PS1.1

Exoplanet characterization is one of the main foci of current exoplanetary science. For super-Earths and sub-Neptunes, we mostly rely on mass and radius measurements, which allow us to derive the mean density of the body and give a rough estimate of the bulk composition of the planet. However, the determination of planetary interiors is a very challenging task. In addition to the uncertainty in the observed fundamental parameters, theoretical models are limited owing to the degeneracy in determining the planetary composition.
 We aim to study several aspects that affect the internal characterization of super-Earths and sub-Neptunes: observational uncertainties, location on the M-R diagram, impact of additional constraints such as bulk abundances or irradiation, and model assumptions.
 We used a full probabilistic Bayesian inference analysis that accounts for observational and model uncertainties. We employed a nested sampling scheme to efficiently produce the posterior probability distributions for all the planetary structural parameter of interest. We included a structural model based on self-consistent thermodynamics of core, mantle, high-pressure ice, liquid water, and H-He envelope. 
 Regarding the effect of mass and radius uncertainties on the determination of the internal structure, we find three different regimes: below the Earth-like composition line and above the pure-water composition line smaller observational uncertainties lead to better determination of the core and atmosphere mass, respectively; and between these regimes internal structure characterization only weakly depends on the observational uncertainties. We also find that using the stellar Fe/Si and Mg/Si abundances as a proxy for the bulk planetary abundances does not always provide additional constraints on the internal structure. Finally we show that small variations in the temperature or entropy profiles lead to radius variations that are comparable to the observational uncertainty. This suggests that uncertainties linked to model assumptions can eventually become more relevant to determine the internal structure than observational uncertainties.

How to cite: Fernandez Otegi, J., Dorn, C., Helled, R., Bouchy, F., Haldemann, J., and Alibert, Y.: Impact of the measured parameters of exoplanets on the inferred internal structure., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4617, https://doi.org/10.5194/egusphere-egu21-4617, 2021.

We are now entering an era of rocky exoplanet detection. To determine whether an exoplanet is ‘Earth-like’, we must estimate not only its mass, radius and insolation, but also its geological composition. These geological constraints have wide ranging implications, not least for a planet’s subsequent evolution and habitability.

Polluted white dwarfs which have accreted fragments of planetary material provide a unique opportunity to probe exoplanetary interiors. We can also learn about their formation histories, including the geological process of core-mantle differentiation.

Cr, Ni and Fe behave differently during differentiation, depending on the conditions under which it occurs. This alters the Cr/Fe and Ni/Fe ratios in the core and mantle of differentiated bodies. The pressure inside the body is a key parameter, and depends on the body’s size.

In our work, we present a novel approach for modelling this behaviour and use it to gain crucial insight into the sizes of exoplanetary bodies which pollute white dwarfs.

How to cite: Buchan, A., Bonsor, A., Shorttle, O., Wade, J., and Harrison, J.: Chromium, Nickel and Iron as clues to the formation histories of exoplanetary bodies, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12909, https://doi.org/10.5194/egusphere-egu21-12909, 2021.

We investigate how the bombardment of terrestrial planets by populations of planetesimals left over from the planet formation process, asteroids from the main belt and comets affects the evolution of their atmospheres, through both impact induced atmospheric mass loss and volatile delivery. This work builds on previous studies of this topic by combining prescriptions for the atmosphere loss and mass delivery derived from hydrodynamic simulations with results from dynamical modelling of a realistic population of impactors.

The effect on the atmosphere predicted by the hydrodynamical simulations performed by Shuvalov (2009) as a function of the impactor and system properties are incorporated into a stochastic numerical model for the atmospheric evolution. The effects of rare but destructive giant impacts, that can cause non-local atmosphere loss, are also included using the prescription from Schlichting et al. (2015). The effects of aerial bursts and fragmentation of impactors in the atmosphere are included using a prescription based on the work of Shuvalov (2014). These effects are found to be relevant for hot and dense atmospheres analogous to the present day conditions on Venus.

We compare the impact induced atmosphere evolution of Earth, Venus and Mars using impact velocities and probabilities inferred from the results of dynamical models of the population of left over planetesimals in the early solar system from Morbidelli et al. (2018), the population of asteroids from Nesvorny et al. (2017a) and comets from Nesvorny et al. (2017b). We use realistic size distributions for these populations based on the main belt asteroids and trans-Neptunian objects. The effect of the variation in the distribution of the impactor material through their bulk density and volatile fraction is investigated, as is the effect of varying the initial conditions assumed for the atmospheres of Earth, Venus and Mars.

Our results for the Earth are discussed in light of observational constraints regarding the composition of the material delivered as the late veneer. The results for Venus and Mars are compared to those for the Earth and considered in comparison to observational evidence regarding the past climate of these worlds. A holistic view of the results for all three planets allows constraints on the past atmospheres to be inferred, in the absence of other atmospheric effects.

How to cite: Sinclair, C. and Wyatt, M.: Atmospheric Evolution due to impacts during the final stage of planet formation, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2628, https://doi.org/10.5194/egusphere-egu21-2628, 2021.

Introduction. Post-accretionary impact bombardment is part of planet formation and leads to localized, regional [e.g., 1-3], or even wholesale global melting of silicate crust [e.g., 4]; less intense bombardment can also create hydrothermal oases favorable for life [e.g, 5]. Here, we generalize the effects of late accretion bombardments to extrasolar planets of different masses (0.1-10M_⊕). One example is Proxima Centauri b, estimated at ~2× M_⊕ [6]. We model a 0.1M_⊕ “mini-Earth”and “super-Earth” at 10M_⊕, the approximate upper limit for a “mini-Neptune” [7]. Output predicts lithospheric melting and subsurface habitable volumes.

Methods. The model [1,2] consists of (i) stochastic cratering; (ii) analytical thermal expressions for each crater [e.g., 8,9]; and (iii) a 3-D thermal model of the lithosphere, where craters cool by conduction and radiation.

We analyze impact bombardments using our solar system’s mass production functions for the first 500 Myr [10]. Surface temperatures and geothermal gradients are set to 20 °C and 70 °C/km [2]. Total delivered mass for Earth is 7.8 × 10²¹ kg, and scaled to other planets based on cross-sectional areas, with 1.7 × 10²¹ kg for mini-Earth, 1.2 × 10²² kg for Proxima Centauri b, and 3.6 × 10²² kg for super-Earth. The impactors' SFD is based on our main asteroid belt [11]. Impactor and target densities are set to 3000 kg m^-3 and planetary bulk densities are assumed to be 5510 kg m^-3, omitting gravitational compression [7]. Impactor velocity was estimated at 1.5 × v_esc for each planet, with 7.8 km s^-1 for mini-Earth,  16.8 km s^-1 for the Earth, 21.1 km s^-1 for Proxima Centauri b, and 36.1 km s^-1 for super-Earth.

Results. We assume fully formed crusts, so melt volume immediately increases due to impacts. Super-Earth reaches a maximum of ~45% of the lithosphere in molten state, whereas mini-Earth reaches a maximum of only ~5%.  This is due to much higher impact velocities and cratering densities on the super-Earth compared to mini-Earth. We also show the geophysical habitable volumes within the upper 4 km of a planet’s crust as the bombardment progresses. Impacts sterilize the majority of the habitable volume on super-Earth; however, due to its large total volume, the total habitable volume is still higher than on other planets despite the more intense bombardment in terms of energy delivered per unit area.

References: [1] Abramov, O., and S.J. Mojzsis (2009) Nature, 459, 419-422. [2] Abramov et al. (2013) Chemie der Erde, 73, 227-248. [3] Abramov, O., and S. J. Mojzsis (2016) Earth Planet Sci. Lett., 442, 108-120. [4] Canup, R. M. (2004) Icarus, 168, 433-456. [5] Abramov, O., and D. A. Kring (2004) J. Geophys. Res., 109(E10). [6] Tasker, E. J. et al. (2020). Astronom. J., 159(2), 41. [7] Marcy, G. W. et al. (2014). PNAS, 111(35), 12655-12660. [8] Kieffer S. W. and Simonds C. H. (1980) Rev. Geophys. Space Phys., 18, 143-181. [9] Pierazzo E., and H.J. Melosh (2000). Icarus, 145, 252-261. [10] Mojzsis, S. J. et al. (2019). Astrophys. J., 881(1), 44. [11] Bottke, W. F. et al. (2010) Science, 330, 1527-1530.

How to cite: Mojzsis, S. J. and Abramov, O.: Thermal consequences of impact bombardments to silicate crusts of terrestrial-type exoplanets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13084, https://doi.org/10.5194/egusphere-egu21-13084, 2021.

One of the most important questions in planetary science is the origin of the current Venus atmosphere, its relationship and coupling to Venus’ geologic and geodynamic evolution, andwhy it is so different from that of the Earth. We specifically address the following question:Does the eruption of the total volume of extrusive volcanic deposits observed in the exposed geologic record of Venus contribute significantly to the current atmosphere through volatile release during emplacement of the extruded lavas? To address this question, we used the observed geologic and stratigraphic record of volcanic units and features, and their volumes, as revealed by Magellan (1; their Fig. 26 and Table 5).  We converted the volumes of the main volcanic units to lava/magma masses using a density of 3000 kg m^-3. Next, we chose the upperthickness values, and added the contributions from allof the units; summing the values of the "total eruptives" gives the absolute upper limit estimate of the mass of documented volcanics that could contribute to the atmosphere, 7.335 x 10²⁰ kg. We then compare this with the current mass of the Venus atmosphere (4.8 x 10²⁰ kg). We find that in order to make the current atmosphere from the above volcanics, the magma would have to consist of 65.4% by mass volatiles, which is, of course, impossible. We conclude that the grand totalof the currently documented volcanics can not have produced other than a very small fraction of the current atmosphere.

Exsolution of volatiles during volcanic eruptions is significantly dependent on surface atmospheric pressure (2-3). However, the total volumeof lava erupted in the period of global volcanic resurfacingis still insufficient to produce the CO₂atmosphere observed today, even if the ambient atmospheric pressure at that time was only 50% of what it is today. Therefore, a very significant part of the current CO₂atmosphere must have been inherited from a time prior to the observed geologic record, sometime in the first ~80% of Venus history. Furthermore, the total volumeof lava erupted in the stratigraphically youngest period of the observed record (1) is insufficient to account for the current abundance of SO₂ in the atmosphere; thus, it seems highly unlikely that current and recently ongoing volcanism could be maintaining the currently observed ‘elevated’ levels of SO₂ in the atmosphere (4).  In addition, because of the fundamental effect of atmospheric pressure on the quantity of volatiles that will be degassed, varying the nature of the mantle melts over a wide range of magma compositions and mantle fO₂ appears to have minimal influence on the outcome.  We conclude that the current Venus atmosphere must be a “fossil atmosphere”, largely inherited from a previous epoch in Venus history, and if so, may provide significant insight into the conditions during the first 80% of Venus history.

(1) Ivanov and Head (2013) Plan. Space Sci. 84, 66; (2) Gaillard & Scaillet, 2014, EPSL 403, 307; (3) Head & Wilson, 1986, JGR 91, 9407;(4)Esposito, 1984, Science 223, 1072.

How to cite: Head, J., Wilson, L., Ivanov, M., and Wordsworth, R.: Contributions of Volatiles to the Venus Atmosphere from the Observed Extrusive Volcanic Record: Implications for the History of the Venus Atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13030, https://doi.org/10.5194/egusphere-egu21-13030, 2021.

In mantle convection studies, two-dimensional geometry calculations are predominantly used, due to their reduced computational costs when compared to full 3-D spherical shell models.  Although various 3-D grid formulations [e.g. 1, 2] have been employed in studies using complex scenarios of thermal evolution [e.g., 3, 4], simulations with these geometries remain highly expensive in terms of computational power and thus 2-D geometries are still preferred in most of the exploratory studies involving broader ranges of parameters. However, these 2-D geometries still present drawbacks for modeling thermal convection. Although scaling and approximations can be applied to better match the average quantities obtained with 3D models [5], in particular, the convection pattern that in turn is critical to estimate melt production and distribution during the thermal evolution is difficult to reproduce with a 2D cylindrical geometry. In this scope, another 2D geometry called “spherical annulus” has been proposed by Hernlund and Tackley, 2008 [6]. Although steady state comparison between 2D cylindrical, spherical annulus and 3D geometry exist [6], so far no systematic study of the effects of these geometries in a thermal evolution scenario is available. 

In this study we implemented a 2-D spherical annulus geometry in the mantle convection code GAIA [7] and used it along 2-D cylindrical and 3-D geometries to model the thermal evolution of 3 terrestrial bodies, respectively Mercury, the Moon and Mars. 

First, we have performed steady state calculations in various geometries and compared the results obtained with GAIA with results from other mantle convection codes [6,8,9]. For this comparison we used several scenarios with increasing complexity in the Boussinesq approximation (BA).

In a second step we run thermal evolution simulations for Mars, Mercury, and the Moon using GAIA with 2D scaled cylinder, spherical annulus and 3D spherical shell geometries.In this case we considered the extended Boussinesq approximation (EBA), an Arrhenius law for the viscosity, a variable thermal conductivity between the crust and the mantle, while taking into account the heat source decay and the cooling of the core, as appropriate for modeling the thermal evolution. A detailed comparison between all geometries and planets will be presented focussing on the convection pattern and melt production. In particular, we aim to determine which 2D geometry reproduces most accurately the results obtained in a 3D spherical shell model. 

Aknowledgments: The authors gratefully acknowledges the financial support and endorsement from the DLR Management Board Young Research Group Leader Program and the Executive Board Member for Space Research and Technology.

References: [1] Kageyama and Sato, G3, 2004; [2] Hüttig and Stemmer, G3, 2008;  [3] Crameri & Tackley, Progress Planet. Sci., 2016; [4] Plesa et al., GRL (2018); [5] Van Keken, PEPI, 2001; [6] Hernlund and Tackley, PEPI, 2008; [7] Hüttig et al, PEPI 2013; [8] Kronbichler et al., GJI, 2012; [9]  Noack et al., INFOCOMP 2015.

How to cite: Fleury, A., Plesa, A.-C., and Hüttig, C.: Thermal evolution of terrestrial planets with 2D and 3D geometries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7311, https://doi.org/10.5194/egusphere-egu21-7311, 2021.

The vigour and style of mantle convection in tidally-locked super-Earths may be substantially different from Earth's regime where the surface temperature is spatially uniform and sufficiently cold to drive downwellings into the mantle. The thermal phase curve for super-Earth LHS 3844b suggests a solid surface and lack of a substantial atmosphere. The dayside temperature is around 1040 K and the nightside temperature is around 0 K, which is unlike any temperature contrast observed at present day for planets in the Solar System. On the other hand, the thermal phase curve of super-Earth 55 Cnc e suggests much hotter temperatures with a nightside temperature around 1380 K and a substellar point temperature around 2700 K. Both super-Earths have therefore temperature contrasts between the day- and nightside of more than 1000 K and we infer that this may also lead to a dichotomy of the interior mantle flow. 
We run 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 modelled with an upper mantle, a perovskite-layer and a post-perovskite layer. 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). For LHS 3844b we find that the surface temperature dichotomy can lead to a hemispheric tectonic regime depending on the strength of the lithosphere and the heating mode in the mantle. In a hemispheric tectonic regime, downwellings occur preferentially on one side and upwellings rise on the other side. We compare these results to the case of 55 Cnc e, where large parts of the surface could be molten. At first order we expect that a magma ocean could homogenise the temperatures on the planet's surface and therefore reduce the likelihood of hemispheric tectonics operating on 55 Cnc e.
For LHS 3844b, the contribution of the interior flux to the thermal phase curve is on the order of 15-30 K, and therefore below the detecting capabilities of current and near-future observations. However, for hemispheric tectonics, upwellings might lead to preferential melt generation and outgassing on one hemisphere that could manifest as a secondary signal in phase curve observations. Such signals could also be produced on hotter planets such as 55 Cnc e where parts of the surface are hot enough to melt.

How to cite: Meier, T. G., Bower, D. J., Lichtenberg, T., J. Tackley, P., and Demory, B.-O.: Exploring the convection in super-Earths: Comparing LHS 3844b with 55 Cnc e, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8910, https://doi.org/10.5194/egusphere-egu21-8910, 2021.

Since the discovery of a potentially low-mass exoplanet around our nearest neighbour star Proxima Centauri, several works have investigated the likelihood of a shielding atmosphere and therefore the potential surface habitability of Proxima Cen b. However, outgassing processes are influenced by several different (unknown) factors such as the actual planet mass, mantle and core composition, and different heating mechanisms in the interior.
We aim to identify the critical parameters that influence the mantle and surface evolution of the planet over time, as well as to potentially constrain the time-dependent input of volatiles from mantle into the atmosphere.

To study the coupled star-planet evolution, we analyse the heating produced in the interior of Proxima Cen b due to induction heating, which strongly varies with both depth and latitude. We calculate different rotation evolutionary tracks for Proxima Centauri and investigate the change in its rotation period and magnetic field strength. Unlike the Sun, Proxima Centauri possesses a very strong magnetic field of at least a few hundred Gauss, which was likely higher in the past. 
We apply an interior structure model for varying planet masses (derived from the unknown inclination of observation of the Proxima Centauri system) and iron weight fractions, i.e. different core sizes, in the range of observed Fe-Mg variations in the stellar spectrum. 
We use a mantle convection model to study the thermal evolution and outgassing efficiency of Proxima Cen b. For unknown planetary parameters such as initial conditions we chose randomly selected values. We take into account heating in the interior due to variable radioactive heat sources and latitute- and radius-dependent induction heating, and compare the heating efficiency to tidal heating.

Our results show that induction heating may have been significant in the past, leading to local temperature increases of several hundreds of Kelvin (see Fig. 1). This early heating leads to an earlier depletion of the interior and volatile outgassing compared to if the planet would not have been subject to induction heating. We show that induction heating has an impact comparable to tidal heating when assuming latest estimates on its eccentricity. We furthermore find that the planet mass (linked to the planetary orbital inclination) has a first-order influence on the efficiency of outgassing from the interior.

Fig 1: Local induction heating and resulting temperature variations compared to a simulation without induction heating after 1 Gyr of thermal evolution for an example rocky planet of 1.8 Earth masses with an iron content of 20 wt-%.

How to cite: Noack, L., Kislyakova, K., Johnstone, C., Güdel, M., and Fossati, L.: Interior heating and outgassing of Proxima Cen b: Identification of critical parameters, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2299, https://doi.org/10.5194/egusphere-egu21-2299, 2021.

The large iron core of Mercury and the low iron contents on the surface inferred from MESSENGER data suggest the presence of a magma ocean after accretion. We modeled the lifetime of an early Hermean magma ocean as well as the structure and loss rates of an atmosphere that is sourced by degassing. We use a large range of initial conditions including several bulk compositions associated with varying degrees of differentiation, the inclusion of carbon and hydrogen degassing volatiles such as CO2 and H2O, as well as considering a larger proto-Mercury size. After obtaining the magma ocean lifetime and volatile vapor pressures, the result is passed on to further models to obtain metal oxide vapor pressures, a complete atmospheric photochemical speciation and ultimately the mass loss rate of the atmosphere.

We show that magma ocean cooling times are sensitive to the size of the planet and the efficiency of radiative heat transfer in the atmosphere. A volatile-free proto-Mercury radiating as a blackbody with its present-day size would cool down within 400 years from an assumed initial surface temperature of 2500 K to an early crust formation threshold of 1500 K. In contrast it takes 9000 years for a volatile rich proto-Mercury with a greenhouse atmosphere and a mantle size representing Mercury before the occurrence of a mantle stripping event. Volatile-rich cases reach massive atmosphere pressures, whereas volatile-free cases are dominated by Si, Na, K, Mg, and Fe species degassed from the magma ocean and end up at a maximum pressure of 0.1 bar at 2500 K. There is however only a small difference in the atmospheric extent, as the absence of volatile species in the thin metal oxide atmosphere causes it to become extended to a degree, where an upper atmosphere height comparable to the volatile cases is reached. In terms of mass loss we found that upper atmospheric loss due to photoionization is highly efficient in the environment of a young Sun, ionizing 100% of the particles reaching Mercury’s exosphere. This leads to loss rates of up to 10⁶ kg/s, which are however diffusion limited by the supply from the homopause, reducing them by 2-3 orders of magnitude. In regards to Na and K loss, we found that a thin, volatile-free atmosphere is most efficient with its extended structure allowing for large loss rates as well as the high Na and K mixing ratio.

How to cite: Jäggi, N., Gamborino, D., Bower, D. J., Sossi, P., Wolf, A., Vorburger, A., Oza, A. V., Wurz, P., and Galli, A.: Early Mercury’s magma ocean atmosphere, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4011, https://doi.org/10.5194/egusphere-egu21-4011, 2021.

One of the fundamental questions for planetary science is how surfaces of other planets similar to the rocky bodies in our solar system look like. What is the rock structure like? Will there be water? Are there any active atmospheric cycles? How can these different conditions be detected?

The current space missions and ground based instruments allow the detection of specific gas species and some cloud compositions in atmospheres of giant exoplanets. With instruments  installed in the near future and space crafts currently being build or planned, these kind of observations will be available for planets with smaller sizes and an overall rocky composition. We aim to further understand the connection of the conditions of the upper atmosphere with the conditions on the crust of the planet (temperature, pressure, composition).

Our equilibrium chemistry models allow us to investigate the expected crust and near-crust-atmosphere composition. With this, we investigate the conditions under which liquid water is actually stable at the surface of a planet and not incorporated in hydrated rocks. Based on this crust - near-crust-atmosphere interaction we build an atmospheric model, which allows us to investigate what kind of clouds are stable and could be present in atmospheres of rocky exoplanets. This allows us to predict what clouds on other planets could be made of. Potential detection of cloud condensates and the high altitude gas phase can constrain the overall surface conditions on those planets. 

How to cite: Herbort, O., Woitke, P., Helling, C., and Zerkle, A.: From clouds to crust - Cloud diversity and surface conditions in atmospheres of rocky exoplanets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9745, https://doi.org/10.5194/egusphere-egu21-9745, 2021.

Introduction: Small cones are common on Mars. Many cones form subparallel chains several kilometers in length. Their origin is discussed in many papers, however, the mechanism of their formation is not explained [1].

In the present paper, we deal with a small region in Chryse Planitia ( ~38^o13′ N and ~319^o25’ E). The region is covered by lacustrine deposits.

    On Mars, chains of small cones occupy vast areas. Therefore, we try to explain the existence of the chains by specific conditions on Mars. We focus on the hypothesis connecting the formation of cones with the loss of water from the regolith due its instability. See e.g. [1], [2], [4], [5].

Mechanism of cones formation: We consider 3 mechanisms of cone formation: (i) a grains’ ejection, (ii) from mud or fluidized sand and (iii) explosive formation. The (iii) and (ii) are possible with additional heat sources only.

    Assuming that only heat of melting was used for vaporization, then only ~13% of liquid water will be vaporized, If the outgassing effect is to be regolith without water, then there must be also other heat sources. Therefore we consider two coexisting factors required for cones formation: (1) the presence of water in the regolith and (2) some additional heating, e.g. magma intrusion.

    The formation of a chain of cones is possible in two situations:

(a) above a linear structure containing water and areal heating. Outcrops of aquifers could serve as linear sources of volatiles.

(b) above a linear source of magmatic heat and the areal aquifer. A dike could serve as linear source of heat.

Conclusions and future plans;

1) Considered cones could be a result of outgassing of regolith due to pressure drop.

2) Subparallel chains of cones were formed along the outcrops of volatile-rich sediments.

3) Numerical modeling indicates that small magma intrusions may not be enough for completely degassing some aquifers.

Acknowledgments: This study was supported by statutory project of Institute of Geophysics of University of Warsaw. We are also grateful to prof. W. Kofman and dr. J. Ciążela for their remarks.

References

[1] Fagents, S., Thordarson, T., (2007) The Geology of Mars: Evidence from Earth-Based Analogs, ed. Mary Chapman. Cambridge Univ. Press. [2] Brož,, et al. (2019) JGR: Planets. 124, 703–720. [3] Rotto, S., Tanaka, K. L. (1995) Geologic/ geomorphologic map of the Chryse Planitia: region of Mars. USGS. [4] Barlow, N.G. (2010) GSA Bulletin (2010) 122 (5-6): 644–657. [5] Brož, P., et al. (2020) Nature Geoscience. 13, 403–407.

How to cite: Czechowski, L., Zalewska, N., Zambrowska, A., Ciazela, M., Witek, P., and Kotlarz, J.: The mechanisms of formation of some small cones in Chryse Planitia on Mars, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15422, https://doi.org/10.5194/egusphere-egu21-15422, 2021.

Silicate weathering is a key step in the carbonate-silicate cycle (carbon cycle) that draws down

CO2 from the atmosphere for eventual burial and long-term storage in the planetary interior. This process is thought to provide an essential negative feedback to the carbon cycle to maintain temperate climates on Earth and Earth-like. We implement thermodynamics to determine weathering rates as a function of surface lithology (rock type). These rates provide upper limits that allow estimating the maximum rate of weathering in regulating climate. We model chemical kinetics and thermodynamics to determine weathering rates for three types of rocks inspired by the lithologies of Earth's continental and oceanic crust, and its upper mantle. We find that thermodynamic weathering rates of a continental crust-like lithology are about one to two orders of magnitude lower than those of a lithology characteristic of the oceanic crust. Our results show that the weathering of mineral assemblages in a given rock, rather than individual minerals, is crucial to determine weathering rates at planetary surfaces. We show that when the CO2 partial pressure decreases or surface temperature increases, thermodynamics rather than kinetics exerts a strong control on weathering. The kinetically- and thermodynamically-limited regimes of weathering depend on lithology, whereas, the supply-limited weathering is independent of lithology. Our results imply that the temperature-sensitivity of thermodynamically-limited silicate weathering may instigate positive feedback to the carbon cycle, in which the weathering rate decreases as the surface temperature increases. 

How to cite: Hakim, K., Bower, D. J., Tian, M., Deitrick, R., Auclair-Desrotour, P., Kitzmann, D., Dorn, C., Mezger, K., and Heng, K.: A Lithology-based Silicate Weathering Model for Earth-like Planets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3273, https://doi.org/10.5194/egusphere-egu21-3273, 2021.

Mercury is characterized by a globally low reflectance associated with remarkably low iron contents. Among several proposed hypothesis, to date, the most convincing explanation of the low reflectance of Mercury invokes mixing of an ancient graphite-rich crust with overlying volcanic materials via impact processes and/or assimilation of carbon into rising magmas during secondary crustal formation (e.g. Peplowski et al.2016). Even though until now graphite has not been directly observed, there are strong evidences suggesting its presence on Mercury’s surface (e.g. Denevi et al.2009; Peplowski et al.2011). The actual presence of graphite within Mercury soil may have several implications, e.g. on the late accretion history of Mercury (Hyodo et al.2021; Murchie et al.2015) or on hollow formation (Blewett et al.2016). Moreover, silicates are often associated to carbon phases in some achondrites (e.g. ureilite, Nestola et al.2020, and references therein). Evaluating in a systematic way the effect of graphite on visible and near-infrared spectroscopy of mafic mineral absorptions is thus of interest to improve our understanding of Mercury remote sensing data, and to make progress in our capability to associate carbon-rich stony meteorites to their parent bodies. Mixing graphite with silicate materials is thought to basically decrease the contrast of reflectance spectra of these materials (Murchie et al.2015). Nevertheless, systematic works addressing the influence of graphite-silicate mixtures on their reflectance spectra are still lacking. Here we mixed microcrystalline graphite with a suite of silicate materials and measured their VNIR reflectance spectra. We selected three silicate end-member compositions, namely: 1) a synthetic glass with chemical composition close to the one inferred for of the volcanic products emplaced in the Mercury’s northern volcanic plains (Vetere et al.2017), 2) a Mg-rich Gabbronorite with FeO < 3% (Secchiari et al.2018) and 3) a hawaiitic basalt (Pasquarè et al.2008). To decouple the effect of granulometry and graphite content, we produced and analyzed different granulometric classes (ranging between <50 μm and 250μm) for each end-member. In a second stage, we selected three granulometric classes (<50 μm, 75-100 μm and 150-180 μm) for each end member and we added graphite producing different samples with graphite – silicate weight ratio between 0-5% (0%, 1%, 2%, 3%, 4% and 5%) in order to encompass the inferred graphite content in Mercury’s surface (Klima et al.2018). The results of our work confirm that graphite strongly decreases the contrast of the reflectance spectra of the silicate-graphite mixtures and, in most cases, has a negligible effect on the shift of the absorption bands. However the slopes of the reflectance spectra are greatly affected by the graphite content, which tends to decrease the slope of the spectra. Our systematic study will allow to gain a better understanding of the reflectance spectra of materials mixed with opaque phases in meteorites, space-weathered surfaces and rocky planetary bodies. In particular, this investigation is expected to have a strong impact on the interpretation of reflectance measurements of Mercury. Acknowledgments: Part of this research was supported by ASI-INAF Simbio-sys agreement. E.B. and C.C. are supported also by ASI-INAF 2018-16-HH.0 (Ol-BODIES) agreement.

How to cite: Bruschini, E., Carli, C., Capaccioni, F., Vincendon, M., Buellet, A.-C., Vetere, F., Secchiari, A., Ferrari, M., Perugini, D., and Montanini, A.: The effects of graphite and particles size on reflectance spectra of silicates, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2704, https://doi.org/10.5194/egusphere-egu21-2704, 2021.