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Question
In the cases of Jupiter and other gas giants, how can we tell where the atmosphere ends and the solid surface begins?
To find the "edges" of a planet, we usually use a method called "radio occultation." Using this method, a spacecraft will broadcast radio waves towards the Earth while passing behind the target planet. The radio signal will pass through the atmosphere with only minor changes in its character, but it cannot pass through the solid planet. The time at which the signal disappears tells us the location of the actual edge of the solid part of the planet and, therefore, its radius. Other important measurements can be obtained using this method as well. For example, when a spacecraft passes by a planet, its trajectory will be deflected by a small amount due the planet’s gravitational pull, and the amount of the deflection allows us to determine the planet’s mass. From the mass and the radius, we can also determine the average density of the planet.
March 11, 2002
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