Io has fascinated astronomers since Galileo Galilei discovered it in 1610. In 1979, NASA's Voyager 1 spacecraft captured a volcanic eruption on the moon. Subsequent missions to Jupiter have discovered more plumes and lava lakes. Scientists now consider Io the most volcanically active world in the solar system, but concrete data on its eruptions have been limited.

In May and October 2023, Juno flew by Io, coming within approximately 21,700 miles (35,000 kilometers) and 8,100 miles (13,000 kilometers), respectively. JIRAM, among Juno's instruments, provided detailed observations of the intriguing moon.

JIRAM, designed to capture infrared light from Jupiter, also studied Io, Europa, Ganymede, and Callisto during Juno's extended mission. The JIRAM images of Io revealed bright rings around the floors of numerous hot spots.

"The high spatial resolution of JIRAM's infrared images, combined with the favorable position of Juno during the flybys, revealed that the whole surface of Io is covered by lava lakes contained in caldera-like features," said Alessandro Mura, a Juno co-investigator from the National Institute for Astrophysics in Rome. "In the region of Io's surface in which we have the most complete data, we estimate about 3% of it is covered by one of these molten lava lakes."

Fire-Breathing Lakes
JIRAM's data not only highlight Io's abundant lava reserves but also provide a glimpse into subsurface activities. Infrared images of several Io lava lakes show a thin ring of lava at the edges, between the central crust and the lake's walls. The recycling of melt is implied by the lack of lava flows on and beyond the rim, suggesting a balance between erupted melt and subsurface circulation.

"We now have an idea of what is the most frequent type of volcanism on Io: enormous lakes of lava where magma goes up and down," said Mura. "The lava crust is forced to break against the walls of the lake, forming the typical lava ring seen in Hawaiian lava lakes. The walls are likely hundreds of meters high, which explains why magma is generally not observed spilling out of the paterae and moving across the moon's surface."

JIRAM data suggest that most of these hot spots have a rocky crust that moves cyclically due to central magma upwelling. The crust touching the lake's walls causes friction, deforming and breaking, exposing the lava below.

Another hypothesis is that magma wells up in the middle of the lake, forming a crust that sinks along the rim, exposing the lava.

"We are just starting to wade into the JIRAM results from the close flybys of Io in December 2023 and February 2024," said Scott Bolton, principal investigator for Juno at the Southwest Research Institute in San Antonio. "The observations show fascinating new information on Io's volcanic processes. Combining these new results with Juno's longer-term campaign to monitor and map the volcanoes on Io's never-before-seen north and south poles, JIRAM is turning out to be one of the most valuable tools to learn how this tortured world works."

Juno executed its 62nd flyby of Jupiter, which included an Io flyby at an altitude of about 18,175 miles (29,250 kilometers), on June 13. The 63rd flyby of the gas giant is scheduled for July 16.

Research Report:Hot rings on Io observed by Juno/JIRAM

Related Links
Juno at NASA
The million outer planets of a star called Sol