The Volcanic Moon

Jupiter's moon Io, is one of the most exotic places in the solar system. It is the most volcanic body known, with lava flows, lava lakes, and giant calderas covering its sulfurous landscape. It has billowing volcanic geysers spewing sulfurous plumes to over 500 kilometers high. Its mountains are much taller than those on Earth, reaching heights of 16 kilometers (52,000 feet).

Io orbits closer to Jupiter's cloud tops than the moon does to Earth. This places Io within an intense radiation belt that bathes the satellite with energetic electrons, protons, and heavier ions. As the Jovian magnetosphere rotates, it sweeps past Io and strips away about 1,000 kilograms (1 ton) per second of volcanic gases and other materials. This produces a neutral cloud of atoms orbiting with Io as well as a huge, doughnut shaped torus of ions that glow in the ultraviolet.

The torus's heavy ions migrate outward, and their pressure inflates the Jovian magnetosphere to more than twice its expected size. Some of the more energetic sulfur and oxygen ions fall along the magnetic field into the planet's atmosphere, resulting in auroras. Io acts as an electrical generator as it moves through Jupiter's magnetic field, developing 400,000 volts across its diameter and generating an electric current of 3 million amperes that flows along the magnetic field to the planet's ionosphere.

Discoveries on Io

On January 7, 1610 Galileo Galilei observed three pinpoints of light strung out in a line next to Jupiter. The next evening, these stars seemed to have moved the wrong way, which caught his attention. Galileo continued to observe the stars and Jupiter for the next week. On January 11, a fourth star (Ganymede) appeared. After a week, Galileo observed that the four stars never left the vicinity of Jupiter, appeared to be carried along with the planet, and changed their position with respect to each other and Jupiter. Finally, Galileo determined that what he was observing were not stars, but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the still heretical Copernican solar system and showed that everything did not revolve around the Earth.

In 1676 a Danish astronomer Ole Romer was able to make the first accurate measurement of the speed of light by using eclipse timings of the Galilean satellites with Jupiter's shadow. Another discovery was made by Pierre-Simon de Laplace during the late 1700s when he deduced that the orbital periods of Io, Europa, and Ganymede are nearly in a perfect 1:2:4 ratio. In 1920 this knowledge paved the way for the first estimate of the satellites' masses within an accuracy of 20%. Finally in 1979, the Voyager spacecraft flew past the Jovian system, took high-resolution pictures of the moons, and conducted experiments that provided the first accurate measurements of the moon's dimensions and mass. These in turn were used to calculate the mean density of Io (3.5 g/cm3), Europa (3.0 g/cm3), Ganymede (1.9 g/cm3), and Callisto (1.8 g/cm3).

Activity on Io


Two of the biggest telescopes on Earth have let astronomers capture some of the best views of long-term volcanic activity on Jupiter's moon Io.

Io is the most volcanically active body in the solar system. Using two of the world's largest telescopes, the 10-meter Keck II and 8-meter Gemini North, both located in Hawaii near the summit of Mauna Kea, astronomers from the University of California, Berkeley, tracked 48 volcanic hotspots over more than two years, from 2013 through 2015.

The telescopes' adaptive optics systems use adjustable mirrors to cancel out the blurring effects of Earth's atmosphere. That means the telescopes can tease out features just a few hundred miles apart on Io, which is never less than 365 million miles away (587 million kilometers) from Earth. Io itself is about 2,200 miles (3,540 km) across.

"Of Io's hundreds of active volcanoes, we have been able to track the 50 that were the most powerful over the past few years," said Katherine de Kleer, a UC Berkeley graduate student who led the observations, in a statement.

De Kleer and Imke de Pater, a UC Berkeley professor of astronomy and Earth and planetary science, observed the heat of active eruptions and cooling lava flows. They were able to determine the temperature and the total power output of individual volcanic eruptions on Io.

Io Facts