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V. Cataldo, L. Wilson (PSRG, Lancaster University, UK.)
We recently modelled steady eruptions of gas and magma into the the Pillan and Pele volcanic plumes on Io (1). Here we investigate how different exit pressures and mass fractions of SO2 gas within the erupted gas-magma mixture affect the maximum angle of deflection of the exiting flow from the vent. The Pillan and Pele plumes appeared highly symmetrical in Voyager and Galileo images (2, 3). Angles too close to the horizontal would not yield symmetrical plume shapes and ring-like deposits like those of Prometheus and Pele (2). We consider vent pressures ranging from 2 kPa to 2 Mpa, similar to values used to model geyser-like eruptions on Io (4). The volcanic flow exiting the vent at both Pillan and Pele will be highly overpressured relative to the very low ambient pressure (10-7 Pa). The Prandtl-Meyer angle (5) through which the unconstrained flow turns depends on the exiting flow Mach number (M) and the specific heat ratio (gmix) of the gas-magma mixture which behaves as a pseudogas with heat transfer and thermal equilibrium between phases (4). At both Pillan and Pele increasing exit pressures reduce the amount of gas required to get a given angle. The lower the gas amount within the erupted mixture, the higher the eruption temperature (1). At Pillan, the best fit to Galileo data is for exit pressures from 20 kPa to 200 kPa. At the upper pressure, as little as 8 wt The very high temperature values detected by Galileo at Pillan (1800 K) are thus consistent with our results. At Pele, the best fit with Galileo data occurs for exit pressures of ~200 kPa and SO2 gas masses (as high as 30-40 wt existence of higher amounts of gas in such a plume has been inferred since the Voyager era by many authors (2, 6).
REFERENCES: [1] Cataldo & Wilson (1999). XXX LPSC. [2] Strom & Schneider (1982) "Satellites of Jupiter". [3] McEwen et al. (1998) Icarus, 135, 181; [4] Kieffer (1982) "Satellites of Jupiter"; [5] Zucrow & Hoffman (1976) "Gas Dynamics", John Wiley, New York; [6] Johnson et al. (1995) GRL, 22, 3293.