Io, Jupiter’s fifth moon, is the most volcanically active body in the solar system. Io’s surface is dotted with hundreds of volcanoes, some of which spew sulfur plumes hundreds of kilometers into the air.
This volcanic moon is the third-largest Galilean moon of Jupiter and the closest to the planet. It is at the center of a gravitational tug-of-war between Jupiter and two nearby Jupiter moons, Europa and Ganymede. According to NASA, these tidal forces generate heat that is responsible for Io’s intense volcanic activity.
Io’s surface is changing at an incredible rate. Volcanic fissures pour lava onto the moon’s surface, filling impact craters and creating new floodplains of liquid rock. Although Io’s exact composition is unknown, NASA says it is likely molten sulfur and its compounds, or silicate rocks. Additionally, the moon’s thin atmosphere is made up mostly of sulfur dioxide.
Io’s surface is dotted with volcanoes and lava lakes. In this image, NASA’s Galileo spacecraft captured a picture of Io during a volcanic eruption. NASA/JPL/DLR
Io is about 4.5 billion years old, about the same age as Jupiter. Distance from Jupiter: Io is Jupiter’s fifth moon. Its average orbital distance is about 422,000 km (262,000 mi). Io orbits Jupiter every 1.77 Earth days. Io is tidally locked, so the same side always faces Jupiter.
Io’s average radius is 1,821.3 km (1,131.7 mi), making it slightly larger than Earth’s Moon. Io is slightly elliptical in shape, with its long axis pointing toward Jupiter. Among the Galilean satellites, Io is the third largest, behind Ganymede and Callisto but ahead of Europa, in both mass and volume.
Io’s average surface temperature is about minus 130 degrees Celsius (minus 202 degrees Fahrenheit), which results in sulfurous snowfields. However, Io’s volcanoes can reach temperatures of 1,649 degrees Celsius (3,000 degrees Fahrenheit). Io is often described as a celestial body of fire and ice.
According to the space science website The Nine Planets, at closest approach to Earth, when Jupiter and Earth are on the same side of the sun, the distance from Earth to Io may be as little as 588 million kilometers (365 million miles). However, at maximum distance, the distance may be as much as 968 million kilometers (601 million miles).
Io’s volcanic activity was first discovered by NASA’s Voyager missions in 1979. The moon’s volcanism is driven by powerful tidal forces.
Because Io orbits Jupiter in an elliptical orbit, Jupiter’s gravity varies depending on how close the moon is to the gas giant. This gravitational fluctuation creates a constant tug and pull on the moon’s interior in different directions, causing Io’s surface to bulge up to 100 meters (330 feet) high, according to NASA. This movement causes Io’s rocks to rub against each other, generating massive amounts of heat — 20 times more than Earth’s.
Io. NASA
If Io were Jupiter’s only moon, its orbit would likely have “stabilized” into a circle long ago, but the constant gravitational pull of Io’s outer neighbors Europa and Ganymede prevents this from happening. Io cannot escape this constant game of gravitational tug-of-war and the subsequent heating of the planet.
According to the European Space Agency (ESA), Io’s surface is mostly made up of sulfur and sulfur dioxide. Patches of sulfur dioxide frost have also been found on the surface, as well as hundreds of volcanoes.
Io’s sulfur dioxide atmosphere is extremely thin, with a surface pressure of about one billionth that of Earth’s atmosphere, according to the ESA.
Jupiter’s moon Io may be small (about the size of Earth’s moon) compared to the planet (more than 1,300 Earth-sized planets could fit inside Jupiter), but Io still has a huge impact on its parent planet, Jupiter.
Io’s orbit crosses Jupiter’s powerful magnetic field lines, turning Io into an electrical generator. According to NASA, Io can generate 400,000 volts along its path, creating an electrical current of 3 million amperes. That current then bounces back along Jupiter’s magnetic field lines, causing thunderstorms in the planet’s upper atmosphere.
Jupiter’s rotation causes magnetic forces to strip about a ton of material from Io every second. The material ionizes and forms a doughnut-shaped cloud of radiation called a plasma torus. Some of the ions are pulled into Jupiter’s upper atmosphere, causing the auroras. An example of this activity was spotted by the Hubble Space Telescope, which in 2018 found the influence of Io and another Jupiter moon, Ganymede, on Jupiter’s auroras. A 2022 study found new evidence of volcanoes on Io and electrical currents driving Jupiter’s auroras.
Io also has a collapsing atmosphere, according to observations from the Gemini North telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph (TEXES). The gaseous envelope of sulfur dioxide freezes as Io passes through Jupiter’s shadow each day.
A composite image of Jupiter’s auroras taken by the Hubble Space Telescope’s Spectrograph. NASA, ESA and J. Nichols (University of Leicester)
When Io returns to sunlight, the freezing sulfur dioxide turns back into gas. Scientists had long suspected this phenomenon, but it wasn’t until this study, which captured Io’s atmosphere in the dark for the first time, that researchers found supporting evidence.
Io was the first moon of Jupiter, discovered by Italian astronomer Galileo Galilei on January 8, 1610. He had discovered the moon the day before, but was unable to distinguish Io from Europa, another moon of Jupiter, until the following night, according to NASA.
This discovery, along with the discovery of three other moons of Jupiter, marked the first time a moon had been found orbiting a planet other than Earth. Galileo’s discovery eventually led to the understanding that the planets orbit the Sun, rather than our solar system orbiting the Earth.
Galileo first named this moon Jupiter I. In the mid-19th century, the moon was renamed Io. In Greek mythology, Io was a priestess of Hera (wife of Zeus) and the daughter of Inachus, king of Argos. Zeus (the equivalent of the Roman god Jupiter) fell in love with her, but turned her into a cow so that his wife Hera (or Juno) would not catch him with her.
Although no missions have been sent to Io, several spacecraft have flown by Jupiter and observed its moons. NASA’s Pioneer 10 was the first to arrive in 1973, followed by Pioneer 11 in 1974. NASA’s Voyager 1 and 2 probes returned to the system, taking spectacular photographs during their flybys in 1979.
Between 1995 and 2002, NASA’s Galileo spacecraft made several flybys of Io, giving us the closest images to date of the volcanic moon. In 2000, Cassini studied Io as it flew by Saturn.
In 2012, photographs were taken of Jupiter’s volcanic moon Io.
Two sulfur eruptions observed on Jupiter’s volcanic moon Io. Project Galileo, JPL, NASA
Io’s Volcano Plume. This frame from a five-frame series taken by NASA’s New Horizons mission captures the giant plume from Tvashtar on Jupiter’s moon Io. From this vantage point, only the top of the plume is visible. The plume’s source is 130 kilometers (81 miles) below the edge of Io’s disk, on the far side of the moon. New Horizons took this image during its flyby on March 1, 2007, en route to Pluto. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
Io and Europa meet again. This beautiful crescent image of volcanic Io and quieter Europa is a composite of two New Horizons images taken on March 2, 2007. Io steals the show with a beautiful display of volcanic activity. NASA Applied Physics Laboratory/Johns Hopkins University/Southwest Research Institute
Views of Io from Galileo. Observations by the Galileo spacecraft: On the left is a three-color global view of Io taken on July 3, 1999 (orbit 21) with a spatial resolution of 1.3 km per pixel. The corresponding infrared image on the right was taken on October 16, 2001, during daylight at a wavelength of 4.7 μm and has a spatial resolution of 30 km per pixel. The near-infrared image shows thermal emissions from active volcanoes. NASA/JPL/University of Arizona
Bright and young eruptions on Io. Observations of several bright and young eruptions on Jupiter’s moon Io, detected at short wavelengths (~2.1 mm) at the top and at long wavelengths (~3.2 mm) at the bottom since 2004, using the 10-meter WM Keck telescope (May 2004, August 2007, September 2007, July 2009), the 8-meter Gemini North telescope (August 2007), and the ESO 8-meter VLT-Yepun telescope (February 2007) and their adaptive optics systems. F. Marchis
Quiet Io activity observed in 2010 and 2011 showed a few quasi-permanent eruptions in the Lp band (at wavelengths of ~3 μm) [bottom] and no bright flares or young eruptions in the K band (at wavelengths of ~2 μm). F. Marchis
Simulations of Io observations using the WM Keck telescope and its current adaptive optics system, the next-generation adaptive optics system installed on the WM telescope (KNGAO), and the Thirty Meter Telescope (TMT) equipped with its adaptive optics system called (NFIRAOS). F. Marchis
The first global map of Io, Jupiter’s volcanic moon. This first-ever complete map of Jupiter’s volcanic moon Io, released March 19, 2012, was created using data and images from NASA’s Galileo spacecraft (which studied Jupiter and its moons from 1995 to 2003) and the Voyager mission in 1979. Color images from Galileo were superimposed on higher-resolution monochrome images. U.S. Geological Survey
Jupiter’s moon Io, as seen by NASA’s Galileo spacecraft. Io, the most volcanic body in the solar system, is visible in this composite image taken by NASA’s Galileo spacecraft in 1996. The smallest recognizable features are 2.5 kilometers (1.5 miles) across. NASA/JPL/University of Arizona
In 2020, it was discovered that volcanoes are what feed the gaseous atmosphere on Jupiter’s moon Io.
Io’s atmosphere is known to be mostly sulfur dioxide, but the source of this gas has long been a subject of debate. Io’s atmosphere is about a billion times thinner than our planet’s, allowing scientists to peer down into the moon’s surface, which is dotted with lava lakes, liquid rock, and sulfur ice. The work was described in a new study accepted for publication in the Planetary Science Journal
NASA’s Juno orbiter, scheduled to end in July 2021, has mostly observed Jupiter itself, so instead of using an orbiter, scientists have been studying Io with radio telescopes including the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, as well as optical and infrared telescopes like the Keck telescopes in Hawaii.
De Pater and her colleagues looked at 30 years of observations of the moon’s atmosphere and found that every 42 hours as Io orbits Jupiter, the moon’s atmosphere becomes highly unstable. In the new study, the scientists found that on March 20, 2018, when Jupiter eclipsed the moon, causing a temporary drop in temperature, sulfur dioxide levels appeared to drop sharply.
This composite image shows Jupiter’s moon Io against the backdrop of a Hubble Space Telescope image of Jupiter. A new study shows how sulfur dioxide in the moon’s atmosphere is produced by volcanic activity. ALMA (ESO/NAOJ/NRAO), I. de Pater et al.; NRAO/AUI NSF, S. Daniello; NASA/ESA
The researchers determined that the temperature drop likely caused the gas to essentially collapse back onto the moon’s surface. While Io’s average surface temperature is about minus 202 degrees Fahrenheit (minus 130 degrees Celsius), this temperature drop would have dropped it to minus 270 degrees Fahrenheit (minus 168 degrees Celsius), which is below the freezing point of sulfur dioxide.
The team further confirmed these findings when, in September 2018, after the Moon emerged from a cold eclipse, sulfur dioxide emissions returned to their previous levels. (This rebound happened quickly, in just 10 minutes.)
“As soon as Io gets into sunlight, the temperature rises and all that SO2 ice sublimates into gas, and the atmosphere recovers in about 10 minutes – faster than models predicted,” de Pater said.
The researchers also found evidence of “hidden volcanoes,” or volcanoes that do not emit smoke or other particles, by observing low levels of sulfur dioxide in the moon’s atmosphere. In July, the team also noted the presence of abundant sulfur dioxide in Io’s atmosphere, but only over active volcanoes.
Using this data and watching Io move in and out of eclipse, the team was able to determine how much of the moon’s sulfur dioxide is caused by volcanic activity. “When Io is in Jupiter’s shadow and out of direct sunlight, it’s too cold for the sulfur dioxide to condense on Io’s surface. At this time, we can only see sulfur dioxide that is of volcanic origin. So we can accurately determine how much of the atmosphere is affected by volcanic activity,” study co-author Statia Luszczyk of Columbia University said in the same statement.
The team hopes to continue studying Io, its atmosphere and volcanic activity to unravel the world’s mysteries, including the temperature of its lower atmosphere.
In 2021, the Juno spacecraft received a private radio transmission from Jupiter’s closest moon, Io, which has high volcanic activity.
NASA’s Juno spacecraft is “listening” to radio emissions from Jupiter’s volcanic moon Io, allowing researchers to figure out what’s causing the strange radio waves.
Of all the planets in the solar system, Jupiter has the largest and most powerful magnetic field, extending so far that some of the planet’s moons orbit within it. Because Io is closest to the planet, the moon is “caught in a gravitational tug-of-war” between Jupiter and the other two large moons, according to NASA. These opposing gravitational forces cause intense internal heating, which has led to hundreds of volcanic eruptions on the moon’s surface.
The volcanoes spew a ton of gases and particles into space every second, NASA said in a statement. Some of that material breaks down into electrically charged ions and electrons, which then rain down on Jupiter through the planet’s magnetic field. Electrons caught in the magnetic field accelerate toward Jupiter’s poles, generating what scientists call decameter radio waves (also known as decameter radio emissions, or DMEs) along the way.
This conceptual image shows Jupiter’s large, powerful magnetic field and how it links Io’s orbit to Jupiter’s atmosphere. NASA/GSFC/Jay Friedlander
When the spacecraft is in the right listening position, Juno’s Waves instrument can pick up those radio waves, said Yasmina Martos of NASA’s Goddard Space Flight Center. The researchers used Juno’s data to determine where in Jupiter’s massive magnetic field the radio emissions are coming from. The data sheds light on the behavior of the huge magnetic fields generated by gas giants.
The radio waves come from what can be described as a hollow cone, where conditions are ideal: a certain magnetic field strength and electron density, the research team said. The signal rotates like a lighthouse, and Juno only picks up the signal when the “light” hits the spacecraft, NASA said in a statement.
The radio data also showed that the electrons creating these radio waves were emitting huge amounts of energy, 23 times greater than the researchers expected. The team said these electrons could also be coming from other sources, such as the planet’s magnetic field or the solar wind.
In late 2022, the scientific world was waiting for a NASA probe to explore the solar system’s most active volcanic world, Jupiter’s Io. After studying Jupiter’s moons Ganymede and Europa, NASA’s Juno probe was preparing to set its sights on the third of the giant planet’s four major moons, the mysterious volcanic moon Io.
The Juno probe, which has been orbiting Jupiter since 2016, received commands from NASA mission control to image Io on December 15, 2022. Io, which is roughly the same size as the moon, was expected to remain Juno’s focus for the next year and a half as the probe made a total of nine flybys of the moon, including two that came within 930 miles (1,500 kilometers) of its surface. By comparison, the Orion spacecraft flew by the moon during the recently concluded Artemis 1 mission at a much closer altitude of just 80 miles (130 kilometers).
The flybys will allow scientists to “conduct the first high-precision monitoring campaign of the magma-covered moon Io, studying Io’s volcanoes and how volcanic eruptions interact with Jupiter’s powerful magnetosphere and auroras,” NASA said in a statement.
Juno has already taken a brief look at Io. In June 2022, the satellite passed the moon at a distance of about 80,000 km.
Jupiter’s volcanic moon Io, as captured by the Juno probe earlier this year. NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM
Io is very different from the ice-covered moons Ganymede and Europa, whose frozen surfaces are thought to harbor oceans of water that scientists believe could support primitive life.
Io’s surface, by contrast, is covered in lava lakes erupting from hundreds of volcanoes scattered across the moon’s surface. According to NASA, some of the lava geysers erupt tens of kilometers into the air. Although Io is unlikely to harbor life, unlike Ganymede and Europa, scientists are still eager to gain a detailed understanding of the moon, which is the most tectonically active body in the entire solar system.
The Io observations are part of Juno’s extended mission, which began in 2021.
“The team is very excited that Juno’s extended mission will include studying Jupiter’s moons,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute in San Antonio. “With each close flyby, we’ve learned a tremendous amount of new information.”
While Juno’s sensors were primarily designed to study the gas giant, they are also proving to be outstanding at studying Jupiter’s moons, Bolton added. The 2021 Ganymede flyby sparked a flurry of papers about the moon’s surface, magnetic field, interior structure, and interactions with Jupiter’s magnetosphere. The September 2022 Europa flyby provided the first-ever 3D view of the icy world’s frozen shell.
Measurements made with Juno’s Microwave Radiometer (MWR) allowed the team to peer beneath the ice sheets of Europa and Ganymede for the first time. The instrument was able to measure the structure, purity, and temperature of the moons’ water ice and their oceans at depths of up to 24 km.
A powerful, months-long volcanic eruption rocked Jupiter’s moon Io in 2022. Io is the most volcanic body in the solar system, producing emissions every year, the largest of which occurred in the fall of 2022.
A powerful volcanic eruption has been observed on Jupiter’s moon Io. The eruption was observed in the fall of 2022 using the Io Input/Output Observatory (IoIO) by Planetary Science Institute (PSI) Senior Scientist Jeff Morgentaler.
Io, one of Jupiter’s largest moons, is considered the most volcanic body in the Solar System, with its extreme conditions and annual volcanic outbursts caused by the enormous gravitational influence of its parent planet.
The gravity of Jupiter, the solar system’s most massive planet, and Jupiter’s two other large moons create powerful tidal forces inside Io. This stretches and squeezes Io, the innermost of Jupiter’s four large moons, causing violent volcanic activity.
A coronagraph image of sodium emissions from a volcanic eruption on Io. Image courtesy of the Planetary Science Institute’s Io Input/Output Observatory (IoIO). Jeff Morgentaler, PSI
The PSI-operated IoIO telescope is located near Benson, Arizona, and has been monitoring volcanic activity on Io since 2017. Using coronagraphic technology that dims the light coming from Jupiter, the instrument is able to image faint gases near the gas giant.
This allowed Morgentaler to notice an increase in the brightness of sodium in the cloud or “nebula” around Jupiter that began between July and September 2022 and did not end until 2023.
The ionized sulfur surrounding Jupiter in a doughnut shape called the Io plasma torus also brightened in the fall of 2022. However, this enhancement was less pronounced than the brightening of the Io plasma torus observed during previous outbursts.
“This could tell us about the composition of the volcanic activity that caused the ejecta, or it could tell us that the torus is more efficient at shedding material as more material enters it,” Morgentaler said in a statement.
These additional IoIOs, located at different locations around the globe, could help astronomers continue observing Jupiter’s moon from Earth during breaks caused by adverse weather conditions. More IoIOs could also provide more time to observe Io’s highly dynamic plasma torus and Jupiter’s sodium nebula.
In addition to studying Jupiter and the elements surrounding it, IoIO observes the sodium “tail” that follows Mercury and planets outside the solar system, called exoplanets, as they pass across the disks of their stars.
In 2022, it was revealed that Jupiter’s auroras are caused by a magnetic tug-of-war with volcanic eruptions on its moon Io. Jupiter’s auroras are caused by a cosmic tug-of-war game fueled by volcanoes on the planet’s innermost moon, Io, according to new research.
NASA’s Juno spacecraft and the Hubble Space Telescope have found new evidence that Jupiter’s rapid rotation and emissions of sulfur and oxygen from volcanoes on Io, the most volcanically active world in the solar system, create an electrical current system that drives the powerful auroras seen around the gas giant’s poles, the University of Leicester said in a statement. The findings were published on January 5, 2022, in the journal Geophysical Research: Space Physics.
“We’ve had theories for over two decades linking these electrical currents to Jupiter’s powerful auroras, and it was incredibly exciting to finally test them by finding this connection in the data,” said Jonathan Nichols, lead author of the study from the University of Leicester. “When we put them together, I nearly fell out of my chair seeing how obvious the connection was.”
An artist’s impression of the tug-of-war effect that occurs when ejected material from Jupiter moves along Jupiter’s magnetic field lines back towards the planet’s poles and through the planet’s upper atmosphere. Emma Bunce/Stanley Cowley/Jonathan Nichols/University of Leicester
Jupiter is more than 11 times wider than Earth and rotates on its axis every about 9.5 hours. Jupiter’s moon Io, which orbits at an average distance of about 422,000 kilometers (262,000 miles), is home to more than 400 active volcanoes that spew lava tens of kilometers into the air. The plumes fall into Jupiter’s orbit and are converted into electrically charged matter, or plasma, the statement said.
Juno’s Magnetic Field Investigation module, which measures Jupiter’s magnetic field from orbit, provides detailed insights into Jupiter’s outer plasma environment and the electric currents that flow through it, while Hubble’s spectrograph measures the brightness of Jupiter’s auroras.
“These exciting results from Jupiter’s auroral studies demonstrate the power of combining Hubble’s ground-based observations with Juno’s measurements,” said Scott Bolton, NASA’s Juno principal investigator. “The images provide the big picture, while Juno studies the object up close. Together, they make a great team!”
Jupiter’s rapid rotation pushes away much of the material ejected from it, and as the material moves outward, its rotation rate slows. However, Jupiter tries to keep this material spinning at the same rate as its own by using electrical currents flowing through the planet’s upper atmosphere and magnetosphere, the region influenced by the planet’s magnetic field, the statement said.
In turn, this creates an electromagnetic “tug of war” between the system of electrical currents and the material in the magnetosphere. As the material moves along Jupiter’s magnetic field lines back toward the planet’s poles, it passes through the planet’s upper atmosphere and interacts with gases, creating bright auroras.
“Finding this connection is very exciting because it helps us understand how the magnetic field works not only on Jupiter, but also on planets orbiting other stars for which we have previously applied the same theories, and now with new confidence,” Nichols said in a statement.
In 2023, NASA’s Juno probe took close-up photos of lava lakes on Jupiter’s moon Io. The infrared images from the solar-powered spacecraft have fueled debate about the inner workings of Jupiter’s hottest moon.
New data from NASA’s Juno probe has provided a more complete picture of the distribution of lava lakes on Jupiter’s moon Io and insight into the volcanic processes taking place there. These results were obtained thanks to the Jovian Infrared Auroral Mapper (JIRAM) instrument, provided by the Italian Space Agency and capable of “seeing” in the infrared range. On June 20, 2023, the researchers published a paper on the latest volcanic discoveries made by Juno in the journal Nature Communications Earth and Environment.
In May and October 2023, Juno flew past Io at distances of about 35,000 and 13,000 kilometers, respectively. Among Juno’s instruments that got a good look at this fascinating moon was JIRAM.
The JunoCam camera on NASA’s Juno spacecraft captured two volcanic plumes rising above the horizon of Jupiter’s moon Io. The image was taken on February 3 from a distance of about 3,800 kilometers (2,400 miles). NASA/JPL-Caltech/SwRI/MSSS, image processing by Andrea Luck (CC BY)
Designed to detect infrared light (invisible to the human eye) emanating from deep within Jupiter, JIRAM probes the atmosphere 50 to 70 kilometers (30 to 45 miles) beneath the gas giant’s cloud tops. However, during Juno’s extended mission, the mission team also used the instrument to study the moons Io, Europa, Ganymede, and Callisto. JIRAM’s images of Io revealed bright rings surrounding the bottom of numerous hot spots.
“The high spatial resolution of the JIRAM infrared images, combined with Juno’s favorable position during the flybys, revealed that Io’s entire surface is covered in lava lakes located in caldera-like features,” said Alessandro Mura, a Juno co-investigator at the National Institute for Astrophysics in Rome. “In the region of Io’s surface for which we have the most complete data, we estimate that about 3 percent of its surface is covered by one of these molten lava lakes.” (A caldera is a large depression formed when a volcano erupts and collapses.)
JIRAM data from the Io flyby not only demonstrates the moon’s abundant lava reserves, but also provides insight into what might be going on beneath the surface. Infrared images of several of Io’s lava lakes show a thin circle of lava at the boundary between the central crust that covers most of the lava lake and its walls. The lack of lava flows at the edge of the lake and beyond indicates melt recycling, suggesting a balance between melt erupting into the lava lakes and melt circulating back into the underground system.
Infrared data acquired on Oct. 15, 2023, by the JIRAM instrument on NASA’s Juno spacecraft shows Chors Patera, a lava lake on Jupiter’s moon Io. The team believes the lake is mostly covered by a thick molten crust with a hot ring around the edge where lava from Io’s interior is in direct contact with space. NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM/MSSS
“We now have an idea of what type of volcanism is most common on Io: huge lava lakes where magma moves up and down,” Mura said. “The lava crust is forced to break against the lake walls, forming the typical lava ring seen in Hawaiian lava lakes. The walls are likely hundreds of meters high, which explains why magma does not typically flow out of paterae” — bowl-shaped formations created by volcanic activity — “and move across the moon’s surface.”
JIRAM data suggests that much of the surface of Io’s hot spots is made up of rocky crust that cycles up and down as a single continuous surface due to a central upwelling of magma. According to this hypothesis, as the crust touches the lake walls, friction prevents it from sliding, causing it to deform and eventually collapse, exposing the lava just beneath the surface.
An alternative hypothesis remains valid: magma rises in the middle of the lake, spreads and forms a crust that drops along the edge of the lake, exposing lava.
“We’re just beginning to explore the results from JIRAM, which was collected during the close flybys of Io in December 2023 and February 2024,” said Scott Bolton, Juno principal investigator at the Southwest Research Institute in San Antonio. “The observations provide exciting new insights into Io’s volcanic processes. When combined with Juno’s long-term program to monitor and map volcanoes at Io’s previously unobserved north and south poles, JIRAM is becoming one of the most valuable tools we have for understanding how this volatile world works.”
In 2023, NASA’s Juno mission uncovered the heart of volcanic fury on Jupiter’s moon. Scientists working on NASA’s Juno mission to Jupiter have discovered that volcanoes on Jupiter’s moon Io are likely powered by their own chambers of bubbling hot magma, rather than a magma ocean. The discovery solves a 44-year-old mystery about the origins of the subterranean formations that power the moon’s most striking geologic features.
The paper on the source of Io’s volcanism was published in the journal Nature on December 12, 2023, and its findings, along with other scientific research on Io, were discussed at a press conference in Washington at the annual meeting of the American Geophysical Union, the country’s largest gathering of Earth and space scientists.
Roughly the size of the Moon, Io is known as the most volcanically active body in our solar system. The moon has about 400 volcanoes that continually spew lava and jets, which contribute to the formation of a crust on its surface.
Although the moon was discovered by Galileo Galilei on January 8, 1610, volcanic activity there was not discovered until 1979, when imaging specialist Linda Morabito of NASA’s Jet Propulsion Laboratory in Southern California first spotted a volcanic plume in an image taken by the agency’s Voyager 1 spacecraft.
“Ever since Morabito’s discovery, planetary scientists have wondered how the volcanoes were fed by lava from below the surface,” said Scott Bolton, Juno’s principal investigator at the Southwest Research Institute in San Antonio. “Was there a shallow ocean of hot magma feeding the volcanoes, or was their source more localized? We knew that data from Juno’s two very close flybys could give us some insight into how this tortured moon actually functioned.”
This five-frame sequence shows a giant plume erupting from Io’s Tvashtar volcano, extending 330 kilometers (200 miles) above the fiery moon’s surface. The footage was captured over eight minutes as NASA’s New Horizons mission flew past Jupiter in 2007. NASA/Johns Hopkins Applied Mathematics Laboratory/SwRI
The Juno spacecraft made extremely close flybys of Io in December 2023 and February 2024, coming within about 930 miles (1,500 kilometers) of its surface. During these encounters, Juno communicated with NASA’s Deep Space Network, acquiring high-precision dual-frequency Doppler data that was used to measure Io’s gravity by tracking its effect on the spacecraft’s acceleration. The mission’s data on the moon’s gravity during these flybys led to a new paper revealing more details about the effects of a phenomenon called tidal flexing.
Io is extremely close to the giant planet Jupiter, completing a full orbit around the gas giant in an elliptical orbit every 42.5 hours. As the distance changes, so does Jupiter’s gravitational pull, causing the moon to inexorably shrink. The result: an extreme case of tidal flexing—tidal friction that generates internal heat.
“This constant flexing creates a tremendous amount of energy that literally melts parts of Io’s interior,” Bolton said. “If Io has a global magma ocean, we knew that its tidal deformation would be much greater than that of a more rigid, mostly solid interior. So depending on how Juno probes Io’s gravity field, we might be able to determine whether there’s a global magma ocean lurking beneath its surface.”
The Juno team compared the Doppler approach data from the two flybys with observations from previous Juno missions and ground-based telescopes. They found tidal deformation consistent with the absence of a shallow global magma ocean on Io.
“Juno’s discovery that tidal forces don’t always create global magma oceans doesn’t just prompt us to rethink what we know about Io’s interior,” said lead author Ryan Park, a Juno co-investigator and head of the Solar System Dynamics Group at JPL. “It has implications for our understanding of other moons like Enceladus and Europa, and even exoplanets and super-Earths. Our new findings offer the opportunity to rethink what we know about planet formation and evolution.”
In 2023, the Juno mission was making its 51st flyby of the gas giant Jupiter and its moons when it captured stunning images of the ‘ragged moon’ Io. NASA’s Juno mission made its 51st close flyby of Jupiter, capturing stunning images of its moon Io.
Jupiter and Io as seen by NASA’s Juno probe during a close flyby on May 16, 2023. NASA/JPL-Caltech/SwRI/MSSS Image processing: Kevin M. Gill (CC BY)
On May 16, 2023, Juno flew past Io, coming closer to Jupiter’s volcanic moon than ever before. The spacecraft passed within 35,500 kilometers (22,060 miles) of the moon’s surface.
If Jupiter’s moon looks placid in these images (which NASA’s Planetary Science Division tweeted May 18), that appearance could be deceiving. Io isn’t just the most volcanic moon orbiting a gas giant; it’s also the most volcanic world in the entire solar system.
The Juno spacecraft observed Jupiter’s moon Io in visible and infrared light during its flyby on May 1, 2023, to capture this composite image showing hot spots on the surface of the solar system’s most volcanically active world. NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM
Some of the detailed visible-light images Juno and its JunoCam camera captured show Io’s reddish surface, pockmarked by volcanoes. An infrared image of the moon taken earlier during a May 1 flyby by JIRAM (Jovian InfraRed Auroral Mapper) shows the source of these scars, revealing multiple pockets of volcanic activity on the moon’s surface.
NASA estimates that Io, Jupiter’s third-largest moon and the gas giant’s fifth-largest moon, is home to hundreds of actively erupting volcanoes located about 422,000 kilometers (260,000 miles) from its surface. These volcanoes can shoot lava tens of kilometers (miles) into Io’s thin, waterless atmosphere, which itself is roughly the size of Earth’s moon.
This photo of Io was taken by NASA’s Juno probe during its close flyby of Jupiter on May 16, 2023. NASA/Kevin M. Gill
Jupiter’s large moons will be closely monitored further when the JUpiter Icy Moon Explorer (JUICE) arrives in 2031. Juice will study the oceanic moons Europa and Ganymede to see if the conditions are right for life.
In early 2024, NASA’s Juno probe detected active volcanic eruptions on Jupiter’s volcanic moon Io. The images of the solar system’s most volcanic body were taken as Juno approached Io closer than any other spacecraft in the past 20 years.
NASA’s Juno spacecraft has made its second flyby of Jupiter’s moon Io, capturing the solar system’s most volcanic body in stunning detail. The spacecraft also allowed camera operators to see a side of Io they hadn’t seen in 35 years.
NASA’s Juno probe has captured stunning images of Jupiter’s volcanic moon Io (left), including close-ups of jets erupting from its surface (right). (left) NASA/JPL-Caltech/SwRI/MSSS. Image processing: Emma Välimäki; (inset) NASA/JPL-Caltech/SwRI/MSSS/AndreaLuck
The images of Io were taken on February 3, 2024, when Juno came within 930 miles (1,500 kilometers) of the surface of Jupiter’s moon.
The close approach, which matched Juno’s final flyby of Io on December 30, 2023, allowed the spacecraft’s JunoCam instrument to capture several of the moon’s volcanic features, including actively erupting plumes, lava lakes, and high mountain peaks, as well as the shadows they cast on Juno’s surface.
One of Juno’s most dramatic images was taken at the end of the flyby on February 4, showing two plumes emanating from the volcanic moon. The plumes were visible when the spacecraft was 3,852 kilometers (2,393.5 miles) above Io, according to NASA.
The image of two plumes, the source of which is not shown, indicates ongoing volcanic activity on this Jupiter moon.
Volcanic emissions from Io, captured in this image by Juno on February 4, 2024. NASA/JPL-Caltech/SwRI/MSSS/AndreaLuck
Io is about the size of Earth’s Moon, but unlike the Moon, it is dotted with hundreds of actively erupting volcanoes that can spew material tens of miles into the Jupiter moon’s thin, waterless atmosphere.
In another image of Io taken immediately after the flyby, this time on February 5, the Jupiter moon looks truly psychedelic. The image was created using data from multiple filters, and Io’s dark left side glows with greens and reds, while the illuminated right side takes on a corresponding fiery orange hue.
Several fractal filters/layers show IO rendered in psychedelic colors. NASA/SwRI/MSSS/MikPetter
Another image taken by JunoCam on February 5 shows the side of Io that is permanently facing Jupiter, the “sub-Jovian hemisphere.”
For the first time since Voyager 1’s flyby of the Jupiter system in March 1979, Io’s sub-Jovian hemisphere has been shown in detail. NASA/SwRI/JPL/MSSS/Jason Perry
This is the first time Io’s sub-Jovian hemisphere has been observed since the Voyager 1 spacecraft flew through the Jupiter system in 1979, according to NASA.
In this image, the sub-Jovian hemisphere is illuminated by light reflecting off Jupiter’s cloud tops. This illumination allowed JunoCam to capture a series of changes on Io’s surface caused by volcanic activity.
This includes a change in the shape of the complex flow field in the Kanehekili region at the left center of the image, which contains two persistent hot spots and an active volcanic plume. Just east of Kanehekili, named for the Hawaiian god of thunder, Juno discovered a new lava flow.
NASA’s spacecraft, now in its third year of an extended mission after its primary mission ended in July 2021, will never get as close to Io as it did this time. Each subsequent flyby after February 3 will bring it closer and closer to the volcanic moon.
At its next close approach, Juno will be at an altitude of about 10,250 miles (16,500 kilometers), and the final flyby will bring NASA’s spacecraft within about 71,450 miles (115,000 kilometers) of the volcanic moon.
Juno’s extended mission to explore Jupiter and its moons will last until September 2025, after which NASA will deliberately send the probe into the gas giant’s atmosphere.
In 2024, NASA’s Juno probe revealed an aerial view of a mountain and lava lake on Io. Images from the solar-powered spacecraft offer a close-up look at intriguing features of Jupiter’s hellish moon.
Scientists on NASA’s Juno mission to Jupiter have transformed data collected during two recent flybys of Io into animated movies showing two of the Jovian moon’s most striking features: a mountain and a nearly glassy lake of cooling lava. Other recent scientific results from the solar-powered spacecraft include polar cyclones and the abundance of water on Jupiter.
The new results were announced on April 16, 2024, by Juno Principal Investigator Scott Bolton during a press conference at the European Geosciences Union General Assembly in Vienna.
In December 2023 and February 2024, Juno made extremely close flybys of Io, coming within about 930 miles (1,500 kilometers) of the surface and obtaining the first close-ups of the moon’s northern latitudes.
“Io is literally dotted with volcanoes, and we captured several of them in action,” Bolton said. “We also got some great close-up images and other data from a 200-kilometer (127-mile) lava lake called Loki Patera. These crazy islands, located in the middle of a potentially magmatic lake surrounded by hot lava, show amazing detail. The specular reflections our instruments captured suggest that parts of Io’s surface are as smooth as glass, reminiscent of volcanic obsidian glass on Earth.”
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Maps created from data collected by Juno’s Microwave Radiometer (MWR) show that Io not only has a relatively smooth surface compared to Jupiter’s other Galilean moons, but its poles are also cooler than mid-latitudes.
During the extended Juno mission, the spacecraft gets closer to Jupiter’s north pole with each flyby. The change in orientation allows the MWR instrument to improve the resolution of Jupiter’s north polar cyclones. The resulting data allows the poles to be compared at different wavelengths, showing that not all polar cyclones are created equal.
“Perhaps the most striking example of this discrepancy is the central cyclone at Jupiter’s north pole,” said Steve Levin, Juno project scientist at NASA’s Jet Propulsion Laboratory in Southern California. “It’s clearly visible in both infrared and visible images, but its microwave emission is nowhere near as strong as other nearby cyclones. This tells us that its subsurface structure must be very different from that of other cyclones. The MWR team continues to collect more and more microwave data with each orbit, so we plan to develop a more detailed 3-D map of these interesting polar cyclones.”
One of the mission’s primary scientific goals is to collect data that will help scientists better understand the abundance of water on Jupiter. To do this, the Juno science team is not looking for liquid water. Instead, they are aiming to quantify the presence of oxygen and hydrogen molecules (the molecules that make up water) in Jupiter’s atmosphere. An accurate estimate is critical to unraveling the mystery of how our solar system formed.
This animation, created using data collected by the JunoCam camera aboard NASA’s Juno spacecraft during flybys in December 2023 and February 2024, is an artist’s impression of a feature on Jupiter’s moon Io that the mission’s science team has nicknamed “Spike Mountain.” NASA/JPL-Caltech/SwRI/MSSS
Jupiter was likely the first planet to form, and it contains most of the gas and dust that did not make up the Sun. The abundance of water also has important effects on the gas giant’s meteorology (including the direction of Jupiter’s winds) and its internal structure.
In 1995, NASA’s Galileo probe provided the first data on the amount of water on Jupiter during the spacecraft’s 57-minute descent into its atmosphere. But the data raised more questions than they answered, revealing that the gas giant’s atmosphere was unexpectedly hot and, contrary to computer models, waterless.
“The probe did amazing science, but its data were so different from our models of Jupiter’s water abundance that we considered the site where it sampled to be an anomaly. But before Juno, we couldn’t confirm that,” Bolton said. “Now, with recent results from MWR data, we’ve determined that the abundance of water near Jupiter’s equator is about three to four times greater than the solar abundance of hydrogen. This definitively proves that the Galileo probe’s entry site was an anomalously dry, desert-like region.”
This animation is an artist’s concept of Loki Patera, a lava lake on Jupiter’s moon Io, created using data from the JunoCam camera on NASA’s Juno spacecraft. Loki, with its many islands inside, is a basin filled with magma and surrounded by molten lava. NASA/JPL-Caltech/SwRI/MSSS
The results support the idea that during the formation of our solar system, water ice may have been a source of enrichment of heavy elements (chemical elements heavier than hydrogen and helium accreted by Jupiter) during the formation and/or evolution of the gas giant. The formation of Jupiter remains a mystery, as Juno’s studies of the gas giant’s core indicate very low water content, a mystery that scientists are still trying to solve.
Data obtained during the remainder of Juno’s extended mission could prove useful because it will allow scientists to compare the abundance of water on Jupiter near the polar regions with the equatorial region and shed further light on the structure of the planet’s tenuous core.
NASA’s Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, California, manages the Juno mission for Scott Bolton, principal investigator at the Southwest Research Institute in San Antonio. Juno is part of NASA’s New Frontiers program, which is managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. The Italian Space Agency (ASI) funded the Jovian Auroral Infrared Mapper project. The spacecraft is built and operated by Lockheed Martin Space in Denver.
In 2024, NASA’s Juno probe discovered a huge new volcano on Jupiter’s moon Io. Comparisons with Galileo data showed the presence of new huge volcanoes and lava flows.
Researchers have discovered a huge new active volcano on Jupiter’s moon Io by comparing images taken by two NASA missions more than a quarter of a century apart.
The images of the new volcano were taken by NASA’s Juno spacecraft and its JunoCam camera as it flew past Io, the solar system’s most volcanic body, on February 3 this year. The images were taken on Io’s night side, illuminated only by reflected sunlight from Jupiter.
A side-by-side comparison of JunoCam data from February 2024 with images of the same region taken by the Galileo spacecraft in November 1997 reveals a new volcanic structure on the surface of Jupiter’s moon Io. NASA/JPL-Caltech/SwRI/MSSS
A comparison with images of the same area south of Io’s equator taken by the Galileo spacecraft in November 1997 showed that there were no previous volcanic features in that location, meaning that the massive object and its aftermath appeared at some point within the last 27 years.
The results of the study confirming Io’s volcanic activity were presented at the European Planetary Science Congress (EPSC) in Berlin.
“Our latest JunoCam images show many changes on Io, including this large and complex volcanic feature that appears to have formed out of nothing since 1997,” Michael Ravine, advanced project manager at Malin Space Science Systems, Inc., which designed, developed and operates JunoCam for NASA’s Juno mission, said in a statement to Europlanet.
The JunoCam image shows a reddish area on the volcano’s eastern side, thought to be sulfur that was initially ejected into space and then returned to Io’s surface. To the west are two dark lava flows about 100 kilometers long, surrounded by two gray, circular deposits.
A comparison of JunoCam data from February 2024 with images of the same region taken by the Galileo spacecraft in November 1997 (grayscale inset) reveals a new volcanic structure on the surface of Jupiter’s moon Io. NASA/JPL-Caltech/SwRI/MSSS/Europlanet
JunoCam recorded a total of nine plumes associated with active volcanic features on Io, as well as other changes such as new lava flows and other surface deposits, during three flybys in 2023 and 2024, according to the statement.
Juno was launched in 2011 and reached the Jupiter system in 2016. The spacecraft completed its primary mission in July 2021 but has an extended mission through September 2025.
In 2025, NASA’s Juno spacecraft observed the most powerful volcanic event ever observed on a moon of Jupiter. Even for Jupiter’s moon Io, the most volcanic body in the solar system, the recent volcanic event witnessed by NASA’s Juno spacecraft exceeded all expectations.
The event was just one eruption centered on a newly discovered hot spot on Io that is estimated to be larger than Lake Superior on Earth. The as-yet-unnamed region is so powerful that it produces six times the energy produced by all of Earth’s power plants combined.
In this annotated image taken by the JIRAM infrared imager aboard NASA’s Juno spacecraft on Dec. 27, 2024, as it flew past the moon of Jupiter, a huge hot spot larger than Earth’s Lake Superior can be seen to the right of Io’s south pole. NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM
The team behind these discoveries used data collected by Juno, which flew by Io in December 2023 and February 2024, coming within about 1,500 kilometers of the Jovian moon’s surface. In particular, a large volcanic hotspot was spotted during Juno’s final flyby of Io on December 27, 2024, when the spacecraft came within 74,400 kilometers of Io’s surface and aimed its infrared instrument at the moon’s southern hemisphere.
“During its extended mission, Juno made two very close flybys of Io,” said Scott Bolton, the mission’s principal investigator, of the Southwest Research Institute. “And while each flyby provided data about the ailing moon that exceeded our expectations, the data from this final — and farther — flyby simply blew us away.”
The reason Io is so volcanic has much to do with its parent planet, Jupiter, the largest world in the solar system. As Io orbits the gas giant every 42.5 hours, Jupiter’s immense gravity creates powerful tidal forces that interact with the giant moon. This compresses and squeezes Io, causing the moon’s interior to heat up due to friction. The result is molten lava that erupts onto Io’s surface through some 400 volcanoes that dot the moon. This lava carries with it plumes of gas and ash that fill Io’s atmosphere.
Images of Io taken in 2024 by the JunoCam camera on NASA’s Juno spacecraft show large and noticeable surface changes (arrows) near the south pole of Jupiter’s moon. NASA/JPL-Caltech/SwRI/MSSS Image processing: Jason Perry
What’s more, the lava eruptions produce searing heat that Juno can observe from space in infrared light using its Jovian Infrared Auroral Mapper (JIRAM) instrument. JIRAM allows scientists to collect data on Io’s “weather layer,” located about 50 to 70 kilometers (30 to 45 miles) below the giant moon’s southern cloud tops.
This led to the discovery of a previously unknown region of Io, which is subject to extreme volcanic activity over an area of 40,000 square miles (100,000 square kilometers).
“JIRAM detected an extreme infrared emission event — a huge hot spot — in Io’s southern hemisphere that was so intense that it filled our detector,” said Alessandro Mura, a Juno scientist at the National Institute for Astrophysics in Rome. “However, we have evidence that we actually detected several hot spots close together emitting radiation at the same time, indicating the presence of a vast system of magma chambers beneath the surface. The data confirm that this is the most intense volcanic eruption ever recorded on Io.”
Juno didn’t just capture this turbulent region in infrared light. It was also photographed by the JunoCam spacecraft, which operates in visible light. This allowed the team to compare what the region looked like during the latest flyby with images from the two previous flybys of Io. They found changes in the coloration of the surface around the region, associated with volcanic activity and the development of hot spots.
This image shows Io and the Juno spacecraft, looking down at its south pole and the largest volcanic eruption in the history of the solar system, marked by a ring. Robert Lee (created with Canva)/NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM
Eruptions of this size and intensity are associated with different long-term characteristics. Other large eruptions on Io have left behind traces of rock debris ejected by volcanoes, small lava flows from fissures, and volcanic deposits rich in sulfur and sulfur dioxide.
“While it’s always exciting to see events that rewrite history, this new hotspot has the potential to do much more,” Bolton said. “This intriguing feature could improve our understanding of volcanism not only on Io, but on other planets as well.”
In 2025, scientists suggested that Earth’s Moon may have once been as hellish as Jupiter’s supervolcanic moon Io.
The Moon was a volcanic desert for millions of years, covered in constant eruptions from mountains and even the Earth itself. New research suggests that the Moon’s orbit could have turned it into a molten monster over tens of millions of years. The result could be comparable to Jupiter’s moon Io, the most volcanic body in the solar system.
Early in the history of the solar system, a massive protoplanet slammed into the young Earth. The collision mixed matter and then formed two separate bodies that would become the Earth and the Moon. They orbited almost on top of each other, but over time the Moon slowly drifted away, solidifying as it moved.
Eventually, the Sun’s gravity began to pull on the Moon more strongly. Even though it was farther away from the Sun, both bodies pulled on the Moon equally.
The resulting gravitational swirl stirs the Moon’s interior, melting rocks and creating a layer of magma that erupts onto the surface.
A new study suggests that the moon’s surface may have been resurfaced by lava flows about 4.35 billion years ago. The lava flows would have filled all existing craters and helped restore the age measured from lunar rocks and lunar zircons.
Determining when the Moon formed is difficult enough. There are a number of methods, from studying samples brought back to Earth to studying zircons in those rocks to trying to figure out how the planets and their moons moved billions of years ago. But all of these methods can give conflicting answers.
Determining the age of lunar rocks was one of the first ways scientists tried to determine the age of the moon. Samples returned by NASA’s Apollo program were studied and measured. Today, new samples continue to arrive, now from China’s ongoing Chang’e program. These are new objects to study and test.
An illustration of the young Earth-Moon system. Mark Garlick/Science Photo Library/Getty Images
The second piece of evidence for the moon’s age comes from zircons found in the rocks themselves. Zircons are silicate minerals found in rocks on both Earth and the moon, and are among the oldest objects in the solar system. Although rocks can melt and reform, zircons themselves are strong enough to resist melting at all but the highest temperatures.
Both dating methods can be challenging. Zircons are more accurate, but require incredibly precise measurements, so scientists have only been able to study them in the last two decades. Moon rocks, on the other hand, are harder to interpret and give different ages. Together, the two methods suggest that the moon is about 4.35 billion years old.
Scientists also rely on dynamic models. Studies of Earth show that it accumulated a lot of iron-related metals after it formed its iron core. These metals settled in the mantle, making up about half a percent of Earth’s mass after the core was complete, and are called the late mantle. But these metals would have been consumed while the material was still floating around in the chaotic, cluttered early solar system, and by 4.35 billion years ago, the material was gone.
With new results published in Nature in late 2024, Nimmo and his colleagues suggest that the moon’s 4.5 billion-year-old age is consistent with measurements of younger lunar rocks. When the moon was in a tug-of-war between Earth and the sun, melting would have caused lunar rocks and zircons to melt and reform, yielding a younger age that doesn’t necessarily reflect the birth of the moon itself.
Over the course of tens of millions of years, volcanic activity could have blanketed the moon. But it would not necessarily have manifested itself in the form of giant eruptive mountains. On Earth, volcanic material is rich in silicate, which thickens the lava and allows it to accumulate into giant mountains. On the moon, the lava was probably thinner and seeped through the crust, spreading out over its edges rather than accumulating on itself.
“There was certainly volcanic activity, but there were no volcanoes that we know of,” Nimmo said.
The constant seepage may have prevented the magma ocean layer from forming because the newly formed liquid was not trapped in the mantle. As it melted and became more buoyant, it would have seeped upward into the crust. Some of it would have burst out, but some might have simply filled in, forming outward troughs similar to those seen on Earth.
At that time, Earth was much closer to the moon than it is today and would probably have been twice as big in the lunar sky. “There would have been glowing lava flows and maybe some volcanic eruptions everywhere,” Nimmo said.
According to Nimmo, a state like Io’s could only last a few tens of millions of years.
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