Just out from Space Science Reviews:
Lodders, K. Bergemann, M. Palme, H. Solar System Elemental Abundances from the Solar Photosphere and CI-Chondrites art. 23 s11214-025-01146-w
What’s the Solar System made of? Sounds like a simple question to ask, but a hard question to answer. How could we possibly, say, inventory all bodies of the Solar System for their compositions, including the outer Solar System, including their interiors? Surely, this is an impossible task! Impossible, it sounds… to the untrained. But we don’t need to sample all parts of all bodies, when the Sun is over 99.8 percent of the mass of the entire Solar System. If we know the Sun’s composition, then all other bodies are not just rounding errors by comparison, but possibly even in the noise.
And the solar values are noisy, indeed. We can’t actually go to the Sun proper, much less take a sample and come back. Instead, spectroscopists study the light from the Sun, and deduce what elements must have formed that spectrum we see in our spectroscopes. More recently, we have placed spacecraft outside the Earth’s protective bubble (the magnetopause) to sense the solar wind- hot material shed by the Sun. Using a mass spectrometer or other instrument, we can- start, at least- measuring the elements that are in that solar material.
There are noise issues with these measurements. Recording a light spectrum is indirect, and requires assumptions about elements. Recording a mass spectrum of the solar wind makes the assumption that the solar wind is representative of the body of the Sun. We now know that the details of plasma physics makes some elements and isotopes biased up in the solar wind, some biased down. Therefore, we want every possible means to gauge the Sun, to serve as complementary checks on all the other means.
The third- and least noisy- values for the elements in the Sun comes from CI meteorites. The carbonaceous chondrite meteorites are richer in volatiles and organics than all others; the CI group of carbonaceous chondrites, in turn, are the richest within all the carbonaceous chondrites. When the elemental assay of CI chondrites was compared to the (assumed) solar element spectrum, the result (except for hydrogen, helium, etc.) was startlingly close. In other words, the CI chondrites (and to a lesser extent other carbonaceous chondrite meteorites) are made of star stuff, to a startling degree.
And now, here’s the relevance for this blog: there are just five recognized CI chondrite meteorites; they’re fragile and rare. Only five such meteorites made it through Earth’s atmosphere, and been recognized and recovered. Until now. The Hayabusa2 mission to asteroid (162173) Ryugu has succeeded, bringing over 5 grams of the asteroid’s regolith (surface rubble) back to our labs. This includes fresher subsurface grains. Our labs then found Ryugu is, pretty much, a giant ball of CI-chondrite material. And yet, that material got to Earth in a sealed capsule. Knowing that carbonaceous chondrite asteroids/meteorites contain volatiles, the Japanese space agency JAXA built the sample capsule with dual redundant hermetic seals. It was built with extreme cleanliness, and contamination control procedures. And the recovery procedures and capsule opening were done to prevent earthly recontamination of the precious sample. In other words, the Ryugu data is not just CI-like; in some ways, it’s better than CI.
We now have six known CI samples. Well known. Combined with solar spectra, and the solar wind data, we can put serious decimal places on the abundances of solar elements. In turn, those solar numbers can put all other bodies of the Solar System- all other bodies- in their context as subsamples.