisation product.  

How to cite: Ofierska, W., Schmidt, M., Sossi, P., and Liebske, C.: Crystallisation of the upper lunar magma ocean and implications for KREEP and crust formation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1746, https://doi.org/10.5194/egusphere-egu22-1746, 2022.

09:06–09:13
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EGU22-4890
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ECS
Marie-Theres Herret et al.

The application of the short-lived radiogenic 182Hf/182W-system (t1/2 = 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.

09:13–09:20
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EGU22-5850
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ECS
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Highlight
Julia Marleen Schmidt and Lena Noack

The radiogenic elements K, Th, and U are large contributors to the heating inside a terrestrial planet. Because they act incompatible in solid mantle rocks, they prefer to gather in partial melt, which is generally less dense than the surrounding material and rises upwards. While rising, the melt transports the radiogenic heat sources and other incompatible elements towards the surface, where over time they accumulate inside the crust. The amount of the transported incompatible elements is heavily dependent on their degree of incompatibility in mantle rocks and therefore their mineral/melt partition coefficients. Despite the fact that partition coefficients can change by multiple orders of magnitudes from 0-15 GPa along a peridotite solidus (Schmidt and Noack, 2021), they were generally taken as constant in mantle evolution models due to a lack of high-pressure models and experimental data.

Based on the thermodynamic approach of Blundy et al. (1995), Schmidt and Noack (2021) modelled partition coefficients for sodium in clinopyroxene/melt from 0-15 GPa. As sodium has a very low strain in the M2 lattice site of clinopyroxene and is therefore very compatible, its partition coefficients can act as a reference to model the other elements from. In this study, we take the approach of Schmidt and Noack (2021) to model the partition coefficients of the above-mentioned heat producing elements and volatiles at local P-T conditions for partial melting events inside the mantle of terrestrial planets. We insert local bulk partition coefficients for an adequate mantle rock composition into a 1D interior evolution model of Mars. By comparing the results of the redistribution to models with constant partition coefficients, we can assess the impact of the locally calculated partition coefficients on the accuracy of models which deal with the thermal evolution of a planet and the enrichment of heat producing elements and volatiles inside the crust.

Blundy, J. et al. (1995): Sodium partitioning between clinopyroxene and silicate melts, J. Geophys. Res., 100, 15501-15515.

Schmidt, J.M. and Noack, L. (2021): Clinopyroxene/Melt Partitioning: Models for Higher Upper Mantle Pressures Applied to Sodium and Potassium, SysMea, vol 13 nr 3&4, to be published.

How to cite: Schmidt, J. M. and Noack, L.: Applying locally calculated partition coefficients for radiogenic heat sources and volatiles to interior evolution models of terrestrial planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5850, https://doi.org/10.5194/egusphere-egu22-5850, 2022.

09:20–09:27
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EGU22-490
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ECS
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Highlight
Virginia Strati et al.

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.

09:27–09:34
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EGU22-5002
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ECS
Tim Bögels and Razvan Caracas

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.

09:34–10:00
Compositional evolution discussion

Mon, 23 May, 10:20–11:50

Chairpersons: Lena Noack, Paolo Sossi

10:20–10:25
Disk and planet formation

10:25–10:35
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EGU22-3311
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ECS
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solicited
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Highlight
Miki Nakajima et al.
  • The Apollo lunar samples reveal that Earth and the Moon have strikingly similar isotopic ratios, suggesting that these bodies may share the same source materials. This leads to the "standard" giant impact hypothesis, suggesting the Moon formed from a partially vaporized disk that was generated by an impact between the proto-Earth and a Mars-sized impactor. This disk would have had high temperature (~ 4000 K) and vapor mass fraction of ~20 wt %. However, impact simulations indicate that this model does not mix the two bodies well, making it challenging to explain the isotopic similarity. In contrast, more energetic impacts, such as a collision between two half Earth-sized objects, could mix the two bodies well, naturally solving the problem. These impacts would produce much higher disk temperatures (6000-7000K) and higher vapor mass fractions (~80-90 wt%). These energetic models, however, may have a challenge during the Moon accretion phase. Our analyses suggest that km-sized moonlets, which are building blocks of the Moon, would experience strong gas drag from the vapor portion of the disk and fall onto Earth on a very short timescale. This problem could be avoided if large moonlets (>1000 km) form very quickly by the process called streaming instability, which is a large clump formation mechanism due to spontaneous concentration of dust particles followed by gravitational collapse. We investigate this possibility by conducting numerical simulations with the code called Athena. Our 2D and 3D hydrodynamic simulations show that moonlet formation by streaming instability is possible in the Moon-forming disk, but their maximum size is approximately 50 km, which is not large enough to avoid the strong gas drag. This result supports the Moon formation models that produce vapor-poor disks, such as the standard model. We will further discuss implications for moons in the solar system and extrasolar systems (exomoons). 

How to cite: Nakajima, M., Atkins, J., Simon, J. B., and Quillen, A. C.: Moon Formation via Streaming Instability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3311, https://doi.org/10.5194/egusphere-egu22-3311, 2022.

10:35–10:42
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EGU22-7919
Arthur Briaud et al.

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.

10:42–10:49
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EGU22-10146
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ECS
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Highlight
Haiyang Wang

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.

10:49–10:56
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EGU22-7484
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ECS
Rob Spaargaren et al.

The study of exoplanets can provide a more general understanding of planetary systems and terrestrial-planet evolution. How terrestrial exoplanets differ from Earth has so far mostly focused on planet size and orbital distance. In contrast, bulk planet composition has gained much less attention, even though it controls key physical properties of planetary interiors, and thus interior dynamics and long-term evolution. Bulk planet composition is related to core size as well as mantle chemistry and mineralogy. To better understand the variability of interior properties among terrestrial exoplanets, we attempt to constrain the range of bulk terrestrial exoplanet compositions. 

To constrain the compositional range of terrestrial exoplanets, we use the compositional link between rocky planets and their host stars. At least in the Solar System, planetary building blocks (chondrites) correspond to the devolatized star (Sun) composition. Accordingly, we apply devolatilization to stellar compositions in the galactic neighbourhood (i.e., within 500 pc) according to the approach of Wang et al. [1]. These bulk compositions are then split into core and mantle reservoirs by considering interior oxygen fugacity during core formation equal to that of Earth. 

We find compositional ranges of molar mantle Mg/Si-ratios from 0.9 to 2.0, core sizes between 18 and 35 wt%, and mantle molar MgO+FeO+SiO2 abundances between 88 and 94 mol%. We summarize our results by defining 20 end-member compositions that represent the full range of bulk terrestrial exoplanet compositions in the Solar neighbourhood. A Gibbs energy minimization algorithm, Perple_X, shows that these planets all have mantles dominated by Fe-Mg-Si minerals, such as olivine, pyroxene, bridgmanite and periclase. The relative abundances of these minerals control mantle viscosity, where Mg-rich minerals (periclase) are weaker than Si-rich minerals (olivine, bridgmanite). We continue by simulating mantle dynamics using a 2D geodynamic model. Most of our end-member planets have a lower mantle viscosity than Earth, and their mantles are more fertile than Earth's. Accordingly, we find that mantle cooling is more efficient than for Earth for most Earth-sized exoplanets in the solar neighborhood. Future work is needed to further constrain the coupled interior-atmosphere evolution of Earth-like exoplanets, and how bulk planet composition affects it. 

[1] Wang, H.S., Lineweaver, C.H., Ireland, T.R. (2019). The volatility trend of protosolar and terrestrial elemental abundances. Icarus, 328, 287-305 

How to cite: Spaargaren, R., Ballmer, M., Mojzsis, S., and Tackley, P.: The effects of terrestrial exoplanet bulk composition on long-term planetary evolution , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7484, https://doi.org/10.5194/egusphere-egu22-7484, 2022.

10:56–11:03
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EGU22-1433
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Highlight
Gregor Golabek and Martin Jutzi

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.

11:03–11:10
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EGU22-5270
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ECS
Antoine Schneeberger et al.

How volatiles were incorporated in the building blocks of planets and small bodies in the protosolar nebula remains an outstanding question. Some scenarios invoke the formation of planetesimals from a mixture of refractory material and amorphous ice in the outer nebula while others argue that volatiles have been incorporated in clathrate or pure condensate forms in these solids. Here we study the fate of volatiles (H2O, CO, CO2, CH4, H2S, N2, NH3, Ar, Kr, Xe, and PH3) initially delivered in the forms of amorphous ice or pure condensates to the protosolar nebula. We investigate the radial distribution of these volatiles via a transport module coupled with an accretion disk model. In this model, multiple icelines are considered, including the condensation fronts of pure condensates, as well as those of clathrates when enough crystalline water is available at given time and location. Our simulations show that a significant fraction of volatiles can be trapped in clathrates only if they have been initially delivered in pure condensate forms to the disk. Under specific circumstances, volatiles can be essentially trapped in clathrates but, in many cases, the clathrate of a given species coexists with its pure condensate form. Those findings have implications for the compositions of giant planets and comets.

How to cite: Schneeberger, A., Mousis, O., Aguichine, A., and Lunine, J.: Evolution of the reservoir of volatiles in the protosolar nebula , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5270, https://doi.org/10.5194/egusphere-egu22-5270, 2022.

11:10–11:17
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EGU22-6692
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ECS
Artyom Aguichine et al.

The formation mechanism of Jupiter is still uncertain, as multiple volatile accretion scenarios can reproduce its metallicity [1-4]. The Galileo mission allowed in situ measurements of the abundances of several elements (Ar, Kr, Xe, C, N, S and P), which exhibit a uniform enrichment of 2 to 5 times the protosolar abundance, and a subsolar abundance has been measured for O. Recent measurements for N and O by the Juno mission confirmed the supersolar abundance of N, but indicated that the abundance of O may also be supersolar [5]. Elemental abundances measured in the Jupiter's atmosphere are key ingredients to trace the origin of various species.
Here, we investigate the possible timescale and location of Jupiter's formation using measurements of molecular and elemental abundances in its envelope. To do so, we use a 1D accretion disk model to compute the properties of the protosolar nebula (PSN) that includes radial transport of trace species, present in the form of refractory dust, a mixture of ices and their vapors, to compute the composition of the PSN [6]. We focus on the radial transport of volatile species by advection-diffusion combined with the effect of icelines, computed as sublimation/condensation rates. Initialy, the disk is uniformly filled with H2O, PH3, CO, CO2, CH4, CH3OH, NH3, N2, H2S, Ar, Kr and Xe [6,7], corresponding to the main bearers of C, N, O, P, S, Ar, Kr and Xe.
As the PSN evolves, solid particles drift inward due to gas drag. Volatile species are thus efficiently transported to their respective icelines, where they sublimate. This results in supersolar abundances of volatile elements in the inner part of the PSN. We find that the composition of Jupiter’s envelope can be achieved by accretion of enriched gas only, or a mixture of gas and solids, depending on the viscosity of the PSN. In both cases, the composition of the PSN matches the one measured in Jupiter’s envelope in timescale that are compatible with a formation by core accretion or gravitational collapse.

[1] Gautier, D., Hersant, F., Mousis, O., et al. 2001, ApJL, 550, L227.
[2] Mousis, O., Ronnet, T., and Lunine, J. I. 2019, ApJ, 875, 9.
[3] Öberg, K. I. and Wordsworth, R. 2019, AJ, 158, 194.
[4] Miguel, Y., Cridland, A., Ormel, C. W., et al. 2020, MNRAS, 491, 1998.
[5] Li, C., Ingersoll, A., Bolton, S., et al. 2020, Nature Astronomy, 4, 609.
[6] Aguichine, A., Mousis, O., Devouard, B., and Ronnet, T. 2020, ApJ, 901, 97.
[7] Lodders, K., Palme, H., & Gail, H.-P. 2009, Landolt Börnstein, 4B, 712

How to cite: Aguichine, A., Mousis, O., and Lunine, J.: Formation of Jupiter's envelope from supersolar gas in the protoplanetary disk, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6692, https://doi.org/10.5194/egusphere-egu22-6692, 2022.

11:17–11:24
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EGU22-3776
Ludovic Huguet and Renaud Deguen

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.

11:24–11:31
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EGU22-6591
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ECS
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Highlight
Kar Wai Cheng et al.

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.

11:31–11:50
Disk and planet formation discussion

Mon, 23 May, 13:20–14:50

Chairpersons: Ingo Stotz, Simone Pilia

13:20–13:25
Planetary Geodynamics

13:25–13:35
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EGU22-6028
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solicited
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Highlight
Nicolas Coltice

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.

13:35–13:42
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EGU22-6651
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ECS
Mohamed Ismail and Maxim Ballmer

      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.

13:42–13:49
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EGU22-1140
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ECS
Sebastian Ritterbex and Taku Tsuchiya

            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.

13:49–13:56
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EGU22-9006
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ECS
Joshua Guerrero et al.

The long-term evolution of the mantle is simulated using 2D spherical annulus geometry to examine the effect of heterogeneous conductivity on the stability of primordial thermo-chemical reservoirs. Conductivity of the mantle is often emulated in numerical models using purely depth-dependent profiles (e.g., taking on values between 3 and 9 W/m-K). This approach is meant to synthesize the mean conductivities of mantle materials at their respective conditions in-situ. However, because conductivity depends also on temperature and composition, their role in the conductivity of the mantle is masked. This issue is significant because dynamically evolving temperature and composition introduce lateral variations in conductivity, especially in the deep-mantle. Minimum and maximum variations in conductivity are due to the temperatures of plumes and slabs, respectively, and depth-dependence directly controls the amplitude of the conductivity (and its variations) across the mantle depth. Our simulations allow assessing the consequences of these variations on mantle dynamics, in combination with the reduction of thermo-chemical pile conductivity with iron composition, which has so far not been well examined. 

First, we examine the effect of depth (D)-dependence employing a linear profile and vary the bottom-to-top conductivity ratio. We find that increased conductivity ratio acts to reduce pile temperature. Greater conductivity in the lower mantle helps to efficiently extract heat from piles (at rates sufficient to overcome or suppress temperature increases due to enrichment in HPEs). This reduction in thermal buoyancy stabilizes the piles and may play a major role in organizing thermo-chemical reservoirs into two distinct piles. 

Next, the combined effects of temperature (T) and composition (C) are examined. A positive feedback occurs when the reduced conductivity of piles inhibits its cooling and the resulting increase in temperature further reduces its conductivity. Consequently, the augmented thermal buoyancy destabilizes piles (i.e., greater topography or enhanced erosion). Furthermore, the combined T and C-dependences can greatly underestimate typical mantle conductivities if D-dependence is also underestimated. By increasing the amplitude of D-dependence, the destabilizing effects of T and C-dependence can be suppressed. 

Finally, mineral physics data is employed to emulate a more realistic depth-dependent profile for the upper and lower mantle. Depth-dependence is no longer a linear profile and values range from 3 to 27.5 W/m-K. Buoyancy ratio and the enrichment in heat-producing elements in piles are examined for this conductivity model to determine potential evolution scenarios of primordial thermo-chemical piles. We find that this model produces stable piles for periods exceeding the age of the Earth. When B is reduced from 0.23 to 0.15, piles are destabilized earlier (by approx 1 Gyr) for cases with lesser depth-dependence. HPE enrichment in piles increases their temperature over time (and further reduces their conductivity). For HPE enrichment 10 times the mantle heat production, two distinct piles are formed with moderate topography. For greater enrichment, the piles become unstable and material becomes entrained by thin plume conduits.

How to cite: Guerrero, J., Deschamps, F., Li, Y., Hsieh, W.-P., and Tackley, P.: The effect of heterogeneous conductivity on the long-term thermo-chemical evolution of the lower mantle: implications for primordial reservoirs, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9006, https://doi.org/10.5194/egusphere-egu22-9006, 2022.

13:56–14:03
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EGU22-4128
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Highlight
Lena Noack and Caroline Brachmann

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.

14:03–14:10
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EGU22-4758
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ECS
Callum Watson et al.

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 cases3,4, 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.

5 Roy, A., Wright, J.T. & Sigurðsson, S. 2014 Earthshine on a young moon: Explaining the lunar farside highlands. The Astrophysical Journal Letters 788 (2), L42.

6 Golabek, G.J., Keller, T., Gerya, T.V., Zhu, G., Tackley, P.J. & Connolly, J.A.D. 2011 Origin of the martian dichotomy and tharsis from a giant impact causing massive magmatism. Icarus 215 (1), 346–357.

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.

14:10–14:17
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EGU22-9683
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ECS
Tobias G. Meier et al.

Super-Earth LHS 3844b is a rocky exoplanet with a radius around 1.3 Earth radii. Its thermal phase curve suggests that the dayside temperature is around 1040 K and the nightside temperature is around 0 - 700 K, indicating inefficient atmospheric heat circulation. Therefore, this planet most likely lacks an atmosphere. In a previous study, we have shown that such a strong surface temperature dichotomy can lead to a so-called hemispheric tectonic regime. In such a regime, a cold downwelling forms preferentially on one side and hot upwellings are getting pushed towards the other hemisphere. 
GJ 486b is a super-Earth that is very similar to LHS 3844b in terms of size and it is currently unknown whether this planet has an atmosphere. In this study, we are investigating under which circumstances hemispheric tectonics can operate on GJ 486b. We also investigate the stability of hemispheric tectonics. 

We run 2D geodynamic simulations of the interior mantle flow using the mantle convection code StagYY. The models are fully compressible with an Arrhenius-type viscosity law where the mantle is mostly composed of perovskite and post-perovskite. The lithospheric strength is modelled through a plastic yielding criteria and the heating mode is either basal heating only or mixed heating (basal and internal heating). 
We use general circulation models (GCMs) of potential atmospheres to constrain the surface temperature assuming different efficiencies of atmospheric heat circulation. 

We find that a hemispheric tectonic regime is also possible for surface temperature contrasts with moderate heat redistribution. The location of the strong downwelling depends on several factors such as the surface temperature contrast and strength of the lithosphere. By reducing the temperature contrast, the location of the downwelling becomes less stable and it can start to move from one side towards the other over very long timescales (Gyrs). Our results show that hemispheric tectonics could operate on tidally-locked super-Earths, even if the surface temperature contrast between the dayside and nightside is not as strong as for LHS 3844b. Upwellings that rise preferentially on one hemisphere could lead to generation of melt and subsequent outgassing of volatiles on that side. Imprints of such outgassing on the atmospheric composition could possibly be probed by current and future observations such as JWST, ARIEL or the ELT. 

How to cite: Meier, T. G., Bower, D. J., Lichtenberg, T., Hammond, M., and Tackley, P. J.: Exploring hemispheric tectonics on tidally-locked super-Earths, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9683, https://doi.org/10.5194/egusphere-egu22-9683, 2022.

14:17–14:24
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EGU22-12336
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ECS
Yue Zhao et al.

Mercury’s high core mass ratio means that its core evolution could have strong implications for its mantle dynamics, surface geology, and the generation of a dynamo. Radial contraction, present-day magnetic field, ancient crustal magnetisation, and early extensive volcanism are some of the observations that are controlled by the thermal evolution of Mercury’s interior and therefore influenced by the core.

The low intensity and lack of small-scale variations in Mercury’s present-day magnetic field can be explained by a convective liquid below a thermally stratified core layer where heat is transported conductively. Numerical studies confirmed the plausibility of a sub-adiabatic heat flow at the core-mantle boundary, giving rise to the thermally stratified layer. Investigating the conditions leading to the formation of the thermally stratified layer, and its evolution, is of crucial importance for our understanding of Mercury’s geological and geophysical history.

We couple mantle and core thermal evolution to investigate the conditions under which the thermally stratified layer is formed in the liquid core, and to study the interactions between the core and the mantle. Events such as the cessation of convection in the mantle may strongly influence the core-mantle boundary heat flow and affect the thickness of the thermally stratified layer in the core. Our results highlight the importance of coupling mantle evolution with that of the core, taking into account processes such as melting in the mantle and solidification of an inner core, and the effects of a sub-adiabatic core-mantle boundary heat flow.

How to cite: Zhao, Y., Deproost, M.-H., Knibbe, J., Rivoldini, A., and Van Hoolst, T.: Evolution of the thermally stratified layer in the outer core of Mercury, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12336, https://doi.org/10.5194/egusphere-egu22-12336, 2022.

14:24–14:31
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EGU22-12407
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ECS
Jiacheng Tian et al.

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.

14:31–14:50
Planetary Geodynamics discussion