Constraining the accretion time of Ryugu&#8217;s parent body from the chronological record of samples returned by Hayabusa2
Authors: Neumann Wladimir , Bouvier Audrey , Trieloff Mario 
Published: July 2024

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 &#8776;3-3.9 Ma, &#8776;3-3.8 Ma, &#8776;3.7 Ma, and &#8776;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&#8217;s parent body: an accretion time of &#8776;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 &#8776;2.8 Ma, which is still consistent with the result by [4]. However, in this case the parent body radius is >10 km.&#160;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.&#160;DiscussionThese results suggest accretion times for Ryugu&#8217;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 &#8776;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., &#8776;5 km for Ryugu samples (assuming Mn-Cr age from [8]) and &#8776;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&#8217;s material originates.&#160;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.