See Io, our most volcanic moon, erupt like never before

In our Solar System, many planets are volcanically active.

This June 26, 1991 photo of the Mt. Pinatubo eruption in the Philippines shows an enormous volcanic ash cloud towering above the landscape. Although ash clouds are common, the possibility of a high-temperature ash cloud surge is potentially far more deadly and unpredictable.

Earth, Mars, and Venus all have active volcanoes, driven by internal processes.

These two images of the same region of the surface of Venus, taken by the Magellan spacecraft in 1990 and 1992, show evidence of a changing landscape: consistent with a volcanic eruption resurfacing and adding material to part of the imaged landscape depicted here. The resurfacing, or covering-over of previous craters, is extremely strong evidence for such a phenomenon. Io, however, was the second world in the Solar System (after Earth) that humanity discovered to be volcanically active.

But the most volcanically active world is Jupiter’s innermost large moon: Io.

When one of Jupiter’s moons passes behind our Solar System’s largest planet, it falls into the planet’s shadow, becoming dark. When sunlight begins striking the moon again, we don’t see it instantly, but many minutes later: the time it takes for light to travel from that particular moon to our eyes. Here, Io re-emerges from behind Jupiter, the same phenomenon that Ole Rømer used to first measure the speed of light, while Europa and Ganymede hover on the right.

Io experiences tidal forces ~10,000 times stronger than Earth’s.

When two objects of a finite size gravitationally attract one another, the gravitational force on different parts of the object are different from the average value. This effect causes what we see and experience as tidal forces, which can get extremely large at short distances. Jupiter’s moon Io, with a large physical size but located very close to our Solar System’s most massive planet, Jupiter, experiences the largest tidal forces in the Solar System: some 10,000 times stronger than Earth experiences due to our Moon and Sun combined.

This heats and cracks the giant moon, leading to rapid, frequent resurfacing events.

This 1997 Hubble Space Telescope image of Jupiter and its innermost Galilean moon, Io, shows a mound rising from Io’s surface at lower-right: an eruption from the volcano Pillan, which was thought to be dormant before this eruption was spotted. The ejecta consists of sulfur dioxide and moves at speeds of up to 2880 kph (1800 mph).

It’s the only known Solar System world without craters: evidence of its surface’s youth.

This image of Jupiter’s innermost Galilean moon, Io, comes courtesy of NASA’s Voyager 1 spacecraft and was taken on March 4, 1979, representing our first close-up view of this volcanically active world. Io is seen from a distance of about 862,000 kilometers (500,000 miles) here, exhibiting both explosive volcanic events that launch enormous plumes into the air as well as lava flowing across the surface from more mundane vents.

Two spacecraft from the 20th century imaged it up close: Voyager 1 and Galileo.

On July 3, 1999, NASA’s Galileo spacecraft made its closest-ever approach to Jupiter’s moon Io, coming within 130,000 kilometers (80,000 miles) of its surface. A series of images were taken, converted to false color, and tiled together to make this hemispherical mosaic of Io, with North on that world pointing “up” in this view. The resolution of this is 1.3 kilometers (0.8 miles) per picture, and was taken with the Solid State Imaging system. Just 25 years later, Io no longer looks like this in detail, as its rapidly changing surface topography is driven by volcanic activity and near-constant resurfacing.

Those images revealed a pock-marked surface: craterless but littered with volcanoes.

This NASA Galileo view of Jupiter’s moon Io is a curiosity, as it shows the enormous volcanic feature Pillan Patera, at the lower-left surrounded by an orange colored ring, with a black dark spot toward its upper-right that was not present just 5 months earlier. Rapid changes in the appearance of Io are due to its enormous amount of volcanic activity and constant resurfacing, which is why practically no craters ever exist on (or remain on) Io for very long.

As tidal forces form cracks, internal lava fills in any lowland regions.

This image shows a volcanic feature on Io, imaged up close by the Galileo spacecraft during 7 close encounters from 1995-2003. The yellowish color here is due to sulfur, while active lava flows can be seen unambiguously even from space. Tidally induced tectonics can theoretically support life just as well as plate tectonics can.

Active volcanoes create “hotspots” visible even from afar.

This image of Jupiter’s moon Io was taken from the ground here on Earth by the Large Binocular Telescope Observatory. Europa is the dark shadow seen transiting across the face of Io, while two bright hot spots, including the large Loki Patera at the top, represent the largest and brightest among Io’s volcanoes. Although this image was taken in 2015, the large Loki Patera still remains present today.

But the best views always come from up close.

This image illustrates NASA’s Juno mission, which sought to map out Jupiter among a variety of other science goals, on its extended mission: lasting from 2021-2025, where it is now working to find out details such as how Io’s volcanic engine works and whether a global magma ocean exists beneath Io’s rocky surface.

NASA’s Juno mission presently captures Io with increasing sharpness.

During different orbits of NASA’s Juno mission, closest approach to Jupiter, or perijove, takes it ever closer to Jupiter’s innermost Galilean satellite, Io, as well. On February 3, 2024, perijove (which will be #58) takes Juno to within its closest approach of Io of all: to within about 1500 km (930 miles) of its surface. The closer it comes to Io, the more detailed and higher-resolution its images are.

As Juno nears perijove — the closest point to Jupiter in its orbit — it frequently approaches Io.

This image of Io from NASA’s Juno spacecraft, taken in August of 2023, shows the largest volcanic caldera on Io in more than 20 years: Loki Patera. It is very likely that in under a century, none of the features presently on active Io will remain, as the surface is constantly being replaced and turned over with volcanically active violence.

At very close approach, you can make out Io’s surface features.

This monochrome image of Jupiter’s innermost large moon, Io, was taken with a green light filter aboard NASA’s Juno spacecraft. The right portion of the image is in direct sunlight, but the left portion is illuminated by reflected sunlight off of Jupiter: Jupiter-shine, just as the non-illuminated portion of our own Moon is still lit up by Earthshine.

What appears to be mountains, lowlands, and flow channels, however, are all driven by lava.

This color-balanced image of Jupiter’s moon Io, taken with NASA’s Juno spacecraft, shows the moon in close to true color, with volcanically active mountains, calderas, and features resembling lava flows all visible. Toward the lower-right of the image, just past the shadow line marking the day/night boundary, an erupting volcanic plume can be seen.

Features include:

This high-contrast color map of Io’s sun-lit hemisphere (to the right) showcases a variety of surface features, including high-peaks active volcanic mountains, deep, depressed volcanic calderas, and a wealth of surface features showing evidence of lava flows: both recent and ongoing. Most of Io’s surface is smooth, especially the lowland regions that have been filled in by new lava flows.

• volcanic mountain peaks,

Seen with the Sun illuminating it from the right, Io’s illuminated hemisphere contains shadows that extend off toward the left (and down) with respect to this orientation, showcasing the height of the mountainous regions highlighted here. Io, remarkably, shows no evidence of cratering, as its surface is constantly filled in by new lava flows across this world.

• depressed calderas,

This image showcases a few of the volcanically active regions on the surface of Io, as imaged by NASA’s Juno spacecraft. Calderas and surface discolorations due to recent and even current lava flows are prominent in this region of Io’s terrain.

• and cooled, solidified lava flows.

Although there are multiple instruments aboard NASA’s Juno mission, the infrared-sensitive JIRAM camera is most useful for identifying temperature differentials across this world’s surface, showcasing dozens of hot, glowing regions corresponding to current volcanic activity at the surface.

Through Juno’s eyes, all these still-hot features on Io are alight, volcanically glowing.

In May of 2023, during its 51st dive through perijove, NASA’s Juno spacecraft captured this series of views of its innermost large moon, Io, clearly showing the volcanic features alight on the surface of our Solar System’s most volcanically active moon.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.