V O L C A N O E S    O N    I O

Vocabulary: Effusive vs. Explosive, viscosity, silica, tidal heating Material: pencil, ruler, and a calculator (optional)

INTRODUCTION:
The only place active volcanic eruptions have been observed beyond the Earth is on Io, one of Jupiter'9s four large moons (a.k.a. the Galilean satellites). Io, the large moon closest to Jupiter, has a surface that is made up of rocky materials. Volcanic plumes, up to 300 km high, were imaged by the Voyager spacecraft (1978-1981). Images also show Io'9s multicolored surface, resulting from a wide range of volcanic flows and sulfur-containing deposits (Figure 1). New images from the Galileo spacecraft provide an opportunity to look for surface changes and to gain a better understanding of the types of volcanic activity on Io. We will use these Galileo images to investigate the styles of volcanism on Io.

A. EXPLOSIVE VOLCANISM ON IO
One of the first images of Io obtained by the Voyager 1 spacecraft in 1979 shows a plume above one of it'9s volcanoes. Since then, both the Voyager and Galileo spacecraft have photographed many volcanic plumes on Io. Plumes are the products of volcanic eruptions in which gas-rich magma is explosively shredded by gases as they escape from the magma. On Io deposits resulting from these plumes are typically red. However, plumes on Io also result from lava flows releasing gases from surface materials. As the front of the lava flow moves with time, the location of the plume also migrates. Figure 2 shows a ~140 km high plume, which Galileo observed erupting from the volcano Pillan Patera in June of 1997.

Gravity vs. gas

Because the Earth'9s Moon has a lower gravitational force compared to Io, volcanic eruptions on the Moon should have been more explosive (with a wider distribution of deposits) than Io (all else being equal). But Io'9s eruptions are more explosive! The red circular feature seen in the left side of Figure 3 is a ring of volcanic deposits surrounding the volcano Pele. The dark deposit which interrupts this ring in the right side of Figure 3 is an Arizona-size deposit from a later explosive eruption. The wider distribution of volcanic deposits on Io compared to the Moon probably indicates that there is something different about the chemical make-up or gas content of Io'9s eruptions that make them more explosive than eruptions on the Earth'9s Moon. Other observations of lava flows on Io support this hypothesis as well.

On the Earth, very fluid low-silica lavas have produced the longest recognizable lava flows. Lavas of similar composition are thought to have erupted on the Moon, Venus, and Mars, based on the length and thickness of the flows. However on Io additional information is available.

Instruments on the Galileo spacecraft have obtained temperatures of erupting volcanic materials on Io, including those around Pillan (Figure 2), Pele (Figure 3), and Lei-Kung (Figure 4). The temperatures in these areas range from 1210-1625'0C (2210-2960'0F), which are much hotter than the hottest eruptions on Earth (1080-1180'0C or 1980-2160'0F). These temperatures are more like those of very low-silica lavas which erupted during the early part of Earth'9s history, over 2 billion years ago. Very hot lavas are very fluid and can easily flow long distances. Alternatively, carbon-dioxide-rich lavas can also have low viscosities that might promote long-distance flow. Although fluid, low-silica lavas on Earth can travel long distances, the fact that very high temperatures were measured on Io near the source of these flows may mean that lavas with unusual compositions (very low silica content, or carbon-dioxide rich) - compared to Earth - may be erupting on Io.

The chemical make-up or composition of lavas strongly influences the types of deposits that they form. On Io, darker materials, which are also very fluid, form long flows that cover large areas producing shield volcanoes or lava plains. On Earth, lighter-colored, silica-rich materials which have high viscosities form short flows that tend to pile up forming domes and steep volcanoes.

Many changes can be noted in these images. Here are a few: The most obvious is the large circular dark feature just to the right (east) of Pele that has partially covered the red ring of sulfur-rich material. At the center of this dark feature is the volcano Pillan Patera which erupted a plume of material (cp., Figure 2). This dark feature is roughly 400 km (250 miles) across. At the center, the material is dense, and may contain lava flows; away from the center the dark material is diffuse, and may be explosive deposits.

D. IO'9S HEAT SOURCE
Measurements from the Galileo spacecraft show that very hot lavas are erupting on Io. What would cause these unusually hot lavas to be produced? The answer is tidal heating. Io is continuously pulled by the massive planet Jupiter on one side, and by the changing gravitational tugs of the large moons Europa, Ganymede and Callisto. This pulling, together with Io'9s elliptical orbit, causes Io'9s interior to flex, which generates large amounts of heat. This heat melts part of Io'9s interior, and produces the very hot lavas that are observed. The effect of tidal heating can be demonstrated with the exercise Flexing Muscles and Moons (http://galileo.jpl.nasa.gov).

E. SUMMARY

With the information obtained from the Galileo spacecraft, scientists are learning more about volcanism on Jupiter'9s moon, Io. Io'9s multicolored surface is made up of volcanic materials, including dark low-silica flows, and several different types of sulfur deposits. Explosive eruptions produce plumes and deposits which travel hundreds of kilometers (farther than on the Earth or the Moon), These great distances may be due to unusual lava compositions and/or relatively high gas contents. Very hot lavas that are erupting from volcanic vents on Io may be similar to lavas that erupted on Earth billions of years ago. These lavas are very fluid and produce very long flows. As we have seen, eruption environments (e.g., gravity), lava compositions and gas contents influence the types of volcanic deposits made. Understanding these differences is key to understanding volcanism on Io, the Earth, and other planetary objects as well.

Volcanoes on Io: Graphics Page

	Figure 1. Global view of Io obtained by the Galileo spacecraft.
	Figure 2. Volcanic plume erupting from Pillan Patera
	Figure 3. Left: Red volcanic ring surrounding Pele. Right: Dark, Arizona-size eruption.
	Figure 4. Lei-Kung volcanic region on Io.

Volcanoes on Io: Student Worksheet

A. Explosive Volcanism: Plumes and deposits

1. Based on the observation of volcanic plumes, like the one seen in figure 2, erupting on Io for a period of over 20 years, and presumably for the age of the solar system, what can you determine about the abundance of gas-rich materials on Io?

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2. Volcanic plumes on Io have a range of heights. For example, the plume from Pillan Patera is 140 km high, while a plume from Ra Patera was measured at about 100 km high. Assuming all other factors being equal, what does this difference in plume heights imply about the distribution of gases beneath Io'9s crust and on its surface (e.g., is it uniform or nonuniform)?

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3. If a plume from Pillan Patera had a height of 250 km three months after the image used in Figure 2 was obtained, what would that imply (again, all other factors being equal) about the change in gas content with time? ________________________________________________________________________________

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1. In addition to explosively deposited deposits, Io'9s surface contains many lava flows. To determine how far some of these lava flows have traveled, use Figure 4, to measure the dimensions of the T-shaped flow (lines A-B, C-D) with a ruler. If 1 cm in the picture = 130 km on the ground, what are the flow lengths in kilometers and miles. (Recall 1 km = 0.62 miles).

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2. The flows in figure 4 are very long (and thin)! Based on this observation, do you think the lava which produced these flows had a high or low viscosity? ________________________________________________________________________________

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1. The left picture in Figure 3 is a Galileo image of Pele taken on April 4, 1997, and the right picture is the same area photographed on September 19, 1997, roughly five and a half months apart. Describe all the changes you see:

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2. Based on what you have learned, list at least two factors that are important in whether a volcanic eruption will be explosive or effusive, and how they affect the distance flows or deposits will travel.

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Volcanoes on Io: Teacher Notes

Answer Key

1. Gas-rich materials have been and continue to be abundant on Io, both on the surface and in the subsurface.
2. The distribution of gases is nonuniform, otherwise all plumes would reach equal heights. This assumes that vent geometries, sizes and ascent rates are similar between volcanoes.
3. This implies that gas abundance changes with time.

1. The distance from A to B is about 600 km or 375 miles. The distance from C to D is about 415 km or 250 miles.
2. These are very low viscosity (fluid) flows. High viscosity flows do not flow easily and result in thick flows or dome-like structures.

Answers will vary. The major changes are listed in the exercise, and include the large, dark circular feature that disrupts the red ring, as well as the presence/absence of white material (SO2 frosts).

The figures used in this exercise, along with the original release captions, may be located at the following URLs:

Figure 1: http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA02309
Figure 2: http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA01081
Figure 3: http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA00744
Figure 4: http://photojournal.jpl.nasa.gov/cgi-bin/PIAGenCatalogPage.pl?PIA00537