This is a composite image of Io and Europa taken March 2, 2007 with the New Horizons spacecraft. Here Io is at the top with three volcanic plumes visible. The 300-kilometer (190-mile) high plume from the Tvashtar volcano is at the 11 o'clock position on Io's disk, with a smaller plume from the volcano Prometheus at the 9 o'clock position on the edge of Io's disk, and the volcano Amirani between them along the line dividing day and night.
Previous theories of how this heat is generated within Io treated the moon as a solid but deformable object, somewhat like clay. However, when scientists compared computer models using this assumption to a map of the actual volcano locations on Io, they discovered that most of the volcanoes were offset 30 to 60 degrees to the East of where the models predicted the most intense heat should be produced.
The pattern was too consistent to write it off as a simple anomaly, such as magma flowing diagonally through cracks and erupting nearby. "It’s hard to explain the regular pattern we see in so many volcanoes, all shifting in the same direction, using just our classical solid-body tidal heating models," said Wade Henning of the University of Maryland and NASA Goddard, a co-author of the paper.
The mystery of Io's misplaced volcanoes called for a different explanation—one that had to do with the interaction between heat produced by fluid flow and heat from solid-body tides.
"Fluids – particularly 'sticky' (or viscous) fluids – can generate heat through frictional dissipation of energy as they move," said co-author Christopher Hamilton of the University of Arizona, Tucson. The team thinks much of the ocean layer is likely a partially molten slurry or matrix with a mix of molten and solid rock. As the molten rock flows under the influence of gravity, it may swirl and rub against the surrounding solid rock, generating heat. "This process can be extremely effective for certain combinations of layer thickness and viscosity which can generate resonances that enhance heat production," said Hamilton.
The team thinks a combination of fluid and solid tidal heating effects may best explain all the volcanic activity observed on Io. "The fluid tidal heating component of a hybrid model best explains the equatorial preference of volcanic activity and the eastward shift in volcano concentrations, while simultaneous solid-body tidal heating in the deep-mantle could explain the existence of volcanoes at high latitudes," said Henning. "Both solid and fluid tidal activity generate conditions that favor each other's existence, such that previous studies might have been only half the story for Io."
The new work also has implications for the search for extraterrestrial life. Certain tidally stressed moons in the outer solar system, such as Europa and Saturn's moon Enceladus, harbor oceans of liquid water beneath their icy crusts. Scientists think life might originate in such oceans if they have other key ingredients thought to be necessary, such as chemically available energy sources and raw materials, and they have existed long enough for life to form. The new work suggests that such subsurface oceans, whether composed of water or of any other liquid, will be more common and last longer than expected, both within our solar system and beyond.
Just as a precisely timed push on a swing will make it go higher, oceans can fall into a resonance state and sometimes produce significant heat through tidal flow. "Long-term changes in heating or cooling rates within a subsurface ocean are likely to produce a combination of ocean layer thickness and viscosity that generates a resonance and produces considerable heat," said Hamilton. "Therefore the mystery may be not how such subsurface oceans could survive, but how they could perish. Consequently, subsurface oceans within Io and other satellites could be even more common than what we've been able to observe so far."
The research was funded by a grant from the NASA Outer Planets Research program.