Juno observes lava lake on Io, provides insight into Jupiter’s water abundance


In December 2023 and February 2024, NASA’s Juno spacecraft, currently in orbit around and investigating Jupiter and the Jovian system, made several close flybys of the innermost of the Galilean moons, Io. During the flybys, Juno came as close as 1,500 kilometers from the surface of Io, during which extensive data and imagery of the moon were captured.

From the imagery and data, scientists were able to make the first up-close observations of the northern latitudes of Io, as well as sharp mountains and lava lakes. Furthermore, Juno’s recent flybys of Jupiter allowed scientists to refine their understanding of Jupiter’s polar cyclones and water abundance.

Io’s lava lakes and sharp mountains

Io is known to be one of the most extreme locations in the solar system, with the moon having the most geologic/volcanic activity of any planetary body in the solar system. Io’s immense volcanic activity has been recorded by several spacecraft, with images showing large plumes of sulfur and sulfur dioxide shooting as high as 500 kilometers above the surface.

Io’s volcanism is largely due to a two-to-one mean-motion orbital resonance with Europa, and a four-to-one mean-motion orbital resonance with Ganymede — meaning that Io completes two orbits of Jupiter with every orbit of Europa, and four orbits with every one orbit of Ganymede. These resonances expand and contract the surface of Io, which then allows Jupiter’s gravity to heat the interior of the moons, providing the heating needed for Io’s extreme geologic activity.

“Io is simply littered with volcanoes, and we caught a few of them in action. We also got some great close-ups and other data on a 200-kilometer-long (127-mile-long) lava lake called Loki Patera. There is amazing detail showing these crazy islands embedded in the middle of a potentially magma lake rimmed with hot lava. The specular reflection our instruments recorded of the lake suggests parts of Io’s surface are as smooth as glass, reminiscent of volcanically created obsidian glass on Earth,” said Scott Bolton, Juno’s principal investigator.

Using the Juno data, the team was able to create two simulations that visualize the characteristics of the lava lake, Loki Patera, and sharp mountains, one of which is Steeple Mountain.

Additional data from the flyby using Juno’s Microwave Radiometer instrument (MWR) shows that Io’s surface is relatively smooth compared to the surfaces of the other three Galilean moons, which are Ganymede, Callisto, and Europa. Furthermore, MWR also found that Io’s poles are colder than the moon’s middle latitudes.

Jupiter’s pole position

Juno successfully inserted itself into orbit around Jupiter in July 2016. The original mission plans had the spacecraft deorbit into Jupiter’s atmosphere after completing 32 orbits of Jupiter, where it would ultimately burn up and disintegrate. However, with the spacecraft remaining in good condition and all of its instruments still operating as expected, NASA awarded Juno and its team a mission extension in 2021 that would have the spacecraft complete 42 additional orbits of Jupiter. Juno is expected to complete its mission extension in September 2025.

Juno’s trajectory for its mission extension brings the spacecraft closer and closer to Jupiter’s north pole with each orbit. This trajectory allows for the MWR instrument to continuously improve its resolution of the planet’s north pole, which is filled with massive polar cyclones. The new data allows scientists to compare the poles and the cyclones in multiple wavelengths, and scientists have found that not all polar cyclones are created equally.

“Perhaps the most striking example of this disparity can be found with the central cyclone at Jupiter’s north pole. It is clearly visible in both infrared and visible light images, but its microwave signature is nowhere near as strong as other nearby storms. This tells us that its subsurface structure must be very different from these other cyclones. The MWR team continues to collect more and better microwave data with every orbit, so we anticipate developing a more detailed 3D map of these intriguing polar storms,” said Juno project scientist Steve Levin of NASA’s Jet Propulsion Laboratory in California.

Jupiter’s northern polar cyclones, seen in infrared by Juno. (Credit: NASA/JPL-Caltech/SwRI/MSSS)

Water abundance within Jupiter

Understanding water abundance within Jupiter is one of the primary science goals for Juno’s mission. However, the team isn’t searching for liquid water, but rather investigating the presence of oxygen and hydrogen molecules — the molecules that make up water — within Jupiter’s massive atmosphere. Getting an estimate of Jupiter’s water abundance is crucial for understanding the formation of the solar system and Jupiter.

Scientists believe that Jupiter was the first planet to form, which means it likely contains most of the gas, dust, and other cosmic material left over from the formation of the Sun and our solar system. Having insight into the abundance of different molecules and materials within the planet gives scientists the chance to record what materials were present during the formation of our solar system.

Water abundance, specifically, is important for Jupiter’s meteorology, including the flow of wind currents, and the planet’s internal structure. Scientists have been trying to measure Jupiter’s water abundance for decades, with NASA’s Galileo mission collecting one of the first datasets on Jovian water abundance in 1995 during the spacecraft’s 57-minute entry into Jupiter’s atmosphere at the end of its mission. However, Galileo’s data created more confusion than clarity, as the spacecraft found that the planet’s atmosphere was hot and void of water.

“The probe did amazing science, but its data was so far afield from our models of Jupiter’s water abundance that we considered whether the location it sampled could be an outlier. But before Juno, we couldn’t confirm. Now, with recent results made with MWR data, we have nailed down that the water abundance near Jupiter’s equator is roughly three to four times the solar abundance when compared to hydrogen. This definitively demonstrates that the Galileo probe’s entry site was an anomalously dry, desert-like region,” Bolton said.

The northern and equatorial regions of Jupiter, imaged by Juno during its 10th flyby of the planet. (Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill)

Juno’s new results on Jovian water abundance suggest very low water abundance—an unexpected result that scientists are still trying to understand. However, these results do support scientists’ theories that during the solar system’s formation, water-ice material was likely a driving force behind heavy element enrichment, the process by which chemical elements heavier than hydrogen and helium were accreted by Jupiter during its formation.

Additional data on Jovian water abundance collected by Juno during the remainder of its extended mission will help scientists compare Jupiter’s water abundance at polar regions and equatorial regions. Additional data will also help reveal the structure of the planet’s core.

Juno’s next flyby of Jupiter, the 61st of the mission, is planned for May 12.

(Lead image: Image of Io taken by Juno on October 15, 2023. Credit: NASA/JPL-Caltech/SwRI/MSSS/Ted Stryk)

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