Like an ocean liner making one last circuit of its ports of call, the Cassini spacecraft is scheduled to make its last visit to one of the moons of Saturn today. Enceladus is a ball of ice about 300 miles in diameter. Its pure white surface hides the moon’s most interesting feature: a global ocean of liquid water. It could be up to 15 or 20 miles deep — several times the depth of Earth’s oceans. Some of it squirts through cracks in the crust, near the south pole. Some of the water falls back to the surface, coating parts of Enceladus with fresh ice. But some escapes the moon and supplies fresh ice to one of Saturn’s rings. Cassini discovered the geysers a decade ago. And it’s kept a close eye on them during more than 20 close encounters with the icy moon. It flew through the geysers in late October, allowing it to measure minerals and other compounds mixed with the water. Today’s pass won’t be nearly as close — about 3100 miles. At that range, Cassini will measure how much heat Encelad
