JunoCam image of Jupiter’s moon Io and the volcanic plume along the day-night boundary.
Image: NASA/SwRI/MSSS

While performing its 17th flyby of Jupiter, NASA’s Juno spacecraft witnessed a volcanic plume erupting from the surface of Io, the most geologically active of the gas giant’s 79 known moons.

As detailed in a Southwest Research Institute press release, the flyby occurred on December 21, 2018. Mission controllers had no less than four instruments honed in on Io in an effort to study the moon’s surface, especially its polar regions. These instruments included the JunoCam, the Stellar Reference Unit (SRU), the Jovian Infrared Auroral Mapper (JIRAM), and the Ultraviolet Imaging Spectrograph (UVS). An hour was budgeted for the survey, and it just so happened that a volcanic eruption occurred during this time.

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NASA’s Galileo spacecraft captured a volcanic explosion on Io back in the late 1990s.
Image: NASA/JPL/University of Arizona

NASA’s Galileo probe captured evidence of volcanic activity on Io over 20 years ago, but this is the first time that Juno has done the same. It was a fortuitous event, to be sure—but not altogether unexpected. Io is the most geologically active object in the Solar System, with its surface and interior regions subject to Jupiter’s intense gravitational influence. Io has more than 400 active volcanoes and much of its surface is bathed in lava.

The natural-light photo captured by the JunoCam (top image) was taken just as Io was beginning to drift into Jupiter’s shadow. The bright plume can be seen along the day-night boundary. Juno was around 300,000 kilometers (186,000 miles) from Io when the photo was taken.

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“The ground is already in shadow, but the height of the plume allows it to reflect sunlight, much like the way mountaintops or clouds on the Earth continue to be lit after the sun has set,” Candice Hansen-Koharcheck, the JunoCam lead from the Planetary Science Institute, explained in the SwRI statement.

Image showing Io soon after it was eclipsed by Jupiter. The brightest spot is likely a penetrating radiation signature, while the other glowing regions show activity from several of Io’s volcanoes. The smaller circled image is the volcanic plume.
Image: NASA/JPL-Caltech/SwRI

Some 40 minutes later, Io was totally immersed in Jupiter’s shadow. Reflected sunlight from the nearby moon Europa allowed for further observations with Juno’s SRU camera (shown above), which works optimally in low-light conditions. In addition to detecting Io’s intense radiation signature, the SRU camera caught evidence of activity from several other volcanoes on Io’s roiling surface.

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A third image taken by JIRAM shows Io’s hot spots.

JIRAM image showing hot temperature regions on Io.
Image: NASA/JPL-Caltech/SwRI/INAF

“Though Jupiter’s moons are not JIRAM’s primary objectives, every time we pass close enough to one of them, we take advantage of the opportunity for an observation,” Alberto Adriani, a researcher with Italy’s National Institute of Astrophysics, told SwRI. “The instrument is sensitive to infrared wavelengths, which are perfect to study the volcanism of Io. This is one of the best images of Io that JIRAM has been able to collect so far.”

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With this data, scientists can now investigate the possible ways in which Jupiter’s other large moons influence surface activity on Io, and how volcanic activity on Io may be influenced by its host planet during an eclipse.

We’re now halfway into the Juno mission, which continues to astound. The probe orbits Jupiter every 53 days, and it will continue to do so for the next three years. More of this, please!