The latest “issue” (all-digital) of Geochemical Perspectives Letters includes (17 Sep 2025- vol. 37):
Wölfer, E. Hellmann, J. Burkhardt, C. et al. Ge, Te, and Zn isotopic link between Ryugu and CI chondrites P. 1 geochemlet.2537
A brief examination of the Ryugu sample returned by Hayabusa2 showed that Ryugu resembles a meteorite group, the CI chondrites. Now, we put a finer point on it. The CI chondrite meteorites are treasured, not just for their rarity, not just for their high content of water and other volatiles, but for the fact that their chemical makeup is like that of the Solar System- the whole Solar System. We can look at the Sun, and deduce what elements are in it, in what proportions. Hydrogen excepted, and helium too to an extent, we had separately found that CI meteorites match up with the Sun’s element levels. Since the Sun is not just the majority of the Solar System’s mass, but >99% of the System’s mass, all other parts of the Solar System are tiny or even irrelevant line items in the Solar System inventory.
That CI meteorites have, secondhand, the element proportions of the broader Solar System- eerily matching the proportions of the broader Solar System- is a scientific gift. The CIs are now used to gauge element levels of everything else, except interstellar material. (And even then, interstellar stuff is declared interstellar when it looks weird relative to the CIs- the CIs define “normal” levels.) A new gift, then, is the Haya2 asteroid material. Our pebbles and grit from Ryugu can be considered CI “meteorites” that were never meteorites, didn’t get heated by entry through our atmosphere, didn’t get contaminated by contact with the ground or even the air. (The Haya2 program made sure to seal the sample capsule, and only open it in a sealed laboratory.) Hence, the Ryugu Reference Project– a plan to curate the sample as a priceless chemical standard, and distribute it for important reference work.
Wölfer et al. now give us a finer point. After the cryogens hydrogen and helium, the heavier gases neon, nitrogen, and oxygen, and some really light mineral-forming elements, there come the “moderately volatile elements” like sodium, sulfur, potassium, cadmium, etc. Sure, they’re solids on Earth, but the cosmochemical term “moderate” refers to the early Solar System, when everything was still forming and it was hotter. The list also includes germanium, tellurium, and zinc, which had not been well-assayed when Haya2 first returned to Earth. Wölfer et al. find that Ge, Te, and Zn levels at Ryugu (or at least, our 5.4g of Ryugu) match the CI chondrites not only in proportion, but in their isotope proportions- no higher-level separation processes had altered the isotope assays of the three.
Not that there was much doubt, but… truly, the 5.4g of Ryugu sample, and to a lesser extent the “contaminated” CI chondrites, are a gift from 4.5 billion years ago. They are a priceless legacy from the birth of the Sun and the Solar System.