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Name: Gary
Status: other
Age: 40s
Location: N/A
Country: N/A
Date: 1999 - 2000

I'm a docent at the California Academy of Sciences and lead school groups and adults on tours focused on Earth and space sciences. During a tour, I mentioned that Jupiter and Saturn had no solid surface, and then later, I mentioned how the surface gravity of Saturn was only slighly more than Earth, even though it's so much larger. Someone asked, "how do you have surface gravity when there is no surface?" And of course, I had no idea. What is the "surface" that astronomers refer to when they measure "surface gravity" on Saturn? And if they picked some other point as the surface, would the force of gravity be different?

Where is the surface of a cloud? Even though you know that a cloud is just a misty fog, from a distance it appears to have well-defined edges. The gas giant planets are the same. Although it's technically impossible to point to a precise place where the "surface" is, you can get pretty close.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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