# Rocks/Rocky objects/Io

This is a true-color image of Io taken by the Galileo probe. Credit: NASA.

In the image at right, "[t]he smallest features that can be discerned are 2.5 kilometers in size. There are rugged mountains several kilometers high, layered materials forming plateaus, and many irregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no landforms resembling impact craters, as the volcanism covers the surface with new deposits much more rapidly than the flux of comets and asteroids can create large impact craters. The picture is centered on the side of Io that always faces away from Jupiter; north is to the top."[1]

## Rocky objects

Io is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres (2,263 mi), the fourth-largest moon in the Solar System. With over 400 active volcanoes, Io is the most geologically active object in the Solar System.[2][3] Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost. Io's volcanism is responsible for many of the satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur.

## Yellows

Io, the most volcanic body in the solar system, is seen in the highest resolution obtained to date by NASA's Galileo spacecraft. Credit: NASA.
This image of Io continues to the left around the rocky object. Credit: NASA.
This composite of images from Galileo and Voyager 1 shows Io's north pole. Credit: U.S. Geological Survey and NASA.
This composite is of Io's south pole. Credit: U.S. Geological Survey and NASA.
In the same way that the Moon always has the same side facing Earth, Io always has the same side facing Jupiter. Credit: U.S. Geological Survey and NASA.

In the image at right, "[t]he smallest features that can be discerned are 2.5 kilometers in size. There are rugged mountains several kilometers high, layered materials forming plateaus, and many irregular depressions called volcanic calderas. Several of the dark, flow-like features correspond to hot spots, and may be active lava flows. There are no landforms resembling impact craters, as the volcanism covers the surface with new deposits much more rapidly than the flux of comets and asteroids can create large impact craters. The picture is centered on the side of Io that always faces away from Jupiter; north is to the top."[1]

"Color images acquired on September 7, 1996 have been merged with higher resolution images acquired on November 6, 1996 by the Solid State Imaging (CCD) system aboard NASA's Galileo spacecraft. The color is composed of data taken, at a range of 487,000 kilometers, in the near-infrared, green, and violet filters and has been enhanced to emphasize the extraordinary variations in color and brightness that characterize Io's face. The high resolution images were obtained at ranges which varied from 245,719 kilometers to 403,100 kilometers."[1]

The second image at right continues around Io to the left.

The third image is of Io's north pole. "[The] new basemap [and the polar images] of Jupiter's moon Io was produced by combining the best images from both the Voyager 1 and Galileo Missions. Although the subjovian hemisphere of Io was poorly seen by Galileo, superbly detailed Voyager 1 images cover longitudes from 240 W to 40 W and the nearby southern latitudes. A monochrome mosaic of the highest resolution images from both Galileo and Voyager 1 was assembled that includes 51 Voyager 1 images with spatial resolutions sometimes exceeding the 1 km/pixel scale of the final mosaic. Because this mosaic is made up of images taken at various local times of day, care must be taken to note the solar illumination direction when deciding whether topographic features display positive or negative relief. In general, the illumination is from the west over longitudes 40 to 270 W, and from the east over longitudes 270 W to 40 W. Color information was later superimposed from Galileo low phase angle violet, green, and near-infrared (756 nanometer wavelength) images. The Galileo SSI camera's silicon CCD was sensitive to longer wavelengths than the vidicon cameras of Voyager, so that distinctions between red and yellow hues can be more easily discerned. The "true" colors that would be visible to the eye are similar but much more muted than shown here. Image resolutions range from 1 to 10 km/pixel along the equator, with the poorest coverage centered on longitude 50 W."[4]

The fourth image is of Io's south pole.

The left image is an animated image showing a 1 Io day of rotation. "In the same way that the Moon always has the same side facing Earth, Io always has the same side facing Jupiter. The movie shows two speeded-up rotations of Io (a single rotation really takes 1.77 days), and begins with a view of the Jupiter-facing hemisphere. With rotation in an easterly direction, after two seconds the volcano Prometheus (on the equator) comes into view. The massive red deposit around Pele (seconds 5-10) is the most distinctive expression of volcanic activity on Io, and just to the north-west is the horse shoe-shaped Loki Patera, the most powerful volcano on Io. The animation was made using a computer program that wrapped the Io mosaic around a sphere to produce a globe. In all, 360 images were used, each differing by one degree in longitude from the previous image."[4]

## Craters

This mosaic of Voyager 1 images covers Io's south polar region. The view includes two of Io's ten highest peaks, the Euboea Montes at upper extreme left and Haemus Mons at bottom. Credit: NASA/JPL-Caltech/USGS.
Changes in surface features in the eight years between Galileo and New Horizons observations are shown. Credit: .

On the left is a mosaic of Voyager 1 images covering Io's south polar region. The view includes two of Io's ten highest peaks, the Euboea Montes at upper extreme left and Haemus Mons at bottom.

The pair of images on the right changes in surface features for the eight years between Galileo and New Horizons observations.

## Volcanoes

The Aug. 29, 2013, outburst on Io was among the largest ever observed on the most volcanically active body in the solar system. Credit: Katherine de Kleer, UC Berkeley.

On the left is a five-image sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface.

Io is the innermost of the four Galilean moons of the planet Portal:Jupiter and, with a diameter of 3,642 kilometres (2,263 mi), the fourth-largest moon in the Solar System. With over 400 active volcanoes, Io is the most geologically active object in the Solar System.[2][3] Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost. Io's volcanism is responsible for many of the satellite's unique features. Its volcanic plumes and lava flows produce large surface changes and paint the surface in various shades of yellow, red, white, black, and green, largely due to allotropes and compounds of sulfur.

## Volcanic activity

These images are of Io obtained at different infrared wavelengths (in microns, µm, or millionths of a meter). Credit: Imke de Pater and Katherine de Kleer, UC Berkeley.
Five-image sequence of New Horizons images showing Io's volcano Tvashtar spewing material 330 km above its surface. Credit: Serendipodous.

"One of Jupiter’s moons has unleashed a series of huge volcanic eruptions over a hellish 2 week period that were so bright they could be studied in detail by ground based observatories."[5]

“We typically expect one huge outburst every one or two years, and they’re usually not this bright.”[6]

“Here we had three extremely bright outbursts, which suggest that if we looked more frequently we might see many more of them on Io.”[6]

“These new events are in a relatively rare class of eruptions on Io because of their size and astonishingly high thermal emission. The amount of energy being emitted by these eruptions implies lava fountains gushing out of fissures at a very large volume per second, forming lava flows that quickly spread over the surface of Io.”[7]

"While recording the eruptions that occurred in the moon’s southern hemisphere on Aug. 15, 2013, the researchers saw the brightest emanate from a caldera called Rarog Patera, which produced a 50 square-mile, 30 foot-thick lava flow — enough lava to cover Manhattan Island. Another eruption that was generated by the caldera Heno Patera produced a flow covering 120 square miles. Both eruptions generated “curtains of fire” as lava blasted from long fissures in Io’s crust."[5]

"Images [on the right are] of Io obtained at different infrared wavelengths (in microns, μm, or millionths of a meter) with the W. M. Keck Observatory’s 10-meter Keck II telescope on Aug. 15, 2013 (a-c) and the Gemini North telescope on Aug. 29, 2013 (d). The bar on the right of each image indicates the intensity of the infrared emission. Note that emissions from the large volcanic outbursts on Aug. 15 at Rarog and Heno Paterae have substantially faded by Aug. 29. A second bright spot is visible to the north of the Rarog and Heno eruptions in c and to the west of the outburst in d. This hot spot was identified as Loki Patera, a lava lake that appeared to be particularly active at the same time."[6]

## Theoretical volcanic Io

This still from five-frame photo sequence by NASA's New Horizons mission captures the giant plume from the Tvashtar volcano on Jupiter's moon Io. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

"Io is Jupiter’s innermost Galilean moon that is 2,300 miles wide, approximately the same size as our moon, and is the only place in the solar system (except Earth) where active volcanoes have been observed. The volcanic activity is driven by powerful tidal interactions with the gas giant that squeeze Io’s interior, heating it up. Like Earth’s volcanoes, molten rock (magma) is then forced through Io’s crust intermittently erupting as volcanoes."[5]

"Io is the most volcanic body in the solar system, boasting activity 25 times that of Earth. Some of Io's volcanoes blast plumes of sulfur and other material 250 miles (400 kilometers) above the moon, which is completely resurfaced every million years or so. On Thursday (April 4), NASA released a video of Io's volcano plumes based on five images snapped by the agency's Pluto-bound New Horizons spacecraft in March 2007."[8] One of these frames is on the right.

"The hundreds of volcanoes on Jupiter's moon Io aren't where they're supposed to be, [...] Io's major volcanic activity is concentrated 30 to 60 degrees farther east than models of its internal heat profile predict".[8]

"The unexpected eastward offset of the volcano locations is a clue that something is missing in our understanding of Io. In a way, that's our most important result. Our understanding of tidal heat production and its relationship to surface volcanism is incomplete."[9]

"This intense activity is ultimately generated by gravitational tugs from Jupiter, with an assist from the nearby moons Europa and Ganymede."[8]

"Io completes two orbits for every one that Europa makes, and four for every one of Ganymede's laps. As a result of this regular timing, Europa and Ganymede have pulled the orbit of Io into an oval, with explosive consequences for the 2,260-mile-wide (3,640 km) moon."[8]

"As Io moves closer to Jupiter, the planet's powerful gravity pulls hard on the moon, deforming it. This force decreases as Io retreats, and the moon bounces back. This cycle of flexing creates friction in Io's interior, which in turn generates enormous amounts of volcano-driving tidal heat."[8]

"Common sense suggests that Io's volcanoes would be located above the spots with the most dramatic internal heating."[8]

"What's causing the disconnect between expected and observed volcano locations remains a mystery. It's possible that Io is rotating faster than scientists think, researchers said. Or models of Io's tidal heating may be missing some components, such as the complications caused by an underground magma ocean."[8]

"Our analysis supports a global subsurface magma ocean scenario as one possible explanation for the offset between predicted and observed volcano locations on Io. However, Io's magma ocean would not be like the oceans on Earth. Instead of being a completely fluid layer, Io's magma ocean would probably be more like a sponge with at least 20 percent silicate melt within a matrix of slowly deformable rock."[9]

## Active regions

“Io has no impact craters; it is the only object in the Solar System where we have not seen any impact craters, testifying to Io’s very active volcanic resurfacing.”[10]

"Io is extremely active, with literally hundreds of volcanic sources on its surface. Interestingly, although Io is so volcanically active, more than 25 times more volcanically active than Earth, most of the long-term surface changes resulting from volcanism are restricted to less than 15 percent of the surface, mostly in the form of changes in lava flow fields or within paterae."[11]

“Our mapping has determined that most of the active hot spots occur in paterae [broad, flat dishes or saucers], which cover less than 3 percent of Io’s surface. Lava flow fields cover approximately 28 percent of the surface, but contain only 31 percent of hot spots.”[10]

“Understanding the geographical distribution of these features and hot spots, as identified through this map, are enabling better models of Io’s interior processes to be developed.”[10]

## Mountains

Galileo greyscale image is of Tohil Mons, a 5.4-km-tall mountain on Io. Credit: NASA.{{free media}}

Io has 100 to 150 mountains, averaging 6 km (3.7 mi) in height and reaching a maximum of 17.5 ± 1.5 km (10.9 ± 0.9 mi) at South Boösaule Montes.[12] The longest is 570 km (350 mi), and the highest is Boösaule Montes, at 17,500 metres (57,400 ft), taller than any mountain on Earth.[12] Mountains often appear as large (the average mountain is 157 km or 98 mi long), isolated structures with no apparent global tectonic patterns outlined, in contrast to the case on Earth.[12] To support the tremendous topography observed at these mountains requires compositions consisting mostly of silicate rock, as opposed to sulfur.[13]

Despite the extensive volcanism that gives Io its distinctive appearance, nearly all its mountains are tectonic structures formed as the result of compressive stresses on the base of the lithosphere, which uplift and often tilt chunks of Io's crust through thrust faulting.[14] The compressive stresses leading to mountain formation are the result of subsidence from the continuous burial of volcanic materials.[14] The global distribution of mountains appears to be opposite that of volcanic structures; mountains dominate areas with fewer volcanoes and vice versa.[15] This suggests large-scale regions in Io's lithosphere where compression (supportive of mountain formation) and extension (supportive of patera formation) dominate.[16] Locally, however, mountains and paterae often abut one another, suggesting that magma often exploits faults formed during mountain formation to reach the surface.[17]

Mountains (generally, structures rising above the surrounding plains) have a variety of morphologies: Plateaus, which resemble large, flat-topped mesas with rugged surfaces are most common.[12] Other mountains appear to be tilted crustal blocks, with a shallow slope from the formerly flat surface and a steep slope consisting of formerly sub-surface materials uplifted by compressive stresses, while both types of mountains often have steep scarps along one or more margins, only a handful of mountains on Io appear to have a volcanic origin, which resemble small shield volcanoes, with steep slopes (6–7°) near a small, central caldera and shallow slopes along their margins.[18] These volcanic mountains are often smaller than the average mountain on Io, averaging only 1 to 2 km (0.6 to 1.2 mi) in height and 40 to 60 km (25 to 37 mi) wide, where other shield volcanoes with much shallower slopes are inferred from the morphology of several of Io's volcanoes, where thin flows radiate out from a central patera, such as at Ra Patera.[18]

Nearly all mountains appear to be in some stage of degradation, with large landslide deposits common at the base, suggesting that mass wasting as the primary form of degradation, with scalloped margins common among Io's mesas and plateaus, the result of sulfur dioxide sapping from Io's crust, producing zones of weakness along mountain margins.[19]

Four morphological types of mountains have been identified.[12][20]

1. Mesa: a mountain with flat top and relatively smooth surface. It may be difficult to distinguish mesas from eroded layered plains. Ethiopia Planum is a good example of this morphological type. Eleven mountains on Io are classified as mesas.
2. Plateau: an elevated plain with a rugged surface. There is no steep or prominent peak on plateau. Iopolis Planum is a good example of this type. About 46% of Ionian mountains belong to this morphological type.
3. Ridge: an elevated structure dominated by one or more linear or arcuate rises. 28 (24%) mountains on Io have been cataloged into this type.
4. Massif: an elevated structure dominated by rugged or complex surface and has one or more peaks. Boösaule Montes and Tohil Mons are good examples.
Basal scarps on Io. This image taken by NASA's Galileo spacecraft during its close flyby of Io on November 25, 1999, shows some of the curious mountains found there. The setting sun to the left exaggerates the shadows cast by the mountains. By measuring the lengths of these shadows, Galileo scientists can estimate the height of the mountains. The mountain just left of the middle of the picture is 4,000 m (13,000 ft) high, and the small peak to the lower left is 1,600 m (5,200 ft) high. Credit: NASA.{{free media}}
Patera and plateau on Io. NASA's Galileo spacecraft acquired the images in this mosaic of Hi-iaka Patera (the irregularly shaped, dark depression at the center of the image) and two nearby mountains on November 25, 1999, during its 25th orbit. The sharp peak at the top of the image is about 11,000 m (36,000 ft) high, and the two elongated plateaus to the west and south of the caldera are both about 3,500 m (11,500 ft) high. The ridges on the northwestern mountain are often seen on Ionian mountains and are thought to be formed as surface material slides downslope due to gravity. Credit: NASA.{{free media}}
Mesa on Io. This example is Tvashtar Mesa. It has a very flat top and a sharp boundary. Credit: NASA.{{free media}}
Mass wasting and layered plain on Io. The shape of Euboea Montes, especially the northern flank's thick, ridged deposit, is interpreted by Schenk and Bulmer as evidence of slope failure along the entire face of the northern flank.[21] The northern portion of the image shows layered crust labeled "layered plain". Credit: NASA.{{free media}}

Several common features of Ionian mountains have been summarized.

1. Basal scarps: basal scarps always appear as an abrupt boundary of Ionian mountains that separate mountains from volcanic plains. Most Ionian mountains are observed to have this feature. The basal scarps are ten to a few hundred meters high. Sometimes, the scarp is resolved in high-resolution images as the margin of a debris apron. An example is Iopolis Planum.[12]
2. Tilted block: thrust faults have been interpreted to bound tilted blocks on Io. Tilted blocks have a polygonal shape and curved crests. One example is Euboea Montes. A terrestrial analogy is the Black Hills of South Dakota.
3. Mass wasting: several types of mass movement deposits have been observed adjacent to Ionian mountains. Downslope movements of blocks have been noted in at least one place, the Euboea Montes. Fan-shaped deposits resembling debris aprons are found at the base of steep slopes. The ridged or crenulated surfaces of some mountains like northern Hi'iaka Montes may be formed by downslope creep of layered rock.[20]
4. Layered crust: the upper Ionian crust may be layered, as suggested by flowing observations: mountain uplifted 17,000 m (56,000 ft) and exposed crust section at Euboea Montes, different colored units exposed on Haemus Mons, a ridged unit atop northern Hi'iaka Montes, and striations on mountains such as Haemus Mons and Tohil Mons.[12][21]
Tilted block is on Io. Credit: NASA-JPL.{{free media}}

Stress plays an important role in the origin of Io's mountains: Folding and faulting form all kinds of topographic features on Io.

1. Overburden stress: on Io, the resurfacing process keeps forming new layers at the surface and pushing older layer downwards. Overburden stress is stress imposed on an older layer of rock by the weight of overlying younger layer of rock. The horizontal stress (${\displaystyle \sigma _{h}}$) generated is less than the vertical overburden (${\displaystyle \sigma _{v}}$) by a factor of ${\displaystyle \nu }$ /(1-${\displaystyle \nu }$ ),where ${\displaystyle \nu }$ is Poisson's ratio (value is 0.25 for rock).[22] The differential stress is (${\displaystyle \sigma _{h}}$)-(${\displaystyle \sigma _{v}}$). This tensile stress is insufficient to cause faulting on Io, because the value is less than Byerlee's rule for rock failure in extension. However, the overburden stress may contribute to the faulting when combined with other stresses[23]
2. Subsidence stress: continuous burial of older crust by younger crust causes older rock to be pushed inward to a sphere with a smaller radius. This subsidence of older crust may imply enormous horizontal compressive stress. This stress is related to resurfacing rate (v), Io's radius (R), subsidence distance (ΔR) and Yong's modulus. The subsidence-generated horizontal stress is equal to E/(1-V)× ΔR/R. This stress is more than enough to cause faulting on Io.[23]
3. Thermal stress: thermal stress is another possible stress source on Io, as increasing temperature in Io's crust can cause expansion of the crust. The total tidal heating generated in Io is dissipated to resurfacing processes and conductive heat flow. The more heat used on resurfacing, the less heat can become conductive heat flow and the less thermal stress is caused by heat. The thermal stress is important as it can be generated wherever and whenever volcanic output is less than tidal heating input.[23]

Many mountains higher than 10,000 m (33,000 ft) have been observed on Io which implies that Io has a thick crust.[24] Part of the heat in Io is transported by advection.[24] Magma from depths rises to the surface through isolated vents, and spreads out and cools at surface.[24] The solid lithosphere subsides under the continuously generated new lava flow.[24] Solid material is heated by conduction at the base of lithosphere and melts again.[24]

The thrust faulting and uplifting of large crust blocks on Io are in a model.[21] Io's crust keeps recycling: violent volcanic activity brings lava to the surface and older, buried layers are forced to subside, the old volcanic crust materials are compressed laterally as they sink.[21]

A later model presents more details:[12]

1. Io consists of a continuous stack of mafic and ultramafic deposits. After new erupted volcanic materials cool down and are buried, the stack of rocks become indurated and form bedrocks.
2. The bedrocks are fractured due to tidal flexing, compression at depth, volcanic intrusion and other mechanisms, and then are broken into large blocks a hundred kilometers across.
3. Products of magmatism like sills, dikes and batholiths may intrude into layers of stacking volcanics to form a composite crust.
4. Occasionally, the large blocks of crust are rotated and thrusted along deep-rooted thrust faults, where they may expose a cross-section of crust to the surface, as at Euboea Montes.
5. Later, these blocks can also be eroded by mass wasting and reburied by subsequent volcanism.
6. At the base of the crust, materials are met again by heat.
7. Compression at depth due to global burial and subsidence can also form ductile deformation like folding of crust.
Resurfacing process on Io. Enormous tidal heating causes Io's highly active volcanic activities. Newly generated surfaces push old surfaces inwards. Major stresses related to this process are labeled in this image.[24] Credit: Lsuanli.{{free media}}
Geodynamic model of Io. Violent volcanic activities cause rapid resurfacing on Io. Newly formed surfaces keep pushing the older layer inwards. As the older layer is squeezed to a smaller sphere, horizontal compressive force cause shortening (horizontal contraction) at the older layer. Credit: Lsuanli.{{free media}}

Paterae and mountains are observed to appear near each other on Io.[25] This observation indicates that these two structures are somehow related.[20] As Io has strong tidal heating and very violent volcanic activities, the interior of Io should be vigorously convecting).[26][27] Localized regions of up-welling and down-welling of mantle material could affect the stress field in Io's lithosphere, where buoyant mantle diapir can locally enhance the compressive stress which may be sufficient for the development of thrust faults.[20] This mechanism would predict curved and circular mountains if it were responsible for the initiating faulting; however, many Ionian mountains are observed to have straight margins.[12] This contradiction indicates that faults exist before the raising of diapirs; thus, diapirs only provide a mechanism for focusing the stresses in Io's lithosphere.[12] Fractures that are not under compression stress induced by underlying diapiring processes could serve as conduits through which the melt erupts onto the surface.[12] Meanwhile, in a global view, an anti-correlation between the distributions of mountains and volcanic centers is observed on Io.[12] This may reflect a global convective pattern: on the hemisphere which is dominated by up-welling, there are more volcanic centers and on the hemisphere that is dominated by down-welling, there are more mountains.[20]

## Hypotheses

1. The volcanoes of Io originate from the current flowing through Io.

## References

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