Explosive Volcanism on Atmosphere-less Bodies - a Comparison of the Moon, Mercury and Io

Introduction: There is abundant observational evidence of explosive volcanism on all three of the large airless silicate-dominated bodies, the Moon [1], Mercury [2] and Io [3]. The lack of atmospheres makes analysis of explosive activity relatively simple. We combine the results of earlier analyses of explosive volcanism to draw inferences about the geologic circumstances of explosive activity on these three bodies and the amounts of volatiles driving the eruptions. Theoretical development: In steady eruptions, classified as Hawaiian or Plinian, most of the energy release driving the eruption speed, and hence vacuum range, of pyroclasts occurs as gases expand from the magma fragmentation pressure, P f , to the pyroclast decoupling pressure, P d. P f is the pressure at which exsolved gas bubbles are close-packed so that they collapse to convert the magmatic foam to a gas stream entraining pyroclastic droplets. If the foam collapses at a gas bubble volume fraction f then for gases obeying the perfect gas law P f is easily shown to be P f = [(1-f) n Q T ρ] / [f (1-n) m] (1) where n is the free gas mass fraction, Q is the universal gas constant, T is the magma temperature, ρ is the magma liquid density and m is the molecular mass of the gas. P d , is the pressure at which the mean free path of the gas molecules becomes about 50% greater than the typical pyroclast size, d, and is given by [4] as P d = (2 1/2 Q T) / (3 π φ 2 N d) (2) where φ is the diameter of the gas molecules and N is Avogadro's number. With good thermal coupling between clasts and gas, the steady eruption speed, U, of the gas and small pyroclasts is given by U 2 /2 = [(n Q T) / m] ln(P f / P d) (3) If the thermal coupling is poor, the eruption speed is given instead by