The Gentle Giants of Ganymede (The Giants series, Book 2)

Long before the world of the Ganymeans blew apart millennia ago, the strange race of giants had already vanished. All that remained of them was a wrecked ship abandoned on a frozen moon of Jupiter. Now Earth's scientists are there, determined to ferret out the secret of the lost race. But when suddenly the Ganymeans return, they bring with them answers that will reveal the secret of our own as well.