Oral presentations and abstracts

Disruption and Reaccumulation: 

Asteroids such as Ryugu and Bennu are likely fragments formed from a larger body that was disrupted in the main asteroid belt [1,2]. Numerical simulations of asteroid disruptions—including the fragmentation phase during which the asteroid is broken up into small pieces and the gravitational phase during which fragments may reaccumulate due to their mutual attractions—lead to a family of rubble piles over a range of sizes [3]. Considering microporous parent bodies of 100 km in diameter, we found that their disruption (Fig. 1) can lead to rubble piles with oblate spheroidal or top shapes [4]. Moreover, assuming that the parent body is hydrated, the various degrees of heating at impact can produce rubble piles with different level of hydration as a result of a single parent body disruption.

We proposed two scenarios where Ryugu and Bennu could originate from the same parent body. In scenario a, Ryugu and Bennu are composed from materials sourced from near the impact point and near its antipode, respectively. In scenario b, Ryugu and Bennu are composed from materials sourced from the parent-body center and near the impact point’s antipode, respectively. The detected signature of  exogeneous material introduces new complexities to the collisional origin of Ryugu and Bennu [5, 6].

Rubble Pile Contamination:

Due to the apparent spectral homogeneity observed on the surfaces of Bennu and Ryugu during the first observational campaigns, our simulations in [4] only considered the fate of material originating from the parent body, assumed to be homogenous in composition. However, subsequent spectral data from the OSIRIS-REx and Hayabusa2 missions show a small fraction of anhydrous silicate material on the surface of the two bodies [5, 6]. The presence of this material can be explained by retention of a projectile on either the parent body or on the rubble piles themselves after their formation.  However, projectile retention efficiencies for impacts of anhydrous silicates on hydrated minerals are poorly constrained [7, 8] for expected impact speeds in the main asteroid belt (~ 5km/s, [9]). Here, we investigate whether the family-forming catastrophic disruption can lead to the incorporation of impactor material in the reaccumulated family members, leading to the small fraction of apparently exogeneous material on their surface.

Figure 1: Outcome of a SPH simulation of the disrup-tion of a microporous 100-km-diameter parent body. Each particle is a fragment. Colors represent the various degrees of impact heating. This outcome is the starting point of the gravitational phase during which the fragments reaccumulate to form rubble piles.

Approach:

We performed a series of numerical simulations of sub-catastrophic and catastrophic disruption of 1- and 100-km-diameter microporous asteroids. We account for both the parent body material and the projectile material in the subsequent gravitational phase when fragments re-accumulate to form the parent-body remnant and smaller rubble-pile family members. As in our previous works, the fragmentation phase was simulated using a Smoothed Particle Hydrodynamics (SPH) hydrocode, and the gravitational phase was computed using the N-body code pkdgrav, including the Soft-Sphere Discrete Element Method (SSDEM) [10]. We then track the surviving materials of both the projectile and the parent body, including their level of heating, as they reaccumulate. For each aggregate, we measure their shapes, the fractions of projectile and parent body materials that compose them, and their associated level of heating.

Projectile material was neglected in previous work because asteroid families appear spectrally homogeneous, suggesting that they are mostly made of the material of their parent body. The advanced observational capabilities of space missions enabled the discovery that this scenario may be more complex.

Outlook: 

Observational analysis of exogenous material on Ryugu and Bennu provide constrains for our numerical simulations. In particular, the total volume and the spectral characteristics of the exogenous material can be measured [5,6,11]. The total volume bounds the required contamination efficiency and/or the total time needed to contaminate the parent body. The spectral analysis shows that Bennu hosts HED-like material whereas Ryugu has ordinary chondrite–like material.

This difference in the spectral signature of exogenous material may render scenario b (outlined above) invalid, as our preliminary calculations show that contamination on large 100-km parent bodies is likely only limited to its outer shell. Thus, it is difficult to form a 1^st generation rubble-pile that has both: i) material from the parent body core, and ii) exogenous material that originated from the contamination of the original parent body’s outer shell. This scenario may be possible if the asteroid is a 2^nd generation object, with its precursor being an approximately 20-km rubble-pile that incorporated material originating from both the  center and exterior of the parent body [12, 13]. Our numerical simulations will provide claraity on the feasibility of these various scenarios. Ultimately, analysis and comparison of the returned samples will provide clarity on the potential shared collisional origin of Ryugu and Bennu, and the prevalence of impact contamination in the Solar System. 

Acknowledgements

This material is based upon work supported by NASA under Contract NNM10AA11C issued through the New Frontiers Program. P.M. acknowledges support from the Centre National d’Études Spatiales and from the Academies of Excellence on Complex Systems and Space, Environment, Risk and Resilience of the Initiative d’EXcellence “Joint, Excellent, and Dynamic Initiative” (IDEX JEDI) of the Université Côte d’Azur. We are grateful to the entire OSIRIS-REx and Hayabusa2 teams for making the encounters with Bennu and Ryugu possible.

References:

[1] Michel, P. et al. (2001) Science, 294, 1696–1700. [2] Walsh, K.J. (2018) ARA&A 56, 593. [3] Jutzi, M., et al. (2019) Icarus 317, 215. [4] Michel, P., Ballouz, R.-L. et al. (2020) Nature Comm. 11, 2655.  [5] DellaGiustina, D.N., et al. (2019) EPSC-DPS2019-1074.  [6] Sugimoto, C., et al. (2019) Asteroid Science in the Age of Hayabusa2 and OSIRIS-REx, 2051. [7] Avdellidou, C., et al. (2016) MNRAS, 456, 2957. [8] Daly, R.T., & Schultz, P. H. (2018) M&PS, 53, 1364. [9] Bottke, W.F., et al. (2005) Icarus, 179, 63.  [10] Ballouz, R.-L., et al. (2019) MNRAS 485, 697. [11] Campins, H., et al. (2020) EPSC. [12] Walsh, K.J., et al. (2020) LPSC 51, 2253.  [13] Sugita, et al. (2019) Science 364, 252.

How to cite: Ballouz, R.-L., Michel, P., Barnouin, O., Walsh, K., Jutzi, M., Tatsumi, E., Barucci, M. A., DellaGiustina, D., Campins, H., Sugita, S., Watanabe, S., Miyamoto, H., Bottke, W., Connolly, H., Yoshikawa, M., and Lauretta, D.: Modeling the contamination of Bennu and Ryugu through catastrophic disruption of their precursors , Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-510, https://doi.org/10.5194/epsc2020-510, 2020.

Ponded craters have been predominantly identified on small, dry planetary bodies like (433) Eros and Itokawa. We identified similar features on Vesta, where loose fragmented ponded materials are present on small crater floors. While the morphological details of the ponded features on Vesta and Eros/Itokawa are similar, their production mechanisms may vary, due to differences in gravity or the insolation environment Previous studies conducted on Vesta have provided evidence for volatile outgassing in some regions. In this study, we investigate the morphology of the ponded crater and possible involvement of volatiles outgassing and its interaction with surface material in producing ponded craters on Vesta.   

Ponded craters have widely received lime light due to its unusual characteristics on Eros revealed by the NEAR Shoemaker mission (Sears et al., 2015, Robinson et al., 2001,2002). In general, ponded craters show a smooth layer of fine-grained material with grain size less then cm (Robinson et al., 2001) partially covering topography of the crater floor (Figure 1). They may also possess varying sized boulders or unconsolidated material (Sears et al., 2015). The depth of the pond is about ~5% of the depth of the original crater (Robinson et al., 2001) on Eros. Other than the distinct morphological impression (smooth and flat floor), ponded crater regolith also shows sharp variation in the spectral signature (Robinson et al., 2001) which can be due to mineral heterogeneity (Robinson et al., 2001), space weathering (Sears et al., 2015, Heldmann et al., 2010, Robinson et al., 2001) or the difference in grain size between regolith and the surrounding region (Heldmann et al., 2010, Robinson et al., 2001).

Based on the evidences on Eros, the formation mechanisms of ponded craters include electrostatic levitation, seismic shaking and/or boulder comminution (Robinson et al., 2001). However, the effects of these mechanisms may vary on other dry planetary bodies with different compositions, gravity or insolation intensity. In our study, we characterize ponded craters on Vesta to understand their formation mechanisms and how interactions with the regolith may have influenced the generation of ponded craters.

Figure 1: A classic example of pond crater on Eros. The crater has diameter of ~0.09km (Robinson et al., 2001). The pond material has sharp boundary, low albedo and flat smooth surface which makes it easy to distinguish from the original carter floor. Image source: Robinson et al., 2001  

For the identification of ponded craters on Vesta, HAMO mosaics (~70 m/pixel) and LAMO mosaics (~20 m/pixel) provided by the NASA Dawn Mission were considered. To extract elevation information, a DTM of HAMO resolution was used (92 m/pixel) (Preusker et al., 2016) prepared from stereo-pairs.

So far, we have identified 10 ponded craters nearby the equator (0°-30°) on Vesta. Overall, the crater floor is fully or partially covered by fine and loose material. The usual diameter of ponds ranges from 0.9 -6.4 km within craters of 1.78- 8.43 km diameter. Most of the identified ponded craters, have clear flat floors in which the fine material is evenly distributed within the bowl-shaped depression (Figure 1) covering the original floor of the crater entirely. An example is given in Figure 2, located at 15°S,189°E. The carter has a diameter of ~8km. By fitting a polynomial shape (e.g. a parabola) to the crater walls, we estimated the original depth of the crater with ~0.66km. The ponded material has filled the original crater surface, producing a shallow depth crater. The material has filled ~0.31km of the crater, which means half of the original crater depth is infilled by the fine material. On Eros ponds have average infilling depths of ~10cm or 5% of the original depth (Robinson et al., 2001). The smaller infilling might be due to the fact that craters on Eros are significantly smaller in comparison with Vesta. However, it is unreasonable to draw any conclusions based on a single example. At the meeting we will present measurements of the rest of the identified ponded sites to understand the overall morphology and discuss formation mechanisms for ponds on Vesta based on our findings.

Figure 2: Example of a ponded crater on Vesta. (a) The ponded material is covering the original surface within central crater region and exhibits a flat and smooth texture. (b) The elevation profile of the crater in (a) highlighting the flat ponded material in red. The ponded material infilled the deeper parts of the crater depression, masking the original shape (dash-dotted line) and generating a flat floor.

 References

[1] Heldamann et al., 2010, Icarus, 206, 685-690. [2] Robinson et al., 2001, Nature, 413,396-400. [3] Robinson et al., 2002, Meteorit. Planet. Sci., 37 1651-1684. [4] Sears et al., 2015, PSS, 117, 106-118.

How to cite: Parekh, R., Otto, K., Jaumann, R., Matz, K.-D., Roatsch, T., Kersten, E., Elgner, S., Krohn, K., and Raymond, C.: Ponded craters on Vesta, Europlanet Science Congress 2020, online, 21 Sep–9 Oct 2020, EPSC2020-656, https://doi.org/10.5194/epsc2020-656, 2020.

Introduction

Sodic plagioclase is common in Earth’s crust and in many differentiated and undifferentiated meteorites. Under high temperature (HT) and high pressure (HP) conditions in asteroidal collisions, sodic plagioclase may transform into either hollandite-structured lingunite [1] or the recently discovered albitic jadeite [2]. When stoichiometric jadeite forms by decomposition of albite, the excess silica forms an SiO₂ polymorph, often stishovite [3]. Albitic jadeite, by contrast, a Na-rich analogue of tissintite [2], is super-silicic, vacancy-rich pyroxene with excess Si coordinated in the octahedral M1 site. Searching for albitic jadeite alongside other P-sensitive mineral assemblages is therefore potentially important for expanding the list of pressure constraints available for impact events.

We report preliminary results on the occurrence of albitic jadeite within shock veins in the L6 ordinary chondrites Ozerki and Chug-Chug-011 (Fig. 1). Ozerki (fell 21^st June 2018 in Russia) is moderately shocked (S4/5) and un-weathered (W0); it was recovered quickly (25^th June 2018) after its fall. Chug-Chug-011 is a find, recovered in 2018 in Antofagasta, Chile; it is weakly shocked (S2), with minor weathering (W1).

Materials and Methods

Polished thin sections of Ozerki and Chug-Chug-011 were carefully examined for shock indicators and HP polymorphs, with intensive focus on the melt veins (MVs). We used optical microscopy, a JEOL JSM-IT300LV scanning electron microscope, a JEOL JXA 8900 electron probe micro-analyzer, and a dispersive confocal Renishaw inVia Reflex Raman microscope (514 nm laser).

Petrography & mineral chemistry

The thin section of Ozerki displays two discrete areas (Fig. 1A); light-colored chondritic and dark-colored impact melt-rich area. We focused on a network of shock veins intruding the light-colored area. The MVs are dark, variable width (40-850 μm), and locally disrupted by angular to sub-rounded clasts. Clasts are more abundant in wider MVs; jigsaw-fit breccia textures are widespread. Clasts, mostly silicate, concentrate in the center of each MV, whereas the margins are rich in metallic segregations and sulfides.

In Chug-Chug-011, three different MVs (~100 μm wide) crosscut the meteorite matrix (Fig. 1B). Elongated silicate clasts oriented parallel to the veins are common in their central domains.

In Ozerki, albitic jadeite forms acicular to dendritic crystallites aggregates (≤ 2 μm) associated with feldspathic glass (Fig. 2A). In Chug-Chug-011, albitic jadeite is found within a composite clast: low Ca-pyroxene surrounds sodic plagioclase (Fig. 2B). Crystallites near the core of the plagioclase show brighter backscatter than those near the rim.

Albitic jadeite in Ozerki yields an empirical formula (Na_0.70Ca_0.15K_0.05□_0.14)(Al_0.82Si_0.10Fe_0.04)Si₂O₆ whereas that from Chug-Chug-011 is variable: (Na_0.57-0.64Ca_0.07-0.07K_0.03-0.05Mg_0.01-0.07□_0.16-0.29)(Al_0.78-0.86Si_0.10-0.18Fe_0-0.05Mg_0-0.13)Si₂O₆, with Ca# [100×Ca/(Ca+Na)] from 10 to 13.

Pyroxene Raman spectroscopy

Raman spectra of the albitic jadeite in Ozerki display five distinct peaks at 376, 526, 698, 986 and 1036 cm^-1 (Fig. 3A). In Chug-Chug-011, the predominant peak is at 698 cm^-1, but there is a noteworthy 1016 cm^-1 peak in addition to the “typical jadeite” 1038 peak. This may be associated either with a diopside-related structure or another high-P clinopyroxene (Fig. 3B).

Discussion and Conclusions

In Ozerki, albitic jadeite was found in the middle of ~70 μm and ~300 μm wide MVs. The presence of equant idiomorphic crystals with 120° triple junctions suggests that these MVs reached peak HT above the liquidus of the matrix. From such conditions, a ~300 μm wide vein surrounded by cold matrix conductively cools and solidifies in ~6.5 ms, which is an upper limit for growth time of minerals in the MV. Albitic jadeite is less dense than lingunite, implying formation from sodic plagioclase at lower pressures. The absence of lingunite suggests maximum pressures below 21 GPa. According to experiments [4] in jadeite-rich compositions (Jd_70-80), jadeite + stishovite + garnet is stable at 13.5-21.5 GPa. However, the absence of stishovite and garnet in our MV may only reflect sluggish nucleation of these phases rather than an insufficient peak P<13.5 GPa [5]. The presence of albitic jadeite, by itself, therefore yields only an upper limit and not a fully quantitative P constraint.

In Chug-Chug-011, high-pressure Na-clinopyroxene [(Na_0.49Ca_0.15K_0.03Mg_0.24□_0.09)(Al_0.62Si_0.04Fe_0.13Mg_0.21)Si₂O_6] is enclosed in a melt pocket included in pyroxene that is in turn entrained in a MV. The bright crystallites near the center of the pocket yield compositions and spectra similar to the HP-sodic clinopyroxene identified by [6]. The backscatter-dark crystallites closer to the pocket margins better match albitic jadeite. Neither phase is yet calibrated for shock pressure. However, the presence of a mixed xieite-chromite spectrum at the rim of another MV in the section suggests higher P conditions, 18-23 GPa (Fig. 3C). The same MV shows minor wadsleyite peaks near its center, requiring gradients over space or time in preserved P and T conditions across the MV.

Acknowledgements

This research received support from European Social Funds and the Greek State (call code EDBM103).

References

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