"Galileo Calling Earth"

Grade Level: Middle School

Description: How is the data from the Galileo spacecraft sent to scientists on Earth?

Objective: Students learn how digital signals are created and returned to Earth, and then apply these concepts to the interpretation of images from the Galileo Orbiter.

Materials: Digital images (To print images, click here); pencils

Vocabulary: Digital signals, charge-coupled device (CCD), pixel, resolution

Except for some journeys to the Moon, all spacecraft sent to explore the solar system have been one-way trips. This means that the information collected, including pictures, has to be sent back to Earth electronically. At first, this would seem to be very easy; after all, television stations beam billions of pictures into homes every day. However, the energy required for TV broadcast is very high and the signal gets very weak after a few tens of kilometers. Not only is the distance in space much greater, but the energy available on the spacecraft is tiny; the total is less than the light bulb in your reading lamp. To solve this problem, cameras on the spacecraft convert pictures into digital signals that can be sent as a series of ones and zeros using very little electrical power. This exercise will show you how these digital images ("pictures by the numbers") work.

Because film is not returned from the spacecraft, we must use some other means to record a picture. Most spacecraft today use cameras somewhat similar to personal video cameras, based on charge-coupled devices, or CCDs. CCDs are electronic "chips" divided into a grid (see Figure 1). Each cell in the grid is called a picture element, or pixel. To keep track of the pixels, the grid is divided into lines and samples. The area to be "pictured" is focused by a lens onto the CCD, which is sensitive to light -- the more light, the higher the electrical charge. The electrical charge of each pixel (identified by line and sample) is measured and recorded in computer memory, then sent back to Earth.

Figure 2 shows an image returned from the Galileo spacecraft's CCD camera. The image is of Europa, one of Jupiter's moons. The area in the box is enlarged to show the pixels of different brightness levels (shades of gray). For simplicity, let us say that only two possible brightness values for each pixel can be returned to Earth, one value for bright pixels and the other value for dark pixels. In computer language, this would be called 1-bit data, meaning 2¹ (2 raised to the first power), which equals 2 possible values (bright or dark). The computer attached to the camera keeps track of the value for each pixel by line and sample. Figure 3 shows a line and sample grid and the value for each pixel (blank square = white, or bright; b = black, or dark). Take a pencil and fill in each pixel labeled with a b. What do you see as the result? What type of landform might this be?

Although this is a crude picture, you should get the idea!

Now, how could we improve the result shown in Figure 3? Instead of having only black or white pixels, how about having some shades of gray? Remember that we still have to send the information on each pixel back to Earth electronically in computer form. Instead of 1-bit coding (2¹ = 2 values), we will use 2-bit coding, or 2² (2 x 2 = 4 values). Now we can have white, black, and two shades of gray. Figure 4 shows a grid "map" on which b = black, g= dark gray, 1 = light gray, and blank = white. Use a pencil and shade in these tones. Now what do you see? Do you see anything that you were unable to detect in the first picture?

We could continue improving the picture by increasing the number of gray levels with 3-bit data (2³, or 2 x 2 x 2 = 8 values), 4-bit data (2⁴, or 2 x 2 x 2 x 2 = 16 values), etc. Most of the pictures you have seen for Jupiter and its moons used 8-bit encoding (2⁸). How many shades of gray does this represent? _________________

In fact, the human eye can separate less than two dozen shades of gray. This means that 8-bit pictures contain much more information than you could detect if you were looking directly at the area. Computer processing of the pictures enables all of the data to be used for analysis.

Can you think of some other ways of improving the picture shown in Figure 4? The grid is rather coarse, meaning there are not many pixels. Figure 5 shows the same scene but with more lines and samples of pixels. The encodement is still the same (2² = 4 levels of brightness). Shade the pixels using the same method as in Figure 4. Now what are you able to see in the picture?

This improvement refers to resolution, meaning the size of the area shown by one pixel. Resolution depends on the size and number of pixels on the CCD chip, the camera lens (telephoto, normal, or wide-angle lens), and the distance to the scene.

Now when you look at a picture from space, see if you can recognize the pixels and remember how the image was returned to Earth!

1. What is meant by the term bit rate? How fast can Galileo send information back to Earth? Why is this figure lower than the anticipated value for the mission?

Target Level: Middle School

Timetable: 35-40 minutes

Vocabulary: Kepler, orbital period, orbital resonance

Kepler's Third Law describes the motions of objects in different sized orbits. If you know how far away an object is from the body it orbits, you can calculate how long it takes for it to make a complete orbit. Although Kepler first described this fact for planets orbiting the Sun, it holds true for all objects in orbit around a central body. The mathematical equation is written d³ µ p², which means that the ratio of an object's relative distance from the central body cubed (usually given in Astronomical Units, or A.U.) is directly proportional to its relative orbital period squared. (usually given in Earth years). In this activity you will use Kepler's Third Law to show how the orbits of the first three Galilean moons are related.

First, we need to standardize the units to be used for distance and orbital period. For simplicity, we'll choose the moon Io, and say that 421,600 kilometers, which is the average distance from Jupiter to Io, is equal to one moon distance unit (M.U.): d = 1 M.U. Then we'll say that the time it takes for Io to go once completely around Jupiter is equal to one orbital period unit (Orb): p = 1 Orb. The answers you get for orbital period will be in terms of the actual values for Io.

Look again at Kepler's Third Law. We can now write it in this form: (M.U.)³ µ (Orb)²

For Io, we see that

d³ µ p²

(1 M.U.)³ µ (Orb)²

1 M.U. µ (Orb)²

Ö 1 M.U. µ Orb, or 1 µ Orb. This is the simplest case.

Now let's try Europa together. The average distance from Jupiter to Europa is 670,900 km. First, how many moon distance units (M.U.) is this?

Next, to determine Europa's orbital period (in Orbs) we have to cube the distance (in M.U.) and then take the square root.

From these calculations, we see that Europa's orbital period is twice as long as the orbital period of Io.

1) In terms of orbits, how often are Io and Europa aligned on the same side of Jupiter?

2) Determine the orbital period for Ganymede, whose average distance from Jupiter is 1,070,000 km.

3) How does the orbital period of Ganymede compare to that of Io and Europa?

4) Are Io, Europa, and Ganymede ever aligned on the same side of Jupiter? How often? What effect might this have on the orbit of Io, or on tidal forces acting on the moons?

5) If we know that it takes Io 1.77 Earth days to orbit Jupiter, how many Earth days does it take Europa and Ganymede to complete one orbit?

As a final step, consult a textbook on Jupiter and its moons to see how accurate your calculations were!

Teacher Notes 

Answers to Questions:

1) Once every Europa orbit.

2) 1,070,000 km X (421,600 km/ 1 M.U.) = 2.54 M.U.

3) It takes Ganymede almost four times as long as Io to complete an orbit around Jupiter, and twice as long as Europa.

4) Yes. Once every Ganymede orbit. The combined gravitational forces of Europa and Ganymede pull the orbit of Io slightly outward, which results in greater tidal flexing.

5) Europa's orbital period is 3.54 Earth days. Ganymede's orbital period is 7.16 Earth days.

Grade Level: Middle School

Description: What force is responsible for the tremendous volcanic activity on Jupiter's moon Io? How does tidal flexing produce heat, and what do the interiors of Io and Europa look like as a result?

Objectives: Students calculate the gravity gradient across Io's surface, and then model the effects of tidal flexing by repeatedly squeezing a rubber ball (applying mechanical energy) and quantifying the result (thermal energy). Finally, students kinesthetically act out the dynamic inner structures of Io and Europa.

Materials: Each group will need two hand-sized flexible rubber balls (see Teacher Notes), scissors or x-acto knife, stopwatch or timer, and a thermometer

Vocabulary: Volcano, plate tectonics, gravity, gravity gradient, orbit, ellipse, tidal flexing, friction, spherical

With over sixty active volcanic regions located so far, Io is now considered the most volcanically active body in our solar system. On Earth, volcanoes are found mainly along the colliding boundaries of great crustal plates. From what we can tell, however, Io has no tectonic plates. What could be the cause of the tremendous volcanic activity on Io? The answer is gravity.

Gravity is a force of attraction between all objects in the Universe. The equation for gravitational attraction between any two objects is given as

In this equation, F is the force of gravity, G is the Gravitational constant (6.67×10^-8 cm³/g-sec²), m is the mass of the first object, m' is the mass of the second object, and R is the distance between the two objects.

That's a lot of numbers! To compare the gravitational attraction between two objects when they are at different distances, we could ignore the values for G, m, and m', because they would stay the same. When we do this, we see that the force of gravity between two objects is simply proportional to 1/R². Stated another way, gravity is a force that weakens with the inverse square of the distance between two objects. When two values like gravity and distance are proportional in some way, it means that when one number changes, the other changes by a known factor. If we double the distance between two objects (2R), the gravitational force changes by a factor of 1/(2)², or 1/4. If we halve the distance (1/2R), the gravitational force changes by a factor of 1/(1/2) ², or 4.

Now that you know how gravity works, let's take a closer look at Io. First, Io occupies an orbit very close to the largest planet in the solar system, Jupiter. Because it is so close, the pull of Jupiter is significantly higher on the side of Io that faces towards Jupiter than on the side that faces away from it. This is called a gravity gradient. An example of a gravity gradient that you are familiar with is ocean tides on Earth. The tidal bulge on the side of Earth that faces the Moon is caused by the proximity of the Moon and its relatively stronger gravitational pull on that side. The tidal bulge on the opposite side of Earth results from that side being attracted toward the Moon less strongly than is the central part of Earth. On planets or satellites without oceans, the same forces apply, but they cause stresses in the solid body.

Second, Io's orbit around Jupiter is an ellipse, which means that its distance from Jupiter changes during a complete orbit. When Io is close to Jupiter, the gravity of Jupiter tries to pull and stretch Io into the shape of an egg. When it is furthest away from Jupiter, Io relaxes to a more spherical shape. Finally, Jupiter has other large moons that exert their gravitational influence on Io, pulling it in other directions still. Think of it as a giant tug-of-war with Io stuck in the middle!

The rising and falling of Io's surface is caused by the same force (gravity) that causes the rise and fall of tides on Earth's oceans, so we call it tidal flexing. This flexing produces a lot of friction and heat, and leads to volcanoes on Io. Tidal flexing also affects Europa, the next moon outward from Io, although the amount of energy produced is much less because of its greater distance from Jupiter. Even so, it may generate enough heat to partially melt ice deep in the crust, which may have resulted in an ocean under the surface! (See "Europa Geology Jigsaw Puzzle".)

Knowing how gravity is related to the distance between two points, we can calculate the relative gravity gradient across Io by comparing Jupiter's gravitational attraction at the center of Io with its gravitational attraction on both the near and far sides of Io. You may wish to draw a diagram showing the proper distances involved:

radius of Jupiter: 71,000 km

Mean Distance from Jupiter's surface to center of Io: 421,600 km

radius of Io: 1,815 km

Distance from center of Jupiter to center of Io = 492,600 km

R = 492,600 km/492,600 km = 1

F µ 1/R²

F µ 1/(1)²

F µ 1, or 100% of the gravitational attraction of Jupiter

Now let's calculate the gravitational force of Jupiter on the side of Io that faces towards the planet (figure 1, point A). The radius of Io is 1,815 km, so we have to subtract this value from the overall distance between Jupiter and Io. Although we have not actually moved Io closer to Jupiter, this change in the value for R will allow us to calculate the approximate proportional change in F, or the Force of gravity, on the near side Io.

Distance from Jupiter to near side of Io = 492,600 km - 1,815 km = 490,785 km

Change in Radius = 490,785 km/492,600 km = .996

F µ 1/R²

F µ 1/(.996)²

F µ 1.008, or 100.8% compared to gravitational attraction of Jupiter at the center of Io

1. Calculate the relative gravitational force of Jupiter on the side of Io that faces away from Jupiter (figure 1, point B).
2. What is the difference in Jupiter's gravitational attraction between the near and far sides of Io?

The values you come up with may not seem very impressive, but remember that Jupiter's mass is much greater than Io's. In terms of pulling and stretching, even a small gravity gradient turns out to be very significant. Remember also that we have not considered the effects of Io's eccentric orbit or the influence of the other Jovian moons. Tidal flexing of the Jovian moons is a complex phenomenon that we are just beginning to understand!

What happens when gravitational forces stretch and bend rock, just as Jupiter and its moons create stresses on Io? Where does the heat actually come from? In this activity we will model the effects of tidal flexing on a solid body.

Procedure 

Take two hand-sized foam rubber balls (or other flexible balls) and cut a single hole, just large enough to insert a thermometer, in each. Measure the starting/standing temperature of one of the balls. Record this on the data table provided.

Have someone hold the first ball but do not flex it. At the same time, have another person simulate the gravitational flexing of Io by alternately squeezing and relaxing the second ball for 5 minutes (or until they get too tired). As soon as time is up, insert the thermometer into each of the balls and record the temperatures on the data table.

Record the differences in initial and final temperatures, and then calculate and record the difference between these two values.

Finally, record the total flexing time (this should be 5 minutes unless you stopped earlier).

1. What was the purpose of measuring the temperature of a ball that was held in your hand, but not subjected to flexing?
2. Determine the rate of heating for the ball that was squeezed.
3. What are the theoretical temperature limits to heating the balls by these two methods?
4. How often is Jupiter's moon Io "stretched?"
5. What are some other ways of heating up a planet or moon?
6. How could scientists get a direct measure of the amount of tidal flexing on Io or Europa?

1. Jupiter's gravitational force on far side of Io

Distance from Jupiter to far side of Io = 492,600 km + 1,815 km = 494,415 km

Change in Radius = 494,415 km/492,600 km = 1.004

F µ 1/R²

F µ 1/(1.004)²

F µ .992, or 99.2% compared to gravitational attraction of Jupiter at the center of Io

2. The difference in Jupiter's gravitational attraction between the near and far sides of Io is about 1.5%.

• Ball selection. The best rubber balls for this activity are made of thick foam, solid throughout, but flexible. You will need to test the balls you select to see if they can be heated to a measurable amount.

1. Measuring the temperature of a ball that was held but not squeezed serves as a control. Heat from your hand may contribute to warming the ball.
2. Rates will vary. Typical results for solid rubber balls are about 1 degree F per minute.
3. Theoretical limit for heating by hand is body temperature, approximately 98.6 degrees F. Theoretical limit for heating by flexing is until the ball melts and flows out of your hand. Of course your hand would be burned in the process. Remember, heat flows both ways!
4. This is determined by Io's orbital period, which is approximately 1.77 Earth days.
5. Solar energy; impacts by objects from space; radioactive decay from within.
6. Scientists would need to measure the diameter of each moon through a complete orbit to see the changes. An instrument such as a laser altimeter could accurately measure the changes in the moon's surface.

Another way to illustrate the effects of tidal flexing is to select small groups of students to act out the dynamic interior structures of Io and Europa. You may wish to have your "student moons" orbit a central point (Jupiter), and respond to its tremendous gravitation at different positions and distances. 

• The Core of Io: madly hot and wild
• Molten Interior of Io: several students standing around the core of Io, facing outward, arms loosely interlocked, relatively mobile
• Sulfur Volcanoes: responding to the gyrations of molten sulfur compounds bursting forth

The tidal flexing is so dramatic that the Molten Interior constantly creates new volcanic activity! 

• The Core of Europa: dynamic, hot, under pressure
• The Rocky Mantle of Europa: several students standing around the Core of Europa, facing outward, arms interlocked, relatively immobile
• The Deep Ice/Ocean of Europa: first as ice, in a similar arrangement of the rocky mantle, several students encircling, arms interlocked, malleable
• The Surface Ice of Europa: several students with hands placed outward, flat, cold, showing an icy surface

Tidal flexing stretches and unstretches the whole group. As long as the whole structure (surface ice, deep ice and rock) is solid, the tidal flexing is a relatively small effect. If heat from the core emanates through the rock enough to sustain a melting of the deep ice, creating an ocean, then the ocean sloshes around inside creating a more pronounced distortion of the shape of Europa.

Grade Level: Middle School

Description: How are craters formed? How do we interpret the origin of craters on distant surfaces like the Jovian moons? What do craters tell us about the age of the surface on which they are found?

Objectives: First, students will first simulate the effects of collision between asteroids, and describe the resulting size distribution. Next, using images of Jupiter's four largest moons obtained by NASA's Galileo spacecraft, students will investigate crater densities and size distributions, and interpret those in terms of relative surface ages and how the craters might have been formed.

Materials: Large plastic or paper bag; large paper plates or a flat desk; hard-baked cookies; graph paper; images of Jovian moons; rulers or calipers for measuring crater sizes

Vocabulary: Asteroid, comet, caldera, crater density, crater size distribution, bolide, planetesimal, saturation, tectonism, ground truth, impactor

Craters are one of the most common and important types of surface features in the solar system. They are found on almost all the solid planets, satellites, and asteroids, but not on gas giants like Jupiter and Saturn since there is no solid surface to preserve them. Craters are also fairly rare on Earth, where weathering, erosion, and other geological processes have removed them. Scientists believe that most craters are formed by the impact of asteroids and comets, but these aren't the only ways that large circular depressions can be made on a surface. For example, some might actually be calderas, created by erupting volcanoes. How do we identify the origin of craters on distant surfaces like the Jovian moons? What important information can crater density, or the number of craters in a given surface area, tell us? Besides observing each and every impact or eruption as it occurs, is there a way to determine the likely culprits of these scars?

The asteroid belt is a region of small planetesimals between the orbits of Mars and Jupiter. Although they range in size from very small to very large, the number of asteroids of different sizes (known as their size distribution) is governed by the fact that when asteroids collide with each other, as they sometimes do, the fragments that are produced are dominated by certain sizes. Which size is more prevalent? To answer this question, let's model the effects of collisions on the size distribution of asteroids!

Place one or two hard-baked cookies in a plastic or paper bag. Seal the bag tightly, and then slap it against the top of your desk three times. When you are done, carefully pour the broken cookie pieces onto a paper plate. Count or estimate the number of fragments for each size range and record your results in the data table below.

1. Create a graph for your data table, with the number of fragments on the y-axis, and their relative sizes on the x-axis.
2. Describe any pattern to the size distribution of your simulated asteroid fragments.
3. Can you think of other common objects that might have a similar size distribution?

It follows that craters produced by asteroid impact should mimic the asteroids' size distribution, and this is exactly what we see on the Earth's Moon; the numbers and sizes of lunar craters closely match that of the asteroids. Could the size distribution of craters on other planetary bodies provide us with clues to crater formation, as well? How might impacts near Jupiter be different from impacts near Earth? In Part 2 of the next activity you will compare the results of your asteroid simulation with real crater data from Jupiter's moons.

If we assume that comets and asteroids strike all regions of a planetary body at approximately the same rate (an assumption that may or may not be correct), every region should have the same number of craters, or crater density. Regions with higher crater densities, therefore, tend to be older than regions with lower crater densities. For example, if you stand in the rain for a long time you will get soaked, but if you make a quick dash to shelter you will only get spattered with a few raindrops. Just as you can get completely soaked, a surface can become totally covered (saturated) with craters after a certain amount of time. After that point it retains roughly the same crater density no matter how many more impacts occur. New craters are simply created on top of older ones.

On the other hand, if the planet or moon is geologically active or has an atmosphere, then processes such as weathering, erosion, tectonism, and volcanism can partially or completely erase craters. Early in the Moon's history, for example, lava flows flooded large portions of the surface. These areas were essentially wiped clean of craters about 3.5 billion years ago, and thus their crater density dates back to the time of those lava flows. In other words, the lava flows reset the crater clock. The lunar highlands, in contrast, were not flooded. Their crater density remained high, and dates back to when they were formed about 4.1 billion years ago.

What other factors might influence crater density? Beginning when the planets formed about 4.5 billion years ago, the amount of debris began to decrease as collisions and impacts swept it up. By measuring the crater density on different areas of the Moon, and then measuring the actual ages of rocks returned from different regions by Apollo astronauts (data referred to as ground truth), scientists can figure out what density of craters corresponds to the actual surface age (in millions or billions of years). There may also be a relationship between cratering rate and distance from the asteroid belt. Mars, for example, is located nearer to the asteroid belt than the Earth is, and may have a rate of crater formation roughly twice that of Earth. We might also want to consider the sizes of the planets; large planets such as Jupiter possess greater gravitational fields that attract more impactors. Higher gravity also means that objects hit with more speed, resulting in larger impact explosions and bigger craters.

Using the images provided, calculate the crater density (craters per area) for each of the Galilean moons by making a simple count of all identifiable craters. If you are not sure a particular feature is an impact crater, assume that it is not. Record your results in the data table below.

1. Describe the surface of each of the Jovian moons in terms of crater density. Are there differences within any of the images?
2. What can you say about the relative ages of the moons? Is there a relationship between relative age and distance from Jupiter? If so, what is it?
3. Have any craters been altered? If so, what forces do you think are responsible?

In this activity you will compare the results of the asteroid collision simulation with real data from Jupiter's moons Ganymede and Callisto. How do the size distributions compare? If they are similar, does it mean that asteroids have caused most of the impact craters on Jupiter's moons? If they are different, what other objects might have caused them?

Using the images provided, determine the crater size distribution for Ganymede and Callisto. A scale bar is provided for each. Measure the diameters of the identifiable craters in the image, and record your results in the data table below. If you are not sure a particular feature is an impact crater, assume that it is not.

1. Create two graphs (for Ganymede and Callisto) from your data table, with the number of craters on the y-axis, and their relative sizes on the x-axis.
2. Describe any patterns to these crater size distributions.
3. How do these patterns compare to that of your simulated asteroid size distribution? To each other?
4. How might you explain these similarities or differences? In other words, how would you interpret the origin of craters on Ganymede and Callisto?
5. What are the problems with using only one image to determine the relative surface age and possible crater origin for an entire planetary body?

After the activity has been completed, explain to students that, unlike craters on the Earth's Moon, most craters on Jupiter's moons are believed to result from the impact of comets! The large gravitational field of Jupiter tends to propel asteroids toward the inner planets, but tends to attract comets toward itself and its moons. Planetary scientists still know very little about the size distribution of comets.

Finally, have students research the following questions:

1. Are comet orbits completely random, or do they cluster like asteroids?

2. Is it possible that comets are fragments of collisions, just like the asteroids?

3. Can you think of any other ways a comet or asteroid might be broken into pieces?

1. Results should approximate a non-linear relationship, with many more small asteroids.
2. There are many more small fragments than large ones.

You may want to discuss the concept of a power law as it relates to size distribution. A power law is a relationship between variables where a decrease by a factor N in one variable results in the increase by a factor of N² in the second variable. Students could be asked to graph their data on a logarithmic scale.

3. Answers will vary. One good example would be rocks in a river.

The images provided for this activity were selected to highlight the differences in the four Galilean moons. In reality, their surfaces are much more varied. You and your students may wish to select other images of each of the moons for examination.

1. Io has no recognizable impact craters, and is continually being resurfaced by volcanic eruptions that cover any craters that might form. Europa has few craters, a smooth surface, and cracks (implying tectonics.) Ganymede has heavily cratered regions, regions with fewer craters that also have cracks and grooves (implying tectonic activity), and regions that appear to have experienced resurfacing. Callisto is heavily cratered and looks like our Moon and Mercury; therefore, it appears inactive.
2. Relative ages (youngest to oldest): Io, Europa, Ganymede and Callisto similar in age. Generally, the further away from Jupiter, the older the moon.
3. Io has volcanism. Europa and Ganymede have tectonic processes that fracture or distort craters. Callisto's craters are mainly destroyed by subsequent impacts, but landslides have also modified the crater walls.

1. Results should show a more even size distribution of craters.
2. Smaller craters are more common than larger ones.
3. The patterns to the size distribution of craters on Ganymede and Callisto are similar but not identical to the simulated asteroid impact results. There are fewer large impacts in the size distribution for the Ganymede image.
4. The craters on Ganymede and Callisto might have been created by comets or some other objects.
5. Assumed the area in the image is large enough to include full range of crater sizes. Region might not be representative of the moon. Region may have been geologically altered. Image resolution may not show smaller craters.

• Compare other planetary or lunar surfaces in terms of cratering rates, surface processes, etc.
• Discuss the impact of Comet Shoemaker-Levy 9 into Jupiter's atmosphere

What's So Hot About Io?

Grade Level: Activity 1, Middle School; Activity 2, High School

Description: Students will model the Near Infrared Mapping Spectrometer (NIMS) instrument by using a sheet of liquid crystal to detect areas of high infrared (IR) radiation (warm tiles or coins hidden beneath a photo of Io's surface). Advanced students then calculate the area and power output of the volcano Prometheus (figure 1) based on the IR data.

Materials for Activity 1: Figure 2; liquid crystal sheet (see Teacher Notes); scissors; three large coins or small ceramic tiles (tiles work best as they retain their heat longer); two small beakers; ice; Bunsen burner; tongs or spoons for removing objects from beakers

Materials for Activity 2: Figure 3; calculators

Vocabulary: Infrared, spectrometer, liquid crystal, wavelength, micron, power, Watt

Images of Jupiter's moon Io that were photographed in the visible part of the spectrum reveal a surface (figure 2) pockmarked with dark spots, many of which are active volcanoes. How do we know which areas are active and which are not? The Near-Infrared Mapping Spectrometer (NIMS) observes the infrared part of the spectrum, measuring the amount of heat that is produced by a moon or planet. The NIMS instrument is ideal for finding hot volcanoes on the surface of Jupiter's moon Io. It has already shown us that volcanoes on Io have the highest recorded surface temperatures of any planetary body in the solar system (one may be as hot as 3,100 degrees Fahrenheit!)

In this activity, you will use a material similar to that on the NIMS detector to find a series of hidden hot spots. The color of a sheet of liquid crystals will change depending on the temperature of objects that are in close contact or nearby the sheet.

First, you will need to make two copies of the image of Io's surface (figure 2). Cut out the pictures, and place one copy on a flat surface such as a table or desk.

Next, heat three coins or tiles to different temperatures: 0° Celsius (32° F), 37° Celsius (98.6° F), and 100° Celsius (212° F). To do this, use a beaker of ice water, the palm of your hand, and a beaker of boiling water. Be sure to follow all safety procedures outlined by your teacher.

When the coins or tiles have reached the proper temperature, have one member of your group (the "infrared detective") turn around and close their eyes. Quickly and carefully place the coins or tiles onto the image of Io on your desk, aligning them with randomly chosen features. Now place the second cut-out image of Io directly on top of the first, so that the coins or tiles in between are aligned with the same features as below. 

Finally, have the "detective" turn back around and try to find the active hot spots by placing the liquid crystal infrared detector on a portion of the image of Io's surface. Once they are located, observe the hot spots for several minutes, noting any changes in size or color.

1. How many "active volcanoes" were you able to detect?
2. Why doesn't your infrared detector "see" the coin or tile at 0° Celsius?
3. How does the area of each hot spot relate to temperature? How might you explain this?
4. Did the color of any hot spot change over time? If so, how does the color of a hot spot relate to its temperature?

In this exercise we are going to determine the area of the Prometheus volcano, one of many volcanoes on Io, and its total power output. The methods described here are used by scientists on the Galileo Project.

Figure 3 shows a spectrum of the hot spot Prometheus observed by NIMS. The x-axis of the graph shows wavelength (measured in microns: a micron is one-millionth of a meter). The y-axis shows the power (in Watts) as measured by NIMS. You can see that the hot spot emits different amounts of energy at different wavelengths. This distribution of energy output (the output spectrum) depends on the temperature of the hot spot. As different temperatures have different curve shapes, we can find the temperature whose curve gives the best fit. For Prometheus, the best fit is obtained with a temperature of 461 K.

Knowing the temperature of the hot spot, we can calculate the area. We determine the energy emitted per unit area, at a selected wavelength, by a body at the temperature of the hot spot. This is our measuring stick: it tells us that a known area at a known temperature at a known wavelength is emitting a certain amount of energy. We can then compare this calculated figure with the actual amount of energy that NIMS has measured from Prometheus and find the area.

Firstly, select a wavelength (L) from the x-axis in figure 3. This will be used throughout the exercise. The amount of energy (P_m) emitted from 1 m² of a body at the temperature of Prometheus (461 K) at our selected wavelength (L) is given by equation 1. The units of P_m are Watts per square meter per micron, or W/m²/mm. Calculate P_m for T = 461 K and wavelength L. By the way, e = 2.718.

equation 1:

Now, we need to find how much power (P) Prometheus is actually emitting at our selected wavelength. Using figure 2, at wavelength L, draw a line from the x axis up to the 461 K curve, and then across to the y-axis to find power P. This is how much energy Prometheus is releasing at wavelength L.

We also know that at wavelength L every square meter (m²) of the surface area of a body at 461 K emits power P_m . So how big does the volcano have to be to emit power P, at P_m per square meter? The area in square meters (A_m) is given by equation 2.

equation 2:

What is the area of the volcano in square kilometers (A_km)? 1 km² = 1,000,000 m².

Total Power Output. We now know the average temperature and area of the Prometheus lava flows. The total power output, P_T, in Watts, is given by equation 3. Find P_T.

equation 3:

This is all from one volcano, and Io has many volcanoes! A light bulb uses 100 W: How many light bulbs does this hot spot represent?

The power generating capacity of the United States is 700,000 MegaWatts (700,000,000,000 Watts: Department of Energy, 1997 summer generation figures). Io emits 10¹⁴ W, much of it from volcanoes. How do these figures compare?

 What's So Hot About Io?

• The Liquid Crystal IR Detective activity is appropriate for elementary and middle school students. Students should work in groups of two or more. Have them take turns being the "infrared detective."
• Review the electromagnetic spectrum, pointing out the major categories of radiation (radio, infrared, visible, ultraviolet, microwave, etc.) Introduce students to different temperature scales, including Kelvin. This will become relevant for question 2 as well as the Prometheus activity.
• Sheets of liquid crystal are available at many science museums and nature stores, or through a science supply catalog.

Answers to Questions:

1. There should be two hot spots visible.
2. The infrared detector is only sensitive to a small range of temperatures. Make sure students understand that, although one of the coins or tiles is at 0 Celsius, it still gives off heat (this is 273 Kelvin, well above "absolute zero").
3. The higher the temperature, the greater the area.
4. Answers will vary.

 Activity 2 - The Power of Prometheus!

• Because of the complex mathematical equations, the Prometheus activity should be reserved for high school students or very advanced middle school students.

It does not matter which wavelength (from the x axis) is chosen, as long as a measurable power output can be determined from the y axis. Suggest that a wavelength between 3.5 and 5 microns is used. Each different value of wavelength will produce a different value of P_m from equation 1. However, when these values are used to calculate area the area values should be the same (about 233 Watts per square meter per micron.) This is because the ratio of values P_m and P for the same wavelength is constant.

What is the area of the volcano in square kilometers (A_km)? We know that something at 461 K emits 233 Watts per m² per micron. This is the energy output for a unit area (1 m²) at the selected wavelength of 5 microns. We know from the figure what the total output from all of Prometheus at 5 microns is, so we can determine the area of Prometheus by asking the question, how many times larger than 233 W (at 5 microns) is the total output (at 5 microns)? This is the number of square meters of the Prometheus volcano. At 5 microns, the power output from the volcano is P_, measured from the y axis and found to be 3.43 x 10⁹ Watts per micron. If radiating at 233 Watts per micron from every square meter of surface, the area of the volcano is P / P_m = 1.47 x 10⁷ m², = 14.7 km².

Find P_T. The figure in this equation, 5.67 x 10^-8 W m^-2 K^-4 is the Stefan-Boltzman constant. The total power from Prometheus is the sum of the power output at all wavelengths. The area has been determined from equation 2. The figure for area in square meters must be used in equation 3. Using a value of 461 K for T and 1.47 x 10⁷ for A_m:

P_T = 5.67 x 10^-8 x (461)⁴ x 1.47 x 10⁷ = 3.77 x 10¹⁰ Watts. This is nearly 40 billion Watts!

How many light bulbs does this hot spot represent? This is equivalent to 377 million light bulbs! The power coming out of the Prometheus volcano could light up every house in the United States (assuming 100 million homes in North America, each home could have 3 or 4 lights on simultaneously).

How do these figures compare? Io generates (from natural sources) about 140 times as much energy as the United States (from all of its power stations, mostly coal, gas and nuclear power plants.) Prometheus, one of many volcanoes on Io, generates about 1/18^th of the power capacity of the United States.

Target Level: Middle School

Timetable: Two 40-minute class periods

Materials: Each student will need two copies of C3 "Wedges" image, one printed on card stock (To print image, click here); Colored pencils; Scissors

Vocabulary: Crosscutting relationships, crust, geologic unit, mid-ocean ridge, plate tectonics, relative age, seafloor spreading, subduction zone; terrain

Although the surface of Europa is frozen solid, there is the possibility that liquid water may exist underneath as lakes or even an ocean. Pictures taken by the Voyager and Galileo spacecraft show areas of the surface where large plates of ice have apparently broken apart and moved away from each other. Some have even twisted and rotated. The easiest way for these movements to occur is if the plates are floating on water, just like ice floes that fracture and shift in the polar seas on Earth. If the plates can be fit back together like a jigsaw puzzle, with their edges matching up neatly, it will provide strong evidence that the material beneath them was once liquid, or at least a warm, slushy ice. Let's try it and see!

Look at the image labeled C3 "Wedges." Differences in surface texture, color or shading show us that particular regions are made of distinct materials, formed in unique ways, or were created at different times. Making a geologic map is an easy way to show these differences. Using a copy of the C3 "Wedges" image, identify three areas of similar texture or shading. For now, avoid the smaller, narrower ridges. Lightly color each of the three surface types using a different color, even if they are not touching on the map (for example, color all smooth bright areas light green). When you are done, each color on your map will represent a unique geologic unit.

Now that you have made a geologic map of the area, can you figure out which unit is oldest and which is youngest? One way to determine relative age is to look for crosscutting relationships. Like a cake that is sliced, the surface rocks may be broken by a fracture or a fault. The unit (or slice) which cuts through the rock (or cake) is always younger than the rock itself. In other words, you can't cut something unless it's already there! Using the color as the name for each geologic unit, arrange them in order of their relative age in the space below.

Oldest-------------------------------------------------------------------------------------------------------Youngest

Now you will reconstruct the landscape to see how it looked at different times in the past. Using scissors, carefully cut up your map into the different geologic units you identified. Each piece should contain one color only. When you are done, place the pieces back together in their correct position in front of you.

Look again at the relative age of each geologic unit on the time line above. Carefully remove from your map those pieces that belong to the youngest unit. This will leave empty space behind. Can you fill the empty space by shifting together the remaining units? What kinds of motion did you have to use to do this? Do any features or ridges match up now that were once separated by the youngest unit? The surface you have created represents Europa at some time in the past.

Remove the pieces for the next youngest unit and shift the remaining pieces to fill the gaps. What types of motion were required this time? Do any features or ridges match up again? Compare this "original landscape" to an uncut C3 "Wedges" image.

As a final step, reverse the process and "rebuild" the surface of Europa one unit at a time. This should give you a clearer picture of some of the types of geologic materials, forces, and events that are responsible for shaping Europa's interesting surface.

1) List the types of motion you used to reconstruct the Wedges landscape.

2) What does this suggest to you regarding the physical properties of the material beneath this region?

3) Construct a simple graph showing the relationship between the brightness of each geologic unit and its relative age. What are some possible explanations for this relationship?

4) If a new geologic unit were to form in this region, would you predict it to be bright or dark?

5) Explain how the youngest geologic units you identified might have formed (i.e., where did the material come from?) Give the evidence for your answer.

6) a. If new ice or crust is being created in this region, and we find that the moon is not expanding or building mountains, what must be occurring elsewhere?

b. Describe two ways in which this might occur.

7) Assuming that Europa's surface is no older than 100 million years old, as many scientists believe, calculate the minimum spreading rate for the dark wedge at left in the image. (Note: you will need to know that the image represents about 150 miles on each side)

Teacher notes are available here.

This activity was adapted from the following source:

Tufts, B.R., Greenberg, R., Sullivan, R., and Pappalardo, R., 1997, Reconstruction of Europan terrain in the Galileo C3 "Wedges" image and its geological implications, Lunar and Planetary Science Conference XXVIII Houston, Lunar and Planetary Institute, p. 1455-1456.

Part B: Determining Relative Age

The youngest unit is composed of the prominent dark wedges that run diagonally from NW to SE. The next youngest unit contains gray bands that run from W to E, and fork several times. The oldest unit contains the bright icy plains.

Part C: Reconstructing the Landscape

As the prominent dark wedge at left is removed, the remaining pieces will have to slide together AND rotate to be aligned. It is very important that students recognize both types of motion. Note that some features that match up as a result of this step are even younger still (small fractures that the students were instructed to avoid), whereas others truly represent old features that were separated by the creation of the wedges.

1) Sliding past (faulting), moving together, rotation

2) The fact that some of the plates had to be rotated suggests that there is a liquid or ductile material supporting them from below.

3) Graph should show a linear relationship. The youngest wedges may have higher ridges that produce greater shadows; Over time, the surface might be coated by a light-colored material such as frost; Darker material in the wedges may sink through the ice with time; Other plausible explanations.

4) It would probably be dark.

5) They probably formed as the result of material filling in the cracks from below. The fact that there is no missing material from the edges of adjacent plates supports this interpretation.

6) a. Crustal plates in other regions would have to be destroyed.

b. The crust might subduct and melt, or evaporate/sublimate into space.

7) At its widest point, the dark wedge is approximately 11 miles wide. Dividing this distance by 100 million years gives a rate of 1.1 x 10 ^-7 miles/year, or .006 inches/year. Answers will vary slightly.

Grades K-4

• Use the Galileo E11 image, "Regional Mosaic of Chaos and Gray Band," instead of the Wedges image.

Grades 9-12

• Have students create a short video or flip-chart showing the motion of the crust in this region over time.
• Have students use image processing software (such as NIH Image) to determine the differences in shading for the three different geologic units. Assign arbitrary ages to the end members, and then determine a rate of shading change. Using this value, determine the relative age for the middle unit. Is it closer in age to the dark wedges or the background plains?

Grade Level: Elementary and Middle School

Description: A game of chance involving gathering the ingredients necessary for life and determining whether life has evolved by attempting to assemble a puzzle at the end of the game.

Objective: Students will learn about the ingredients necessary for life (as we know it) to exist, the sources of these ingredients, the role of large impact events in shaping life, and the potential difficulty of producing life even when these ingredients are present.

Materials: As many copies of the print-outs as there are players (To print images, click here); dice; scissors, a razor knife or a paper cutter; glue, paste or spray adhesive; four plastic baggies or large cups; a large flat surface; Optional: poster board or foam core for mounting gamepieces

Vocabulary: organic compounds, asteroid, comet, trilobite, extinction, evolution, biodiversity, satellite, Chicxulub crater

Life, as we know it, is known only to exist on Earth. Earth, unique among the planets in our solar system, has surface conditions that allow the presence of liquid water. Although life is very diverse, and can exist in extremely harsh environments, all life on Earth requires three basic ingredients to exist: liquid water, an energy source, and organic compounds to use as the building blocks for biological processes.

There are two other worlds in our solar system on which liquid water may exist at fairly shallow depths (<5 kilometers (<3 miles) or so) beneath their surfaces: Mars and Europa, a satellite of Jupiter. On Mars, water would exist as subsurface water, in tiny pores and cracks in rocks. On Europa, liquid water may exist beneath its icy crust. The Galileo spacecraft is taking images of Europa during its 2-year extended mission that are designed to search for clues as to whether or not liquid water is present beneath the ice.

Because of the constant tugging on Europa by Jupiter, Io and Ganymede (other moons of Jupiter), significant internal heat may be generated in Europa (an energy source!) Carbon compounds are present in meteorites, and probably in comets, which have impacted Europa and could be the original source for the organic compounds on Earth. Therefore, the three necessary ingredients for life were, and may still be, present on Europa.

If there is liquid water on Europa, does that mean that life will have evolved there? The chance of finding life in a Europan ocean is extremely difficult to estimate. But it is by no means a certainty that life will exist simply because the ingredients needed to survive are present.

One way to think of this is to compare the formation of life to the baking of a cake. You need to have flour, eggs, sugar and water (and other things like chocolate flavoring, etc.) to bake a cake. Just because you have these ingredients, however, and you combine them in a bowl and add an energy source (like an oven), doesn't mean you're going to get a cake. The ingredients have to be carefully measured, combined, and properly prepared for an edible (and hopefully quite delicious!) cake to result. So it is with the ingredients for life.

The game that follows involves collecting the three ingredients necessary for life, as well as large impact events, in abundances determined by chance (rolling of dice). A typical game will last about 10 to 20 minutes, for 6 to 8 teams, depending on the age and enthusiasm of the students. For most games, the winner will be the group that comes closest to assembling the whole puzzle. Eight or nine is often sufficient to win, with 6 or 8 teams playing. Ten out of twelve pieces right is excellent. Constructing the entire puzzle is quite rare. If precisely the right amounts of these ingredients are combined, then life may result!

The game involves rolling the die twelve or more times to obtain the ingredients necessary for life. Play can be as individuals or as teams.

Gamepieces, provided by your teacher, should be placed in four separate bins or baggies according to life ingredient type. The bins should be numbered 1-4 with a pencil or marker.

The die is cast in turn and the player/team obtains a gamepiece equivalent to the number rolled. A roll of 5 results in loss of turn and the player/team does not draw a gamepiece. A roll of 6 entitles the player/team to select a gamepiece of his/her choice, either from a bin or another player/team (unless that player has left the game with 12 pieces.) If there are no gamepieces left from the bin number that is rolled, the player/team shall re-roll until a valid number is obtained.

Once a player or team collects twelve gamepieces, they are no longer involved in play. When all players have collected twelve gamepieces, you may turn over your gamepieces and attempt to assemble the life puzzle in accordance with the key (now displayed by the instructor). Any player or team that has managed to acquire precisely one of each of the twelve gamepieces (a very difficult task, indeed) will have successfully produced life! The winning group is the one that comes closest to constructing the entire puzzle.

• Gamepieces should be kept in front of each team with the life ingredient sides up.
• You may not flip over the gamepieces to the life puzzle side during play.

The ingredients necessary for life should be briefly reviewed, and the students challenged to think of examples on Earth where, for instance, oxygen isn't necessary for life, nor sunlight. A good example of both would be the 'black smoker' vents where hot water issues from the deep sea floor, creating a unique ecosystem based on the chemical energy obtainable from minerals in the hot water. Tube worm colonies, etc., live in such places.

Mass extinctions produced by large impact events, like the one that produced a huge crater on the Yucatan Peninsula 65 million years ago (Chicxulub crater) and contributed to the demise of the dinosaurs, open the door for increased biodiversity among the animals that survived (like mammals, after the dinosaurs were gone). The water and carbon compound building blocks on the Earth today may also have initially come from impacting asteroid and comet materials. We don't yet know whether carbon compounds are present on Europa, but some amount should have been provided by impacting asteroid and comet material, as well.

The instructor should print out the two image sheets and carefully make as many two-sided copies as there are players or teams, plus one extra (answer key). Be sure the images on each side are perfectly aligned with each other. When this is done, simply cut out the gamepieces on each sheet.

The answer key is especially nice if the sheets are pasted to both sides of posterboard or foam core, available from art supply or craft stores. Showing the completed puzzle at the beginning of the game is useful but optional.

If you are not able to print two-sided copies, print as many individual sheets as you will need, and then paste the two sheets together back to back, so that the life puzzle (trilobite) image is on one side, and the mosaic of life ingredients is on the other. Again, be sure the images on each side are perfectly aligned with each other, and in the same orientation as the key. After pasting, cut out the gamepieces on each sheet.

• After each third roll (after rolls 3, 6, 9 and 12), student teams might be allowed to trade a gamepiece (one for one) with another team in order to have some of each of the ingredients for life.
• The instructor may allow the students to see the arranged gamepiece key (not the trilobite side!) during play, so that the appropriate proportions of each ingredient can be sought out. It will still be difficult to obtain exactly the right combination, because there are multiple examples of each ingredient necessary, with different portions of the trilobite on the back of each gamepiece.

• The level of detail of discussion of the different ingredients necessary for life and the impact hypothesis of mass extinctions can be varied for effective use of this game for a variety of grade levels from Grades 3 through 9.
• By allowing more trades, or making other modifications of the instructor's own choice, the game can be made easier to win. Design your own customized version of the game and have fun!

Discuss probabilities, and how to calculate the of odds of life on other worlds (Drake Equation)

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Galileo K-12 Educational Activities Page

This module was written by Brian Exton (National Optical Astronomy Observatories, Tucson AZ).

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Galileo Solid State Imaging Team Leader: Dr. Michael J. S. Belton

The SSI Education and Public Outreach webpages were originally created and managed by Matthew Fishburn and Elizabeth Alvarez with significant assistance from Kelly Bender, Ross Beyer, Detrick Branston, Stephanie Lyons, Eileen Ryan, and Nalin Samarasinha.

Last updated: September 17, 1999, by Matthew Fishburn

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