Io is the Solar System’s most volcanically active planetary body. However, concentrations of volcanic activity are displaced from where they are expected to be based on models that predict how the satellite’s interior is heated. According to a new study published in the Astrophysical Journal Supplement Series, tides flowing in an underground ocean of molten rock could explain why Io appears to have its volcanoes in the ‘wrong’ place.
This image from NASA’s Galileo spacecraft shows Io. There are rugged mountains several miles high, layered materials forming plateaus, and many irregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no landforms resembling impact craters, as the volcanism covers the surface with new deposits much more rapidly than the flux of comets and asteroids can create large impact craters. The picture is centered on the side of Io that always faces away from Jupiter; north is to the top. Image credit: NASA / JPL / University of Arizona.
Intense geological activity on Io is the result of heat produced by a gravitational tug-of-war between Jupiter’s gravity and the smaller but precisely timed pulls from Europa and Ganymede, two neighboring moons that orbit further from Jupiter.
Io orbits faster than these other moons, completing two orbits every time Europa finishes one, and four orbits for each one Ganymede makes.
This regular timing means that Io feels the strongest gravitational pull from its neighboring moons in the same orbital location, which distorts Io’s orbit into an oval shape. This in turn causes Io to flex as it moves around Jupiter.
As Io gets closer to Jupiter, gas giant’s gravity deforms the moon toward it and then, as Io moves farther away, the gravitational pull decreases and the moon relaxes. The flexing from gravity causes tidal heating – in the same way that you can heat up a spot on a wire coat hanger by repeatedly bending it, the flexing creates friction in Io’s interior, which generates the tremendous heat that powers the moon’s extreme volcanism.
Previous theories of how this heat is generated within Io treated the moon as a solid but deformable object.
However, when researchers 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.
“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,” said Dr Robert Tyler from the University of Maryland and NASA’s Goddard Space Flight Center, lead author on the study.
“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.”
“Viscous fluids can generate heat through frictional dissipation of energy as they move,” said co-author Dr Christopher Hamilton of the University of Arizona, Tucson.
According to the scientists, much of the ocean layer is likely a partially molten slurry or matrix with a mix of molten and solid rock.
As the magma 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,” Dr Hamilton said.
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,” explained co-author Dr Wade Henning of the University of Maryland and NASA’s Goddard Space Flight Center.
“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.”
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Robert H. Tyler et al. 2015. Tidal heating in a magma ocean within Jupiter’s moon Io. ApJS 218, 22; doi: 10.1088/0067-0049/218/2/22
