EXPLOSIVE VOLCANISM IN THE OUTER SOLAR SYSTEM

This paper will discuss the non-effusive types of volcanism on Io and the icy satellites, including plumes on Enceladus and Triton. “Explosive” volcanism in the outer solar system has significant differences from what normally occurs on Earth. The products of these eruptions are also different from the explosive deposits found on Earth, making interpretation often challenging. Traditional definitions of explosive volcanism on Earth involve the sudden release of gases from high viscosity magmas, or interactions of magma with water. Thick atmospheres suppress the ability of gases to expand during ascent, and so suppress explosive volcanism; in contrast, most of the moons in the outer solar system have negligible atmospheres, expanding the range of starting conditions that permit explosive volcanism. However, high magma viscosities such as in terrestrial rhyolites lead to potentially more explosive magmas, and such viscosities are only observed on Earth. On Io, the magma is thought to be silicate (basaltic or ultramafic), but there is no water. Explosive volcanism occurs in the form of gases (mostly sulfur and sulfur dioxide) being released into the near vacuum, and in the form of powerfully gas-driven fire fountains. Explosive release of sulfur dioxide f f rom subsurface deposits due to contact with magma is also thought to be happening. A few large, dark deposits suggest large depositions of ash, but columns of ash have not been directly observed, possible due to the very small solid volume fractions in plumes. On the icy satellites, water is present in the form of surface ice and, at least in some bodies, oceans of liquid water under icy crusts. On those bodies, the “magma” is water mixed with liquidus-depressing constituents such as ammonia, forming cryomagmas. Plumes erupt from the south polar region of Enceladus and may have erupted from other icy satellites. On Triton, plumes up to 8 km high were detected, but these are thought to be produced by gas venting driven by solar heating and the vaporization of subsurface nitrogen. Titan is a special case of an icy body with abundant hydrocarbons and also a dense atmosphere that may inhibit explosive volcanism. Numerous other satellites show evidence of possible cryovolcanic resurfacing, which may have an explosive component.