Contact: Tony Fitzpatrick
Washington University in St. Louis

It takes a lot of stress, and a little chaos, to create some of the tallest mountains in our solar system. That is the theory proposed by earth and planetary scientists at Washington University in St. Louis studying mountain formation and volcanic activity on Io, one of Jupiter’s many moons.

The researchers analyzed images taken by the Galileo and Voyager spacecraft and found that Ioís enigmatic mountains may be the combined result of heating, melting, and tilting of giant blocks of crust.

The origin of Io’s prodigious mountains has intrigued planetary scientists for over 20 years. Io, about the size of Earth’s moon, is the most geologically active body in the solar system, with mountains up to 55,000 feet tall (the summit of Mt. Everest is a meager 29,000 feet). Io’s surface is dotted with active volcanoes spewing plumes of sulfurous gas and emitting vast streams of scorching lava.

The heat released from Io — from lavas as hot as 1,800 Kelvin or 2,800 degrees Fahrenheit — is about 25 to 30 times greater per square foot than the heat released from Earth. This makes Io’s mountains, which are not themselves volcanoes, all the more interesting, because at these temperatures planetary scientists would expect the surface to be liquid or soft, with little topography to speak of.

How, then, can mountains form in such a furnace-like environment? William B. McKinnon, Ph.D, professor of earth and planetary sciences and Andrew J. Dombard, Ph.D., recent Ph.D at Washington University in St. Louis, and Paul M. Schenk, Ph.D., of Houston’s Lunar and Planetary Institute, answer this geological conundrum in the February issue of the journal Geology.

The paper is: “Chaos on Io: A model for formation of mountain blocks by crustal heating, melting, and tilting” The work was funded by NASA’s Planetary Geology and Geophysics and Jovian Systems Data Analysis Programs.

“Two things work in concert to produce Io’s mountains,”says McKinnon “These are compressive stress, due to the general movement or sinking of the crust closer to Io’s center, and thermal stress which is generated when regions of cool crust suddenly become heated.”

The combination of compressive and thermal stresses breaks up the crust and produces irregular, or chaotic, distributions of mountain peaks. Slight changes in the rate of lava flow from Io’s mantle and the heating of cooler crust below the surface create the mountain-forming faults.

“Heat is actually trying to come out from deep in the interior of Io, but the crust is subsiding, or sinking, as new layers of lava are laid down, all on the order of one to several centimeters a year,” says McKinnon. “For this heat, it’s like trying to run up a down escalator; you run in place. But if the escalator slows down, meaning the lava eruptions slow down, then you (the heat) can in fact run to the top.”

The researchers used stereoimaging — a method where three-dimensional objects are reproduced by combining two or more images of the same subject taken from slightly different angles — to reconstruct the physical topography of much of Io’s surface. Maps of all the mountains and volcanoes on Io’s surface were also made.

“The stereo data and high-resolution pictures taken by the ongoing Galileo mission allowed us and others to confirm that Io’s mountains were indeed tilted fault blocks and not volcanoes,” says McKinnon. “You can see sequences of mountains in early, middle, and late stages of collapse; first tall and steep with landsliding, then intermediate, and then basically flat.”

Working out the irregular and chaotic distribution of the mountain peaks — which is quite different from the linear or arcuate patterns found on Earth — allowed the researchers to propose that Io’s mountains are the result of natural disturbances in the surface crust.

Ioís lava makes it to the surface and normally radiates its heat into space (Io has no atmosphere to speak of). Of course, volcanoes are notoriously unstable, so if volcanism falters in one region, the surrounding crust begins to heat.

This causes the crust to expand, generating compressive thermal stress in the crust, which in turn forces the crust apart, forming faults and mountains. This helps explain why concentrations of mountains are seen on Io that are separated from concentrations of volcanoes. The researchers propose that similar events may have occurred on Europa, another satellite of Jupiter, and the early Earth.

“Even though the moons have different surfaces — Europa is ice; Io, rock — they have common geological phenomena,” says McKinnon. “This combination of thermal and compressive stresses could operate in other situations where a body gets a lot of heat. The early Earth was very hot, and therefore could have been more like Io in terms of tectonics and volcanism than the Earth today.”