Abstract
Atmophile elements (carbon, hydrogen, and nitrogen) were thought to accrete late during planetary formation, yet cosmochemical and astrophysical evidence suggests that they were already present while small bodies, such as Vesta, were forming. High-temperature magmatic events on small bodies, however, should have outgassed these ultravolatile elements because gravity would have been insufficient to retain them. In this study, experiments reveal that gas bubbles strongly attach to metal droplets in magma ocean–like mixtures, forming compounds that resist segregation. Early flotation of such compounds would have enhanced mantle siderophile endowment, eliminating the need for late accretion. On small bodies, low gravity would have enabled millimeter- to centimeter-sized compounds to sink, trapping atmophiles in their interiors. This mechanism links core formation to the early entrapment of atmophile elements and provides a framework for reconciling volatile delivery across planetary bodies and planetary accretion models.