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.

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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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