The fate of pyroclasts produced in explosive eruptions on the asteroid 4 Vesta

We evaluate the consequences of explosive activity having taken place during volcanic eruptions on the differentiated asteroid 4 Vesta, which is the likely parent body of the howardite-eucrite-diogenite (HED) meteorites. For a wide range of magma volatile contents, we calculate the eruption speeds and subsequent trajectories of the pyroclastic magma droplets produced. By considering the size distribution and eruption speeds of the droplets, and the mass fluxes at which they are ejected, we show that, under all realistic circumstances, the droplets will have formed lava fountains that were extremely optically dense. As a result, virtually all of the droplets will have reached the surface having suffered a negligible amount of radiative cooling and will have coalesced into lava ponds feeding lava flows. Typically, <1% of the pyroclasts will have undergone enough cooling to allow them to accumulate into a recognizable fall deposit consisting of unwelded or partly welded volcanic glass beads. This result is consistent with the apparent absence of identifiable pyroclastic material in the HED (and other types of) differentiated meteorites.