Jupiter’s moon Io is notable for being the most volcanically active body in our solar system. But Io also has some of the tallest mountains we’ve seen yet, with the tallest rising about 17km above the surrounding terrain – Boösaule Montes is about twice the height of Mount Everest. And, unlike the Olympus Mons on Mars, it is not volcanic. In fact, many of the moon’s highest peaks are not associated with volcanoes. They also do not form chains, but rise as isolated blocks about 100 km wide.
Since Io doesn’t appear to have plate tectonics, it’s not clear what could be building these kinds of spikes. But a new study suggests they were created as a result of volcanism, but only very indirectly. It appears that Io’s volcanism is emptying its interior fast enough to cause intense stresses on its crust.
The researchers involved (Michael Bland and William McKinnon) suggest that the mountains of Io have some terrestrial analogues. “Their morphology, which ranges from peaks and ridges to massifs, mesas and plateaus, is consistent with thrust or tilted fault blocks,” they suggest. These are instances where pieces of the Earth’s crust are broken up and pushed up relative to their surroundings (although clearly not to the same extent as on Io). So what could be causing the excessive tension that pushes them so high?
The authors suggest we can blame the volcanoes. Io pushes out so much material that the existing surface is pushed down with increasing pressure. Meanwhile, some areas under the crust will have emptied material by sending it up through volcanoes. Over time, the combination will create intense compressive stresses on the crust itself. Essentially, the existing crust is a rigid sphere that needs to shrink; making mistakes is the only way to relieve that stress.
Io isn’t the only body to have experienced a shrinking crust. Both Mars and Mercury have condensed a bit over their history as they cooled. But this would have stressed the crust relatively evenly; Io is distinguished in that the tension will be most intense at the base of the crust.
To find out what would happen, the authors created a relatively simple, if computationally intensive, model (a finite element simulation, to be precise) of Io’s crust. The crust itself is modeled as a straight line rather than a curve, and the stress is scaled to increase with depth. The authors omit the deposition of volcanic material on the surface to keep things relatively simple.
The simulations suggest that the stress, while diffuse at first, quickly concentrates on a limited number of failure points, where fractures start at the base of the crust and spread upwards. These faults eventually reach the surface, where they break through creating a small ridge—”the site of burgeoning mountain formation,” according to the authors. There are some mountains on Io (Zal Montes, Hi’iaka Montes, and Mongibello Mons) that look a lot like the simulations, though the authors aren’t sure if that makes sense since similar forces also produce plateaus.
With the stress removed, the rest of the near surface expands, creating features such as basins that are typical of this behavior on Earth. While these have been seen on Io, the authors suggest it’s difficult to get a full picture of features like this because of the amount of material the volcanoes spew out. At the average rate at which new material is deposited on the surface, it would take less than 100,000 years for a basin several kilometers deep to fill.
While their model doesn’t get past the ridge stage, Bland and McKinnon argue that there’s a big factor that could be pushing the faults in different directions: the immense tidal forces Io’s surface experiences as it orbits Jupiter. These can promote the fracture process and cause more complicated fractures. They could also serve to open the errors, which are otherwise under great pressure. This could serve to provide molten material from the mantle with a path to the surface, aiding the process that caused the faults in the first place.
Natural Geosciences2015. DOI: 10.1038/NGEO2711 (About DOIs).