New research out of NASA suggests that subsurface oceans – composed of water or magma – may be more common on tidally stressed moons than previously expected. The oceans appear to last longer than predicted, thus increasing the chances of finding life beyond Earth. Just like the oceans here on Earth, subsurface oceans on other worlds could have tides, and in the case of one Jovian moon, these tides could help solve a volcanic mystery: Why do Io’s volcanoes appear to be “misplaced”? 

Active volcanoes are only found on two worlds in our Solar System – Earth and Jupiter’s moon Io. Trapped in a cosmic tug-of-war between the gas giant and two other Jovian moons – Ganymede and Europa – Io is constantly stretched and squeezed as it orbits Jupiter. As a result, the moon’s interior is heated. The resulting friction fuels Io’s volcanism, making it the most volcanically active body in our Solar System. On Io, lakes of molten rock and lava cover a geologically young surface, adorned with volcanoes known for spewing jets of lava over 400 kilometers (250 miles) high. 

Multiple NASA missions have flown by the Jupiter system, with Voyager being the first to observe the moon’s volcanoes directly. Scientists used computer modeling to predict how tidal heating affects the moon’s interior, hypothesizing that Io’s volcanoes should be located over regions of intense heat. The models were then compared to actual maps of the volcanoes' locations – produced from data collected over the years by multiple space probes and telescopes. The team was surprised to discover that the computer models indicated the volcanoes were not where they should be. Instead of forming directly over the hottest regions, the volcanoes were located 30-60 degrees further to the east. What could account for this displacement? One word: Tides.

"This is the first time the amount and distribution of heat produced by fluid tides in a subterranean magma ocean on Io has been studied in detail," Robert Tyler of the University of Maryland, College Park and NASA's Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "We found that the pattern of tidal heating predicted by our fluid-tide model is able to produce the surface heat patterns that are actually observed on Io."

Computer models originally treated Io’s interior as solid but malleable like clay. However, the team thinks the ocean layer is actually more like a slurry containing both molten and solid rock. As the slurry flows beneath the crust due to gravity, the molten rock heats up as it pushes through the solid pieces. A combination of fluid tides along with solid-body heating deep in the moon’s mantle could be the best explanation for Io’s volcanic activity. 

"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. "This process can be extremely effective for certain combinations of layer thickness and viscosity which can generate resonances that enhance heat production."

The research may play a role in the search for life beyond Earth. Tidally stressed moons, such as Europa or Enceladus, have liquid water oceans lurking beneath their icy surfaces. If life exists in these icy seas, other ingredients – such as an energy source – must have existed long enough for life to form. So the real mystery, according to Hamilton, “may be not how such subsurface oceans could survive, but how they could perish.” 

The research was funded by NASA’s Outer Planets Research program and was published in the June 2015 edition of the Astrophysical Journal Supplement Series.