For years, the motto among astrobiologists — people who look for life in distant worlds, and try to understand what life is, exactly — has been “follow the water.” You have to start the search somewhere, and scientists have started with liquid water because it’s the essential agent for all biochemistry on Earth.

Now they’ve followed the water to a small, icy moon orbiting Saturn. Scientists reported Thursday that Enceladus, a shiny world about 300 miles in diameter, has a subsurface “regional sea” with a rocky bottom.

This cryptic body of water is centered around the south pole and is upwards of five miles deep. It has a volume similar to that of Lake Superior, according to the research, which was published in the journal Science.

The moon’s liquid reservoir had already been inferred from the presence of plumes of water vapor emerging from the south pole. The plumes stunned scientists when they were detected by NASA’s Cassini spacecraft in 2005. This latest report adds the detail of the rocky sea floor, which is significant because the contact between liquid water and rock creates the potential for the kind of interesting chemistry that gets astrobiologists excited.

This bulletin from the outer solar system could boost Enceladus as a possible target of a future robotic space mission. A spacecraft could fly through the plumes and study whatever’s coming out of the moon — something Cassini has done, but with instruments from the previous century that were not designed to look for signs of life.

One of Saturn's smaller moons has an ocean of water beneath 20 miles of ice. The findings help explain the jets of water vapor first observed flowing from fractures in the moon's surface in 2005. (Sapienza Universita di Roma)

To become a target for a new mission, however, Enceladus would probably need to outshine Jupiter’s moon Europa, which also appears to have subsurface ocean and also has plumes shooting water vapor into space.

NASA is putting together preliminary designs for a possible Europa mission, though budgetary pressures for now make any new major, costly venture in the outer solar system problematic.

“I love Mars, but I think the two of them” — Enceladus and Europa — “provide the highest probability of finding extant life,” said Mary Voytek, senior scientist for NASA’s astrobiology program. She said she is “torn” about which moon would be the better destination for a robotic probe.

The description of the subsurface sea on Enceladus is based entirely on indirect evidence. The body of water, if it exists, is covered with at least 20 miles of ice, according to the new report. But there are several lines of evidence that point to its presence.

The first is gravitational: The Cassini spacecraft, which has been exploring the Saturn system for nearly a decade, has made multiple flybys of Enceladus. Faint changes in the wavelengths of radio signals sent back to Earth have enabled scientists to calculate how the moon’s gravity tugs on the spacecraft. These are not simple calculations, because the gravitational effects have to be disentangled from other effects, including the drag on the spacecraft as it flies through the plumes of water vapor.

But ultimately the scientists created a model for the moon’s interior and what appears to be a striking gravitational asymmetry. Around the moon’s south pole, there’s something that’s slightly off, and the calculations seem to be begging for the model of the interior to include some material denser than water ice. Liquid water — about 7 percent denser than ice in those conditions — seems to be the answer.

Another line of evidence is the moon’s shape: It has a shallow dimple, a depression, at the south pole. There’s missing mass. This fits with the hypothesis that there’s denser water down below, deforming the planet’s shape.

“We know the composition of the shell. We know that it’s water ice. So it’s pretty obvious to think that some of the ice is molten and, therefore, if you melt part of the ice, if you transform it, the volume of it reduces, and you create a depression,” said Luciano Iess, a professor of space systems at the University of Sapienza in Rome and the lead author of the Science paper. (The Cassini mission, which included the Huygens probe dropped to the surface of Saturn’s moon Titan, is a joint endeavor of NASA, the European Space Agency and the Italian Space Agency.)

Finally, there are those plumes, which spew water vapor into space. It’s possible to generate such a phenomenon without geysers; instead, you could make plumes by rubbing blocks of ice together. But the plumes could be created by a deep ocean sending water up through cracks and into space.

So could there be life there? That’s highly speculative. Even if, in a general sense, Enceladus has features of habitability, it’s not clear how long the sea has existed, or whether it has ever had the right conditions for the origin of life. The origin of life is its own special mystery. Does it require an evaporating tidal pool bathed in sunshine — what Darwin called a “warm little pond”?

“Liquid water’s not enough — not enough for the origin of life certainly,” said Carol Cleland, a University of Colorado professor of philosophy who has written about astrobiology. For example, “You need an energy source so that you can drive thermodynamically uphill processes.”

Chris McKay, a NASA astrobiologist who has been a major advocate for a new Enceladus mission, says this moon has the major essentials for life as we know it. There’s the liquid water, obviously, and energy from tidal forces, plus such life-friendly elements as carbon and nitrogen, which were detected by Cassini when it flew through the plumes.

“Carbon and nitrogen are the concrete and rebar — you need them to build,” McKay said, describing a partial blueprint for life as we know it.

He said he’s optimistic about the existence of extraterrestrial life, but knows that he and his colleagues have yet to come up with a single sample.

“It’s the occupational hazard of astrobiology to jump to the conclusion that you want to be true. I have to constantly chide myself and my colleagues for doing that,” McKay said.

Chris Chyba, a professor of astrophysical sciences at Princeton, said any discussion about extraterrestrial life is hampered by a lack of a “theory of life.”

“Trying to define it the way you define, say, a chair, is a hopeless project,” Chyba said.

He compared it to the difficulties of Leonardo da Vinci five centuries ago when he tried to describe what water is: “It’s impossible for him to explain what water is because he was trying to do it before there was any theory of molecules or atoms.”