QUESTION:
How can you tell if Jupiter's moons absorb different amounts of heat?

ANSWER from Paul Harvey on Oct.16,1995:
We can measure the temperature of Jupiter's moons (or in fact, the
temperature of any object in the universe) by measuring the amount of Infrared
Radiation coming from the object. Since the temperature of the object is set by
how much heat it receives, we can then measure how much heat the object is
receiving from either the sun, or even some heat on neighboring Jupiter.