How can we tell what's beneath the surface of a moon or planet? A moon might have layers of
different material, or be the same throughout. There might be an ocean underneath. Even though
we can't make a direct measurement, we can measure effects and draw conclusions.
Scientists use the spacecraft's communications system to measure how the frequency of a radio
signal changes by the time it is received on Earth. When Galileo passes close by Jupiter or one
of the large moons, gravity tugs on the spacecraft, changing its speed. By precisely measuring the
change in the radio frequency received on Earth, we can estimate the mass and internal structure
of Jupiter or its moons.
Close to a moon, if Galileo's Magnetometer
detects a magnetic field, it is a likely reflection of what is inside. Magnetic fields have unique
signatures that can be read by the Magnetometer. One kind of field is created when there is a
flowing, liquid metallic substance inside the moon. But a different sort of field results when
Jupiter's huge magnetic field sweeps through a layer of salty water, generating electric currents. It
appears that this is the case on the moon Europa, and perhaps on Callisto as well.
Next: Mission Operations 
