From a mountaineering perspective, Earth doesn’t present much of a challenge when compared to other planetary bodies in the Solar System. Mount Everest, Earth’s tallest mountain whose peak reaches 8.9 km above sea level, may seem like a towering giant from a terrestrial standpoint, but when put in context to the mountains and ridges that dominate the surfaces of some of our Solar System’s planets and moons, it feels more like a modest hill.
Some of the mountain heavyweights in our planetary family are found on Jupiter’s innermost moon Io. Slightly larger than Earth’s Moon, having a diameter of 3,630 km, Io has the unique distinction of being the most volcanically active place in the Solar System. With more than 400 active volcanoes that spew large plumes as high as 200 km into space and produce extensive lava flows which constantly reshape the surface, Io is every geologist’s dream. All this volcanic activity is mainly the result of the immense tidal forces that the neighboring, massive Jupiter exerts on the diminutive satellite.
More specifically, as Io races around the giant planet every 1.76 days it gets squeezed and stretched by the latter’s gravity. In addition, Io is locked in a 4:2:1 orbital resonance with two other nearby Galilean moons, Europa and Ganymede, which means that for every four times that Io revolves around Jupiter, Europa revolves two times and Ganymede revolves once. This gravitational tug-of-war with the massive Jupiter as well as the rest of the Galilean satellites creates a large amount of tidal friction on Io which in turn is converted into heat that keeps the moon’s interior constantly molten. Whereas the same process maintains an underground ocean of liquid water on the neighboring moon Europa, Io’s interior harbors a global ocean of molten rock, or lava instead.
NASA’s Voyager and Galileo missions, which studied the Jupiter system in the late 1970s and mid-1990s respectively, photographed Io’s surface in great detail, capturing amazing views of the moon’s hundreds of volcanoes in action which allowed planetary scientists to study the effects of present-day volcanic activity on another celestial body besides Earth for the first time. But, aside from Io’s trademark volcanic activity, the photographs that were beamed back by Voyager and Galileo revealed another set of standout geologic features on the surface of this unique satellite in the form of mountains. To this day, scientists have identified more than 110 mountain structures on Io, which cover approximately 3 percent of its surface. Yet, these structures are as unique as the moon they sit upon and they bear little resemblance to their terrestrial analogues. For instance, contrary to mountains here on Earth which often form long ridges that can stretch across the horizon, Io’s mountains are surprisingly stand-alone in nature. They appear here and there, peppered across the landscape like isolated blocks that are situated among smooth lava-filled plateaus. Furthermore, and despite Io’s fierce volcanic activity, the moon’s mountains aren’t volcanic in origin or the result of plate tectonics, which is the driving force that reshapes the Earth’s surface.
One other aspect that makes Io’s mountain unique is their height. The mean height of the Ionian mountains is approximately 6.3 km, while the moon’s largest one, Boösaule Montes, towers more than 17 km above the surface—almost twice the height of Mount Everest! Planetary scientists had been puzzling for decades over the mystery of how Io’s gigantic mountain formations, compared to the moon’s size, came to be. One of the most plausible and accepted explanations put forth back in 2001 by a team led by William McKinnon, a professor at the department of Earth and planetary sciences at the Washington University in St. Louis, was that the moon’s solitary mountains are the result of thrust faulting. The latter is a geological process during which two different blocks of a planetary crust move relative to each other with the one being pushed upward over the other, driven by compression forces that have built up underneath inside the crust. On Earth, these faults are caused by the compression forces exerted by plate tectonics, but on Io no such mechanism exists. Instead, plate tectonics is believed to be supplanted on the small moon by the latter’s extensive volcanism.
In order to test this hypothesis, McKinnon teamed up with Michael Bland, a research scientist at the USGS Astrogeology Science Center in Flagstaff, Ariz., who has specialised in combining detailed computer simulations with planetary datasets for better understanding the geological evolution of planetary bodies. As detailed in their study which was published on the Nature Geoscience journal earlier this week, the results of the simulations by McKinnon and Bland were consistent with what has been observed on Io, indicating that thrust faulting is indeed the most plausible explanation for mountain building on the jovian moon. What these simulations showed was that as lava is released on Io from volcanic eruptions, it is placed on the surface causing the latter to sink deep into the crust. As this pre-existing surface material sinks deeper into the moon’s interior it is contracted and squeezed, giving rise to compressive forces that over time build up which in turn build up lithospheric stresses deep inside the moon. When the latter reach the crust’s breaking point, a fault forms locally which then propagates upward, causing large blocks of the moon’s crust to break through the surface. Io’s mountains are the end result of this thrust faulting as the previously sank material is transported back to the surface. “The compressive forces deep in the crust are incredibly high,” says Mckinnon. “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.”
One interesting aspect of this tectonic activity is that Io’s volcanic eruptions act as a release valve for the thermal stresses that build up underground as the pre-existing surface sinks into the moon’s interior. As long as volcanic eruptions take place, they take away from the thermal energy that is stored on Io’s crust which in turn creates lithospheric cracks and faults. If for any reason magma cannot be released to the surface, the underground energy builds up then becomes sufficient to drive mountain formation through thrust faulting. So, in essence, mountains are formed on Io over the areas where volcanic eruptions have temporarily ceased and vice versa. This overall tectonic activity also serves another purpose: stabilising Io’s thermostat and helping the moon to cool.
The researchers’ study helped to address another seeming enigma of Io’s topography: the global distribution of mountains on the moon’s surface which are placed opposite to that of volcanoes and vice versa. “If you look at a big map of Io there are concentrations of mountains and concentrations of volcanoes, and they kind of nest into one another,” says Mckinnon. “Even though mountains and volcanoes are often found together, if you look at all of the mountains and all of the volcanoes, they’re anti-correlated. It’s a peculiarity of Io.” It all has to do with how the thermal stresses on the interior of Io are released. If the underground magma can reach the surface, then it takes away the energy that drives thrust faulting, so the landscape is dominated by active volcanoes. On the other hand, the material that sinks at the base of the lithosphere gets compressed and heated and tends to expand. These stresses tend to halt magma flow, giving rise to thrust faulting that makes mountain formations more likely.
This overall tectonic activity on Io is quite unique from that of other planetary bodies like Mercury, Earth, and the Moon, whose internal heat source is the result of radioactive decay, contrary to Io’s which is caused by tidal heating. In addition, faults on Earth are much shallower in depth and can be found on a global scale, whereas on Io they are very deep and occur locally, resulting in very different mountain forming processes. “Faulting on Earth usually occurs in the relatively shallow subsurface,” explains Bland. “We show that mountain formation on Io is just the opposite. The excessive volcanism causes the stresses to be increasingly large at depth, so faulting initiates there and extends upward. Our work confirms that Io’s mountains are ultimately a result of its prodigious volcanism. Everything is connected.”
Yet, as novel as Io’s geologic processes may seem at first glance, the researchers think that similar ones could have helped to shape the face of the primordial Earth, early on its history. “The same kind of thing could have happened on Earth, when it was very young and entirely covered by a shallow ocean,” argues McKinnon. “Because there was still lots of volcanism, mountains like those on Io might have burst through the ocean. They might have been the first emergent land on Earth.”
“This study is an outstanding example of how computer models can be used to see what is happening under the surface of a planet,” comments Laszlo Kestay, director of the USGS Astrogeology Science Center. “By studying a bizarre world like Io, we see unexpected interactions between geologic processes like volcanism and mountain building, which can ultimately help us better understand our home planet.”
As has been showcased by planetary exploration time and again, the study of other worlds is also a study of ourselves.