STEALTH PLUMES ON IO

We suggest that Io's eruptive activity may include a class of previous ly undetected SO2 geysers. The thermodynamic models for the eruptive p lumes discovered by Voyager involve low to moderate entropy SO2 erupti ons. The resulting plumes are a mixture of solid and gas which emerge from the vent and follow essentially ballistic trajectories. We show t hat intrusion; of silicate magma into buried SO2 deposits can create t he required conditions for high entropy eruptions which proceed entire ly in the vapor phase. These purely gaseous plumes would have been inv isible to Voyager's instruments. Hence, we call them ''stealth'' plume s. Such eruptions could explain the ''patchy'' SO2 atmosphere inferred from recent UV and microwave spectral observations. The magma intrusi on rate required to support the required gas production for these plum es is a negligible fraction of estimated global magma intrusion rates.