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Mountains on Io, Jupiter’s volcanic moon, are formed by a unique geologic mechanism not found elsewhere in the solar system, according to a new study by the U.S. Geological Survey.

Io is a world of extremes: famous for its profuse volcanism, but also covered with some of the highest mountains in the Solar System - a few taller than Mount Everest. Since the Galileo mission in the 1990s, planetary scientists have hypothesized that Io’s mountains formed through thrust faulting, where deep rocks are pushed up and over shallower rock layers. This is a result of compression in the planetary crust, brought on by the near-constant burial of the surface by newly erupted lava.

Results show, for the first time, that these lithospheric stresses do lead to thrust faulting deep in Io’s crust, uplifting large blocks of the planetary surface to form the mountains we see today. Scientists developed these findings using computer models of how the surface of a planet deforms. In addition, modeling suggests that these large faults remove barriers in the crust, providing a pathway for magma to reach the surface. These results help explain the connection between mountains and volcanic crater-like features, called patera. The full report is available online in the journal Nature Geoscience.

“This study is an outstanding example of how computer models can be used to see what is happening under the surface of a planet,” said Laszlo Kestay, USGS Astrogeology Science Center Director. “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.”

An image of Hi'iaka Montes and Hi'iaka Patera on Jupiter's volcanic moon Io. The sharp peak at the top of the image is about 36,300 feet high, and the two elongated plateaus to the west and south are about 11,500 feet high. Hi'iaka Patera is the irregularly shaped, dark depression at the center of the image. Image credit: NASA 

Unlike mountains on Earth, most of Io’s mountains are isolated rather than occurring together in mountain ranges or chains. Io’s mountains are also associated with the volcanic, crater-like features, called patera, at least at local scales. Understanding how these mountains form and how they are related to Io’s volcanism is one of the keys to understanding the overall geologic processes of Jupiter’s moon.

“Faulting on Earth usually occurs in the relatively shallow subsurface,” said Michael Bland, USGS scientist and lead author on the study. “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.”

Galileo image mosaic of mountains on Io.
The distribution of differential stress throughout the lithosphere on Io. Major zones of tensile (pink/white) and compressive (black/brown/red/orange) stresses are indicated. The top panel shows schematically the expected surface features that correspond to the modeled stress distribution.
A regional map of Jupiter's volcanic moon, Io.