Washington:
Tides flowing in a subsurface ocean of molten rock, or magma, could explain why Jupiter's moon Io appears to have its volcanoes in the 'wrong' place and could also have implications for the search for alien life, a NASA study has found.
The research implies that oceans beneath the crusts of tidally stressed moons may be more common and last longer than expected.
"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 lead author Robert Tyler of the University of Maryland and NASA's Goddard Space Flight Centre in US.
"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," said Mr Tyler.
Io is the most volcanically active world in the solar system, with hundreds of erupting volcanoes blasting fountains of lava up to 400 kilometres high.
The is due to the heat produced by a gravitational tug-of-war between Jupiter's massive gravity and other smaller but precisely timed pulls from Europa, a neighbouring moon to Io that orbits further from Jupiter. Io orbits faster, completing two orbits every time Europa finishes one.
This regular timing means that Io feels the strongest gravitational pull from its neighbour in the same orbital location, which distorts Io's orbit into an oval shape.
This modified orbit causes Io to flex as it moves around Jupiter, causing material within Io to shift position and generate heat by friction.
Previous theories of how this heat is generated within Io treated the moon as a solid but deformable object.
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 mystery of Io's misplaced volcanoes called for a different explanation.
The team thinks much of the ocean layer is likely a partially molten slurry 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.
The team thinks a combination of fluid and solid tidal heating effects may best explain all the volcanic activity observed on Io.
The study also has implications for the search for alien 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 study was published in the Astrophysical Journal Supplement Series.