Astronomy 101 Lab - Galilean Moons...I need help answering the following questions!!!!?
II. Io - The Jovian Cauldron
5. Measure the diameter of the volcano Prometheus as seen on
Astronomy 101 lab manual, v. 6.4 page 85 ENCOUNTERS WITH THE GALILEAN
OBJECTIVES AND BACKGROUND
Jupiter's four large moons were first discovered by Galileo in 1610 and examined in close detail
by the Voyager I and II spacecraft in 1979. Launched in October 1989, NASA's Galileo
spacecraft entered orbit around Jupiter on December 7, 1995. Its mission was to conduct
detailed studies of the giant planet, focusing on its largest moons and the Jovian magnetic
The objective of this laboratory is for you to observe some of the pictures of these distant worlds
and to learn what such pictures tell us about their surfaces, interiors, and past histories. ACKNOWLEDGMENTS
This laboratory exercise was modified slightly from one originated by the Sommers-Bausch
Observatory, University of Colorado at Boulder, for their Astronomy 1010 laboratories on the
Galilean satellites. We gratefully acknowledge their permission to use this laboratory exercise. SKILLS/COMPETENCIES Demonstrate the ability to select and apply contemporary forms of technology to
solve problems or compile information.
Evaluate the relevancy of data.
Apply concepts to new situations. MATERIALS Photographs taken from the Spacecraft Galileo of the Galilean moons,
A ruler, and a calculator. If you are provided photographs in the classroom for this laboratory please do NOT mark on
them! Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 86 LAB
This lab is a guided tour of the Galilean moons through photos. Written explanations of each
photo or set of photos is provided. You need to complete the questions at the end of the lab
and turn in only those pages. I: PROPERTIES OF THE GALILEAN SATELLITES Table 1 shows the properties of the Galilean satellites, and, for comparison, the Moon and
(kg/m3 ) Albedo
(P/Pio) Moon 1,738 3,340 0.11 ? ? ? Io 1,821 3,530 0.61 5.9 1.77 1.00 Europa 1,565 3,030 9.4 3.55 2.01 Ganymede 2,634 1,930 0.42 15.0 7.15 4.04 Callisto 2,403 1,790 0.20 26.4 16.69 9.43 Mercury 2,439 5,420 0.12 ? ? ? Object 0.64 Table 1?Properties of Galilean Satellites, Moon, and Mercury Figure 1 (not included in this lab manual, see provided figures) shows a family portrait of
the four Galilean moons. Note that Io and Europa are similar in size to the Moon, whereas
Ganymede and Callisto are comparable to Mercury in size. However, when we look at other
properties, such as density and albedo, the similarities do not hold up. Each of the four Galilean
moons is believed to have some iron at their center, surrounded by rock with perhaps a layer of
water/ice on top. The density of iron is about 8,000 kg/m3, the density of rock is 2,000 to 3,000
kg/m3, the density of water/ice is about 1,000 kg/m3. Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 87 II: IO ? THE JOVIAN CAULDRON Io, the Galilean moon closest to Jupiter, is the most volcanically active body in the solar system.
What makes Io?s volcanic activity so interesting is the mechanism that causes it. For instance,
our Moon?s interior cooled and most volcanic activity stopped over 3 billion years ago. Because
Io is about the same size as the Moon, its interior must have also cooled. So, why has Io?s
volcanic activity not stopped? The answer lies with Jupiter and the other Jovian moons. Their
strong gravitational pull on Io ?stretches and squeezes? the interior of Io, thus melting the interior
and causing volcanic activity. The process?known as tidal heating?is like the gravitational
effect of the Sun and Moon on the Earth, causing the tides of the ocean and crust, although the
force exerted by Jupiter on Io, because of its mass and closeness to Io, is much greater than
that of the Moon and Sun on the Earth. Figure 2 shows Io photographed by the Galileo spacecraft. The image is centered on the side
of Io that always faces away from Jupiter. The color in the image was derived using the nearinfrared, green, and violet filters of the Galileo camera and has been enhanced to emphasize
variations in color and brightness that characterize Io?s volcano-pocked face. The white patches
are sulfur-dioxide frost. The black and bright red materials correspond to the most recent
volcanic deposits, some less than a few years old. .
The active volcano Prometheus is seen just to the right of the center of the disk. To appreciate
the size of the volcanoes on Io, we have to determine the scale of the photograph.
Figure 3 is another image of Io, also taken by the spacecraft Galileo that shows a new bluecolored volcanic plume, named Pillan, extending into space (enlarged in the inset with the
plume. The other inset is an enlargement of Prometheus near the center of the moon). The blue
color of the plume is consistent with the presence of sulfur-dioxide gas erupting from the
volcano, scattering sunlight (similar to water molecules making the Earth?s sky blue). The sulfur
dioxide freezes into ?snow? in Io?s extremely tenuous atmosphere and makes the white patches
on the surface. These eruptions are common on Io and were first discovered when Voyager
passed Io in 1979. Interestingly, they were not discovered by Voyager scientists, but rather by
Linda Morabito, a young member of the Navigation Team. Morabito discovered them when she
had difficulties matching the edge of Io?s image with a circle, and then enhanced the images to
investigate the problem.
These volcanic eruptions on Io are far larger than any found on Earth. Because Io?s atmosphere
is exceedingly thin, there is no air resistance and the gas molecules that erupt from a volcano
follow paths like shells fired from a cannon. These paths are called ballistic trajectories (after
ballista, an engine used in ancient warfare for hurling stones).
The maximum height, H, an object on a ballistic trajectory can reach is given by: v2
2g Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 88 where v is the initial velocity of material exiting the volcano and g is the acceleration of gravity
which is 9.80 m/sec2 on Earth, but only 1.81 m/sec2 on Io, about a factor of 5 less. Scientists are noting many changes that have occurred on Io?s surface since the Voyager flybys
17 years ago, and even a few changes from month to month between Galileo images. Figure 4
shows three images taken in 1997, including one on April 4 (left) and September 19 (center).
The large black area in the September 19th image is material that has been ejected by the
Pillan plume. (For a more detailed description of this comparison, go to
http://photojournal.jpl.nasa.gov/catalog/PIA00744.) III: EUROPA ? WATER WORLD? Jupiter's moon Europa has a density closer to that of rock than water, yet spectroscopic analysis
has shown that the surface of Europa is composed almost entirely of water ice.
The ice seen on the surface can be the top of only a thin layer of water/ice over a rocky interior.
Recent measurements of the effects of Europa?s gravitational pull on the Galileo spacecraft
allow us to put an upper limit of about 170 km for the thickness of the water/ice shell.
In Figure 5, Galileo pictures of Europa show an icy crust that has been severely fractured. The
upper gray image covers part of the equatorial zone of Europa, an area of about 360 by 770 km
(220 by 475 mi, or about the size of Nebraska), and the smallest visible feature is about 1.6 km
(1 mi) across. In the lower image the color has been greatly exaggerated to enhance the
visibility of certain features. The fractures, called linea, and the mottled terrain appear
brown/red, indicating the presence of contaminants in the ice. The blue icy plains are subdivided
into units with different albedos at infrared wavelengths, probably because of differences in the
grain size of the ice. The area shown is about 1,260 km across (about 780 mi or about the size
Figure 6 shows global views of Europa and of the Moon. Europa is only 10% smaller than the
When asteroids and comets collide with planets and moons, the explosions resulting from the
collisions are extremely violent. Matter is excavated from great depths and can be flung wide
distances from the resulting crater. Larger impacts create larger craters and expose materials
from greater depths. The appearance of the crater resulting from such an impact depends,
among many things, upon the material properties of the substrate in which the crater is formed.
In Figure 7 four examples of impact features are shown, two on the Moon and two on Europa. Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 89 Also shown in Figure 7 is the aftermath of two much larger impacts. The Europa impact feature,
known as Tyre Macula, looks very different from the lunar crater, but it still can be identified as
an impact feature from the clusters of surrounding small secondary craters that were created
during the big (primary) impact. At 5.2 AU from the Sun, the temperature on the surface of
Europa is nearly 200 degrees Celsius below the freezing point of water, and its albedo is 0.64.
In contrast, the Moon?s surface is composed primarily of a volcanically derived type of rock
known as basalt. From seismologic experiments carried out during the Apollo missions, we
know that the Moon's crust is very thick and is composed of the same material throughout. This
is why, up to a certain size, most lunar craters look very similar.
Figure 8 shows an image of ?chaotic terrain? that looks as if it were created when the surface
surrounding the ice blocks was slushy and the ice blocks, or ?ice rafts? were free to float about.
The thickness of these ice rafts probably does not exceed their width. (Why not? Think of a
pencil floating in water. In which way would it orient itself?)
If such an ocean does exist, it might be a possible place for life to exist. Life, as we know from
studies of life here on the Earth, can be very hardy and can adapt to survive in many regions.
Life has been found near volcanic vents at the bottom of the ocean where no sunlight can
reach, the frozen lakes of Antarctica, and even inside rocks! IV: GANYMEDE ? THE GIANT MOON
Ganymede is the largest moon in the Jovian system and also in the entire Solar System. In fact,
it is even larger than Pluto and Mercury! Furthermore, the Galileo spacecraft detected a
magnetic field near Ganymede that indicates that the moon has a liquid iron core (similar to that
of the Earth and Mercury). However, the average density of Ganymede is close to that of water
so that the outer layers of Ganymede must be water/ice rather than rock.
Figure 9 shows global pictures of the Moon and of Ganymede. Figures 10 and 11 are close
ups of a darker and lighter region on Ganymede, respectively. When we look closer, any
resemblance to the Moon disappears.
The high-resolution image from the light region on Ganymede covers a region about the same
size as San Francisco Bay, compared in Figure 12. On the left, the resolution is that of the best
images taken by Voyager in 1979 and on the right the images are at the Galileo resolution.
You may remember Comet Shoemaker-Levy-9 making a spectacular display when it
bombarded Jupiter in 1994. A couple of years before, the comet had been broken up into about
21 fragments when it had passed close by Jupiter. When it came back to Jupiter in 1994, the
comet fragments plunged (and exploded) in Jupiter?s atmosphere. It was realized at this time
that comets are probably frequently broken up by Jupiter?s gravity. Sometimes these broken up
comets hit the moons rather than the planet. Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 90 V: CALLISTO?A BATTERED WORLD
Callisto, the fourth Galilean moon, has the dubious distinction of being the most cratered moon
in the solar system. Callisto is a little smaller than Ganymede (about the size of the planet
Mercury) and is apparently composed of a mixture of ice and rock similar to Ganymede. Unlike
the other Galilean moons, Callisto has endured virtually no tidal heating. Callisto's albedo is
about half that of Ganymede but Callisto is still more reflective than the Moon. The darker color
of Callisto suggests that its upper surface is ?dirty ice? or water rich rock frozen at Callisto's cold
surface. The abundance of craters on its surface suggests that its surface is the oldest in the
Galilean system, possibly dating back to final accretion stages of planet formation 4 to 4.5 billion
years ago. And, unlike the other Galilean moons, Callisto's surface shows no signs of
volcanism, tectonics, or other geologic activity, further supporting the hypothesis that Callisto's
surface took its present form long ago, and is hence very old.
Valhalla, the prominent ?bulls eye? type feature in the 1000-km image in Figure 14, is believed
to be a large impact basin, similar to Mare Oriental on the Moon and the Caloris Basin on
Mercury. The ridges resemble the ripples made when a stone hits water, but here they probably
are the result of a large meteorite. The fractured ice surface was partially melted by the impact,
but then resolidified before the ?ripple? could subside.
The four images in Figure 14 show increasing detail as Galileo zoomed in to smaller and
The dark material is probably dust produced by the breakup of micrometeorites (tiny, dust-sized
meteorites). It is probably very similar to the dust on the surface of the Moon. Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 91 I: PROPERTIES OF THE GALILEAN SATELLITES
1. Considering just the average densities in the table above, (a) which Galilean moon would
you expect to have the most ice (i.e., the least rock/iron); and (b) which moon could be all
rock and iron, with no ice on the surface? 2. All of the Galilean moons are ?phase locked? with Jupiter. That is to say, one side of each
moon always faces toward Jupiter, and one side always faces away. Can you think of
another moon in the solar system that is also ?phase locked? with the planet that it orbits?
What is its name? II: IO ? THE JOVIAN CAULDRON
3. The dark spots in figure 2 are volcanoes. Make a rough count of the number of volcanoes in
the image 4. Measure the diameter of Io on Figure 2 (measure in mm). Io is actually 3,630 km in
diameter. Determine the scale factor, S, of the image. By dividing the actual diameter of IO
(in km) by your measurement of the photo (in mm), you are finding the number of ?real? km
in each mm in the photograph.
S (km / mm) = Diameter of Io in km / Diameter of Image in mm Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 92 5. Measure the diameter of the volcano Prometheus as seen on Figure 2, including the bright
ring around it. Use the scale factor above to determine its actual size. What is its actual size
in kilometers? Compare this with the sizes of the volcano Mauna Kea in Hawaii (~80 km
across, about the size of a metropolitan area) and Olympus Mons on Mars (the largest
volcano in the solar system at 700 km across, about the width of Colorado). 6. The scale factors S for the insets of Figure 3 are about 10 km per millimeter. (You can
confirm this by comparing the size of Prometheus in the inset to the size you calculated for it
from the last question.) Measure the height of the plume of Pillan in millimeters, and then
determine its actual physical height. Convert this height to meters (1 km = 1,000 m). 7. Using your value for the height (in m) of the plume, determine the initial vent eruption
velocity of the plume (in m/s). For comparison, 1000 m/s equals about 2,000 mph. (Use the
equation on page 87, but rearrange it to solve for velocity.) Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4
8. page 93 Volcanic eruptions on Earth cannot throw materials to such high altitudes because of
atmospheric resistance and stronger gravity. Ignoring atmospheric friction and considering
only the stronger gravity on Earth, what would the height of the Pillan plume be if it had
erupted from a similar volcano on Earth? (That is, what would H be on Earth if the initial v
were the same, but with the Earth?s value of g?) 9. For comparison, Mount Everest is just over 8 km high, commercial jets fly at about 15 km,
and the Space Shuttle orbits the Earth at 150?500 km. Would such plumes be a hazard to
commercial jets or the Shuttle? 10. Tall plumes (and high velocities of ejected material) are due to the pressures that build up
because of internal sources of heat. Compare the height of the Pillan plume, scaled down to
the Earth?s gravity, with the height of the geyser ?Old Faithful? (around 70 m) and the plume
from the eruption of Mount St. Helens (1,000 m). 11. What does this tell us about the materials that build up inside Io compared with the magmas
that feed volcanoes on Earth? Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 94 12. The diameter of the new black patch of volcanic material in figure 4 is 400 km (half the width
of the state of Colorado). This corresponds to about 1% of the surface area of Io. If such
eruptions occur at random over the surface every 6 months, estimate how many years it
would take to resurface the moon. (Hint: what percentage gets resurfaced every year?
From there figure out how many years to get to 100%) 13. Do you see any impact craters on any of the Io pictures? Explain your observation. III: EUROPA ? WATER WORLD?
14. What percentage of Europa?s radius is its outer water/ice shell? 15. From the two images in figure 5, does it look like the contaminants come from below (e.g.
seeping through the cracks) or from above (e.g. meteorite dust deposited on the surface)?
Explain. Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 95 16. Does our Moon or Europa have the older surface? Explain your reasoning. 17. Based on the appearance of the craters on Europa and on the Moon, is Europa?s ice cold
enough to behave similarly to rock on our Moon? 18. Because larger impacts excavate material from greater depths and thus reveal more of the
interior, what does the peculiar appearance of this large impact structure on Europa suggest
about its crust of ice? Do you think it is frozen solid all the way through? What do you think
happened to the impactor that made Tyre Macula? 19. The image covers an area of 34 km by 42 km (caption is incorrect). Pick the smallest ?ice
raft? that still looks as if it was broken off from neighboring ice blocks. Determine the image
scale (mm per km), and estimate a width for this ice raft in kilometers. From this
measurement, make a rough estimate of the thickness the ice crust (in km) at the time that
the ?chaotic terrain? froze. Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 96 20. The next issue is to find out how long ago the ?chaotic terrain? froze. Could there be just a
thin layer of ice over a liquid ocean today? What is the evidence in Figure 8 that the chaotic
terrain froze over relatively recently? (Millions rather than billions of years ago) 21. What difficulties do you think life in a hypothetical Europan ocean would have to overcome
in order to survive? IV: GANYMEDE ? THE GIANT MOON
22. Superficially (Fig. 9), Ganymede and the Moon look very similar. For the Moon, which is
older, the lighter or darker regions? 23. The dark regions on Ganymede (Fig. 10) are older than the newer, lighter regions (Fig. 11).
The dark material is probably meteorite dust that has accumulated on the surface. Looking
at the close up of the dark region in Figure 10, how do the craters on the icy surface of
Ganymede change as they get older? (Compare them with the ?fresh? lunar craters shown in
Figure 7.) Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 97 24. What do you think happened to create the grooves in the light region in Figure 11? 25. For the cases of (a) San Francisco Bay and (b) Ganymede, describe how the higherresolution images (figure 12) change our interpretation of the surfaces of these two regions. 26. Figure 13 shows a chain of craters on Ganymede that may have been produced by a
broken up comet. How many pieces of comet impacted Ganymede? V: CALLISTO?A BATTERED WORLD
27. Notice how the top two (large-scale) pictures in figure 14 show the surface of Callisto is
saturated with craters so that if another impactor hit, it would probably cover up on old
crater. However, when we go to smaller scales, does the surface remain saturated with
craters? 28. What is the approximate size of the smallest crater that you can distinguish as a crater in the
1-km image? (The scale bar is 1 km.) Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 98 29. As a general rule of thumb, most craters are about 1/10th as deep as they are wide. (That
is, craters tend to have a width-to-depth ratio of 10:1.) If all smaller craters are covered up
with meteorite dust, roughly how thick must the dust layer be? VI: GALILEAN MOONS W RAP-UP Now, you are equipped to make some very general statements about how the characteristics of
the four Galilean moons vary with distance from Jupiter. 30. First, how does the degree of tidal heating vary with distance from Jupiter? 31. How might this lead to the observed differences in the amount of liquid water on these four
moons? Galilean Moons Lab 15 pages Astronomy 101 lab manual, v. 6.4 page 99 32. How might this result in the observed ages of their surfaces? The tidal heating which causes Io?s volcanic activity is due to Io?s orbital resonance with the
other Galilean moons. A moon is in orbital resonance if its orbital period is (or nearly is) a wholenumber multiple of the orbital period of another moon. (E.g., if one moon has twice the period of
another moon, they are in orbital resonance with each other). 33. From the table at the beginning of this exercise, which of the Galilean moons is not in
orbital resonance with any of the other Galilean moons? 34. How does the surface age of this non-resonant moon?s surface compare to the
others? Galilean Moons Lab 15 pages
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