Shortly after the ignition of the Sun, the solar system contained a colossal cloud of dust and gas known as the protoplanetary disk. Within just a few million years, trillions of submillimeter-scale solids formed from this disk and coalesced to create a spectrum of planetary bodies that included asteroids, comets, and ultimately the planets that we recognize today. The mechanisms by which these early solids formed and evolved in the disk as well as the processes by which they interacted with each other and the remaining gas and dust governed the chemical and physical properties adopted by these bodies. As such, these early processes played pivotal roles in establishing the long-term thermochemical evolutions of the different planetary bodies found throughout our solar system. Aggregates of these early solids, as well as fragments of the first planetary bodies that formed in the solar system, exist on Earth today in the form of meteorites. Through novel high-resolution and high-sensitivity techniques, these samples have recently provided several new insights into the evolution of the protoplanetary disk and the formation of the first solids. For instance, a number of measurements have revealed that the stable isotopic compositions of all meteorites fall into two distinct families (Warren 2011; Scott et al. 2018; Kruijer et al. 2019; Kleine et al. 2020; Bermingham et al. 2020). This dichotomy has been used to argue that these extraterrestrial rocks each originate from one of two reservoirs of material that existed in the protoplanetary disk that were spatially separated by a large feature (possibly Jupiter; Kruijer et al. 2017; Brasser & Mojzsis 2020; Lichtenberg et al. 2021) that hindered the exchange of material between these regions. This restricted motion prevented disk-wide mixing and compositional homogenization, allowing the distinct composition of each reservoir to form and be retained over much of the disk’s lifetime. One key observation supporting this dichotomy is the ε94Mo and ε95Mo values (where ε denotes normalized isotopic concentration in parts per 10,000) of iron meteorites (fragments of the metallic cores of melted asteroids), rocky achondrites (fragments of the mantles and crusts of melted asteroids), and chondrites (fragments of unmelted asteroids), which together form two parallel lines when plotted against each other (Budde et al. 2016; Kruijer et al. 2017; Budde et al. 2019). Additionally, the isotopic compositions of a suite of other elements (including ε48Ca, ε50Ti, ε54Cr, ε62Ni, ε100Ru, and △17O values, where △ denotes the normalized non-mass-dependent isotopic concentration in parts per 1000) all form pairs of clusters when plotted against each other, further supporting the existence of separated reservoirs composed of material with isotopically distinct compositions in the protoplanetary disk (Warren 2011; Fischer-Godde & Kleine 2017; Bermingham et al. 2018; Schiller et al. 2018; Nanne et al. 2019; Worsham et al. 2019). In all of these cases, the same meteorite groups systematically share similar isotopic signatures and fall into the same families. As such, all meteorites can be primarily categorized as either noncarbonaceous (NC) or carbonaceous (CC) based on their isotopic compositions (named after the type of chondrite found within each family; Budde et al. 2016).
Citation: Kleine et al. 2020
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