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The two inner jovian satellites - Io and Europa - are among the most interesting planetlike bodies in the Solar System. Io has a hot mantle that continues even today to send volcanic materials to the surface; its closeness to parent Jupiter leads to strong tidal forces that help to account for this activity. For its size, Io has more active volcanoes, some spewing sulphur compounds, than even Earth, the only other spherical body in the S.S. on which current volcanism can be monitored. Europa is quite different: it has a crust largely composed of ice, mixed with rock. Below that cracked crust there is mounting evidence of a fluid shell - probably of water - of moderate thickness which has an analog in the ice-covered Arctic of our Earth where ocean water lies beneath. This suggests a possibility, which NASA hopes to test, of a medium in which some forms of marine life may have developed.

Jupiter's Galilean Satellites: Io and Europa

Jupiter has 16 satellites or moons (we use the term "moon" to describe spherical satellites while "satellite" may apply to an irregularly-shaped body). All but four are small and irregular, and most of those were discovered by telescope from Earth, but Voyager found three. We generally consider the four large ones, known as the Galilean Moons, in honor of the Italian scientist, Galileo, as the most extraordinary, as a group, in the solar system. We introduce these now in the Voyager montage below, created from individual global (full) views scaled, so that each retains its size, relative to the others:

Voyager montage of global views of the moons of Jupiter - from right to left (progressively outward from Jupiter itself: Io, Europa, Ganymede, and Callisto.

Data gathered both by the Voyager and Galileo spacecraft on the densities and nature of the crusts of these four Moons have resulted in general models of the interior of each. Io (upper left), Europa (upper right), Ganymede (lower left) all have cores (denser rock; possibly some solid metal); Callisto may have a small core but its interior is thought to be mainly water ice plus rock (largely undifferentiated).

Cutaway models of the four Galilean satellites of Jupiter; the innermost is Io (right), Europa (upper right), Ganymede (lower left), and Callisto (lower right) showing the interpreted interior of each. The blue indicates possible liquid water. The layer of water is thicker on Callisto than on Europa. Browns indicate rock. All but Ganymede have presumed cores based on densities.

The innermost moon, Io, lies some 422,000 km (262,231 mi) from Jupiter's center. It's about 3,630 km (2256 mi) in diameter, making it just slightly larger than Earth's Moon.

The volcanically active satellite Io, as seen in this Galileo image, with yellows and oranges generally denoting volcanic deposits containing  sulphur. The blacks may be another phase of sulphur or just basaltic lava.

Different parts of Io are portrayed in these three views taken by the Galileo TV camera:


Galileo images of Io as different parts of its surface rotate into view.

The ionian surface can also be displayed in a flat projection (which compresses and distorts polar regions):

Mercator-like projection of the entire ionian surface.

19-54: What common planetary feature seems to be absent on Io? Venture a guess as to what may be the dominant process acting on this satellite. ANSWER

Io displays prominent orange colors, with blackish mottled areas (that has reminded some of a giant pizza with olives). It may well be the most remarkable of all satellites, in that the surface is undergoing constant dynamic change, largely through volcanic processes. Astrogeologists have rated Io as the "most geologically active planetary body in the Solar System", roughly 100x that of Earth on a surficial area basis. What they mean is that it is undergoing more volcanic eruptions, for its size, of any such body (at least 16 volcanoes that are active have been detected); besides Earth, it is the only other object with live volcanoes spewing hot lavas. Although the ionian surface generally has a temperature of -140° C, hot spots with temperatures up to 1200° C attest to volcanically active areas. Io's surface is geologically young (no impact craters have been found on it), constantly undergoing "resurfacing" by volcanic processes.

One Voyager 1 image startled JPL scientists, because it captured a major plume from the eruption of an Ionian volcano that was then tossing material up to 300 km (186 mi) above its surface. This was the first clear indication that Io was a body with (very) active volcanism.

Quasi-natural color Voyager 1 image with a major plume from the eruption of an Ionian volcano clearly visible along the horizon.

The plumes are particularly likely to be detected when their source volcanoes lie at moment on or near the limb of the illuminated/dark parts of the satellite. In this UV image made by the Cassini spacecraft, two plumes from separated volcanoes are visible against the dark background just past the limb terminator. These eruptive plumes are nearly symmetric hemispheres because of lack of strong winds to disperse the ejected materials directionally.

Eruptive plumes from two Ionian volcanoes, seen against the dark edge of the satellite, aas imaged in the UV.

In August 2001, Galileo had a stroke of good fortune: An unknown volcanic source was caught in eruption while the spacecraft was actually looking at a different volcanic site, Tvashtar, whose activity had been persistent earlier. This time Tvashtar was apparently quiescent but a plume, imaged in the infrared, was observed along the illuminated crescent as it rose to 300 km (200 miles) above Io's surface at a point that had not earlier disclosed any discrete volcanic features. This is how this eruption appears in this observational mode; most of the excited particles causing the red color are SO2.

An eruption from a previously unknown volcanic site, rising higher than any seen before on Io, as it is displayed as a bulge in the ionian atmosphere above the surface of this very active satellite.

Other eruptions were "caught in the act" during the fly-bys meaning that, along with signs of young surface deposits, for its size Io is more volcanically active than Earth. A major eruption in progress at a volcano named Prometheus was captured by the Galileo TV camera, as depicted in these two views:

Two views of the volcano Prometheus in eruption, with plumes of ash (?) being ejecta; Galileo images.

Two views of the volcano Prometheus in eruption, with plumes of ash (?) being ejecta; Galileo images.

19-55: Look at the Prometheus images. How does this eruption differ from the usual big ones (typically from stratovolcanoes) on Earth? ANSWER

By comparing images of Prometheus acquired by Voyager in 1979 (left) and Galileo in 1997 (right), several distinguishable major changes are evident:

Views from Voyager (1979) (left) and Galileo (1997) of Prometheus and a neighboring volcano, both showing changes in the intervening 18 years.

The caldera central to the Pele eruption site was imaged after it had discharged sulphurous lava, ash and gases out several hundred kilometers in all directions, forming a heart-shaped halo deposit (SO2 frost), named the Matuike Patera (volcanic field).

The heart-shaped volcano Pele, in a Voyager 2 image.

A thermal sensor on Galileo has taken measurements showing temperatures associated with specific volcanic hot spots (see caption for identities of individual volcanic centers):

Temperature map of Io, showing hot areas around L-K = Lei-Kung Fluctus; L = Loki; M = Mardak; Pi = Pilan; Pe = Pele.

The actual temperatures are mostly low, by Earth volcano standards. The blue denotes temperatures of 90° K (-297° F) and the yellow denotes 170° K (-153° F). But some hot spots have local temperatures up to 1500° K (2240° F).

Another part of Io, overlapping onto the Pele area, has been examined in the infrared by the European Southern Observatory (an Earth-based telescope). The image below was made by using these bands: 1) Blue = 1.6 µm; 2) Green = 2.2 µm; 3) Red = 3.8 µm. The last band singles out areas on Io that are notably warmer than their surroundings.

Earth-based infrared image of part of Io, seen by the ESO telescope.

Galileo has imaged an active or recent lava flow, still hot as suggested by its orange color. This is the first time a "living" flow has been seen on any other planetary body:

An active lava flow on Io; Galileo image.

Some of the Galileo images, when converted to approximations of natural color but with some artistic license, are quite attractive to behold, as is this one of Culann Patera:

A Galileo color composite, with color differences emphasized, of the Culann Patera, an ionian volcano.

At least 100 eruption sites have been identified, but many may be inactive. More than 30 appeared to undergo one or more events during the observation periods. Volcanic deposits dominate the entire ionian surface, some being flows and others, probably fragmental. We can trace many of these to calderas. This is a Galileo view of a typical part of the ionian surface:

Part of the ionian surface showing the wide occurrence of volcanic sites, most probably still active; Galileo image.

Volcanic sites can undergo significant modifications in just a few years, as demonstrated by this series of images taken over the last 18 years from Voyager 1 (upper right) through Galileo (lower right) around the volcanic complex known as Ra Patera (upper left):


Change detection using Voyager and Galileo: Upper left = high resolution image of Ra Patera; Upper right = Voyager 1 in color; Lower left = Galileo color image; Lower Right = Voyager 2; newly appearing are small dark flows below the central vent.

19-56: In general terms, describe the changes at Ra Patera. ANSWER

Standard image processing for enhancement or change detection that we've used in this Tutorial for terrestrial images work just as well for other planetary bodies. Here are three Galileo images of an active volcanic terrain on Io. The left and center were taken four months apart. The right image is a ratio product of the first divided by the second date; the new tonal patterns indicate changes most probably of volcanic deposits over this short interval.

An October 1999 Galileo image of a volcanic area on Io; A February 2000 image of the same area; on the right a ratio image, with the second divided by the first, note new patterns.

An even more recent observation has pinpointed the volcanic complex called Amirani-Maui where new lava is pouring out at a rate of 100 cubic meters. This is producing a lava flow continuum that is at least 350 km long (longer than any equivalent on Earth). The images below, on the right, show areas of new lava cover built up in just 4 1/2 months.

Most of the ionian volcanoes do not appear to be high stratocones such as are displayed as the majestic volcanic peaks observed in the Ring of Fire on Earth. Where some relief is evident, many Ionian volcanoes are more like terrestrial shield volcanoes, often with wide calderas. Haemus Mons is an example:

Haemus Mons, a rising volcanic structure on Io; Galileo image.

Credit: C. Hamilton

The volcanic materials are likely silicates, mixed with large amounts of (native?) sulphur and sulphur compounds. These compounds can account for Io's orange, yellow, and black colors that are typical of this element. Sulphur-rich lakes have been observed. Sulphur dioxide makes up a tenuous atmosphere.

The cause of this (totally unexpected) activity on Io is a process termed "tidal pumping," in which the outer part of this satellite flexes and stretches as much as 100 m (328 ft) as Jupiter and the other moons tug on it. The tidal energies acting on Io induce heating, and the stretching causes it to wobble.

Io has the highest degree of volcanic activity of any planetary body in the solar system. Some of this activity was missed during the earlier imaging of Io in the visible light range. But thermal infrared images have found at a large number of hot spots (at least some being volcanoes) representing new locations; later inspection of visible imagery then found evidence for most of these being associated with volcanism. Two examples illustrate this:

Thermal infrared imagery (right) taken by Galileo that reveals hot spots that are obscure in the visible image (left) of this region of Io..

Named hot spots on Io, with their locations being quite obscure in the visible black and white image on the left

An image made during an October, 2001 Galileo pass by Io shows the Tupan caldera in a false color rendition and in a thermal plot using data from the 4.7 µm thermal channel:

The Tupan caldera.

The present ionian surface is now believed to be quite young since lava flows like those shown above are commonplace. With such activity, the surface is continually being replated with lava cover. Some planetary geologists contend that Io's surface today is similar to that of the early Earth, in its first billion years, and is thus a "living" model of terrestrial conditions not long after its formative years (involving intense planetesimal bombardment and cratering) came to a close and a continuous crust developed. However, the Earth of about 800,000 years of age probably didn't have a sulphur-rich crust.

Amazingly, Io also has the highest mountains on any of the rocky or ice-covered satellites, although these are uncommon. Using both Voyager and Galileo images, stereo has allowed estimates of elevations of various peaks rising from the general surface. Some exceed 10 km (6 miles) in height (relief). One of the more prominent is Tohil Mons, a volcano with a caldera and associated lava flows, that resembles similar volcanoes on Earth:

Galileo image of Tohil Mons.

This mosaic shows the region around Tohil Mons, with its sets of what appear to be individual lava flows:

Mosaic of the Tohil Mons region.

This next image is a perspective re-creation (with added vertical exaggeration) from the above imagery of Tohil Mons; note that the viewing direction is different. The peak is more than 6 km (20000 ft) above the plains.

Tohil Mons, a high mountain peak on Io.

Seen close up, this mountain (~6150 m or 18500 ft high) is seen to have a subsidiary rimmed crater (to its northeast in the above view) with a smooth central filling that appears to be a (solidified?) lava lake.

Tohil Mons close-up.

Recently, Galileo sent back evidence that Io has an iron-rich core whose radius is about half (~900 km) that of the complete satellite. Io is generating a small magnetic field that interacts with (and may be largely caused by) the intense field caused by Jupiter. Io also seems to have an ionosphere. It is surrounded by a vapor cloud made up of hydrogen, sodium, potassium, and ionized sulfur (derived by sputtering off the ionian surface) which has been stretched out to form a torus around its parent planet. A flux tube between Io and Jupiter allows a current of 10 million amps. to flow.

As would be expected on such an active, dynamic planet-like surface, other processes besides volcanism are taking place. This Galileo image shows landslides against the steep front of the Telegonus Plateau:

Landslides at the edge of the Telegonus Plateau, as imaged by Galileo.

Io also has an atmosphere made up of gases and dust ejected from the volcanoes (most eventually escaping to space) and ionized particles. This next Galileo image shows the atmosphere (reddish band near top) and volcanoes in eruption.

Galileo view of Io, showing its atmosphere and materials being erupted from active volcanoes.

Galileo has made a remarkable image of the presence of ionized sodium in its ionosphere. Sodium ions when excited give off visible light (remember sodium vapor lamps) at 5890 angstroms (or 589 nanometers or 0.589 µm). Using a yellow-green filter, its camera captured this view of Io and surrounding space. The yellow is an approximation of the sodium light. In the Io sphere, the Sun has lit the far side, as seen here, but there is a bright white light on the dark side which is sunlight illuminating the plume of ash coming from a Prometheus eruption in progress.

Aura of excited sodium ions giving off a yellow glow, as imaged by the Galileo camera using a UV filter.

Our next moon is Europa (diameter = 3,140 km (1951 mi), 671,000 km (416959 mi) from Jupiter. Below are images (using different filter combinations), as it appears from Galileo:

Two views of the jovian satellite Europa, imaged by Galileo; left image = natural color; right image = special color filters.

Europa's surface reminds some viewers of a giant "cue ball" that has cracked with age. The left view is a natural color image. On the right is a false color image, made by projecting the violet band through blue, the green band through green, and the IR through red.

19-57: What Earthly object with which you may be familiar does Europa remind you of (hint: think of a game you play indoors)? ANSWER

Seen by Voyager close up, the surface is extremely smooth, with few markings (minimal craters). Much of the ice making up the europan surface is blue, similar to fresh ice found in polar regions or in glaciers on Earth. This is typical:

Blue ice on Europa.

Other regions on Europa have a brown coloration. These contain pronounced sets of darker brown "fractures" that divide the crust into polygonal segments. These cracks are only slightly higher or lower (<300 m [984 ft]) than the main surface that appears to be water ice, which may top an ice-water slush or a pure water "ocean" at some depth before grading again into ice and then rocky material.

Enlargement of a Galileo image of the surface markings on Europa; the surface is generally smooth (no noticeable topographic heights), free from craters, and broken by numerous cracks that are filled with darker red-brown material.

Another view in the Minos Linea region of this surface from Galileo has been reprocessed to give a color version in which the ice is blue and the crack-filling material is dark red - a color very close to that of hematite, the non-hydrated form of iron oxide (suggestive, but not proof it is that mineral).

The Minos Linea region of Europa in a  Galileo enlargement using band combinations that render the ice blue (a believable color if it is not contaminated) and the crack-filling a dark red (iron oxide?).

Still another eye-appealing color version of a small part of Europa's surface:

Another color view of the europan surface.

At higher resolution, we can see several orders (size discontinuities) of fracturing. Some are distinctly curvilinear (a common pattern in rapidly cooled liquids, such as those used to make silicate glass). Slivers of dark material are scattered within. On Europa, some interior rocky material may mix with the ice.

The origin of these linear breaks is uncertain. Scientists compare these breaks with polar sea ice on Earth, which experience cracks or "leads" by tension, as water currents cause them to flex. These cracks then reheal. Ridges, formed by compression, are an alternative.

The cracks may be curvilinear as well, as seen in this view of the Theros and Thrace region near the southern pole of Europa.

Linear and curvilinear fractures in the Theros and Thrace regions as imaged from Galileo.

19-58: On what basis can you conclude that Europa's surface is relatively young? ANSWER

Galileo shed more light on this, when it made a close approach (586 km [364 miles]),on February 20, 1997. The picture below shows closely-spaced parallel cracks, with different orientations, in two blocks separated by a high ridge, furrowed in its center.

Galileo image of the surface of Europa showing close-spaced parallel cracks in blocks with different orientations, separated by a high ridge furrowed in its center.

This image shows in addition to cracks, a break-up of the crust that produces what have been called "ice rafts" which rise above the surface. Individual rafts are grooved, but the grooves vary from raft to raft indicating that they floated as breakoffs from grooved terrain or alternatively were reoriented by pushing around from a disrupted terrain.

Ice rafts, with grooves, on the surface of Europa; Galileo image.

These appear to be break-ups of a thin ice cover (with its leads), in which the rafts are free to move about, with additional evidence favoring a liquid water body below. This is now being referred to as the "buried ocean" - some estimates envision this as a continuous subshell of water that may be as much as 100 km in depth that extends over the entired subsurface of Europa. The implication of a liquid "ocean" topped by a frozen crust (analogous to the polar arctic on Earth) has led some scientists to speculate that conditions may be favorable for primitive life in this liquid. This supposition has fueled intense interest in getting even closer approaches to Europa, which planners scheduled in the continuing Galileo observations. Officials are considering a new mission in the early 21st century, specifically to search for more signs of life.

More recent observations shed further light on the variety of terrains on Europa. The scene below, covering an area of 7 km by 4 km (4.3 mi x 2.5 mi), shows a chaos of small plates and wedges in an area known as Conamara Chaos. This December 16, 1997, image reveals chunks of ice, perhaps bound to each other by a matrix of ice mush, including several hillocks rising to 250 m (820 ft). Note the single flat lineated plate at the top. On the left is a straight chasm that is nearly 400 m (1312 ft) across.

Image of the surface of Europa showing a chaos of small plates and wedges in an area known as Conamara Chaos.

Recently, Europa investigators have proposed the possibility of "ice volcanoes" in which viscous ice (perhaps with some water) erupts from vents and flows like a glacier over the surface. Galileo has imaged such flows, as seen in this image in which a flow (in the center) is extending over large cracks.

Grabens in the water ice surface of Europa, with an apparent ice flow overlapping one of the trenches.

Although impact craters, especially larger ones, are quite uncommon on Europa, a few exist, such as this one, not yet destroyed by ice movements, known as Tyre Crater. This sparsity indicates that Europa is an active planet, with continual or periodic replating of its surface in a process affected by the presumed liquid ocean beneath its ice cover.

The Tyre Crater on Europa; color-processed Galileo image.

This last view (Galileo) is again typical of the europan surface, with fractures and grooves, ridges and small domes (up to 8 km [5 miles] in diameter) that are believed to represent salty liquid pushing upward into the crust which may be as much as 13 km (8 miles) thick.

Another view of the europan surface.

Now on to the remaining two galilean moons of Jupiter.

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Primary Author: Nicholas M. Short, Sr. email: