NASA / JPL-Caltech / MSSS This graphic shows the relative sizes of Earth, Mars and Europa, an icy moon of Jupiter.





A British astrobiology conference has revived a years-old debate over the best place to look for life elsewhere in the solar system: Mars, or the moons of Jupiter and Saturn?

"For reasons I don't really understand, the wider solar system and the potential for life there has not been high priority," The Telegraph quoted Robert Pappalardo, a senior research scientist at NASA's Jet Propulsion Laboratory, as saying on BBC Radio 4.

Pappalardo's remarks were occasioned by this week's astrobiology conference at the UK Center for Astrobiology in Edinburgh, Scotland. The center recently established the International Subsurface Astrobiology Laboratory, or ISAL, half a mile (1 kilometer) beneath the surface in Yorkshire's Boulby mine. Biologists will use that facility to see how organisms hold up in extreme environments, learn about life's chemical signatures, and test instruments that could look for those signatures on other worlds.

Someday, one of the worlds may well be Europa, an icy moon of Jupiter. With a diameter of 1,945 miles (3,130 kilometers), Europa is just slightly smaller than Earth's moon, and yet it is thought to contain more water than Earth's oceans beneath a miles-deep layer of ice. Researchers recently suggested that hydrogen peroxide in the ice could serve as an energy supply for simple forms of life in the ocean hidden below.

Europa is the focus of Pappalardo's research, and for months he has been urging NASA to support a $2 billion mission to study Europa at close range. However, proposals for NASA missions to Europa have been losing out, in part because of the cost of missions to Mars. Last week's federal budget proposal for the next fiscal year provides no funding for a Europa mission, but it does fund Mars missions such as Maven (launching this year), InSight (launching in 2016) and a new science rover (launching in 2020).

Kevin Hand (JPL-Caltech) / Jack Cook (WHOI) / Howard Perlman (USGS) If Europa's ocean is 100 kilometers (62 miles) deep, and all that water were gathered into a ball, it would have a radius of 877 kilometers (545 miles). This graphic compares that hypothetical ball of Europan water to the size of the Jovian moon itself, as well as all the water on planet Earth. Europa is thought to have two to three times the volume of water in Earth's oceans.

At February's annual meeting of the American Association for the Advancement of Science, Pappalardo worried that NASA's study of the outer solar system would go "radio-dark" in 2017, when the Cassini mission to Saturn and the Juno mission to Jupiter are both due to end. He continued that theme in this week's BBC interview.

"I worry that if Europa exploration is delayed, but then finally it happens some day, we might look back and say 'Why didn't we do that sooner?' Imagine 50 years from now, we get a lander there and find signs of life. All this time we'll have been looking in the wrong place," he was quoted as saying.

Europa isn't the only moon that intrigues astrobiologists: In the Jovian system, Callisto and Ganymede also have icy shells and may hold hidden oceans. Meanwhile, Cassini has repeatedly observed geysers of water ice rising from the surface of the Saturnian moon Enceladus — suggesting that liquid water and perhaps life may lie beneath the surface. Saturn's largest moon, Titan, has a thick atmosphere and seas of hydrocarbon that some scientists think could harbor a totally alien kind of life.

As for Mars, astrobiologists say hints of life could well lurk beneath the surface. To some extent, the Red Planet has been winning out over Europa and Enceladus because it's easier to get to. Moreover, NASA's vision calls for sending astronauts to Mars and its moons in the 2030s. NASA's robotic missions serve as precursors for those human voyages, as well as steps in a long-term program to learn about life in the universe.

Europa's fans can take heart in the fact that the European Space Agency is planning its own mission to Jupiter's moons: The Jupiter Icy Moons Explorer, or JUICE, is due for launch in 2022 and arrival at the Jovian system in 2030. There's also talk of a sample return mission that would target Enceladus' geysers, and a proposal to drop a boat onto Titan's seas.

So what if all of these worlds — Mars and Europa, Callisto and Ganymede, Titan and Enceladus — turn out to be lifeless? Charles Cockell, who heads the UK Center for Astrobiology, addressed that scenario in an interview with the BBC.

"A lot of people think astrobiology is some sort of hunt for life, and if we don't find life, it will be a big disappointment," Cockell said. "But in fact, that's not the case. The discovery of many lifeless planets across the universe, the discovery that the Earth might be unique as a place for life, would be an astonishing discovery in itself. It would be a very lonely discovery, but it would be an astonishing discovery."

More about the search for life:

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.