On the origin and evolution of the asteroid Ryugu: A comprehensive geochemical perspective

Abstract

Presented here are the observations and interpretations from a comprehensive analysis of 16 representative particles returned from the C-type asteroid Ryugu by the Hayabusa2 mission. On average Ryugu particles consist of 50% phyllosilicate matrix, 41% porosity and 9% minor phases, including organic matter. The abundances of 70 elements from the particles are in close agreement with those of CI chondrites. Bulk Ryugu particles show higher ' 18 O, " 17 O, and C 54 Cr values than CI chondrites. As such, Ryugu sampled the most primitive and least-thermally processed protosolar nebula reservoirs. Such a finding is consistent with multi-scale H-C-N isotopic compositions that are compatible with an origin for Ryugu organic matter within both the protosolar nebula and the interstellar medium. The analytical data obtained here, suggests that complex soluble organic matter formed during aqueous alteration on the Ryugu progenitor planetesimal (several 10's of km), <2.6 Myr after CAI formation. Subsequently, the Ryugu progenitor planetesimal was fragmented and evolved into the current asteroid Ryugu through sublimation.

Rare earth element identification and quantification in millimetre-sized Ryugu rock fragments from the Hayabusa2 space mission

“…3 B) which, together with its larger grain size relative to points 1 and 2, supports its identification as a dolomite grain. This is in agreement with first analyses of other Ryugu samples that have reported a widespread abundance of coarse-grained (~ 100 µm) dolomite that also contains a rhodochrosite (MnCO 3 ) component of ~ 9 mol% (Nakamura et al 2022a ), and is further in line with reports of carbonate grains in other CI and CM carbonaceous chondrites, where the concentration of Mn is known to be significantly higher in dolomite relative to calcite or aragonite (Lee et al 2014 ; Riciputi et al 1994 ).…”

“…The other two apatite spectra are less REE-enriched (30–100 times mean CI values) and display a slight increase from LREE to HREE. Thus, relative to the mean CI concentrations of REE, clear and significant (30–300 fold) enrichments in REE concentration are observed for the three apatite grains, which is in agreement with REE results for other Ryugu samples that found apatite to be the main REE bearing phase (Nakamura et al 2022a ) and are, furthermore, in line with the research by Morlok et al . ( Morlok et al 2006 ) .…”

“…The concentration levels of REEs heavier than Dy (e.g., Ho, Er, Yb) were below the detection limits in the dolomite grain. This is in agreement with LREE enrichments observed in dolomite grains of other Ryugu samples (e.g., C0053, (Nakamura et al 2022a )). All four matrix points of the Ryugu fragments (A0055 Pt4 and C0076 Pt2, Pt3 and Pt4) reveal a slight increase from LREE to HREE, but the two Ryugu fragments appear to differ with regard to their matrix Eu concentrations.…”

“…For this reason, identification of these grains by SR-XRF in such a mm-sized particle without prior invasive sample preparation, such as targeted cutting or polishing, must be achieved indirectly. First analyses of Ryugu material have reported that the main Ca-bearing phases are dolomite and apatite, and that anhydrous Ca-bearing silicate phases such as pyroxene or anorthite are rare (Nakamura et al 2022a ). Given the small grain size of points 1 and 2 and the typical enrichment of REEs within Ca-phosphate phases (Morlok et al 2006 ), it is suggested that the joint Ca and REE signals from points 1 and 2 both originate from apatite phases.…”

The Solar System calcium isotopic composition inferred from Ryugu samples

“…Chemical, mineralogical, petrological, and isotopic analyses of these samples suggest that they are closely related to CI chondrites. In particular, their bulk Cr and Ti isotopic signatures and the chemical abundances of most elements are within the range of CI chondrites (Nakamura E. et al, 2022;Yokoyama et al, 2022). Among meteorites, CI chondrites have chemical compositions that most closely resemble the Sun; therefore, they are the most representative samples of the solar nebula composition with the exception of volatile elements (Palme et al, 2014).…”