The Origins of Cosmogenic Nuclides

Where do the cosmogenic nuclides found in certain meteorites come from? As we have explained, the first source of nuclides are cosmic rays. Their properties show that they can in theory come from different cosmic sources:

• The nucleosynthesis which takes place in the heart of the Sun and other stars
• AGB stars located on the asymptotic branch of the giants, a class of stars at the end of their life
• Supernovae resulting from the explosion of massive stars
• GRB eruptions (Gamma-Ray Bursts)
• The activity of quasars and black holes, including supermassive black holes

Supernovaa and AGB stars generate extremely energetic and powerful stellar winds (streams of particles), rich in heavy elements, including short-lived radionuclides.

Numerical simulations show that under certain conditions, when these winds strike an interstellar molecular cloud that cannot spontaneously collapse, shock waves and stellar wind can compress it to the point that it becomes gravitationally unstable and collapses. The simulations also show that some of the elements carried by the stellar wind, including short-lived radionuclides, mix with the dust of the collapsing cloud.

In the case of meteorites, there are specimens contaminated with isotopes that could only have formed in a supernova. For its effect to have an impact in the solar system, it had to be close, that is to say, less than a few hundred light years from the Sun. We will come back to the effects of a supernova on life on Earth, including in the article on cosmic rays.

In such scenarios, the radionuclides act as fingerprints of the stellar wind. Their presence in meteorites testifies to the contamination of the protosolar nebula, or more generally of the solar system for more recent times, by these radionuclides. We cannot help wondering if a supernova having exploded near the solar system would not be at the origin of the formation of certain isotopes (cf. DN Schramm and RN Clayton, 1978), and by extension if it does not would not have influenced the formation of the first prebiotic molecules.

Another explanation for the presence of short-lived radionuclides in meteorites is their synthesis in the protosolar nebula by the intense radiation of the young Sun in formation. But this mechanism is a little less effective than the idea of the stellar wind in explaining the absolute and relative abundances of short-lived radionuclides.

Currently, no model incorporating either of these explanations is completely satisfactory.

The scenario of the formation of the solar system is certainly incomplete and imprecise in its details (for example the origin of the chondrules, the formation of the pallasites, etc). However, without meteorite specimens and samples of primitive materials taken from asteroids, comets and other planets in the solar system, specialists would have no observational basis to formulate their theories and develop their models. Rightly, Anglo-Saxon researchers have nicknamed meteorites “poor man’s space probes” (cf. J. Olsen, 1973; G. Morrison, 1986), an expression which has become classic and has been adopted by several researchers and institutions ( cf. Christie’s, The European Commission) when they evoke the meteorite study programs.