Ejectas

Ejectas refers to material that has been expelled from a planetary surface during explosive events, most commonly impact cratering or volcanic eruptions. The term describes the fragments, melts, and other debris that are ejected and come to rest at varying distances from the source.

During a hypervelocity impact, energy excavates a crater and ejecta is launched along ballistic trajectories. The ejecta blanket surrounding the crater consists of fractured rock fragments and, in some cases, impact melt that re-solidifies as glass. High-velocity ejecta can form bright rays that extend from the crater and may generate secondary craters beyond the rim. The distribution of ejectas depends on impact angle, velocity, gravity, and the mechanical properties of the target.

Composition and forms: Ejectas include pieces of the target rock, mineral grains, and sometimes melt products such as glass droplets. If the source is volcanic, ejectas can include ash, pumice, and lava fragments. The particles range from large blocks to fine dust, and finer material can travel farther from the source due to ballistic sorting and atmospheric transport on airless or thin-atmosphere bodies.

Scientific significance: Ejecta deposits help scientists interpret surface ages and impact histories, constrain crater dimensions, and model the populations of impactors. Mapping ejecta blankets and rays aids in establishing terrain relationships and stratigraphy on bodies such as the Moon and Mars. Remote sensing and sample analysis are key methods for characterizing ejectas.

Terminology: The standard term is ejecta; ejectas is a less common plural form encountered in some literature, but ejecta is typically used as a mass or collective term.