Target Level: Middle School

Timetable: 10-15 minute demonstration by the teacher

Materials: You will need at least 4 balls of polymer putty (such as Silly Putty_® brand), combined to make one large mass

Understanding the properties of matter, and how forces affect them, is critical to interpreting Europa's geologic history. As you know, matter exists primarily in three states: solid, liquid, and gas. Depending on the forces that act upon them, however, each may act very differently from what we would expect. Unusual environmental conditions, such as extreme temperatures and pressures, can result in the unexpected behavior of materials. Geological forces exerted over very brief or extended periods of time, which are difficult or even impossible for us to observe directly, can also cause materials to act in unusual ways. For example, what happens the moment a meteorite impacts the Earth's surface, or an earthquake occurs? At the other extreme, how does a glacier move, or a mountain rise? Changes in Europa's icy crust, whether on the surface or at depth, may be determined by unique forces or conditions. In addition, those changes may occur over vastly different time scales, from nearly instantaneous, to millions or perhaps even billions of years. If we can observe and describe how material on Europa has behaved, we may be able to determine what caused those changes.

Shape a ball of polymer putty into a short, fat cylinder. Grab each end and pull apart rapidly (you may want to use a partner). Note how the mass snaps very quickly into two short pieces. If force is applied rapidly, the putty undergoes brittle deformation.

Now using the same ball of putty, pull the cylinder apart slowly. You should be able to stretch the mass very far apart, with a narrow piece dangling in between. When force is applied slowly, the putty undergoes ductile deformation.

Examples of brittle deformation on Europa include the many types of fractures seen in the icy crust. On the other hand, some images of Europa show plates of ice that have apparently tilted or rotated, suggesting a ductile layer below.

Roll the putty into a smooth ball. Place the ball on the table, and use your palm to press down on it very slowly. After a few seconds, remove the mass from the table, noting how its shape has changed. Because the force was applied slowly, the putty will remain deformed. This is called inelastic deformation.

Once again, roll the putty into a smooth ball. Find a hard, uncarpeted floor and hold the ball about four feet above the ground. Now drop the ball. It will bounce and return to nearly the same height. After you catch the ball, examine it for deformation (there should be very little). Because the force was applied rapidly, the ball of putty behaved quite differently than before. Although the surface was deformed briefly at the time of impact, it returned to nearly its original shape. This is called elastic deformation.

Impact craters on Europa show evidence of both inelastic and elastic deformation. The smallest craters leave bowl-shaped impressions on the surface. Larger structures, on the other hand, show very little relief, suggesting the surface rebounded after it was impacted.

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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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