THE PLUMES OF IO AND FUTURE EXPLORATION

Nine plumes were initially discovered by the Voyager spacecraft in 1979, including smaller (~50 km high) stable plumes like Prometheus and larger, transient or intermittently-visible plumes like Pele (>300 km high). Galileo, HST, Cassini, New Horizons, Juno, and ALMA have also observed plumes, emanating from a total of 17 volcanic centers, all of which include high-temperature hot spots. Additional plume sites are inferred from new ring-shaped deposits, such as from Surt and Aten Patera. Ground based observations infer the presence of high-entropy eruptions produced by superheating SO₂ vapor at depth, leading to “stealth” plumes. SO₂ and S₂ are thought to be the primary gases driving plumes, but little is known about plume compositions. Many of the small plumes are produced by advancing lava flows that vaporize near-surface frosts, but others are associated with vigorous lava fountaining. Some are associated with lava lakes. The majority of >100 high-temperature hot spots lack detected plumes; perhaps persistent hot spots tend to drive off the volatiles needed for plumes, unless lava flows advance over new frost deposits. The explosion-dominated plumes likely carry particles formed in the lava fountains or entrained from the vent, forming dark surface deposits. The plumes strongly impact Io’s atmosphere and mass loss. Io’s visible plumes typically have a bluish color, indicative of scattering by small particles in sunlight or auroral emissions in eclipse. Many plumes are concentrated near latitudes of +20 and –20 degrees, which may enhance the apparent “rocking” of auroral emissions seen during eclipse as Jupiter rotates with its tilted magnetosphere. Modelling of the gas dynamics associated with plumes and lava lakes is advancing but misses constraining data on the distribution and temperature of the atmosphere and the properties of the interaction with Jupiter's magnetosphere. There are many open questions about Io’s plumes and associated processes. To make a significant advance in understanding Io’s plumes, we need in-situ measurements by a neutral gas mass spectrometer and a dust mass spectrometer in addition to remote sensing designed to study Io during a mission with many close (<500 km) encounters.