SB8

Introduction: Knowledge of the physical and surface properties of most NEAs lags far behind the current rate of their discoveries. Still, asteroid surfaces and internal structures are very diverse, and knowledge derived from a limited number of asteroids typically could not be safely extrapolated to a large number of objects. The situation calls for an alternative approach that permits estimating asteroids' properties for a much larger number of NEAs.

The Demystifying Near-Earth Asteroids (D-NEAs) is the Planetary Society STEP Grant 2021 project aiming to develop a novel method that directly characterises asteroids primarily from ground-based data. In particular, the project's objective is to develop a model to characterise surface thermal properties.

Methodology: The idea is based on the Yarkovsky effect, a non-gravitational phenomenon that causes objects to undergo orbital semi-major axis drift as a function of their size, orbit, and material properties. The effect joins together the asteroid's orbital dynamics, composition, and physical properties. Our idea to derive the surface thermal properties of near-Earth objects is built around these facts.

Theoretical models of the Yarkovsky effect allow predicting the semi-major axis drift, assuming a set of input parameters is available. On the other hand, astrometric observations and orbit determination procedures allow detecting the semi-major axis drift in motion of an asteroid. Therefore, at least one asteroid's property that determines the drift rate could be estimated by comparing the model’s predicted (da/dt) and measured (da/dt)_m magnitude of the effect, as given by Equation 1:

Especially critical are the thermal conductivity uncertainties that span a range of about four orders of magnitude (Delbo et al. 2015). It is also a key to proper estimation of the thermal inertia, which could be diagnostic of surface porosity and cohesion, and, therefore, for the possible presence of the regolith layer at the surfaces.

Results: The first results obtained by Fenucci et al. (2021) are encouraging but also intriguing at the same time. We found that a small super-fast rotator, near-Earth asteroid 2011 PT, should have low thermal inertia (Γ < 100 J m^-1 K^-1 s^-1/2) to maintain the high Yarkovsky drift detected from astrometry.

Future prospect: This exciting result opens the possibility for further studies. There are, however, several essential features that are not included in the preliminary model. To fully exploit the potential of our approach, it is necessary to extend the model by including, for instance, Yarkovsky correction for eccentric orbits, heterogeneity in object’s density, or variable thermal inertia along the orbit (Rozitis et al. 2018). The D-NEAs project will address these issues and develop a robust model which will apply to a much larger number of asteroids.

References

• Delbo, M., Mueller, M., Emery, J.P., Rozitis, B., Capria, M.T.: Asteroid Thermophysical Modeling. Asteroids IV, p.107-128, University of Arizona Press, Tucson, 2015.

• Fenucci, M., Novakovic, B., Vokrouhlicky, D., Weryk, R.J.: Low thermal conductivity of the super-fast rotator (499998) 2011 PT. Astronomy and Astrophysics, id 647, 2021.

• Rozitis, B., Green, S.F., MacLennan, E., Emery, J.P: Observing the variation of asteroid thermal inertia with heliocentric distance. MNRAS, 477, 1782, 2018.

How to cite: Novakovic, B., Fenucci, M., Marceta, D., and Pavela, D.: Demystifying Near-Earth Asteroids, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-159, https://doi.org/10.5194/epsc2022-159, 2022.

Introduction:

To understand the long-term history and age of a planetary surface, it is important to analyze the distribution of impact craters, including the case of fast-spinning asteroids. Two of such asteroids have recently been visited by space probes such as the Hayabusa2 and the OSIRIS-REx spacecraft that targeted the C-type and B-type asteroids (162173) Ryugu and (101955) Bennu, respectively. These NEAs have been found to share similar physical and morphological characteristics, including low bulk density, surface characterized by large blocks and boulders, and top-shape profiles [1, 2]. Moreover, on the surface of both small bodies, several geological features can be identified in detail, such as crater-like features, boulders, linear features, fossae, etc [e.g., 1, 3]. Many crater-like depressions can be identified over the surface strongly skewed towards low equatorial latitudes. This uneven global distribution suggests that different evolutionary and impact history took place at the equatorial ridge with respect to the rest of the body.

Such bodies likely aggregated gravitationally right after the catastrophic collision that originated them [4]. In addition, collisions in the main belt and lately the YORP effect may have contributed to modify their spin rate. For instance, they have been found to be currently slowly spinning up (Bennu) and down (Ryugu) by YORP [5, 6]. The past spinning history of such bodies is unknown, however they may have had spin rates fast enough to cause the detachment of boulders from their surface eventually falling back onto them. Under such conditions, the formation of currently observed crater-like depressions may not be necessarily related to impact events and may rather be the outcome of repeated boulder landing and taking off cycles.

Here, we focus on checking the potential correlation between the formation of depressions and crater-like features with the dynamical behaviour of boulders on Ryugu and Bennu like-asteroids. 

Methodology:

We use the global mosaic of images available for Ryugu and Bennu asteroids created by using data collected from OCAMS and ONCs cameras suites overlapped to shape models [2, 7] to identify and map the characteristic features which we are interested in.

To check our hypothesis on the origin of crater-like features, we developed a two-stage model research, based on: (1) dynamical study of lifted particles due to fast spin, and (2) modelling the movement of big boulders using a N-body Discrete Element Method (PKDGRAV. [8,9]). In the first stage, we analyze the dynamics of particles that can detach from the surface of Ryugu and Bennu at a time they were spinning fast enough so that apparent centrifugal acceleration is sufficient to overcome local gravity. We therefore integrate the equation of motion of sample particles in a non-inertial rotating reference frame [10]. Sample particles -representing blocks and boulders- are initially placed at the center of the triangular facets in the shape model of each asteroid [2, 7]. Their dynamical evolution in time is followed and their trajectory and behaviour is tracked. Particle detachment from the surface at the critical spin state, and the landing latitude on the asteroid surface is also analyzed. In the second stage, we use PKDGRAV to place boulders on the asteroid surface and follow their dynamical evolution at an increasing spin rate.

Summary:

Our model of boulder motion on asteroids Bennu and Ryugu under fast spin conditions shows the formation of depressions and crater-like features due to boulder detaching, landing and/or bouncing off their surface is possible. Also, final landing of boulders at high latitudes is found. This model suggests a potential explanation for the crater abundance observed around the equatorial region in Ryugu and Bennu. In near future, the DART (NASA, 2022) and Hera (ESA, 2026) space missions are scheduled to approach the NEA binary system (65803) Didymos. The primary of such system is also a top-shape fast-spinning asteroid for which we may expect similar features as those studied in Bennu and Ryugu.

Acknowledgements:

LMP, NET and ACB acknowledge funding from the NEO-MAPP project (H2020-EU-2-1-6/870377). LMP acknowledges support from the Margarita Salas UCM postdoctoral grants funded by the Spanish Ministry of Universities with European Union funds - NextGenerationEU.

References:

[1] Lauretta, D. S., et al. (2019). Nature 568, 55–60. [2] Watanabe, S., et al. (2019). Science 364, 268–272. [3] Sugita, S., et al. (2019). Science, 364(6437), 252. [4] Campo Bagatin, A., et al. (2018). Icarus 302, 343–359. [5] Kanamaru, M., et al. (2021). JGR: Planets 126. [6] Hergenrother, C.W. et al. (2019). Nat. Commun. 10, 1291. [7] Barnouin, O.S. et al. (2019). Nature Geoscience 12, 247-252. [8] Richardson, D. C. (2000). Icarus, 143, 45. [9] Schwartz, S. (2012). Granular Matter 14, 263. [10] Trógolo N., et al., (2021). EPSC2021-676.

How to cite: Parro, L. M., Trógolo, N., and Campo Bagatin, A.: Non-impact origin of crater-like features on top-shape near-Earth asteroids, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-726, https://doi.org/10.5194/epsc2022-726, 2022.

Thermal properties of C-type asteroid Ryugu have been investigated through remote sensing using the Thermal Infrared Imager (TIR), on the surface using the radiometer MARA on MASCOT lander, and the analysis of return sample. The global average and the local distribution of thermal inertia were mapped by TIR observations, with the lower thermal inertia than that of typical chabonaceous chondrite meteorites. surface boulders and their surroudings have almost the same thermal inertia of 200 to 400 J m^-2 kg^-1 s-^0.5 (tiu, hereafter), indicating that most of boulders are relatively porous and not completely consolidated and the surroundings are covered with boulders and rocks (not sandy regolith) [Okada et al., 2020), which was confirmed during the descent operations for sampling. Boulders have a variety of thermal inertia, with more than 80 % of boulders with 200 to 400 tiu, while some portions have very low or very high thermal inertias. They are identified as Hot Spots and Cold Spors, because they are exceptionally hot or cold compared with their surroundings (Sakatani et al., 2021). The surface experiment by MARA indicated the simiar thermal inertia for a single boulder (Grott et al., 2019), so that the rough boulders should be the representative material on the asdteroid. The thermal properties of return sample do not seem to be the same, although they are not always representative regaring the physical properties since fragile sample might have been broken during the impact sampling process, during the severe shock and shaking in the return capsule when entry to the Earth surface. The return sample dseem to be more consolidated and flatter surface feature, instead of fragile and porous features. The return sample are more like CI chondrite meteorites with darker, more porous and fragile characteristics (Tada et al., 2021).  The difference of thermal inertia between la arger scale (> 1mm)  and a small scale (<0.1ｍｍ）might attribute to the existing cracks and pores inside of boulder materials. A formation scenario of Ryugu will be shown to explain the history of Ryugu formation or planetary formation. 

How to cite: Okada, T., Tanaka, S., Sakatani, N., Shimaki, Y., Arai, T., Senshu, H., Demura, H., Sekiguchi, T., Kouyama, T., Kanamaru, M., and Ishizaki, T.: Thermal properties of asteroid Ryugu from global, local, and micro-scale and its formation scenario, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1187, https://doi.org/10.5194/epsc2022-1187, 2022.

• ¹Instituto Universitario de Física Aplicada a las Ciencias y a las Tecnologías, Universidad de Alicante. Alicante, Spain (nair.trogolo@ua.es)
• ²Observtorio Astronómico de Córdoba, Universidad Nacional de Córdoba. Córdoba, Argentina.
• ³Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante. Alicante, Spain.
• ⁴Instituto de Astrofísica de Andalucía, CSIC. Granada, Spain.

1. Introduction

(65803) Didymos is a binary near-Earth asteroid (NEA) system and is the target of the  upcoming DART (NASA) and Hera (ESA) space missions. Physical parameters of the system have been determined through radar and optical observations. A shape model for the primary, Didymos, is available, showing a top−shape appearance (see Fig. 1). Didymos is a 780 m asteroid with a satellite, Dimorphos, of 160 m in diameter [1]. Also, the main body is estimated to be an S-type asteroid with a density of 2170 ± 350 kg/m³ [2] and a fast rotation period of 2.2600 ± 0.001 h [3]. Given its spectral type, and considering that its meteorite analogs are ordinary chondrites, with a typical density greater than 3 000 kg/m³, Didymos is probably a gravitational aggregate (rubble-pile) [4]. In the equatorial region of such asteroids, the apparent non-inertial centrifugal force acting on surface particles can overcome the gravitational attraction of the asteroid, resulting in a local net outward acceleration, allowing them to leave the surface at zero speed and dynamically evolve in the system [5][6]. 

In this work we analyze conditions for regolith ejection and following dynamical evolution. In addition, we provide an estimation of the amount of material present in the system environment. The model predictions may be tested with data from the DART and Hera missions, making Didymos a particularly interesting system to study.

Figure 1: Surface gravity in the polyhedral shape model used for Didymos made by 1996 faces and 1000 vertices.

2. Methodology

In order to study the detachment process of particles on asteroid Didymos, we developed a numerical code that integrates the equation of motion of a test particle initially at rest at any position on the surface. We have taken into account the central gravitational field generated by the primary, both gravitational perturbation of the secondary and the Sun, and solar radiation pressure (SRP). Furthermore, since that is a rotating, non--inertial  frame, it is necessary to consider the centripetal and coriolis forces. Initially, sample particles are located at the center of the triangular facets of the asteroid shape model, made by 1996 faces and 1,000 vertices. At each integration step, the detachment condition is studied (that is, the sum of the mentioned forces must be directed outwards). A 3D grid is set outside the asteroid  in latitude, longitude and radial distance. Such a grid identifies 3D cells, where cumulative particle mass density is calculated. Based on particle trajectories, we defined four possible final states: ES1) particles that take off from the surface and land again, ES2) particles that remain in orbit, ES3) particles that are accreted into the secondary, and ES4) particles that escape from the system.

3. Numerical simulations and results

The sample particle differential size distribution follows a power law with a -3.5 slope and particles in the range [5 μm-2 mm] are considered. We analyzed the behaviour of particles during 30 days about  perihelion and aphelion epoch and during a complete heliocentric orbit, equivalent to 2.11 Earth years. We find that orbiting particle mass density at the end of integration time (ES2) depends on the position of the asteroid in its orbit around the Sun, especially at small particle size. The system has a high eccentricity (e=0.38), leading to a 4/9 relationship between total mass around perihelion vs. aphelion (See Fig. 2). This is due to smaller particles being strongly influenced by the SRP during passage at perihelion. That circumstance leads them to either leave the system (ES4) or, after taking off, return quickly to the surface (ES1). In general, about 96% of particles that take off land back onto the surface of Didymos, sometimes even at relatively high latitudes (See Fig. 3). Those landing near the equatorial plane can be ejected again, triggering repeated regolith landing and taking off cycles.

 

Fig. 2. Orbiting mass density versus radial distance after 30 days near perihelion and aphelion epoch. R = 0 m corresponds to the Didymos center, R ∼ 430 m to the surface.	Fig. 3. Projection of particle landing positions on the Didymos surface in the Z-Y plane. Each light blue dot corresponds to a particle, the size of the dot does not represent the real size. This is the simulation result over a complete orbit around the Sun (2.1 years).

 

A crucial parameter in the problem we are analyzing is Didymos bulk density, which is currently known with a wide margin of uncertainty. We studied the relationship between the total mass in orbit after 30 days of evolution and the density of the asteroid, varying both the mass of Didymos and its volume. A lack of particles in the system environment is found only in 6% of simulation runs. This shows an optimistic scenario about observing particles in orbit around the asteroid, which perhaps can be estimated by measurement carried out by the Hera space mission.

References:

[1] Benner, L. A. et al. (2010) Bull. Am. Astron. Soc. 42, 1056.

[2] Fang J., Margot J.L., 2012, AJ, 143, 24

[3] Pravec P., et al., 2006, Icarus, 181, 63

[4] Campo Bagatin, A. et al. (2018) Icarus 302, 343–359. 

[5] Campo Bagatin, A. (2013) LPSC. 

[6] Yu, Y., et al. (2019) MNRAS 484, 1057–1071.

How to cite: Trógolo, N., Campo Bagatin, A., Moreno, F., and Pérez Molina, M.: Lifted particles from (65803) Didymos surface due to its fast rotation, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-1044, https://doi.org/10.5194/epsc2022-1044, 2022.

The km-scale near-Earth object (1566) Icarus has an extremely eccentric orbit with a perihelion of q = 0.187 au and is classified as a potentially hazardous asteroid (PHA). It has been suspected to be the larger component of an asteroid pair, with the smaller object 2007 MK₆, that is dynamically adjacent to the Taurid-Perseid meteor shower (Ohsutka et al., 2007; Kasuga & Jewitt, 2019). The low radar albedo of ~2% and photometric behavior at high phase angles together suggest a high-porosity surface with a  high macroscopic roughness (Greenberg, et al. 2017; Ishiguro, et al., 2017). Delay-Doppler and visible lightcurve observations indicate a retrograde spin with a rapid rotation period of ~2.26 hr (Greenberg, et al. 2017; Warner et al., 2009).

Combining visible spectrophotometry from the 24-Color Asteroid Survey (Chapman et al., 2020) and MITHNEOS near-infrared reflectance spectra (Binzel et al., 2019), we classify Icarus (Figure 1) as a slightly space weathered LL chondrite via a band parameter analysis routine (MacLennan, et al. in prep.). Using archived lightcurve observations of Icarus collected in 1968 and 2015 (Lagerkvist et al., 1993; Warner et al., 2009), and informed by spin axis constraints, we implement a Bayesian lightcurve inversion approach (Muinonen, et al. 2020) to construct a convex shape model of Icarus (Figure 2).

Figure 1. Combined visible spectrophotometry and near-infrared reflectance spectra of Icarus and reflectance spectrum of the LL4 ordinary chondrite Hamlet from the RELAB database.

Figure 2. Convex shape model of Icarus from inversion of lightcurve photometry.

We incorporate thermal infrared data from the Spitzer Space Telescope (IRAC photometry and IRS spectra) and the NEOWISE survey in order to characterize Icarus’s thermophysical properties. We estimate the effective diameter and thermal inertia to be 1.4 ± 0.2 km and 60 ± 40 J K^-1 m^-2 s^-1/2, respectively, with moderate surface roughness. The relatively low thermal inertia is consistent with a high porosity surface and/or a fine-grained lunar like surface. The latter interpretation is in contradiction to the polarization-phase relationship that suggests larger regolith grains (Ishiguro et al., 2007). We attempt to reconcile these different measurement results in our presentation.

The physical characteristics of this extreme object are important for informing various resurfacing processes that have been proposed to be relevant for rapidly rotating objects, near-Sun asteroids, and spectrally-fresh Q-type asteroids (Graves et al., 2018, 2019). We thus consider our results in the context of these resurfacing processes.

References:

Binzel, R.P., et al. (2019) “Compositional distributions and evolutionary processes for the near-Earth object population: Results from the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS)” Icarus, 324, 41–76.

Chapman, C.R., Gaffey, M., and McFadden, L. (2020) 24-color Asteroid Survey V1.0. urn:nasa:pds:gbo.ast.24-color-survey::1.0. NASA Planetary Data System.

Greenberg, A., et al. (2017) “Asteroid 1566 Icarus’s Size, Shape, Orbit, and Yarkovsky Drift from Radar Observations” AJ, 153:108.

Graves, et al. (2018) “Resurfacing asteroids from YORP spin-up and failure”, Icarus, 304 (2018) 162–171.

Graves, et al. (2019) “Resurfacing asteroids from thermally induced surface degradation”, Icarus, 322 (2019) 1–12.

Ishiguro, M., et al. (2017) “Polarimetric Study of Near-Earth Asteroid (1566) Icarus”, AJ, 154:180.

Kasuga, T. & Jewitt, D. (2019) “Asteroid-Meteorite Complexes”, In Meteoroids: Sources of Meteors on Earth and Beyond (Ed. G. Ryabova, D. Asher, & M. Campbell-Brown).

Lagerkvist, C.-I. and Magnusson, P., Eds., (2011) Asteroid Photometric Catalog V1.1. EAR-A-3-DDR-APC-LIGHTCURVE-V1.1. NASA Planetary Data System.

MacLennan, E.M., et al. (in prep) “Empirical characterization of space weathering on ordinary chondrite-like asteroids”.

Muinonen , J. Torppa , X.-B. Wang , A. Cellino , and A. Penttilä (2020) “Asteroid lightcurve inversion with Bayesian inference”, A&A, 642, A138.

Ohtsuka, K. “Apollo asteroids 1566 Icarus and 2007 MK6: Icarus Family Members?” AJ, 668: L71–L74.

Warner, B.D., Harris, A.W., Pravec, P. (2009). “The Asteroid Lightcurve Database”, Icarus 202, 134-146.

How to cite: MacLennan, E., Muinonen, K., Uvarova, E., Granvik, M., Wilawer, E., Oszkiewicz, D., and Emery, J.: Shape, Compositional, and Thermophysical Properties of (1566) Icarus, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-893, https://doi.org/10.5194/epsc2022-893, 2022.

We only have indirect information on the internal material properties and structure of small icy bodies in the Solar System, coming from the analysis of meteorites and observations of near-Earth comets. Examination of comets can provide information on the composition of trans-Neptunian objects as they originate primarily in the Kuiper Belt and the Oort Cloud. 

In order to infer the internal structure we need to know the dominant heat sources in the small icy bodies to determine the extent of heat production at each stage of their evolution, and their internal transformations they may have come through. The dominant internal heat sources of small icy bodies during their evolution are:

• accretion heat from the formation process;
• radioactive heating in the silicate component due to the decay of the short-lived radiogenic isotope ²⁶Al (in our Solar system);
• exothermic chemical reactions such as serpentinization;
• differentiation energy released when the nucleus is formed;
• tidal heating by large moons or in binary systems.

In this work, we focused on the early evolution, and our heat evolution model is based on the serpentinization reaction (Farkas-Takács et al. 2022). It takes into account the accretion energy as the initial temperature and the radiogenic decay of ²⁶Al as the internal heat source during the process.

The serpentinization processes require the existence of Mg-pyroxenes (enstatite, MgSiO₃), Mg-rich olivine (forsterite, Mg₂SiO₄), and liquid water in the interior of the planetesimal. The following reaction shows the stoichiometric equation of the formation of serpentinite (Mg₃Si₂O₅(OH)₄):

Mg₂SiO₄ + MgSiO₃ + 2H₂O -> Mg₃Si₂O₅(OH)₄

We examined what changes occur when the initial temperature distribution is inhomogeneous and the surface is warmer (Fig. 1) compared to the homogeneous initial temperature distribution. The serpentinization process is faster in the core independently of the initial conditions due to the higher lithospheric pressure.

Figure 1: Thermal evolution of a planetesimal of R=240km with an outward temperature gradient at the start, represented by curves with 'normal' colors.  The 'pale' colors correspond to the same object/layer, but assuming a homogeneous temperature distribution at the start of the calculations.

We studied another case when the initial temperature distribution was inhomogeneous and the formation occurred in two steps (Fig. 2). In this test, an object formed with a radius of 160km, and τ_f=10,000yr later the formation was completed and the object reached a final radius of 320km. During the time between the two formation/accretion events, the object may have warmed up both from radiogenic decay and serpentinization. We assumed the first formation even t_s=0, the maximum ²⁶Al heat production date, and three ²⁶Al half-life later. The final object is built on the warm core in the second accretion event.

Figure 2: The temperature evolution obtained in models with a two-phase formation scenario. The first formation/accretion event is followed by another event in τ_f=10,000 yr, and the first formation event occurs early, at the maximum of the radiogenic heat production of ²⁶Al (t_s=0, left side) and on the right the first formation event occurs three ²⁶Al half-life later (t_s=3).

We also examined what the critical temperature is which is needed to effectively start the serpentinization reaction with 10 different olivine-to-water ratios. The test object had 400 km radius and the process started when the radiogenic heat production of ²⁶Al was at its maximum (t_s=0). The critical temperature is strongly dependent on the pressure as expected (Fig. 3 left panel), and this dependence can also be seen in serpentinization time (Fig. 3 right panel). The reaction time at a given pressure is also strongly dependent on the composition.

Figure 3: The critical temperature (left panel) and the serpentinzation time (right panel) as a function of the pressure in case of 10 different olivine to water ratio (the colors show the different compositions).

In the case of a high olivine-to-water ratio, the reaction is fast because the water runs out quickly, thus it can achieve only a little temperature increase during the process (Fig. 4). We would expect the fastest reaction with the highest temperature rise at the stoichiometric rate but since a significant part of the process takes place below the freezing point of water, which has yet to melt, and the amount of interfacial liquid water involved in the reaction depends on the amount of rock and the temperature.

Figure 4: Temperature increase (ΔT) during the serpentinization process as a function of the olivine to water ratio under the same pressure.

Our results show that the serpentinization process can proceed under a wide range of initial conditions in the planetesimals in the early Solar System and can efficiently reform the interior of these bodies.

How to cite: Farkas-Takacs, A. and Kiss, C.: Dependence of serpentinization efficiency on the initial conditions of planetesimal formation in the early Solar System, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-977, https://doi.org/10.5194/epsc2022-977, 2022.

Accretion processes in protoplanetary disks produce a diversity of small bodies. In our solar system, such bodies played a crucial role in potentially multiple reshuffling events and in both early and late accretion of planets. Application of thermo-chronometers to meteorites provides precise dating of the formation or cooling ages of various mineralogical components. Nucleosynthetic isotopic anomalies that indicate a dichotomy between non-carbonaceous (NC) and carbonaceous (C) meteorites and precise parent body (PB) chronology can be combined with planetesimal thermal evolution models to constrain the timescale of accretion and dynamical processes in the early solar system.

Achondrite parent bodies are considered to have accreted early and mostly in the NC region. By contrast, late accretion in the C region produced mostly undifferentiated objects, such as the parent body of CR1-3 chondrites that could have formed as late as 4 Ma after solar system formation (Schrader et al., 2011). However, presence of more evolved CR-like meteorites suggests also an earlier accretion timing. Observations of C-type NEA and laboratory investigations of CC meteorites indicate a high porosity of C-type asteroids. The boulder microporosity derived for Ryugu (Grott et al., 2019) is substantially higher than for water-rich carbonaceous chondrites, such as CI and CM meteorites, and could indicate distinct thermal evolution paths. Aqueous alteration of Ryugu’s and CI and CM samples suggests accretion times not dramatically different from that of the aqueously altered CR parent body.

In a previous study, we constrained size and accretion time of Ryugu’s parent body using a numerical model for the evolution of the temperature and porosity (Neumann et al., 2021). These calculations indicate a size of only a few km in radius and an early accretion within ≲2-3 Ma after CAIs. By contrast, calculated properties for CI and CM parent bodies obtained by fitting carbonate formation ages indicate radii of ≈20-25 km and accretion times of ≈3.75 Ma after CAIs. In the present study, we fitted thermo-chronological data available for the C chondrite Flensburg and for CR-related meteorites (CR1-3, NWA011, NWA 6704, and Tafassites, see Ma et al., 2021) that range from altered chondrites to basaltic achondrites to constrain the accretion times of their respective parent bodies. We present modeling evidence for a temporally distributed accretion of parent bodies of CR-like meteorite groups that originate from a C reservoir and range from aqueously altered chondrites to highly equilibrated chondrites and partially differentiated primitive achondrites. The parent body formation times derived range from <1 Ma to ≈4 Ma after solar system formation, with ≈3.7 Ma, ≈1.5-2.75 Ma, ≲0.6 Ma, and ≲0.7 Ma for CR1-3, Flensburg, NWA 6704, and NWA 011, respectively. This implies that accretion processes in the C reservoir started as early as in the NC reservoir and produced differentiated parent bodies with carbonaceous compositions in addition to undifferentiated C chondrite parent bodies. The accretion times correlate inversely with the degree of the meteorites' alteration, metamorphism, or differentiation. The accretion times for the CI/CM, Ryugu, and Tafassites parent bodies of ≈3.75 Ma, ≈1-3 Ma, and ≈1.1 Ma, respectively (Ma et al., 2022, Neumann et al., 2021), fit well into this correlation in agreement with the thermal and alteration conditions suggested by the meteorites (Fig. 1).

Figure 1: Parent body accretion times (colored patches) and meteorite metamorphic ages (data points) for the C reservoir modeled incl. Tafassites, Ryugu, and CI/CM PB (Ma et al., 2021, Neumann et al., 2021). Younger Mn-Cr carbonate ages (circles) of CR1-3 (blue) and CI/CM (yellow) than for Flensburg (grey) result in moderately younger accretion age for the CR1-3 and CI/CM PBs. NWA 6704 (cyan) and NWA 011 (green) have older Pb-Pb whole-rock ages (squares) than Tafassites (light blue), resulting in earlier accretion than the Tafassite PB. The NWA 011 and NWA 6704 Al-Mg (triangles) and Mn-Cr (circles) data support an early PB formation. While Tafassites also include a very late, i.e., young, phosphate Pb-Pb age, their older iron-silicate Hf-W (light blue triangle) and Mn-Cr ages cause a shift to an intermediate PB formation time between NWA 011 and NWA 6704 on one hand, and Flensburg, CR1-3, and CI/CM PBs on the other. The youngest Pb-Pb age has its major effect on the large Tafassites PB size. Younger CR1-3 chondrule formation age (blue diamond) supports the PB accretion at ≈3.7 Ma. All accretion times correlate inversely with the meteorite petrologic type and the degree of metamorphism or melting.

Grott M. et al. (2019) Nature Astronomy, 3, 971-976.

Ma N. et al. (2021) Goldschmidt 2021, https://doi.org/10.7185/gold2021.3852.

Neumann W. et al. (2021) Icarus, 358, 114166.

Schrader D. L. et al. (2011) GCA, 75, 308-325.

How to cite: Neumann, W., Trieloff, M., Ma, N., and Bouvier, A.: Temporally distributed accretion of chondritic and differentiated meteorite parent bodies in the C reservoir of the early solar system, Europlanet Science Congress 2022, Granada, Spain, 18–23 Sep 2022, EPSC2022-176, https://doi.org/10.5194/epsc2022-176, 2022.