Cosmogenic Nuclides

Abstract

Cosmogenic nuclides are produced by the interaction of energetic elementary particles of galactic cosmic radiation (GCR) and their secondaries with atomic nuclei in extraterrestrial or terrestrial material. In extraterrestrial samples cosmogenic nuclides produced by energetic particles emitted by the Sun (SCR) are also detectable. Cosmogenic nuclides usually are observable only for noble gas isotopes, whose natural abundances in the targets of interest are exceedingly low, with some radioactive isotopes having half-lives mostly in the million-year range, and a few stable nuclides of elements such as Gd and Sm whose abundance is appreciably modified by reactions with low-energy secondary cosmic-ray neutrons. In solid matter, the mean attenuation length of GCR protons is on the order of 50 cm. Therefore, cosmogenic nuclides are a major tool to study the history of small objects in space and of matter near the surfaces of larger parent bodies. A classical application is to measure “exposure ages” of meteorites, that is, the time they spent as a small body in interplanetary space. In some cases, the previous history of the future meteorite in its parent-body regolith can also be constrained. Such information helps to understand delivery mechanisms of meteorites from their parent asteroids (mainly from the main belt) or parent planets, and to constrain the number of ejection events responsible for the meteorites in collections worldwide. Cosmogenic nuclides in lunar samples from known depths of up to ~2 m serve to study the deposition and mixing history of the lunar regolith over hundreds of million years, as well as to calibrate nuclide production models. Present and future sample return missions rely on cosmogenic nuclide measurements as important tools to constrain the sample’s exposure history or loss rates of its parent-body surfaces to space. First measurements of cosmogenic noble gas isotopes on the surface of Mars demonstrate that the exposure and erosional history of planetary bodies can be obtained by in situ analyses. Exposure ages of presolar grains in meteorites provide at present the only quantitative constraint of their presolar history. In some cases, irradiation effects of energetic particles from the early Sun can be detected in early solar system condensates, confirming that the early Sun was likely much more active than later in its history, as expected from observations of young stars. The increasing precision of modern isotope analyses also reveals tiny isotopic anomalies induced by cosmic-ray effects in several elements of interest in cosmochemistry, which need to be recognized and corrected for. Cosmogenic nuclide studies rely on the knowledge of their production rates, which depend on the elemental composition of a sample and its “shielding” during irradiation, that is, its position within an irradiated object, and for meteorites their pre-atmospheric size. The physics of cosmogenic nuclide production is basically well understood and has led to sophisticated production models. They are most successful if a sample’s shielding can be constrained by the analyses of several cosmogenic nuclides with different depth dependencies of their production rates. Cosmogenic nuclides are also an important tool in Earth sciences, although this is not a topic of this article. The foremost example is 14C produced in the atmosphere and incorporated into organic material, which is used for dating. Cosmogenic radionucuclides and noble gases produced in situ in near-surface samples, mostly by secondary cosmic-ray neutrons, are an important tool in quantitative geomorphology and related fields.