: General Characteristics

• ’s innermost (first) moon Volcanism on Io • Vital statistics – Radius = 1821 km • Slightly larger than Earth’s Moon – Gravity = 1.8 m s-2 GLY 424/524 – Density = 3530 kg m-3 April 17, 2002 • Fe or Fe + FeS core • Silicate mantle?

Io: History of Exploration Io Volcanism

• Earth-based observations • Based on tidal resonances with Jupiter & – Telescopic: discovered by in 1610 Europa, Peale et al. (1979) predicted active – Thermal (mostly from Mauna Kea volcanism observatory): minimum night-time temp was – Smith et al. (1979) confirmed it too high for a “cold” body • Only “other” body in the solar system with • & 2 confirmed active silicate volcanism – 1979 – Voyager 1 observed 9 active eruptions – Active volcanic plumes observed – (4 months later) observed 8 – Colors indicative of

Eruption plume “The called shot” Peale et al., 1979 Voyager 1 & 2

• Launched in 1977 • Originally designed to study: – Jupiter – Saturn – Saturn’s rings – Large moons of both planets • Favorable alignment of outer planets (once every 175 years) allowed flybys of Neptune & Uranus too

1 Eruption plumes on Io as imaged by Voyager 1

Io Heating

• Resonant orbit with Jupiter, Europa, Ganymede – Io goes ‘round 2x every Europa orbit – Europa goes ‘round 2x every Ganymede orbit – Creates a ~100 m “tide” on Io • Stretching and heating by friction • Continually generates melt in the interior • No available data on radioactive heating – Unlikely – Other bodies of similar size are stone-cold dead

Voyager Io Results • Volcanically active! Io Exploration: Galileo – NO impact craters ANYWHERE • Launch repeatedly delayed; final launch – Eruption plumes 1989 – £450 km tall – Changes observed between Voyager 1 and 2 • Swung by Earth/Moon system twice & • 4 months apart Venus once on its way to Jupiter system • 1 plume shut down • Orbit insertion: 1995 • Surface coloration suggests sulfur • Goals – some suggestions of steep topography – Closer look at all the Galilean satellites inconsistent with sulfur – Jupiter atmosphere probe

2 Jupiter atmosphere probe

More O2 discovered than any models predicted

Galileo: Problems & Solutions

• Main antenna never properly unfurled • Drastically reduced data return rate to Earth • Solutions: – Collect all data; store on on-board tape recorder; selectively playback tape recorder – Collect significantly less data than anticipated – Variations of both solutions used

Galileo: Instruments

• NIMS – Near-infrared imaging spectrometer – Looks for “hot spots” • SSI – Solid-State Imaging system – Camera – Resolution (for both) depends on orbit altitude

3 Other Observations Io: Surface Characteristics • • Max topography = 15 km – Can pick up eruption plumes – Pure sulfur too weak to support that topography • Infrared telescope on Mauna Kea, HI – Must be silicates involved somehow – Can monitor temperatures of hot spots • Surface temperatures • Limited resolution • Results are model-dependent – 85 K (night) • Cassini Spacecraft – 125 K (day) -9 – Meant to study Saturnian system • Atmosphere pressure 10 bar – Swung by Jupiter on the way; imaged some • Composition

eruption plumes on Io – SO2 frost overlying silicates (?)

Io Volcanic Styles References • Promethean

– = type locale • Kesztheyli et al., 2001, Imaging of volcanic activity on – Long-lived, steady eruptions Jupiter’s moon Io by Galileo during the Galileo Europa • Produces compound flow field Mission and the Galileo Millennium Mission, JGR 106:33,025. • Takes years to decades • Davies et al., 2001, Thermal signature, eruption style, and • Pillanian eruption evolution at and Pillan on Io, JGR – Pillan = type locale 106:33,079. – Short-lived, high-effusion-rate eruptions • Peale, S.J. et al., 1979, Melting of Io by tidal dissipation, Science 203:892. • Large pyroclastic deposits • Smith, B.A. et al., 1979, The Jupiter system through the • Open-channel or open-sheet flows eyes of Voyager 1, Science 204:951. • lakes (e.g., Loki)

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