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

Abstract

ContextObservations 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 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. DiscussionThese 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 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.