When the Space Age began more than 50 years ago, explorers were eager to visit the planets of the solar system.  As the years have passed, however, astronomers have realized that the moons of the solar system may be even more interesting.

Many of these moons are ‘water worlds’ – and planetary scientists, like golden retrievers, always follow the water.

“On Earth, where there is water, there is life,” says Brian Day of NASA’s Solar System Exploration Research Virtual Institute. “It doesn’t matter if it’s boiling hot like the bubbling acid hot-springs in Yellowstone National Park or frigid like the waters of the Arctic.

Consider Enceladus, a tiny moon floating just outside Saturn's rings. This little wonderland featuresa vast underground ocean that could be friendly to microbial life. That ocean is capped by a thick crust of ice. Yet, NASA’s Cassini spacecraft found Enceladus busily puffing plumes of water vapor, icy particles, and organic compounds out through fissures in its frozen outer shell. Cassini has actually flown through these plumes a few times, sniffing out their composition.

Day says, “The exciting results from Cassini have researchers designing possible future missions that would provide more detailed analyses of Enceladus’ water and look for potential signatures of life.”

Titan, another of Saturn’s moons, is shrouded in a thick atmosphere of nitrogen and methane. There is evidence that Titan, like Enceladus, has a sea of water beneath its icy shell. But Titan’s frigid surface is mosaicked with lakes of liquid methane and ethane.  

Day notes, “It's the only body in the solar system other than Earth with substantial liquid on its surface.”

The seas of Titan intrigue astrobiologists.  Here on Earth, our seas are teeming with life—and indeed the seas may be where life began. What kind of life might arise in the alien seas of Titan? The exotic environment of hydrocarbon seas could teach astrobiologists a lot about the basic chemistry critical to the formation of life.

More water worlds with icy shells are found at Jupiter. Europa, Ganymede, and perhaps Callisto also show strong evidence of liquid water oceans beneath their global ice crusts. Day would most like to ‘dive in’ at Europa.

“This is a big world –much bigger than Enceladus and even bigger than any of the ‘dwarf planets’. Europa’s ocean is a significant body of water with amazing potential for life.”

In fact Europa has twice as much liquid water as Earth, and like the water of Enceladus, it might be accessible.

“There is some evidence that Europa may have periodic eruptions of plumes of water, similar to those seen on Enceladus. We’ve even seen apparent icebergs on the surface that are tipped up as if sitting in slush. All of these provide us with tantalizing clues of what lies beneath – an ocean of liquid water.”

Meanwhile, orbiting Jupiter not far from Europa, is a completely different kind of satellite: 

“Io,” says Day, “is the most volcanically active body in our solar system.”

Io gets its fiery warmth from tidal heating—that is, a back and forth stretching of the moon’s interior caused by Jupiter’s intense gravity.

“Each moon is stretched and pulled differently, causing varying rates of tidal heating. Orbiting more closely to Jupiter than the other Galilean moons, Io is stretched the most –hence its volcanism. Europa, next closest to Jupiter, is stretched less, followed in order by Ganymede and Callisto.”

Day continues, saying “Tidal heating is a source of energy that can melt ice and expand the potential for life. It creates a ‘habitable zone’ among the moons of Jupiter, with Europa sitting squarely in the middle.”

Planets are exciting, but there’s an even greater number of amazing moons in the solar system.

Says Day: “The lure is irresistible, and the potential is amazing. It’s time for us to go!”

For more news about amazing moons—and their planets—stay tuned to science.nasa.gov.