New from Meteoritics and Planetary Science:
Jurewicz A.G.J. Amarsi A.M. Burnett D.S. Differences in elemental abundances between CI chondrites and the solar photosphere .14272
MacArthur, J. Joy, K. Jones, R.H. et al. Curation and classification procedures for the UK Antarctic meteorite collection .14273
I bring up meteoritics in this blog… but not too much. It’s true meteorites (certainly the macroscopic ones) are samples of asteroids, and we really think the micrometeorites/interplanetary dust particles are a mixture of various comet and asteroid bits. But I try to cut myself off, because it’s easy to follow a rabbit down its rabbit hole. Given these samples, and the extreme decimal places we can get with modern lab instruments, it’s easy to chase decimal places down to assumption and speculation and oblivion.
But one thing is not a matter of assumption and speculation: the group of meteorites we call “CI chondrites” is, well, amazing. It’s clear the CI chondrite meteorites formed in the outer Solar System (beyond, oh… the middle/outer Main Belt, or Jupiter). These meteorites were never baked dry by the heat of the inner Solar System. But this describes lots of chondrite meteorite groups, like CM and CR meteorites. What we then learned, after we took better spectra of the Sun, is that the CI group represents the Solar System composition to an amazing degree. That is, as best as we can assay the composition of the Sun (by taking spectra of the Sun’s “surface”), CI meteorites embody that composition. The exceptions are hydrogen, helium, and a few other gas elements (“atmophiles”), but even then the CI group levels are closer to the Sun here than the other meteorite groups are. In short, CIs are like having “baby pictures” of the Solar System. The cloud that collapsed to form what we now call our Solar System (the “presolar nebula”) left this CI material as small, preserved witness samples. They also contain primitive organic compounds, which may have jump-started life where they land.
There are only five official CI meteorites (Orgueil, Alais, Ivuna, Tonk, Revelstoke), and the very similar examples we call “CY” (found in Antarctica), but now we have Ryugu. The Hayabusa2 mission returned >5 grams of pebbles/dust from asteroid (162173) Ryugu, and analysis shows it closely resembles CI material, not other meteorite groups. The difference is that Haya2 sealed the samples into a collection cylinder- hermetically sealed them- while the CI meteorites were heated by atmospheric entry, then sat on the ground and in Earth’s air for however long it took to recover them. The Ryugu material, small as it seems, thus acts like a ‘sixth CI chondrite’ of exceptional, unprecedented cleanliness and preservation state. Haya2 actually collected two samples, one from an artificial crater it blasted into the Ryugu surface, so some of that crater material was not sitting, exposed to space, for very long.
All in all, Ryugu, CI, and to an extent CY and other meteorites preserve the state of the presolar nebula like nothing else… or do they? They lost much of their H, He, Ne, and to an extent atmophile elements like N; that’s understandable. Now Jurewicz et al. report minor losses/retentions of the rocky minerals- lesser differences between CIs and what we assume to be the presolar blend. Nothing shocking, I’d say, compared to loss of N, but it’s important to temper ourselves and not just label all element levels as default presolar.
Which takes us to MacArthur et al. All the CI meteorites were found in ‘temperate’ countries (not Antarctica). The original meteorites that seem to form a “CY” group were recovered by Antarctic science expeditions. (Lately, we have found examples in warmer places.) If these are close to CI, and thus important, then they’re also too important to damage, contaminate, misidentify, waste, etc. All these samples experienced a fiery entry and air exposure, sure, so they’re not as pristine as the Ryugu material. But the Antarctic deep freeze means significantly less weathering than, say, landing and exposure in France or Tanzania. Let’s take care of these important space materials.