Catching up with Geochimica et Cosmochimica Acta, vol. 382, for Oct:
Sugawara, S. Fujiya, W. Kawasaki, N. et al. Update on the 53Mn-53Cr ages of dolomite in the Ivuna CI chondrite and asteroid Ryugu sample p. 40 .08.013
Anand, A. Spitzer, F. Hopp, T. et al. Isotopic evidence for a common parent body of IIG and IIAB iron meteorites p. 118 .07.025
It has been established, by multiple authors of multiple papers, that the Ryugu sample from Hayabusa2 is like the CI meteorites. They represent material from the same origins, with the same makeup: heavily altered by water, yet with lots of volatile materials, and our closest match to the composition of the Sun and protosolar nebula. In other words, of all the materials we have in hand, CI-chondrite meteorites (such as the Ivuna meteorite) and now Ryugu grains best represent the overall mixture of the Solar System. And of the two, Ryugu grains didn’t get baked by passage through the atmosphere at hypervelocity. Nor did they sit on the ground before collection, exposed to dirt, air, dust and spores, etc. No, the Ryugu sample was put in a sterile metal canister, with hermetic seals, only opened back in a Japanese high-security cleanroom. So, what else is different between what we had, and what we now got? What, possibly, happened to CI meteorites while they entered, sat, and acclimated- did they get weathered on Earth, even a little, versus what happened in space, over 4+ billion years of Solar System history? Let’s be sure, let’s check.
Speaking of meteorites, opposite the scale of the wet, fragile CI chondrites are the iron achondrites. These are heavily heat altered, being the chunks from a core somewhere. Yes, early in the Solar System, one or more ‘asteroids’ (whatever you call, depending) grew large enough to retain heat, melt itself, and segregate- a process we call “differentiation.” Differentiated bodies, while molten, had their dense metals fall into a core, and their lighter rocks float up to a crust. The mantle, in between, is bulk material that makes a third category. At some point, one or more of those big bodies suffered a collision; one so catastrophic, the body was destroyed down to the core, with pieces scattering to space. Yes, iron meteorites are samples of ‘planet’ cores, our only samples. The diversity of iron compositions in these meteorites, without Earth or Mars core material (and not Venus- ha!), puzzles us to this day. We know we have entire categories of ‘irons’, and it appears, subcategories. What heads or tails are we supposed to make of the puzzle pieces? Some of the early classifications have already been revised or discarded. Now, per Anand et al., there appears to be a ‘fit’ to two of the pieces. Using strategic isotopes as tracers of Solar System history, subcategories within the II meteorite group have some sort of relationship to each other. In the case of carbonaceous chondrites, we see some categories are altered versions of other carbonaceous chondrites; other groups may be deeper or shallower samples of the same parent body, with the depth causing different effects. What of group-II irons?
In both cases, strategic elements and their isotopes had been identified, and now sought in these samples. Chromium forms stable chromium oxides; the irons also have usable nickel for this purpose. All meteorites have oxygen, which has three possible isotopes, and numerous stable mineral forms to be recovered. Our previous studies have led us to now: questioning the details and individual pieces of our Solar System, and our history.