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, whic h is the likely parent body of the howardite-eucrite-diogenite (HED) m eteorites. For a wide range of magma volatile contents, we calculate t he eruption speeds and subsequent trajectories of the pyroclastic magm a 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 h ave formed lava fountains that were extremely optically dense. As a re sult, virtually all of the droplets will have reached the surface havi ng suffered a negligible amount of radiative cooling and will have coa lesced into lava ponds feeding lava flows. Typically, <1% of the pyroc lasts will have undergone enough cooling to allow them to accumulate i nto a recognizable fall deposit consisting of unwelded or partly welde d volcanic glass beads. This result is consistent with the apparent ab sence of identifiable pyroclastic material in the HED (and other types of) differentiated meteorites.