Planets

How to Form Io's Mountains? Just Squeeze!

On Earth, mountains form along tectonic boundaries, but Io, Jupiter's volcanic moon, is awash with molten rock and dotted with solitary mountains -- how did they form?

Jupiter's volcanic moon Io is full of mysteries, including how its mountains were formed. They have puzzled scientists for decades because they look nothing like mountains on Earth.

At home, we see mountains grow in ranges that can stretch across thousands of miles. But on Io, the more than 100 cataloged mountains mostly grow in isolation. What mysterious tectonic forces are at play here?

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Io is so active that it's hard to look at the tectonics from space; molten lava coats the surface at an incredible rate of five inches per decade. So to answer the question, a new study used simulations to figure things out.

A close-up of Mongibello Mons at sunset. The mountain is about 8.6 kilometers (5 miles) high. Io has mountains that are as high as 10 miles above the plain, which is taller than Earth mountains. | NASA/JPL/University of Arizona

"The planetary community has thought for a while that Io's mountains might be a function of the fact that it is continuously erupting lava over the entire sphere," lead author William McKinnon, a planetary scientist at Washington University in St. Louis, said in a statement. He co-wrote a paper about this in 2001.

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"All that lava spewed on the surfaces pushes downward and, as it descends, there's a space problem because Io is a sphere, so you end up with compressive forces that increase with depth."

The new work simulates this hypothesis, but focuses on the fact that Io's compression gets stronger as you go deeper into the moon. This creates strain in a single fracture created deep inside of Io and then erupting to the surface, creating a cliff. The scientists also suggest this could explain why so many recent eruptions are found near the mountains.

The south polar region of Io as seen by Voyager 1. This includes the mountain Haemus Mons, which is 10 kilometers (32,000 feet) high. It is visible at bottom. | NASA/JPL/USGS

"The compressive forces deep in the crust are incredibly high," McKinnon said. "When these faults breach the surface, those forces are released, and the entire stress environment around the fault changes, providing a pathway for magma to erupt."

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The simulations also may show why Io has some mountains that split over time. That's because the lava inside the crust creates both compression and high temperature. Heating rocks causes them to split because they have nowhere to expand to. This is especially prevalent when the volcano is not erupting and carrying away the lava, reducing the pressure.

The work was published in Nature Geoscience.

Jupiter's moon Io is a hotbed of volcanic activity and much of its geology remains a mystery.

This past week

, scientists found extensive evidence of a magma ocean underneath the surface of Io, a moon of Jupiter. It now looks like all four moons discovered by Galileo in the 1600s -- Io, Europa, Callisto and Ganymede -- could have oceans of some kind, magma or liquid water. Although they may have dramatically different qualities -- from the hellish lava flows of Io to the potentially life-giving sub-surface ocean of Europa -- these oceans are all driven by tidal interactions with Jupiter. While NASA studied the Jovian moons in detail in the 1990s and early 2000s with the Galileo mission, scientists have been anxious for another visit to Jupiter that would be longer than just a flyby.

NASA's Juno spacecraft

may get distant glimpses of the moons after it arrives in 2016. For a close-up view, space fans will need to wait for a

European mission called JUICE (JUpiter ICy moons Explorer)

, which should launch in 2022 and arrive in 2030.

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Starting with the most extreme, Io is a volcanic planet, but what has intrigued scientists for decades is the volcanoes appear to be offset from where computer models predict. A newly released study suggests that a magma ocean -- a great conductor of heat --

could explain why the hotspots are in a different location than expected

. This follows on from research in 2011 from the Galileo spacecraft, showing that there likely is a magma ocean under the moon.

Europa's surface is covered with cracks, something the Voyager spacecraft noted in the 1970s and that the Galileo spacecraft looked at up close in the 1990s and 2000s. It turns out these cracks are not in spots where they are expected to be, based on how Europa's crust flexes as it moves around Jupiter. But if there was an ocean underneath, this would explain them. Also, the surface of Europa is young, indicating something is resurfacing it. In 2013, scientists spotted a watery plume erupting from Europa but

have not been able to view another one since

. It's unclear whether the ocean is liquid water and if it has the potential to support life, although films like The Europa Report are already tackling the subject.

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Earlier this year, the Hubble Space Telescope carried out observations of Ganymede. The moon has its own magnetic field and is also affected by Jupiter's magnetic environment. By looking at how two types of aurora from these magnetic fields move on the planet,

the scientists concluded that it must have some kind of ocean underneath

-- likely saltwater and, like Europa, potentially habitable for basic life forms.

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In 1998, scientists found evidence of an ocean under Callisto after being pointed there by observations of Europa. When the Galileo spacecraft found evidence of electrical currents being conducted by a liquid ocean at Europa,

scientists looked for the same phenomenon

at Callisto and struck gold -- they found varying electrical currents that could be explained by ocean activity. Callisto, however, is further from Jupiter and does not get the tidal energy Europa gets. That tidal energy may make life more favorable on Europa.

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There are other bodies in the solar system that could have oceans, most prominently Enceladus (a moon of Saturn). We've seen plumes at Enceladus and there has been talk of a small ocean on the moon, but now there's something even more intriguing: a global ocean. Earlier this month, observations from the Cassini spacecraft showed the moon's orbit around Saturn wobbles in a way consistent with an ocean under its ice shell. Another strong possibility is Titan (a moon of Saturn), which is already known to host vast lakes of liquid methane and ethane.

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