Jupiter Moon Io: 20 Years of Research

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Io is a moon that orbits Jupiter, the largest planet in our solar system. It is known for its intense volcanic activity, which continuously releases volcanic gases into space

For over 20 years, a research team from Tohoku University and other institutions has been monitoring these emissions. They developed a small, custom-built 10-centimeter telescope made from various components for long-term observation and tracking changes. It is specifically designed for observing Io.

This approach is unfeasible with larger, more expensive telescopes or spacecraft. Moreover, it recently enabled the team to publish new findings in the American Astronomical Society journal.

Observing Io

With a diameter of about 3,600 kilometers, Io is similar in size to Earth's moon. It was the first extraterrestrial body where volcanic activity was discovered. Over 200 of its volcanoes are identified, bearing names like Loki, Pele, and Amaterasu. Io's intense volcanism is thought to result from Jupiter's powerful tidal forces, which cause the moon to expand and contract, generating internal magma.

Over the years, American spacecraft — including Voyager, Galileo, and Juno — have approached Io. Large telescopes on Earth have also contributed to its observation. However, these powerful instruments cannot focus solely on Io, as they are also needed for various other research purposes.

*Eruption activity at Io's Loki volcano (bluish-white area), captured by Voyager 1 (Courtesy of NASA/JPL/USGS).*

Sodium

In contrast to larger instruments, Tohoku University's small telescope is dedicated solely to observing Io. The research team primarily focuses on sodium, an element found in volcanic gasses released from Io into space.

Sodium is ejected alongside sulfur during large-scale, plume-like eruptions, where volcanic material spreads out above Io like an umbrella.

Through a process called resonant scattering, sodium atoms absorb and reemit sunlight, making them glow. This glow's brightness is proportional to the number of sodium atoms present — the more atoms, the brighter the glow. Although this light is too faint for the naked eye, an optical telescope equipped with a highly sensitive CCD camera can capture it.

The area where sodium disperses in space is vast, appearing from Earth to cover an area equivalent to ten full moons. Currently, the research team monitors a section of this region that covers about five full moons across, which includes Io.

Four Eruptions in Two Years

This small telescope, referred to as a wide-field sodium imager, sits atop Mount Haleakalā on Maui, Hawaii. Operating at an altitude of 3,050 meters since 2007, the current model is now in its third generation.

The latest findings are based on data collected over 26 months, from July 2017 to September 2019. During this period, the team detected evidence of four large-scale eruptions on Io. Professor Fuminori Tsuchiya of Tohoku University noted, "Volcanic activity was occurring more frequently than expected."

At the same time, NASA's Juno spacecraft observed auroras on Jupiter. Scientists think Io's volcanic activity may influence these auroras in some way. Professor Tsuchiya expressed optimism about future discoveries. "By comparing our data with Juno's, we may uncover a connection between Io's volcanic activity and Jupiter's auroras," he stated.

20 Years of Creative Observation

This small, custom-built telescope includes a CCD camera paired with a telephoto lens. It also has a mask to block Jupiter's bright light. The setup repurposes a second-hand telephoto lens and CCD camera from the lab, while the mask comes from a commercial neutral density filter. Together, they create a highly cost-effective alternative to spacecraft or large telescopes.

Mizuki Yoneda, now a lecturer at Sanyo Gakuen University, was involved in constructing the telescope during his time at Tohoku University.

Reflecting on the project, he remarked, "We crafted it with ingenuity. It's been in use for a long time and is highly cost-effective." Looking back on years of observations, he added, "We can monitor volcanic eruptions on Io from onset to decline. Some insights have only emerged after 20 years of accumulated data. This approach allows us to be the first to spot new findings," he added, highlighting the unique value of this long-term research.

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Read the article in Japanese.

Author: Shinji Ono

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