1. The student will be able to recognize the landforms associated with impact cratering by conducting a simulation.
2. The student will be able to relate knowledge of geologic processes to features found on the outer planet satellites.
3. The student will be able to compare geologic processes of the outer planet satellites by analyzing satellite imagery.

planets farthest from the sun. Io is unusual in being a rocky world, and in the predominance of active volcanism (Unit IV) which shapes its surface today. Ganymede and Enceladus show widespread evidence for past volcanism, but the "lava" that once flowed on these satellites is comprised of ice, rather than rock. Ganymede and Enceladus show clear evidence for tectonism, expressed as grooves and ridges. Tectonism has affected Io and Rhea to lesser degrees, and its expression will probably not be apparent to students from the images supplied in this exercise. The most observant students, however, may notice small grooves on Rhea and Io. The abundant cliffs on Io may be tectonic in origin, or they may be related to sublimation of a volatile material and subsequent collapse, a form of gradation. In general, gradational processes on outer planet satellites are not apparent at the scale of Voyager images and are not specifically addressed in this exercise. However, in discussing the morphologies of craters on Rhea, it would be useful to point out to students that the principal cause of crater degradation on that satellite is the redistribution of material by impact cratering. New craters pelt older ones, creating ejecta and redistributing material to subdue the forms of fresh craters over time. To a lesser extent, mass wasting probably acts to modify crater shapes through the action of gravity.

ENGAGEEXPLORE	Place the tray on the drop cloth. Fill the tray with sand, then smooth the surface by scraping the ruler across the sand. Sprinkle a very thin layer of the colored sand over the surface (just enough to hide the sand below) using the flour sifter. Divide the tray (target area) into four square shaped sections, using the ruler to mark shallow lines in the sand. In one section produce a crater. Drop an intermediate sized steel ball bearing straight down (at 90-degree vertical) into the target surface.   Hold the ball at arms length from sand surface (70 to 90 cm) facing straight down into the tray. Do not remove the projectiles after launch. Journal: Make a sketch of the plan (map) view and of the cross section view of the crater. Be sure to sketch the pattern of the light-colored sand around the crater. This material is called ejecta. Label the crater floor, crater wall, crater rim, and ejecta on the sketch. Divide the target into four sections. Find the mass of each projectile and enter the values in a data table (shown below). Produce four craters by dropping 4 different sized steel ball bearings from a height of 2 meters above the target surface. Measure the crater diameter produced by each impact. Enter the projectile mass and resultant crater diameters in the table below. Calculate the kinetic energy of each projectile and enter the values in the table (provided). Remove the projectiles and smooth the target surface with the ruler. Divide the target into three sections. Produce three craters by dropping 3 identical size, but different mass, projectiles from a height of 2 meters above the target surface. Find the mass of each projectile and enter the values in the table below. Measure the crater diameter produced by each impact. Enter the projectile mass and resultant crater diameters in the table below. Calculate the kinetic energy of each projectile and enter the values in the table (provided). Journal Write: According to the data collected, explain any relationship you may have found between kinetic energy of the projectile and crater diameter mass and crater size

EXPLORE And EXPLAIN

Students will now investigate a variety of landforms (including craters) which have been observed on satellites in the outer portion of the solar system. Read the introductory material in the packet and examine the table of information. Journal Write: Ganymede From what you have read, and using the information in the table, write a paragraph giving a description of Ganymede. Examine Figure 13.1a, which shows part of the Sippar Sulcus region of Ganymede, and compare what you see to the geomorphic sketch map of the area (Figure 13.1b). Notice that the surface of Ganymede can be divided into two principal geologic units, bright terrain and dark terrain. The dark terrain is believed to be a mixture of ice and rock, while the bright terrain is probably composed mostly of ice. Discuss with your team the similarities and differences of the bright and dark terrains. Use your knowledge and the documents of satellite images provided to answer the questions. List and describe the many characteristics of the bright and dark terrains. Be as detailed as possible. Include factors such as albedo, number of craters, general surface appearance, and other characteristics that are apparent. Which of Ganymede's two principal terrain types is older? What is the evidence for this? What is the age of the ejecta for the crater marked "A" relative to the bright and dark terrain? What is the evidence for this? Many researchers believe that both volcanism and tectonism shaped the bright terrain of Ganymede. What is some evidence that this is true? All the craters you can see in Figure 13.1a probably formed by the impacts of comets or asteroids. Many show small central pits, created as a result of impact into an icy target. Four craters that show unusual morphologies are indicated in Figure 13.1b with the letters A through D. Describe the shapes and characteristics of these interesting craters. Include the dimensions of each crater using the scale bar, and also describe the characteristics that make it peculiar compared to most other craters on Ganymede. Enceladus From what you have read, and using the information in the table, write a paragraph giving a description of Enceladus. Make a geological sketch map of Enceladus. Use as a guide the map of Ganymede in Figure 13.1. Tape a piece of acetate over the photograph of Enceladus, Figure 13.2. Trace the outline of the satellite. You will find that it is simple to trace the satellite's limb, but the terminator is not as clearly defined. Outline the most prominent craters on the satellite. You will have to decide which craters should be included. Locate and describe two unusual looking craters. Grooves on Enceladus are probably tectonic features; next map their locations. Draw a thin line along each groove you see, and place a dot near the center of each line to indicate it is a groove. Where do grooves (and the ridges between them) occur on Enceladus? The surface of Enceladus can be divided into three different types of terrain. Think about the features you have mapped so far, and decide on how to divide the surface into three terrains. Decide on names that describe your units. (For example, "cratered terrain.") Draw boundaries around the different units. There might be only one patch of each unit, or there could be more than one patch. To complete the map of Enceladus, label the units with the descriptive names that you have given them. List the names of your three units. Following each name, describe the characteristics of each unit as you defined it in making your map of Enceladus. Which is the oldest of these three major units on Enceladus? Which is the youngest of these three major units on Enceladus? Provide the evidence for your prediction. Rhea From what you have read, and using the information in the table, write a paragraph giving a description of Rhea. Examine Figure 13.3, which shows a part of Rhea's surface. What is the principal geologic process that has shaped this part of Rhea? Give evidence from the data for your response. Notice the morphologies (shapes) of the craters that you see in Figure 13.3. The 60-km crater A shows a sharp and distinct morphology, with steep and well-defined slopes. On the other hand, the 65 km crater B is more difficult to identify, as it is rounded and indistinct. Keep in mind that the cratered surface of Rhea seen is probably about 4 billion years old. Lay a piece of acetate over Figure 13.3, taping it at the top. Trace the rectangular outline of the photo, and also trace and label the scale bar. With a solid line, trace the outline of crater A, and label the crater "sharp." Next locate and trace the outline of crater B, but this time use a dashed line. Label this crater "subdued." Locate one additional sharp crater, outlining it with a solid line. Find an additional subdued crater, and outline it with a dashed line. The terms "sharp" and "subdued" are descriptive terms, used to describe the morphologies of craters. Sharp-appearing craters are sometimes referred to as "fresh," while subdued-appearing craters are commonly referred to as "degraded." What do the terms "fresh" and "degraded" imply about how a crater's morphology changes with time on Rhea? Notice that some of Rhea's craters, including crater A, show central peaks. These peaks form upon impact, due to rebound of the floor during the "modification stage" of the cratering process. On the acetate, outline as many central peak craters as you can confidently identify. Continue outlining each sharp crater with a solid line, and each subdued crater with a dashed line. Put a dark dot in the middle of each central peak crater that you identify. Are central peaks more recognizable in sharp or subdued craters? Based on your answer to the above, what can you infer about how the topography of a central peak changes over time? Central peaks form only in craters above a certain diameter. This "transition diameter" from simple, bowl-shaped craters to more complex, central peak craters depends on surface gravity and material properties, so it is different for each planet and satellite. Estimate the transition diameter for craters on Rhea based on the smallest central peak craters that you are able to identify. Io From what you have read, and using the information in the table, write a paragraph giving a description of Io. Examine Figure 13.4, which shows part of the surface of Io. The very high albedo material is considered to be sulfur dioxide frost. When the two Voyager spacecraft flew by Io in 1979, they photographed nine actively erupting volcanoes. Examine the feature in the far northeast corner of the image, which shows a central depression with relatively low albedo (dark) material radiating from it. What process created this feature? What is the evidence or rationale for your response? Describe in detail the shape of the central depression. Use a labeled sketch. Examine the other craters and surface features seen in Figure 13.4. What is the principal process shaping the surface of Io? List some observations that support your answer. Education Elements: IMAGES This is a collection of many of the best images from NASA's planetary exploration program. The collection has been extracted from the interactive program "Welcome to the Planets" which was distributed on the Planetary Data System Educational CD-ROM Version 1.5 in December 1995. It has also been updated with the addition of more recent images. http://pds.jpl.nasa.gov/planets/

EXTEND

Consider the surface of Rhea (Figure 13.3) in comparison to the surfaces of Ganymede (Figure 13.1) and Enceladus (see Figure 13.3 and your sketch map). Journal Write: Compare the general appearance of the surface of Rhea to the surfaces of Ganymede and Enceladus. What do the differences in surface appearance suggest about the geological history of Rhea as compared to the histories of Ganymede and Enceladus? Contrast the characteristics of craters that you see on Io to those on Rhea. List at least four differences between Io's volcanic craters and Rhea's impact craters. Construct a chart to rank the relative importance of impact cratering, volcanism, and tectonism on the four outer planet satellites that you have studied, based on the images you have seen. Use numbers from 1 (for the satellite most affected) to 4 (for the satellite least affected). Education Elements: BACKGROUND This site deals with volcanism and impact on Mars. http://curator.jsc.nasa.gov/antmet/marsmets/volcanism.htm

EVALUATE

Journal Write: Create a systems diagram which relates geologic processes and landforms found on the outer planet satellites. Use the information you have learned in this activity and previous activities to compare geologic processes of the inner planets (including Earth) to the geologic processes of the outer planets. Use the Science Rubric to guide your response. GT Extension What’s On Mars and How Do We Know? NASA GSFC. Available: http://edmall.gsfc.nasa.gov/inv99Project.Site/Pages/mola.abstract.html Students will review tectonic and erosional features prior to a treasure hunt to find them on the Martian surface using new data from the MOLA instrument on the Mars Global Surveyor (MGS) spacecraft. They will be able to ground truth their work to online images.

Clear acetate or overhead projector transparencies (2 sheets per student or group), overhead projector markers (colored markers can be used for added clarity)

http://spacelink.nasa.gov/Instructional.Materials/NASA.Educational.Products/.index.html#EG

What’s On Mars and How Do We Know? NASA GSFC. Available:

http://edmall.gsfc.nasa.gov/inv99Project.Site/Pages/mola.abstract.html