Stochastic-ballistic eruption plumes on Io

Some active volcanoes on Io are associated with bright annular deposits. He re we characterize the dimensions of the annulus observed at Prometheus. As suming that relative brightness in images is directly related to areal part icle concentration on the surface, we develop a model describing emplacemen t of particles whose motion is controlled by stochastic processes near the vent and ballistic transport beyond. Stochastic processes are expressed as probability distributions for the important transport variables. By varying the distribution parameters, high particle concentration annuli on the sur face come and go. For isotropic ejection from the stochastic region with a fixed energy, subsequent ballistic transport to the surface produces singul arities in the areal concentration at r = 0 and r = r(max). This areal conc entration of particles features peaks corresponding to the singularities. T runcation of the ejection cone such that particles with a single energy are ejected isotropically between 0 and some maximum angle theta(0) increases the relative importance of the peak near r(max). Extrapolating the model wi th a narrow Gaussian energy distribution introduces enough dispersion in th e areal concentrations to produce broad annuli. Varying combinations of the truncation angle and relative standard deviation for the energy distributi on changes the shape and magnitude of the surface deposit. A truncation ang le of 75 degrees and a relative standard deviation of 0.08 produce a symmet ric annulus closest in shape and size to that observed at Prometheus. From examination of the energetics associated with thermalized particles, we fin d that many molecular compositions are admissible as annulus constituents a t Prometheus.