What is Callisto?  Why is it important?  Well, just read a little bit!  Callisto is one of 16 moons of Jupiter!  Callisto is the 3rd biggest moon in the Solar System, but 2nd biggest moon in the Jovian System.  Cool!  It is about equally made up of ice and rock.    How about that?!  Callisto was discovered in 1610 by Galileo Galilei.  It is about as large as Mercury, but only a third of the mass!  Callisto also contains water ice, carbon, even organics!  Callisto is heavily cratered and is very old.  But beautiful!  It seems as if Callisto evolved under the influence of impacts.  Whoa!  So Callisto was most likely shaped by impacts on the surface.  On a very small scale, Callisto's surface seems to have frost deposits.  That's probably what gives Callisto it's small sparkle!  Callisto has an thin atmosphere made up of carbon dioxide.  The likely presence of an ocean on Callisto predicts that it can harbor life!  Amazing! 
 

Big Facts

The exact date Callisto was founded was January 7, 1610

Another name for Callisto is Jupiter IV

It takes about 16.6 days for Callisto to orbit around Jupiter

The surface pressure on Callisto is about 7.5 pbar[6]

The apparent magnitude is 5.65 (opposition)[5]

Callisto has no axis tilt