Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Rachel Wolfe
Io: Moon in Torment
Io is referred to as one of the four “Galilean satellites,” named after Galileo Galilei, who discovered Io in 1610. Io is the third largest of Jupiter’s moons,
just a little larger than Earth’s moon. It also has the dubious privilege of
being the 5th closest moon to Jupiter. So far, 62 moons have been
found around Jupiter. The other “Galilean satellites” are Europa, Ganymede, and
Callisto. Shown here are Jupiter, Io, Callisto, Ganymede, and Europa.
Io is often described as resembling a giant pizza.
Io is the most active volcanic body in the Solar System, 30 times more active than Earth, with volcanoes constantly erupting; well over 100 volcanoes have been documented. Io’s volcanoes are also the hottest in the Solar System, varying up to 2,700 degrees Fahrenheit. Oddly enough the temperatures between volcanoes usually range from minus 298 to minus 280 degrees Fahrenheit, regardless of latitude. The following map is measured on the Kelvin version of temperature measurement, with 90 degrees Kelvin equating to minus 298 degrees Fahrenheit, and 100 degrees Kelvin equating to minus 280 degrees Fahrenheit. It is a map of Io’s South Pole at nighttime. The three largest volcanoes are listed, with Pi standing for Pillan, Pe standing for Pele, and Ba standing for Babbar.
Photo Credit: NASA/JPL/Lowell Observatory
The volcano Tvashtar, mapped on the picture of Io, was first seen by the
spacecraft Galileo on the 26th of November 1999. Galileo took another
picture on the 22nd of February 2000. The pictures below show that
the lava had shifted vents during these few months, and that the volcano was
still active. In late December of 2000, the spacecraft Cassini, en route to
Saturn, discovered that Tvashtar was still active. This surprised the scientists
involved in the Cassini project, who expected the length of Tvashtar’s eruption
to be similar to that of Earth’s volcanoes.
To the delight of NASA’s scientists, in 2007 Tvashtar proved that it was still
active. On the 28th of February 2007, the spacecraft New Horizons
captured a spectacular eruption. The volcanic debris extended 200 miles into the
atmosphere. It can be seen in the picture below. This picture was obviously
taken at a different angle than the picture from Galileo.
However, there is a more notorious volcano on Io. It is the largest and most
powerful volcano in the Solar System, and has been known and ceaselessly active
since 1979. This volcano, named Loki, is a source of heated debate, as scientist
debate whether it is an active lava lake. Why is this important? Scientists
believe that Io, especially the lava lakes, could tell them a lot about what the
earliest form of Earth looked like. Scientists know that Loki does not behave
the way terrestrial lava lakes do. As of 2004, scientists believed that Loki and
its fellow lava lakes act more like a mid-oceanic ridge, like the Southern East
Pacific Rise. On Earth plate tectonics make these ridges long and narrow.
However, there are no plate tectonics in place on Io, as Io’s surface is
constantly changing. Thus, while an eruption along Earth’s mid-oceanic ridge is
forced into shape, on Io there is no crust, which enables the eruptions to form
a circular pattern. The caldera is constantly being refilled with new lava, and
could easily swallow the entire state of Maryland. The picture below is Loki in
infrared. The picture on the left is Loki shown in the visible spectrum. The two
pictures on the left reflect the temperature changes that occurred in one day.
The yellow color represents 188 degrees Fahrenheit, red is 314 degrees
Fahrenheit, and white is 1,052 degrees Fahrenheit.
The reason for the ceaseless volcanism on Io is that the surface of Io is constantly changing. Io’s orbit around Jupiter is irregular and elliptical. This is due to the forces of the larger moons Europa and Ganymede. Io is not only exposed to the forces of Europa and Ganymede, but also to the gravitational pull of Jupiter itself. Jupiter pulls Io in one direction, while Europa and Ganymede pull in the other direction.
The fact that the gravitational pulls on Io are constantly changing means that
the solid surface of Io can bulge out, or collapse, a total of 330 feet. These
changes are called tides. They are the reason that Io lacks tectonic plates. For
the sake of comparison, the maximum difference between high tide and low tide on
Earth is 16 feet.
The changes on the surface mean that it can be difficult for scientists to find
a volcano again. The crust is constantly breaking and re-forming as Io orbits
around Jupiter. The opposing pulls mean that a volcano can erupt anywhere the
ground weakens. These “wandering volcanoes” are something that cannot be found
on Earth. The change in position of Tvashtar’s lava vent mentioned earlier is a
case in point.
Nearly half of Io’s mountains sit next to volcanoes. While plate tectonics on
Earth force pressure to rise in a lateral direction, Io, as mentioned before,
has no plate tectonics. Lava flows straight up to the surface, and spreads out.
The new outpourings of lava continuously press down on increasing amounts of old
lava. Eventually the pressure causes the area to break, creating a thrust fault.
This creates a tall mountain, and opens a new vent for the lava to flow to the
surface. The mountain Tohil Mons, shown below, is a good example of such a
mountain. Tohil Mons is 18,000 feet high. The pit right next to the peak of
Tohil Mons hints at old volcanic activity. Furthermore, there are two volcanic
craters directly east of the peak of Tohil Mons; a small one near the foot of
the mountain, and a larger crater at the very edge of the picture.
NASA/JPL/University of Arizona
As can be expected, Io’s volcanic gases do not have the same molecular composition as those found on Earth. For instance, scientists have found silicon monoxide, silicon dioxide, iron monoxide, sodium chloride, and potassium chloride. Silicon monoxide is the prevalent gas found over volcanic eruptions. In an odd twist, Astronomers have found silicon monoxide in other places in space, most notably in cooling stars.
If that was not confusing enough, scientists have found that Io is also blanketed by plains of sulfur-dioxide rich white snow. Bizarrely, this snow seems to appear under the lava flow, away from the main volcano. The heat of the volcano turns the solid sulfur into a gas. It feeds the volcano’s plume, and then falls to the ground in frozen crystals. Heat from the many volcanoes on Io redistributes the snow. What really leaves the scientists scratching their heads is the fact that this material is all over Io. More specifically, it appears at every location the scientists have taken closer looks at. The scientists also do not understand why this volatile snow has not been redistributed to the colder polar areas, or been spun off into space. Shown below is a picture a lava flow near the volcano Prometheus. The white areas near the lava flow are the sulfur-dioxide rich snow.
Photo Credit: NASA/JPL/University of Arizona