Io, the innermost large moon of Jupiter, is about the same size and density as Earth's Moon. Io is the most volcanically active body known in the Solar System. Eruptions are so common and so large that the entire surface can be buried under 100 meters of material every 1 million years (it takes submarine volcanoes about 80 million years to resurface about two-thirds of the Earth). Impact craters, which are common on many planets and moons, are absent on Io because of the frequent volcanic eruptions bury them.

These enhanced (false) color views of Io highlight details of the surface. Some areas on Io are truly red and are closely associated with very recent explosive eruptions and volcanic plumes. The most prominent red oval surrounds the volcano Pele (far right). Galileo images courtesy of NASA's Jet Propulsion Laboratory. The following pages provide an overview of many aspects of Io's volcanoes.

Inside Io

Io has differentiated into layers. Understanding the number and composition of the layers is still being studied. Most scientist agree there is a core surrounded by mantle. A simple 2-layer (metallic core and a silicate mantle) model suggests the core is about 17-20 percent of Io's mass and has a radius of about half of Io's radius (Anderson et al. 1996). Another model (that assumes a pure iron core) suggests the core is 11-14 percent of Io's mass and has a radius of about one-third of Io's radius. Schubert (1997) used a three-layer model to suggest the presence of a thick (100-250 km) outer layer on Io. Several lines of evidence indicate that Io's metallic core is at least partly molten. Some models call for Io to have silica-rich crust about 40-60 km thick. The crust would be made of alkali-rich minerals, probably feldspars and nepheline. Much of the mantle may be pure forsterite (magnesian-rich olivine).

The enormous gravitational forces of Jupiter cause heating within Io. Most of this heating is concentrated in the asthenosphere, estimated to be 50-100 km thick. Additional heating occurs deep in the mantle. Melting is probably located at the base of the lithosphere. Based on the amounts on energy released by Io's volcanoes each part of the interior has probably been remelted at least 100 times over the satellite's history.



Two Galileo images of Io. These images reveal that the topography is very flat near the active volcanic centers such as Loki Patera (the large dark horseshoe-shaped feature near the top right edge in the left-hand image) and that a variety of mountains and plateaus exist elsewhere. Image courtesy of NASA/JPL.

The surface of Io has three common features:


  • Mountains

    The mountains of Io are rugged and isolated. They are separated by the plains and the mountains and plains cover about 2% of the surface. Individual mountains can be up to 100 km long and have relief of 9 km. The mountains do not appear to be volcanic in origin (although they are mantled in sulfur) and are thought be older than both the plains and volcanoes. The mountains have been modified by both tectonic and erosional processes. The presence of the mountains suggest Io has a rigid lithosphere, possibly up to 30 km thick.


  • Plains

    About 40% of Io is covered by plains with low relief and light and dark areas. The plains are probably layers of pyroclastic material erupted from volcanoes and possibly lava flows of different compositions or ages. Layering can be seen on the edges of some plains. Other plains contain plateaus with smooth tops and escarpments from 150 to 1700 m high. The escarpments are evidence for erosion.


  • Volcanoes

Only about 5 percent of Io is covered by volcanic vents. About 500-700 volcanic centers have been identified but, over the last decade, most of the energy has been released at only four centers. The energy is released from these centers at enormous rates. Carr (1997) reported that 356 calderas had been identified in the Voyager and Galileo coverage. The largest volcanoes have diameters of more than 250 km and are closer to the equatorial region. Volcanoes at higher latitudes tend to be smaller, less than 100 km in diameter. The random distribution of Io's volcanoes suggests a lack of mantle convection, which is partially responsible for linear hot spot tracks and island arcs on Earth.

Paterae, low-profile volcanic shields are the most common type of vent. Their flows can cover large areas and reach lengths of 700 km. Such long flows suggest high eruption rates and/or low viscosity material. Some patera have summit calderas with relief of 1-2 km from floor to rim. The images above compare the 1979 Voyager 1 image of Loki Patera with Galileo images taken in 1996. The patera is at the center of the images. A dark fissure is just above and right of the patera. Voyager observed an eruption from this fissure in 1979.

Ra Patera covers an area of 760 x 480 km and has numerous long, narrow flows that radiate from the summit. These views of Ra Patera show changes seen on by Voyager 1 (upper left and upper right), Galileo (bottom right), and Voyager 2 (bottom left). The Galileo images reveal the detailed morphology of new deposits. Dark materials are interpreted as the overflow of lava flows from the caldera. New bright deposits, also though to be lava flows, cover an area of about 40,000 square kilometers and surround the dark materials. Images courtesy of The Jet Propulsion Laboratory and NASA. Maasaw Patera, another shield volcano with summit caldera, has been compared to Volcan Alcedo in the Galapagos. The patera are thought to be made mostly of silicate lava flows with interbedded sulfur lava flows and pyroclastics.

Io may have calderas with active lava lakes and fissures erupting silicate lava flows. These calderas may be up to 200 km in diameter and are located on the surface of the plains. Volcanic plumes originate in some calderas.

Eruption Styles

Explosive eruptions have been observed on Io and there is indirect evidence for effusive eruptions.


  • Explosive

    Galileo color images showing two volcanic plumes on Io. A plume erupting over Pillan Patera was captured on edge of the moon (see main image and inset at upper right). The plume was 140 kilometers (86 miles) high. The Galileo spacecraft will pass almost directly over Pillan Patera in 1999 at a range of only 373 miles (600 kilometers). The second plume is erupting over Prometheus, seen near the center of the moon and near the boundary between day and night and the inset at lower right. In the inset image, the shadow of the plume can be seen to the right of the vent. The plume is about 45 miles (75 kilometers) high.

    Two types of eruption plumes have been observed: Prometheus-type and Pele-type.

    Prometheus-type Pele-type
    Plume heights 50-120 km up to 300 km
    Plume character optically
    thick, dark jets
    Deposits bright halos, 200-600 km diameter dark halos, 1000-1500 km in diameter
    Eruption Velocities about 500 m/s up to 1000 m/s
    Duration months to years days to months
    Location common near equator restricted longitudes
    of associated
    'hot spots'
    about 450 K about 600 K

    Ejection velocities for explosive eruptions are estimated to be 500 to 1,000 meters per second. Plume diameters can be as much s 1,000 km. In December 1996, Pele's plume had at a height of 460 km. Most of the plume-producing eruptions are near the equator (between 30 degrees north or south). Two of the eruption sites, called Pele and Loki, are associated with calderas. Explosive eruptions can continue for at least a few days but some wane after a few hours. Sulfur dioxide gas may be the driving force of the explosive eruptions.


  • Effusive

    High-resolution image of part of Io showing lava flows and other volcanic features on Io.

    Earth-based monitoring of thermal emissions on Io have been interpreted as eruptions of surface lava flows. In 1996, two effusive eruptions produced about 3 square km of lava at eruption rates of 10,000 to 1,000,000 square meters per second. Eruption temperatures were greater than or equal to 1130C (Strawberry and others, 1997). Eruptions on Io may produce pahoehoe and aa flows, possibly as overflow from lava lakes or from fissure eruptions.

Sulfur vs. Silicate

The size and density of Io are about the same as the Earth's Moon. Since pieces of the Moon have been directly sampled and found to be made of silicate minerals (minerals with silicon and oxygen), scientists have suggested that most of Io is also made of silicates. In contrast, the red color of Io (the reddest object in the Solar System) and spectra from the surface indicate that sulfur is present.


Sulfur is an unusual substance. It boils at temperatures higher than about 275 C and can remain molten down to 120 degrees C (lava flows made mostly of silica solidify at about 1000 degrees C). Thus, magmas on Io can be generated at much lower temperatures and lava flows can remain molten to much lower temperatures.

The variation of viscosity with temperature for molten sulfur is also unusual:

Viscosity (poises) Temperature ( C ) Color
50-500 275-220 black
500-1000 220-175 red
1000-1 175-150 red-orange
1-0.08 150-140 orange
0.08-1 140-50 yellow

Based on a graph in Rothery (1992).

As temperature decreases the viscosity initially increases. At about 175C, the viscosity drops dramatically almost 4 orders of magnitude while the temperature drops only 40C. This means that lava flows actually get more fluid as they cool (the opposite is true of the silicate lava flows of Earth).

All of the colors listed above have been observed on Io and are best explained by the eruption of sulfur. The color changes with cooling help to map the eruption temperatures of volcanic products on Io. Surface compositions probably consist of: sulfur at various temperatures, anhydrous mixtures of sulfur allotropes with sulfur dioxide frost, and sulfurous salts of sodium and potassium.


Some planetary geologist believe silicate AND sulfur volcanism occur on Io. The presence of mountains with 9 km of relief suggests silicate material is involved because sulfur and its compounds does not have enough strength to support such features. Likewise, the relief along the edges of plains and within patera calderas could not develop if the surface material was all sulfur. The large size of the calderas on Io requires that the crust is strong and at least 10-20 km thick. A more likely scenario is a thin veneer of sulfur or sulfur compounds over a crust of silicate rocks. Silicate volcanism is probably high-volume low viscosity (basalt) lava flows erupted from low shields and possible fissures.


The presence of hot spots (not to be confused with hot spots on Earth) also supports the presence of silicate volcanism. Hot spots are active or recently active volcanic regions on Io. They are recognized by high thermal emissions. Using a Near-Infrared Mapping Spectrometer (NIMS), 30 hot spots have been detected. The NIMS image on the left shows Loki Patera on February 21, 1997, as Galileo made its sixth orbit. The image on the right was taken on March 12, 1997, using the Infra-Red Telescope Facility (IRTF) on Mauna Kea, Hawaii. The image shows the enormous amount of heat generated at Loki during an eruption.


The Galileo image on the left shows volcanic hot spots on Io's darkside. Io was in Jupiter's shadow when the image was taken. This is the highest-resolution image ever acquired of hot spots. The mosaic of Voyager images on the right shows the locations of the hot spots seen in the Galileo image. Image courtesy NASA/JPL.

The hot spots (volcanic centers) are named after mythological figures associated with fire and thunder: Janus, Hi'iaka, Zal, Gish Bar, Sigurd, Monan, Altjirra, Amirani, Maui, Malik, Tupan, 9606W, Prometheus, Culann, Zamana, Volund, Aidne, Fo, Sethlaus, Rata, Lei-Kung, Isum, Marduk, 9611A, Kurdalagon, Mulungu, Pillan, Pele, Daedalus, W. Pele, and Loki. Lopes-Gautier and others (1997) lists the latitude and longitude of these hot spots. The temperatures range from about 100 to 333 degrees Celsius over areas of 192 to 3 square km. All of these hot spots are within 50 degrees of the equator. Hot spots at higher latitudes, if the exist, may be detect by later orbits of the Galileo probe. Ten of the hot spots detect in 1979 were still active in 1997.

High resolution images of hot spots using Galileo's CCD system are interpreted to be caldera floors (actively convecting lava lakes) and/or possibly pahoehoe lava flows. The small areas are thought to have temperatures of at least 725 degrees Celsius or higher (Note: these temperatures are higher than the boiling point of sulfur in a vacuum). These images are some of the best evidence for active silicate volcanism (possibly basalt) on Io.

Studying Io


Voyager 1 and Voyager 2

The Voyager probes obtained images of about 35% of Io at a resolution of 5 km. In some areas resolution was as good as 0.5 km. These images allowed geologic maps of Io to be constructed. Geologists could recognize mountains, plains, and volcanic vents, and the relative ages of these features. Nine eruption plumes were discovered during the Voyager 1 mission. Voyager 2 arrived four months later. Voyager 1 image taken on the morning of March 5, 1979 at a range of 377,000 kilometers (226,200 miles).


Galileo was launched in 1989 and entered orbit around Jupiter on Dec. 7, 1995. Project Galileo: Bringing Jupiter to Earth describes the spacecraft, mission, images and results.

Future observations: Io is the most volcanically active body in the solar system. Scientists hope to learn more about the fiery satellite when Galileo continues its studies over the next two years, during a mission extension known as the Galileo Europa Mission. The extended mission will include eight additional encounters of Europa, four of Callisto, and two close Io flybys in late 1999, depending on spacecraft health. Galileo will pass very close to Pillan Patera in the first of the two Io flybys, so high- resolution images can be acquired over a small portion of this area.

Earth-based Telescopes

The NASA Infrared Telescope Facility (IRTF), on Mauna Kea, Hawaii, collects infrared images of Io when the satellite is observable. This data set is in support of images collected during the Galileo mission. The images can detect eruptions and, under the right conditions, the active volcano. This telescope provides better time coverage and time resolution of volcanic activity on Io compared to Galileo observations.

The images from the telescope have detected very hot events (>1220 C) that lasted days or weeks (Spencer and others, 1997). Some events are located at known calderas. Loki, Io's most powerful single volcano, appears to have periods of increased activity that last several months. Interestingly, not all hot events are associated with plumes or surface changes.

The Infrared Telescope Facility at Lowell Observatory in Arizona has detected thermal emissions on Io caused by violent silicate eruptions, possibly from fire fountains or the overflowing of lava lakes.

Hubble Space Telescope


The Hubble Space Telescope (HST) has been used to observe Io. For example, between March 1994 and July 1995 a major brightening (eruption) was observed at Ra Patera. HST has also observed eruption plumes from Pele such as the one shown in the above image. Plume height is about 400 km. These observations allow estimates of the plume density, composition, and mass and detection of rapid changes in plumes.

Other Io Information



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