A Probable Scenario for the Formation of Primitive Asteroids in the Main Asteroid Belt

The overall heliocentric distribution of primitive asteroids with a low-temperature mineralogy of Tholen taxonomic classes (C, F, B, G, D, and P) has a maximum near the outer boundary of the main asteroid belt (MAB) and reaches the orbit of Jupiter at a heliocentric distance of 5 AU. The features of primitive asteroids suggest that they were likely influenced by the line of water ice at a heliocentric distance of 4 AU, Jupiter’s core formation, and smaller stone-ice bodies in Jupiter formation zone for the first 1-2 million years since CAIs origin. According to recent isotopic studies of the most ancient iron meteorites, the region beyond this ice line in the early solar system was a water-rich carbonaceous chondrite (CC) reservoir. Gravitational perturbations caused by rapidly growing Jupiter, starting from its mass equal to several Earth masses, led to ejection of smaller bodies from its accretion zone in all directions including zone near heliocentric distances of 2-3 AU in an isotopic reservoir of dry, non-carbonaceous chondrites (NC), corresponding to the present MAB. Larger in size and high-velocity stone-ice bodies from Jupiter zone were probably fragmented in collisions with mostly silicate planetesimals near heliocentric distances of 2-3 AU, but they probably transported considerable amounts of water ice, organic matter, and hydrated silicates. At the same time, within 1–2 million years after CAIs formation, these bodies underwent thermal metamorphism and water ocean formation due to heating caused by the decay of short-lived radioactive isotopes, mainly ^26 Al, and subsequent metal core segregation within 1. 5–4 million years after CAIs.