Constraining the accretion time of Ryugu’s parent body from the chronological record of samples returned by Hayabusa2

Context

Observations by Hayabusa2 revealed that Ryugu is dominated by aqueously altered material spectrally similar to CI and CM chondrites [1]. In the context of accretion times suggested for water-bearing CI, CM, CR, and Flensburg chondrites of ≈3-3.9 Ma, ≈3-3.8 Ma, ≈3.7 Ma, and ≈2.7 Ma [2-5] after the formation of Ca-Al-rich inclusions (CAIs), respectively, this could imply a relatively late accretion in the carbonaceous reservoir of the protoplanetary disk. Porosity evolution modeling [4] suggested two options for Ryugu’s parent body: an accretion time of ≈2-3 Ma, a radius of <10 km, and a throughout highly porous interior or a radius of up to hundred km, but a high porosity only in a surface layer, from which Ryugu’s material was produced. Later studies support a small parent body [6,7].

 

Data

Conclusive constraints on the parent body age were expected from Hayabusa2 sample analysis. Recent lab work confirms aqueous alteration and indicates a strong similarity of Ryugu’s material with CI chondrites [8]. In particular, thermo-chronological investigations that provide precise dating of the formation or cooling ages of various mineralogical components after the parent body accretion have been carried out [7-9]. However, there are still strongly conflicting results on Mn-Cr dating of carbonates, specifically dolomite. While [7] infer dolomite formation within 1.8 Ma after CAIs and suggest a <10 km radius planetesimal, which, therefore, accreted even earlier, [8] infer 5.2 Ma after CAIs, allowing a much later accretion than within 1.8 Ma and a much larger parent body radius than 10 km. In particular, ages inferred spread from 0.4 Ma or 0.8 Ma after CAIs at a closure temperature of 283 K [7], over 2.6 Ma at 288 K [9], to 5.2 Ma at 310 K [8]. The age derived by [8] is equal within uncertainties with the Mn-Cr dolomite age of CI chondrites of 5.5 Ma [3].

 

Results

The meteorite record provides only weak accretion time estimates from chondrule or mineral phases’ formation ages, and no information about the parent body sizes. However, thermal evolution and differentiation modeling provides a valuable tool that can be combined with the sample chronology derived from lab work in order to constrain the parent body accretion time, size, internal structure, and the depths samples originate from [e.g., 10].

We fit thermal evolution models to the thermo-chronological data of CI chondrites and Ryugu in separate parent bodies and also of Ryugu and CI data assuming the same parent body. Our fits indicate radically different accretion times for different Mn-Cr Ryugu sample ages. The Ryugu parent body accretion times derived range from 0 Ma for the data from [7] to 3 Ma for the data from [8], while the CI PB accretes between 2 and 2.8 Ma after CAIs and has a radius of >10 km (Figure 1). Accretion times obtained for Ryugu for the data from [8] and [9] agree with the interval of 2-3 Ma derived by [4]. Parent body radius estimates of >2 km or >4 km can be provided only for fits to the data from [9] and [8], respectively. However, bodies that fit the data from [7] or [9] best, experience too strong heating in the interior and are largely dehydrated.

Fitting Ryugu and CI data in one and the same parent body produces a good fit quality only for the late Mn-Cr age from [8]. In this case, the accretion time is ≈2.8 Ma, which is still consistent with the result by [4]. However, in this case the parent body radius is >10 km.

 

Figure 1: Quality of the fit of thermal evolution models to the CI dolomite Mn-Cr data. Exceptionally good fits are obtained for an accretion time interval of 2-2.8 Ma after CAIs and a radius of >10 km.

 

Discussion

These results suggest accretion times for Ryugu’s parent body and for the CI parent body that are closer to that of the parent body of the Flensburg chondrite and are earlier by ≈0.7 Ma than the accretion of the CR parent body [5] as well as previous estimates for the CI and CM parent bodies [2-4].

Only weak estimates of the parent body size of >2 km or >4 km for the Ryugu data from [9] and [8] are marginally consistent with a parent body radius of <10 km suggested by [4] if a throughout porous interior is required. An estimate of >10 km for the Ryugu [8] and CI [3] data fitted in one object implies an alternative structure suggested by [4] and [11] with a consolidated interior and a porous surface layer. Shallow layering depths, e.g., ≈5 km for Ryugu samples (assuming Mn-Cr age from [8]) and ≈12 km for CI samples fitted in a planetesimal radius of 100 km that accretes at 2.7 Ma are consistent with this structure. A deeper layering for CI samples is consistent with lower porosities of CI chondrites than for Ryugu samples. In such a case, high-porosity material represented only a small outer fraction of the parent body volume from which Ryugu’s material originates.

 

References

[1] Sugita S. et al. (2019) Science 364, eaaw0422.

[2] Fujiya W. et al. (2012) Nature Communications 3, 627.

[3] Fujiya W. et al. (2013) EPSL 362, 130-142.

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

[5] Neumann W. et al. (2024) Scientific Reports, under revision, arXiv:2302.13303.

[6] Tang, H. et al. (2023) The Planetary Science Journal 4, 144.

[7] McCain K. A. et al. (2023) Nature Astronomy 7, 309-317.

[8] Yokoyama T. et al. (2023) Science 379, eabn7850.

[9] Nakamura T. et al. (2023) Science 379, eabn8671.

[10] Neumann W. et al. (2023) The Planetary Science Journal 4, 196.

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