Jupiter moon 'holds magma ocean'
Io is the most volcanic world in the Solar System and scientists think they now have a better idea of why that is.
The moon of Jupiter erupts about 100 times more lava on to its surface each year than does Earth.
A re-assessment of data from Nasa's Galileo probe suggests all this activity is being fed from a giant magma ocean under Io's crust.
Researchers tell Science magazine that this blisteringly hot reservoir is probably some 50km (30 miles) thick.
And that figure is a minimum. It could be much, much thicker, says the study's lead author, Krishan Khurana, who is affiliated to UCLA's Institute of Geophysics and Planetary Physics.
"When scientists first started looking at the images of Io from the Pioneer and Voyager spacecraft in the late 70s, the moon appeared so alien," he told the BBC.
"Right away, the scientists were asking questions; and one of the questions was, 'why are volcanoes present all over the surface?' Well, it's because there's a giant aquifer of magma present right beneath the crust. That's what our study is telling us."
Io's volcanism is driven by its parent planet - Jupiter. The great gas giant's enormous bulk produces colossal tides on the moon that squeeze and pull its body, melting its rocks.
The distribution of volcanoes on Io is quite different to that on Earth, however. They are everywhere, whereas on Earth the volcanoes tend to be collected at the boundaries of tectonic plates, the huge slabs of cold rock that cover our planet's surface.
Nasa's Galileo probe, following up the observations of the Pioneer and Voyager spacecraft, made seven close passes of the moon.
Readings from its magnetometer instrument indicated the moon was dramatically distorting Jupiter's magnetic field - but what was going on inside Io to produce the effect was not clear.
It has taken several years to work through the problem and identify the solution, and it comes down to the nature of the rock in the moon and how it behaves when it melts.
Dr Khurana explained: "The data was available almost seven or eight years ago. However, we could not at that time explain what we were seeing.
"Later experiments in mineral physics found out that when ultramafic rocks, which are rocks very high in magnesium and iron - when those are melted, their conductivity shoots up by orders or magnitude. And it is that very high conductivity that can create the type of signature we have seen. So, we needed mineral physics to catch up with our data."
Tests have shown that the signatures detected by Galileo are consistent with a rock like lherzolite, an igneous rock rich in silicates of magnesium and iron. You find this rock, for example, in Scandinavia.
The picture emerging of Io is of a world that apes a body considerably bigger in size.
Its magma ocean layer is at least 50km thick, and probably makes up at least 10% of the moon's mantle by volume. Its temperature probably exceeds 1,200C.
This aquifer sits under the crust, some 50km down. The mantle - the moon's interior mid-layer - probably extends for a further 700-800km. And at the core? Gravity measurements suggest it is made of iron and possibly liquid - much like the Earth.
"The moon in size is only about one-fortieth the volume of the Earth; in mass it's only one-sixtieth," said Dr Khurana.
"And yet because of the tremendous amount of heat generated by tides that Jupiter raises on this very small moon, its internal structure is very similar to the Earth or a bigger planet that has a lot of tectonics on it."