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Fig. 6.5. Sources of Volcanic Plumes Near Prometheus. Prometheus is the "Old Faithful" of Io's many active volcanoes. A plume has been seen here everytime the viewing conditions were favorable for the Voyager and Galileo spacecraft. However, the location of the vent that is the source of the plume is still a mystery. The lava flow extends 90 km from the source. Bright patches probably composed of sulfur dioxide can be seen in several places along the flow's margins. Galileo spacecraft image. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.5. Sources of Volcanic Plumes Near Prometheus. Prometheus is the "Old Faithful" of Io's many active volcanoes. A plume has been seen here everytime the viewing conditions were favorable for the Voyager and Galileo spacecraft. However, the location of the vent that is the source of the plume is still a mystery. The lava flow extends 90 km from the source. Bright patches probably composed of sulfur dioxide can be seen in several places along the flow's margins. Galileo spacecraft image. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.6. A Voyager 1 image taken in March 1979 looking straight down at Pele, one of Io's most active plumes. The heart-shaped feature is where the material in the pl ume falls to the surface. (Credit: NASA/JPL/Caltech)

1997. Even the Cassini spacecraft detected a plume over Pele as it passed Jupiter on its way to Saturn [319]. Of the total resurfacing caused by plume activity Pele, because of its size and activity, accounts for over 40% of it. While some areas of Io, such as that around Pele, were resurfaced continually, much of Io remained unchanged during the course of the Galileo mission. Approximately 83% of Io's visible surface never changed during the Galileo era [320].

While evidence suggests that plumes normally eject SO2 and sulfur, evidence has also been found of silicate deposits. In fact, taking images with red, green, and violet filters, Galileo detected deposits of SO2, sulfur, and silicate [321]. In July 1997, the Galileo spacecraft found a 200 km tall plume rising from Pillan Patera. Later, a gray colored plume deposit was seen around Pillan, indicating the ejected material had been mostly silicate, not just sulfur [322]. Some plume deposits are irregular in shape, while others such as those from Prometheus, produce well-defined rings [323]. E

Although the giant plumes do not contribute much to the resurfacing of Io, £

Scientists believe that they contribute significantly to the escape of dust from c ¡X

Io into the Jovian environment because of the high velocity at which the dust is o ^

ejected. And, even though the plumes are tenuous, they easily supply enough mass *> ®

to account for the flux of dust escaping Io. These plumes provide a connection ,0

between the geologic activity on the surface of the moon and the flux of materials ^ ®

into space. Plumes eject dust and gas directly and help to sustain the tenuous JZ O

atmosphere that is eroded by impacting charged particles [324]. The repeated eruptions of smaller SO2 dominated plumes do contribute significantly to Io's resurfacing rate [325]. The majority of eruptions deposit materials that can reach distances between 50 and 350 km. A much smaller class of giant eruptions can deposit materials as far away as 350-800 km.

Ultimately, Galileo data allowed scientists to conclude that there are two distinct types of plumes. Smaller plumes produce near-circular rings typically 150-200 km in radius, white or yellow in color, that can be contaminated with silicates, and that frequently coat their surroundings with frosts of fine-grained SO2. Galileo observed a much smaller number of larger plumes, which produced oval orange or red, sulfur rich rings with maximum radii in the north-south direction that typically range in radius from 500-550 km [326].

While plumes may be the most exciting events on Io, surface changes are also caused by other events. Numerous small-scale changes have been documented using Voyager and Galileo spacecraft data. Many paterae were seen to brighten or darken during the Galileo mission. One assumption is that these changes were caused by thermal activity, heating the surfaces of the paterae, causing the existing SO2, a bright substance, to sublimate away. Another assumption is that the thermal activity heated the SO2 surrounding the paterae, causing it to liquify the ice, allowing it to flow onto the paterae floor flooding and brightening it. Or that fresh lava flowed from the interior due to thermal heating, darkening the surface of the paterae with new material [327] (Fig. 6.7). High resolution imaging of Amirani showed that new lavas covered an area ~620 km2 during a 134-day interval. The lavas apparently erupted from 23 separate locations across flow fields. Lava flows were also detected at Prometheus and numerous other locations. Scientists believe the Galileo data revealed the overturning of lava lakes, evidenced by the eruption of fresh lava onto surfaces already covered by lava. Thus, we see three main classes of surface changes: volcanic plume deposits, patera color or albedo changes, and SO2 seepage [328].

j— g Fig. 6.7. The Galileo spacecraft caught Tvashtar Catena in active eruption on the surface of Io in

^ W November 1999. The molten lava was so hot that it saturated, or over-exposed, Galileo's camera.

The lava appears to be producing fountains up to 1.5 km above the surface! Several other lava flows can be seen on the floors of the calderas, with the darkest flows probably the most recent. (Credit: NASA/JPL/University of Arizona/PIRL)

j— g Fig. 6.7. The Galileo spacecraft caught Tvashtar Catena in active eruption on the surface of Io in

^ W November 1999. The molten lava was so hot that it saturated, or over-exposed, Galileo's camera.

The lava appears to be producing fountains up to 1.5 km above the surface! Several other lava flows can be seen on the floors of the calderas, with the darkest flows probably the most recent. (Credit: NASA/JPL/University of Arizona/PIRL)

Even from a single site, eruption styles can be diverse. During the Galileo mission, Ishtar Catena exhibited many eruption styles, including a curtain of lava fountains, extensive surface flows, and a ~400 km high plume. These events occurred over a relatively short period of time, ~13 months [329].

After the Voyager missions, it was thought that most of the volcanic flows were due to sulfur flows. However, with the Galileo mission scientists began to realize that the temperatures of flows were so high that they had to be silicate based. Many of the flows observed were too hot to be sulfur based, since sulfur would vaporize at ~700 K. Galileo measured a lava flow at Prometheus at 1,100 K that had to be silicate rich [330]. On the other hand, Emakong Patera is surrounded by lava flows that are notable for being yellow-white. It is thought that this lava flow had been moderately hot sulfurous lava rather than very hot silicate. So, there is till evidence for sulfur flows, but silicate now seems to be more prominent [331]. One important result of the Galileo mission is that Io's paterae appear to be persistent lakes of lava [332].

During the Galileo era episodic brightenings that have been interpreted to be SO2 seepage occurred at several locations around Io; specifically at Haemus Montes, Zal Montes, Dorian Montez, and the plateau to the north of Pillan Patera. These are regions with pronounced topography adjacent to active volcanic centers [333]. A brightening of a bright halo around Haemus Mons apparently occurred between Voyager 2 and Galileo's first orbit around Jupiter. This brightening was attributed to possible SO2 seepage from the base of the massif [334].

There were also many smaller-scale changes in color or albedo that were confined to patera surfaces; such as those located at Gish Bar, Itzamna, Camaxtli, Kaminari, Reiden, Pillan, Dazhbog, and Amaterasu. Most of these locations are recognized hot spots, and in some cases the paterae later produced major eruptions that changed the surrounding surfaces for hundreds of kilometers [335]! Often, Io's volcanoes give notice they are about to erupt by a darkening of their caldera. Such events are common on Io, along with patera brightenings and color changes. This may signify transient heating within a caldera [336].

As with Earth, lava flows on Io can be associated with extremely high temperatures, and there may be a correlation between extremely high temperature and plume activity. There have been four well documented eruptions with temperatures exceeding 1,400 K on Io: Pillan, Masubi, Pele, and Surt. We can probably assume there have been many other events simply not caught by Galileo or other observational means. Scientists believe that such high temperatures may indicate the presence of m lava fountaining, which is driven by volatiles that also produce large plumes in Io's te tenuous atmosphere [337]. Galileo observed a spectacular eruption at Tvashtar, an c ¡X

eruption occurring along a ~25 km long fissure that produced a fire-fountain of lava o ^

reaching 1 km in height! This eruption was also observed from Earth with the IRTF *> Q

and Keck AO systems. These ground based observations helped establish the ~36 h ,0

duration and 1,300-1,900 K temperature of the fire-fountain event [338]. ^ ®

The most surprising thing learned during the Galileo mission may be that there ThSa were only a small number of volcanic centers that visibly altered their surround- TS

ings. Out of over 100 active volcanoes and 450 paterae considered young and potentially active, only 28 produced noticeable surface changes more than a few tens of kilometers in extent. Few of the high temperature thermal events detected by Galileo were accompanied by plume events, and even the powerful eruptions of Loki left no visible mark on the surrounding terrain [339] (Fig. 6.8).

Mass wasting processes act on Io; slumping and landslides occur in close proximity to each other, thus there is spatial variations in material properties over distances of several kilometers (Fig. 6.9). However, even though there is a lot of evidence for mass wasting events, the floors of paterae lying close to mountains are relatively free of debris. Thus, the rate of volcanic resurfacing seems to dominate [340]. Apparently, Io is so volcanically active that it is resurfaced at a rate of ~1 cm per year [341, 342]. In fact, Io is the most active known planetary body in terms of luminosity and resurfacing rate, even more active than the Earth [343]!

Io's volcanic activity does not tend to build mountains or domes and in fact there are few mountains or mountain ranges on Io. We do not see the mountain building associated with plate tectonics as we see on Earth. But Io does have mountains. One of them is Haemus Mons, a ~10 km high mountain surrounded by a bright halo of SO2 [344]. At its base, Haemus Mons appears to span 200 km [345].

Io's mountains are isolated, stand alone, and do not appear to form mountains ranges (Fig. 6.10). Thus, they do not appear to be the result of large-scale techton-ism as on Earth. Io's mountains are interesting because they rise directly off the plains without foothills. Their origin is a mystery, since sulfur does not have the strength to form peaks [346]. Galileo images show its mountains to be collapsing under their own weight. They must therefore be a relatively recent creation [347].

Paul Schenk and Mark Bulmer, using stereo images taken by Voyager in 1979, postulate that mountains like Euboea Mons are individually faulted and uplifted blocks of crustal material [348] (Fig. 6.11). The mountains are massifs that could

Fig. 6.8. A Galileo spacecraft image showing active volcanic centers on Io. Loki Patera is the large, dark horsehoe shaped feature close to the terminator toward the south pole of the moon. The big, reddish-orange ring in the lower right is formed by material deposited from the eruption of Pele, Io's largest volcanic plume. North is at the top of the picture. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.8. A Galileo spacecraft image showing active volcanic centers on Io. Loki Patera is the large, dark horsehoe shaped feature close to the terminator toward the south pole of the moon. The big, reddish-orange ring in the lower right is formed by material deposited from the eruption of Pele, Io's largest volcanic plume. North is at the top of the picture. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.9. Slumping Cliff on Io. The Galileo spacecraft caught this image of a mountain named Telegonus Mensa on Io showing outward slumping. The cliff is slumping due to gravity. The sun illuminates the surface from the upper right. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.10. A Galileo spacecraft image reveals details around a peak named Tohil Mons, which rises 5.4 km above Io's surface. Few of Io's mountains actually appear to be volcanoes. However, the shape of the pit directly to the east of Tohil's peak suggests a volcanic origin. (Credit: NASA/JPL/ University of Arizona/PIRL)

Fig. 6.10. A Galileo spacecraft image reveals details around a peak named Tohil Mons, which rises 5.4 km above Io's surface. Few of Io's mountains actually appear to be volcanoes. However, the shape of the pit directly to the east of Tohil's peak suggests a volcanic origin. (Credit: NASA/JPL/ University of Arizona/PIRL)

have been upthrusted recently - thus they are not necessarily ancient. The underlying structure of the mountains appears to be rigid silicate blocks. These blocks are probably thrust upward and then tilt. The angular mountains appear to be younger than the rounded ones. There may be structural relationships between mountains and calderas on Io. The absolute ages of Io's mountains are unknown [349].

High-resolution images taken by Galileo also reveal unexplained ridges on otherwise flat terrain. Some sites of ridge formation can be attributed to down-slope motion of loose material. But, this cannot explain the ridges seen on plains that are relatively flat. These ridges are similar to dunes on Earth and Mars. Earth and Mars have atmospheres with enough density to allow particle transport and thus dune formation. But the atmospheric pressure on Io is too low to do this. Io's winds cannot transport particles to form dunes [350].

These ridge, or dune-like, features are quite common on Io, since 28% of the Galileo high-resolution images show them. The regions with abundant ridges are rich in volatiles, dominated by SO2. Thus, it is speculated that the presence of a blanket of volatiles is necessary for ridge formation [351]. Therefore, since Io lacks an atmosphere with enough density, scientists conclude that the formation of these ridges is consistent with formation from tidal flexing of Io by Jupiter, in the presence of a substantial volatile rich deposit on the surface [352].

Previous modeling supported the hypothesis that Io has a massive metallic core in which a magnetic field may be produced [353]. With the Galileo flyby of Io, the moment of inertia could be determined, allowing the model of Io's interior to be

Fig. 6.1 1. Mountains of Io. The Galileo spacecraft caught this image of mountains of lo. The mountain just left of center in the image is 4 km (13,000 ft) high, and the small peak to the left is 1.5 km (5,000 ft) high. These mountains seem to be in the process of callapsing. (Credit: NASA/JPL/ University of Arizona/PIRL)

Fig. 6.1 1. Mountains of Io. The Galileo spacecraft caught this image of mountains of lo. The mountain just left of center in the image is 4 km (13,000 ft) high, and the small peak to the left is 1.5 km (5,000 ft) high. These mountains seem to be in the process of callapsing. (Credit: NASA/JPL/ University of Arizona/PIRL)

Fig. 6.12. This is a cutaway view of the possible internal structure of lo. The moon has a metallic iron, nickel core. The core is surrounded by a rock or silicate shell, which extends all the way to the surface. (Credit: NASA/JPL/Caltech)

refined. The low value that resulted from the flyby indicates that Io possesses a two-layer structure, with a metallic core most likely of iron and iron-sulfide, enclosed by a partially molten silicate mantle, over which there is a volcanically active crust, or lithosphere (Fig. 6.12). The Galileo data suggested that the core would be 36-52% of the moon's radius [354]. Io is so hot that its silicate lithosphere is very thin, and the magma is never very far from its surface [355].

6.1.2 Europa

Europa is the second closest Galilean moon to Jupiter, with an orbital period of 3.551 days [356]. It is the smallest of the Galilean moons with a diameter of

3,130 km and a mean density of 2.989 ± 0.046 [357]. Europa is intriguing for a number of reasons, not the least of which is that it may have a liquid ocean beneath its surface that scientists think might harbor some form of life!

The surface temperature on Europa is 120-131 K [358] and its atmosphere is tenuous, at best. The materials present on the surface are the products of radiation transformations, cryovolcanism, impact and gardening events that have occurred over time. Chemical alteration of the ice has been shown to produce condensed hydrogen peroxide within some icy surface regions. An atmosphere composed mostly of atomic and molecular hydrogen with some atomic and molecular oxygen from radiation processing of surface ice has also been detected [359].

As early as 1994, the Hubble Space Telescope had detected oxygen emissions from Europa [360]. The Cassini spacecraft Ultraviolet Imaging Spectrograph g X

(UVIS) showed the presence of an extended oxygen atmosphere in addition to ^O

a bound molecular oxygen atmosphere. The UVIS observations also indicated > £

the presence of atomic hydrogen and possibly other elements. Europa's water ice =

surface undergoes charged particle bombardment. This erodes the surface due to <d £

'sputtering' and to a lesser extent sublimation. 'Sputtering' produces hydrogen and ^^

oxygen. The hydrogen is lost to space leaving the oxygen behind, thus the thin, bound oxygen predominant atmosphere and escaping hydrogen [361]. A torus of energetic neutral atoms has also been associated with Europa [362].

The surface of Europa has been characterized as 'billiard ball' smooth (Fig. 6.13). There is so little topographical relief that the vertical range of its surface is confined to a few hundred meters. Europa is covered with surface ice and though relatively

Fig. 6.13. This color composite view shows a view of the moon Europa in natural color (/eft) and in enhanced color (right). The bright white and bluish part of Europa's surface is composed mo stly water ice. The brownish, mottled regions on the right side of the moon may be covered by hydrated salts and an unknown red component. The yellowish mottled terrain on the left side is caused by some other unknown component. North is to the top of the image. (Credit: NASA/JPL/University of Arizona/PIRL)

Fig. 6.13. This color composite view shows a view of the moon Europa in natural color (/eft) and in enhanced color (right). The bright white and bluish part of Europa's surface is composed mo stly water ice. The brownish, mottled regions on the right side of the moon may be covered by hydrated salts and an unknown red component. The yellowish mottled terrain on the left side is caused by some other unknown component. North is to the top of the image. (Credit: NASA/JPL/University of Arizona/PIRL)

smooth, this surface presents albedo, color, and texture variations [363]. This surface ice is thought by some to be a mixture of salt brines [364].

At low resolution, the surface can be classified into two main terrain types, ridged plains and mottled terrain. High-resolution Galileo images reveal that the mottled terrain consists predominantly of chaos regions. There is debate regarding the mechanisms that form these features, but the most popular model is that convection in the underlying ice shell deforms the surface, producing chaos regions, pits, and domes [365].

Figueredo and Greeley actually identify five principal terrain types: plains, bands, ridges, chaos, and crater materials. These are thought to result from tectonic fracturing and lineament building, cryovolcanic reworking of surface units with possible emplacement of sub-surface materials, and impact cratering [366] (Fig. 6.14). Miyamoto, et al., describe additional surface features as domes, £ platforms, irregular uplifts, and disrupted micro-chaos regions. Some of these

£ features show positive elevations reaching 100-200 m or more, and have surface tex-

c tures bearing no relation to the surrounding terrain, These features appear to have

O ^ obscured and spread over the pre-existing surface as a viscous flow; that is, as though

•> V there has been a fluid emplacement of ice or slush on the surface. These features are

,0 rarely disrupted by tectonic structures such as ridges and must therefore be some of q © the youngest features on Europa's surface [367].

.C O Four types of plain are seen. 'Undifferentiated plains' are smooth, gradational with

^ adjacent terrain, and are cut by numerous linear features. 'Bright plains' are located towards high latitudes, and are crisscrossed by a variety of linear features. 'Dark

Fig. 6.14. The Galileo spacecraft caught this southern hemisphere image of Europa in February 1997, showing the southern extent of the "wedges" region, an area of extensive disruption. South of the wedges, the eastern extent of the Agenor Linea is also visible. Thera and Thrace Macula are the dark irregular features southeast of Agenor Linea. North is to the top. (Credit: NASA/JPL/Caltech)

Fig. 6.14. The Galileo spacecraft caught this southern hemisphere image of Europa in February 1997, showing the southern extent of the "wedges" region, an area of extensive disruption. South of the wedges, the eastern extent of the Agenor Linea is also visible. Thera and Thrace Macula are the dark irregular features southeast of Agenor Linea. North is to the top. (Credit: NASA/JPL/Caltech)

plains' resemble the light plains but are darker. 'Fractured plains' give the appearance of being shattered and bear curved gray streaks and numerous brown spots [368].

The various linear features are what stand out when first looking at Europa. The 'triple bands' comprise a bright stripe, possibly a ridge, running down a dark band (Fig. 6.15). They run for thousands of kilometers but rarely exceed 15 km wide. They often start or end near dark circular spots, or brown mottled terrain. These may be caused by stresses induced by the moon's orbital eccentricity. The 'dark wedges' that are seen can be 300 km long and may be 25 km wide at the open end. Since they cut across older features, they appear to be the result of fractures, splitting and spreading the surface ice, with the gaps being subsequently refilled with slush that froze again [369] (Fig. 6.16).

Ridges are by far the most common linear feature on Europa. A double ridge feature consists of a central trough bounded by a ridge pair. In some instances, double ridges taper and continue as single ridges, troughs, or fractures. Single E

ridges are usually smaller than there double counterparts and form relatively £

short segments [370]. There are examples of linear features and ridges that show c ¡X

evidence of filling in or flooding, specifically swamping of preexisting ridges and o ^

grooves by a fluid that has erupted onto the surface. This is the first evidence *> ®

of ice flows on any of the Galilean moons [371]. Another exciting discovery ,0

were features that scientists referred to as 'icebergs.' Galileo revealed that some ^ ®

of Europa's surface had been fractured into polygonal 'rafts' of ice that were JZ O

individually 3-6 km across (Fig. 6.17). The chaos around these features was stained brown, suggesting the presence of minerals of endogenic origin. Scientist Ron Greeley said the blocks of ice were "similar to those seen on Earth's polar seas

Fig. 6.15. This high-resolution image of Europa taken by the Galileo spacecraft shows a dark, relatively smooth region at the lower right corner that may be a place where warm ice has welled up from below. The image also shows two prominent ridges that have different characteristics; the youngest ridge runs from left to top right and is about 5 km wide. The ridge has two bright, raised rims and a central valley. It overlies, and therefore must be younger than, a second ridge running from top to bottom on the left side of the image. (Credit: NASA/JPL/Caltech)

Fig. 6.15. This high-resolution image of Europa taken by the Galileo spacecraft shows a dark, relatively smooth region at the lower right corner that may be a place where warm ice has welled up from below. The image also shows two prominent ridges that have different characteristics; the youngest ridge runs from left to top right and is about 5 km wide. The ridge has two bright, raised rims and a central valley. It overlies, and therefore must be younger than, a second ridge running from top to bottom on the left side of the image. (Credit: NASA/JPL/Caltech)

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