Name: Gary Flatow
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?
The surface gravity is for the point on the planet where the optical
density is 1.0. Since there is no real surface, this is what is used as
the effective radius for such measurements.
Nathan A. Unterman
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Update: June 2012