Poster presentations and abstracts

Boulders on small Solar System bodies provide a window into the interior. They may be created by spallation during large impacts and therefore are typically found in and around fresh craters. Boulders typically survive for millions of years, until they are gradually eroded into dust by exposure to the space environment. From its vantage point in the lowest mapping orbit, the NASA Dawn spacecraft was able to distinguish boulders on the surface of Ceres at a resolution of 35 m. We study the properties of Ceres’ global boulder population: distribution over the globe, spectral properties, and size-frequency distribution. A positive identification of boulders requires at least 3 pixels, so the boulders in our sample are larger than 105 m. Boulders of such large size are also known as megaclasts or superblocks (Bruno & Ruban, 2017). We compare the Ceres boulder population with that of Vesta, Dawn’s previous target (Schröder et al., 2020).

We identified a total of 4423 boulders on the surface of Ceres with a diameter larger than 3 image pixels (105 m). All boulders are associated with impact craters. The number of boulders per crater is only weakly correlated with crater size, mostly because the largest craters in our sample have fewer boulders than expected. These craters, Azacca, Ikapati, Occator, show evidence of large scale flows that may have destroyed or obscured the majority of their boulders. The number of boulders for craters of the same size is larger on Ceres than on Vesta. Another difference with Vesta is that the boulder density on Ceres decreases dramatically towards the equator. Complicating matters is the fact that boulders may be better visible towards Ceres’ poles because of the higher solar incidence angle. We carefully evaluated the influence of incidence angle on visibility, and found that it cannot explain the large differences in boulder density. Our finding can be understood if the erosion rate of boulders is linked to the degree of solar insolation, which scales with the cosine of the latitude and is therefore minimal at the poles. We also evaluated the average boulder lifetime, by comparing the density of boulders in and around craters with their age, as estimated by crater counting. The typical lifetime of meter-sized boulders on Vesta and Ceres should be similar, based on the expected impactor velocity and density distributions (Basilevsky et al., 2015). Instead, we find that boulders appear to live shorter on Ceres than on Vesta, suggesting that they erode faster. On Ceres, water ice appears to be abundant just meters below the surface (Prettyman et al., 2017; Schmidt et al., 2017). We hypothesize that Ceres boulders contain a significant fraction of water ice, which makes them more susceptible to erosion by solar insolation than rocky boulders.

References
Basilevsky, A. T., Head, J. W., Horz, F., & Ramsley, K. (2015) Survival times of meter-sized rock boulders on the surface of airless bodies. Planetary & Space Science 117, 312-328, doi: 10.1016/j.pss.2015.07.003
Bruno, D. E., & Ruban, D. A. (2017) Something more than boulders: A geological comment on the nomenclature of megaclasts on extraterrestrial bodies. Planetary & Space Science 135, 37-42, doi: 10.1016/j.pss.2016.11.006.
Prettyman, T. H., N. Yamashita, M. J. Toplis, H. Y. McSween, N. Schörghofer, S. Marchi, W. C. Feldman, J. Castillo-Rogez, O. Forni, and D. J. Lawrence (2017) Extensive water ice within Ceres’ aqueously altered regolith: Evidence from nuclear spectroscopy. Science, 355: 55–59. doi: 10.1126/science.aah6765.
Britney E. Schmidt, Kynan H. G. Hughson, Heather T. Chilton, Jennifer E. C. Scully, Thomas Platz, Andreas Nathues, Hanna Sizemore, Michael T. Bland, Shane Byrne, and Simone Marchi (2017) Geomorphological evidence for ground ice on dwarf planet Ceres. Nature Geoscience, 10:338–343, doi: 10.1038/ngeo2936.
Schröder, S. E., Carsenty, U., Hauber, E., Schulzeck, F., Raymond, C. A., & Russell, C. T. (2019) The boulder population of asteroid 4 Vesta. Submitted to Earth & Space Science.

How to cite: Schröder, S., Carsenty, U., Hauber, E., Raymond, C., and Russell, C.: The Icy Boulders of Ceres, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-596, https://doi.org/10.5194/epsc2020-596, 2020.

1. Introduction

The surfaces of the airless solid bodies of the solar system are exposed to the action of the space weathering (i.e., solar wind, cosmic ray and solar flare, and micrometeoroid bombardment) [1].

Solar wind causes the formation of layered rims up a few tens of nanometres [2]. Cosmic rays of solar flares produce tracks up to some millimetres depth in the regolith [3]. The effects of the bombardment by micrometeoroids range from the melting and vaporization to shock deformation of the regolith’s grains, causing their fragmentation or the formation of pit-shaped microcraters [1].

Enstatite is the second most abundant silicate mineral on asteroid surfaces. Its shock deformation features were reported in numerous meteorites [4]. However, there are no natural reports on the effects of micrometeoroid bombardment. To reproduce such effects, gem-quality oriented enstatite crystals were irradiated by femtosecond pulsed laser [5]. A strong control of the shock direction and a rare occurrence of (100) clinoenstatite lamellae compared to shocked meteorites have been demonstrated [5].

Aimed to study the effects of micrometeoroid bombardment on enstatite, four enstatite-rich Itokawa grains allocated from JAXA have been investigated. Micrometeoroid impacts occur stochastically and unfortunately, none of the four requested grains showed microcraters. Despite that, the samples were sliced by focused ion beam (FIB) and then studied by analytical transmission electron microscopy (TEM).

Here, we report on the microstructures of these grains and discuss them in terms of parent body and space weathering history.

2. Samples

RA-QD02-0205 (#205). It is a prismatic grain (55x43x15 µm³). It is constituted by a single crystal of enstatite (En_74.2±3.0Fs_24.5±3.0Wo_1.7±0.8) that is pervasively crossed by (100) clinoenstatite lamellae. Such lamellae are decorated by dislocations. The grain shows a discontinuous solar wind damaged polynanocrystalline rim (up to 50 nm). The density of solar flare tracks (SFT) is <10⁷ tracks/cm².

RB-QD04-0092 (#092). It is a flat grain (26x29x12 µm³) consisting of olivine (Fo_71-78) and enstatite (En_77.9±1.4Fs_21.1±1.9Wo_1.1±0.5). Olivine shows three localized [001] dislocation-rich areas. Enstatite shows pervasive (100) clinoenstatite lamellae that are decorated by dislocations in three distinct areas. One of these areas is spatially related to the dislocation-rich area in olivine. The grain shows a continuous solar wind damaged rim (40-70 nm). SFT have been found in both minerals and their density is in average 9.6x10⁸ tracks/cm².

RB-CV-0192 (#192). It is a trapezoid prism (base width 19x8 µm²; height 10 µm). Its volume is mainly occupied by enstatite (En_76.3±2.7Fs_23.0±2.8Wo_1.0±0.4) and diopside (En_49.7±2.8Fs_7.5±1.2Wo_43.1±3.5). Adjacent to them, olivine, plagioclase, and a Ca-phosphate were also detected. Except for the enstatite, none of the minerals shows microstructures. Enstatite is pervasively crossed by (100) clinoenstatite lamellae. The solar wind damaged polynanocrystalline rim varies from absent to 50 nm. The density of the SFT is <3x10⁸ tracks/cm².

RB-CV-0144 (#144). It is the smallest sample (17x12x5 µm³). It is characterized by a smooth and a rough side. Enstatite (En_78.9±3.0Fs_20.5±3.4Wo_1.1±1.1) and diopside (En_50.4±2.8Fs₅._7±2.4Wo_43.9±3.7) constitute #144. Both minerals are pervasively crossed by (100) lamellae. Among the (100) lamellae in enstatite possible Guinier-Preston zones have been identified [6]. Subgrains and misoriented portions are common in enstatite and diopside. The smooth side is rimmed by a solar wind damaged polynanocrystalline and layered rim (up to 60 nm); on the rough side such rim is absent. The SFT density is ~1.4x10⁸ tracks/cm².

3. Discussion

Due to the similar chemical composition and mineralogy, the four studied grains likely formed in the same parent body, namely an equilibrated LL ordinary chondrite.

The four grains share the occurrence of (100) clinoenstatite lamellae in enstatite. Despite that, they are texturally different. The (100) is the weakest slip plane of orthopyroxene; hence, such lamellae can form in different geological contexts. The ongoing TEM study aims to resolve the origin of the different lamellar textures and to discuss them in terms of parent body evolution and impact processing.

Before and/or after their fragmentation to the present form, the grains resided at or close to Itokawa’s surface for ~10² to 10⁴ yr as estimated from the SFT densities [3]. The direct exposure on the asteroid surface based on the thickness of the solar wind damaged rims (<70 nm) was <3x10³ yr [7]. Rough grain surfaces and absent rims can be indicative of a recent break-up of the samples.

Being exposed on the asteroid surface, regolith grains are also exposed to the impact of micrometeoroids (primary and/or secondary) [8,9]. None of the studied grains shows microcraters, however, localized deformation features ascribable to micrometeoroid impacts were found in #092, i.e., localized [001] dislocations in olivine and dislocations in potential glide configuration associated with the pervasive clinoenstatite lamellae. A similar superposition of microstructures was observed beneath a secondary microcrater in a diopside-rich Itokawa grain [8]. A better understanding of superposed pyroxene microstructures in Itokawa particles and meteorites may provide natural evidence of micrometeoroid bombardment of enstatite. This could become an important part of assessing the recent regolith history of Itokawa and the targets of ongoing and future sample return missions.

Acknowledgements

The authors are grateful to JAXA for providing the Hayabusa samples. Financial support was provided by the DFG and FSU Jena (LA 830/14-1 to FL.; FA 1599/1-1 to AF; DRM/2016-01 to DH).

References

[1] Bennett, C.J., et al. (2013). Chemical Reviews 113:9086-9150. DOI: 10.1021/cr400153k.
[2] Harries, D., et al. (2018). EPSC 2018 Abstract #368.
[3] Berger, E.L. and L.P. Keller (2015). 46^th LPSC Abstract #1543.
[4] Leroux, H. (2001). European Journal of Mineralogy 13:253-272. DOI: 10.1127/0935-1221/01/0013-0253.
[5] Schmidt, D., et al. (2019). Geochemistry 79:125542. DOI: 10.1016/j.chemer.2019.125542.
[6] Nord, G. L. (1980). Physics and Chemistry of Minerals 6:109-128. DOI: 10.1007/Bf00311049.
[7] Christoffersen, R. and L.P. Keller (2015). 46^th LPSC Abstract #2084.
[8] Harries, D., et al. (2016). Earth and Planetary Science Letters 450:337-345. DOI: 10.1016/j.epsl.2016.06.033.
[9] Matsumoto, T., et al. (2018). Icarus 303:22-33. DOI: 10.1016/j.icarus.2017.12.017.

How to cite: Fazio, A., Harries, D., and Langenhorst, F.: Report on four enstatite-rich regolith grains from the Itokawa asteroid, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-645, https://doi.org/10.5194/epsc2020-645, 2020.

Dawn mission sensors detected pervasive Mg and NH₄ phyllosilicates mixed with a dark mineral component, probably magnetite, on Ceres’ surface, and observed Na and Mg carbonates locally associated to impact structures [1-4]. Ceres’ crust is mainly composed by different phases of silicates, water and salts. Stephan et al. [5] suggest that the NH₄-phyllosilicate is also one of the most representative components in the crust, while the distribution of water as ice or liquid is dependent on the depth. Recent models show that Ceres precursors and the differentiated crust have suffered aqueous alteration and porosity reduction during its evolution, in which silicates and water have physically and chemically interacted [6].

To understand the exchanges between water and the rock particles we are performing a set of experiments simulating the thermal evolution of two systems: 1) montmorillonite clays in liquid water; 2) montmorillonite clays in brine solutions.

NH₄-montmorillonite is obtained in the laboratory by cation substitution method [7] from the montmorillonite (Gonzales County, Texas, USA) ((Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O). The resulting smectite was checked and characterized by XRD, IR and Raman spectroscopy.

In the first set of experiments 1.5 wt% of both, the original and the NH₄-montmorillonite, were suspended in liquid water and placed into a pressure cell. In order to simulate the conditions in the ice-rich crust, systems were cooled down to 263 K for 24 hours. After that, the samples were heated up to room temperature.

During the heating of our first tests with pure water, just when the ice started to melt at 272 K, we observed shifts from 1 to 2.8 bar in the case of the montmorillonite, and to 2.6 bar when working with the NH₄-enriched clay.

In the second set of experiments, the protocol was repeated, but the original montmorillonite was suspended in an eutectic solution of NaCl (23 wt %). It also showed a pressure shift near the eutectic temperature of the solution 251 K from 1 to 1.5 bar.

We interpret these pressure shifts as the effect of a positive volume change of the system, in which the reduction of the water volume by melting is overcompensated by the smectite swelling, even at the low clay quantities we are using in these experiments. When the phyllosilicate freezes, the interlayer distance is reduced [8] and the molecules of water release. This effect is reversible if the clay is in an aqueous environment. The number of molecules inserted between layers depends on the cation in the clay. The Na⁺ present in the original montmorillonite has the capability to incorporate more than 12 molecules of water [8]. Experiments done so far with NH₄-smectites suggest that its facility to swell is lower in the NH₄-montmorillonite than in the original montmorillonite [9].

From the laboratory results, we can argue that the interaction between water-smectite during thermal evolution of Ceres’ crust could yield interesting geological effects such as the clay dehydration by freezing, the precipitation of salts from brines when swelling occurs or the generation of stresses by the deformation of the materials.

References: [1] Ammannito et al., 2016. Science, 353, issue 6503 aaf4279. [2] De Sanctis et al., 2015. Nature, 528, 241-244. [3] Longobardo et al., 2017. Icarus, .318, .205-211. [4] Stein et al, 2019. Icarus, .320, 188-201. [5] Stephan et al., 2017 Icarus 318 , 111-123 [6] Neumann et al., 2020 Astronomy & Astrophysics 633, A117  [7] Gautier et al., 2010. Applied Clay Science, 49 (3), 247-254. [8] Madsen F. T. and Muller-Vonmoos. 1989 Applied clay science 4 (2), 143-156. [9] Norrish and Rausel-Colom, 1962 Clay minerals bull. 5, 9-16.

How to cite: Ercilla Herrero, O., Fernandez-Sampedro, M. T., Muñoz-Iglesias, V., and Prieto-Ballesteros, O.: Smectite-water exchanges at the Ceres crust, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-542, https://doi.org/10.5194/epsc2020-542, 2020.

We studied 5 fall Ordinary Chondrites of different groups (H4, H5, LL5, LL6, L3.6) and an Antarctic meteorite (H5), in order to investigate possible compositional differences between the chromites present in the bulk and the chromites formed within the fusion crust. We report here the composition of about 50 chromites measured within the bulk and 70 chromites found in the crust.

Chromites found in the bulk are usually anhedral and relatively large in size (several tens of micrometers), whereas chromites formed within the crust are consistently smaller (few micrometers in size) and can display anhedral, or subhedral to euhedral habit.

The Mg# and Al# determined for all the chromites found in the bulk show a fair agreement with data reported in the literature for chromite compositions in ordinary chondrites (Bunch et al., 1967; Ramdohr, 1967; Rubin, 2003; Wlotzka, 2005), which display a small scatter of the Al# (ca.0.13±0.025) and a large variation of the Mg# (from 0.05 to 0.30).

When compared with the ones found in the bulk, chromites found within the fusion crusts generally exhibit similar values of the Al#; however, they display a much larger scatter of the Mg# and, usually, also larger average Mg# (up to 0.65) than their conterparts in the bulk.

Chromite in the fusion crusts are often associated to magnetite dendrites made up by magnetite octahedral crystals 200-400 nanometers wide; occasionally, other spinel group minerals can be found, as magnesiochromites and magnesioferrites. In most of the samples studied, several chromite crystals are mantled by magnetite crystals, whereas no magnetite crystal has been found mantled by chromites. Textural data so far collected suggest a crystallization sequence in the fusion crust: Olivine, Chromite, Magnetite.

References:

Bunch T.E., Keil K. and Snetsinger K.G. (1967). Chromite composition in relation to chemistry and texture of ordinary chondrites. Geochimica et Cosmochimica Acta, 31, 1569-1582.

Ramdohr P. (1967). Chromite and chromite chondrules in meteorites-I. Geochimica et Cosmochimica Acta, 31, 1961-1967.

Rubin A.E. (2003). Chromite-Plagioclase assemblages as a new shock indicator; implications for the shock and thermal histories of ordinary chondrites. Geochimica et Cosmochimica Acta, 67, 2695–2709.

Wlotzka F. (2005) Cr spinel and chromite as petrogenetic indicators in ordinary chondrites: Equilibration temperatures of petrologic types 3.7 to 6. Meteoritics and Planetary Science, 40, 1673-1702

How to cite: Bellesi, M., Manzetti, F., Pratesi, G., and Giuli, G.: Chromites in Ordinary chondrite fusion crusts, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-886, https://doi.org/10.5194/epsc2020-886, 2020.

Introduction: Asteroid belt observations, meteorite investigations and models converge on the paradigm of the ubiquity of melting in planetesimals. Partial melting of bodies that surpassed the melting temperature led to the weakening of the rock by the interstitial melt, facilitating solid-state convection due to a decrease of the viscosity. Further melting can produce liquid-like layers with suspended particles, i.e. magma oceans. Models^[1] indicate that ²⁶Al-heated bodies experienced differing melting degrees. In fact, transient magma oceans are likely for those that accreted early and massive (Vesta^[2]), consistent with the meteorite record^[3]. Partially molten layers can occur in the interior of undifferentiated bodies and in silicate mantles of differentiated ones. Attempts to model and to quantify the effects of convection in planetesimals remain rare. They are typically restricted to parametrized approaches in a magma ocean, neglecting convection before a magma ocean develops and after it crystallizes, and in the layers below a shallow magma ocean. However, while convection in a solid planetesimal is less likely, it may set in for little partial melt due to the weakening of solid material by the interstitial melt.

Here, the possibility of solid-state convection in planetesimals is investigated and an initial comparison of a 3D convection model with a conductive case for a Vesta-like body is presented. Implications for theoretical studies of meteorite parent bodies are discussed.

Planetesimal convection: The convective heat transport can be described with the Nusselt-Rayleigh relation with the Nusselt number Nu, the Frank-Kamenetskii parameter Θ, the Rayleigh number Ra, and parameters a, c, and β. The strength of the convection increases with Ra that depends strongly on the layer thickness and inversely on the viscosity, while the latter depends exponentially on the inverse temperature via an Arrhenius relation. If partial melting occurs, the viscosity decreases with the melt fraction χ according to η(χ)~exp(-α_ηχ) (where α_η is a constant that depends on the creep regime), e.g. by a factor of 10^-4 for χ=0.25^[4]. Further melting up to the rheologically critical melt fraction χ_RC of 0.25-0.5 marks a transition to liquid-like behavior, resulting in a magma ocean. The likelihood of convection is estimated with the (internal) Rayleigh number Ra_Q as a function of the layer thickness for different values of the viscosity (Fig. 1). In general, with decreasing viscosity and increasing layer thickness, Ra increases and convection sets in for Ra_Q≥Ra_c≈10³. An increased heat transport will result in a temperature and melt evolution that may differ from what is predicted by conventional models and have important consequences: (1) A more efficient cooling of a partially molten planetesimal. (2) Producing high amounts of melt or a magma ocean may be more difficult. (3) Convection in a partially molten mantle may sufficiently cool the core to generate a magnetic field in the absence of a magma ocean.

Figure 1. Ra_Q for different viscosities for a heat production at 2.1 Myr after CAIs suggest that planetesimals as small as some asteroids and H chondrite and Acapulco-Lodran parent bodies^[5,6] could convect at small degrees of melting (figure from [4]).

Vesta:  A 3D full sphere mantle convection model GAIA^[7] was used. It solves a set of differential equations based on the principle laws of conserving mass, momentum, and energy. Here, stagnant-lid regime and a free-slip boundary condition at the surface of a purely internally heated sphere were considered for a Vesta-like body with a radius of 260 km and constant material properties. A purely internal heat production matching an accretion time at three half-lives of ²⁶Al after CAIs guarantees that the melt fraction in the interior remains below χ_RC. An unddifferentiated structure was assumed for simplicity^[2].

Conductive and convective models are compared: Fig. 2 shows the temperature (top) and a combined temperature and velocity field (bottom) (top: conduction, bottom: convection). A Rayleigh number of 2×10⁶ used is consistent with η(χ)≈10¹⁹ Pa s for χ≤0.1, a layer thickness of ≥120 km, and a heat production of 3.33×10^-8 W kg^-1. Both the temperature distribution and the convection velocity differ. The convection model produces plumes that are lacking for the conductive case. The maximum temperature is higher for the conductive case showing a substantial reduction if convection is considered. This reflects effective heat dissipation by the redistribution of warmer and colder material by convective currents.

Conclusions: 3D convection modeling is needed to address abundant questions relating to planetesimal evolution. Ra estimates imply that convection is likely and should cool effectively early accreted and sufficiently large bodies. If one of these conditions is not fulfilled, it can be compensated by other factors, e.g., despite Ceres' weak present-day heat production, convection could be possible due to low-viscosity clay minerals mantle composition^[8].

Figure 3. Temperature (top) and of temperature and velocity (bottom)^[4] of a conductive case (top) and a convective one at Ra_Q =2×10⁶ (bottom).

Efficient cooling could cause a feedback on the melting, delaying or potentially preventing the differentiation. However, convection could also help to initiate melt separation^[9] and to overcome permeability problems^[10]. Another exciting question is the possibility of convection in the mantles of differentiated bodies that may sufficiently cool the underlying cores to initiate their thermal convection and power dynamos, consistent with the remnant magnetization in basaltic meteorites^[11]. More generally, convection is relevant to studies that link models to geochemical data, building on the assumption of an “onion-shell” structure, where it will create lateral inhomogeneities and lead to different parent body properties.

References: [1] Neumann W. et al. (2012) A&A, 543, A141. [2] Neumann W. et al. (2014) EPSL, 395, 267. [3] Righter K., Drake M. J. (1997) MPS, 32, 929. [4] Neumann W. (2019) MNRAS, 490, L47. [5] Henke S. et al. (2012) A&A, 545, A135. [6] Neumann W. et al. (2018) Icarus, 311, 146. [7] Hüttig C. et al. (2013) PEPI, 220, 11. [8] Ruesch O. et al. (2019) Nat. Geosci., 12, 505. [9] Rushmer T. et al. (2000) Origin of the Earth and Moon, UAP, Tucson. [10] Ghanbarzadeh et al. (2017) PNAS, 114, 13406. [13] Weiss B. et al. (2008) Science, 322, 713.

How to cite: Neumann, W.: Modeling Solid-State Convection in Asteroids and Meteorite Parent Bodies, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-726, https://doi.org/10.5194/epsc2020-726, 2020.