Io's Magma Ocean Provides a View

"On January 1610, after observing the night sky with a simple telescope, Galileo Galilei wrote in a letter to the Tuscan court: But what exceeds all wonders, I have discovered four new planets and observed their proper and particular motions, different among themselves and from the motions of all the other stars; and these new planets move about another very large star [Jupiter] like Venus and Mercury, and perchance the other known planets, move about the sun."