THERMAL HISTORY OF IMPACT EJECTA DURING ATMOSPHERIC DESCENT: TEMPORAL AND SPATIAL PATTERNS

Widespread ejecta deposits are evidence for the catastrophic consequences of large meteorite impacts. The characteristics of impact spherule layers (e.g., thickness, size distribution) and those of their component spherules (e.g., size, shape, texture, geochemistry) have been observed to vary vertically and horizontally within a layer. Some of these differences can be attributed to heterogeneous conditions of formation from melted and vaporized target and projectile material. However, hypervelocity ejecta that fall back to the surface must first pass through the Earth’s atmosphere and the resulting heat can modify the final spherule population.

Previous models of ejecta-atmosphere interactions were limited to distal scenarios and employed a simplified scheme of ejecta delivery to the upper atmosphere that combined observed spherule densities with simple ballistic models. Recent impact hydrocode studies constrain the input parameters for ejecta reentry and allow us to model, using a two-dimensional two-phase fluid flow code, ejecta-atmosphere interactions over a range of impact scenarios. Heat generated by the deceleration of hypervelocity spherules is either exchanged between ejecta and air phases or lost as thermal radiation. In distal scenarios, where entry mass flux is low (<10^-2 kg m^-2 s^-1), the spherules reach temperatures near those predicted by micrometeorite reentry theory, in which peak temperature is a function of spherule size and velocity. Smaller particles (<500 µm; e.g., Chicxulub microkrystites) do not re-melt, but larger particles (e.g., Archean spherules) may reach their melting points. With increasing proximity to the impact location, spherule mass flux and size increase. At higher mass fluxes, the decelerated spherules form a dense spherule-air cloud above highly compressed atmosphere where heat cannot radiate efficiently. Despite lower ballistic velocities, peak spherule temperature rises with increasing mass flux, exceeding expected values and resulting in melting or even evaporation of silicate spherules. The onset of this mass flux dependence leads to very different thermal histories for spherules arriving to distal and proximal sites, as well as for spherules arriving at different times to a single site.