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:

• Which alien worlds are most livable?
• Maybe we are alone, after all 
• Cosmic Log archive on astrobiology

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.

NASA / JPL-Caltech / SSI

NASA's Cassini orbiter captured this view of Saturn on June 15, from a distance of about 1.8 million miles (2.9 million kilometers). The rings' shadow runs across the planet's sunlit side. The speck in the lower left corner is Enceladus, a 313-mile-wide (504-kilometer-wide) moon of Saturn.

NASA's Cassini sent back this big, beautiful, black-and-white picture of Saturn — but what's that little white speck in the corner?

The image, unveiled by Cassini's imaging team on Monday, shows tiny Enceladus at lower left. It's just 313 miles wide (504 kilometers wide), and yet it shines brightly from a distance of 2 million miles or so. Enceladus is arguably as intriguing as Saturn, and here's why: The icy moon has geysers of water spouting up from cracks in its surface, suggesting that there's a deep ocean and perhaps even some sort of life down below.

To get a more imaginative view of Enceladus, check out this posting on the io9 blog, featuring an illustration from "Planetfall: New Solar System Visions," a big, beautiful, full-color coffee-table book by Michael Benson. NPR's Robert Krulwich showed off the same image earlier this month on his Krulwich Wonders blog.

Enceladus is just one of the moons of the solar system that's been soaking up the spotlight lately: Also this month, NASA's Curiosity rover watched Mars' two moons, Phobos and Deimos, pass over the sun's disk during a series of mini-eclipses. The rover won't see such a sight again for 11 months or so. Here's a smooth animation of Deimos' transit from Nahum Chazarra on UnmannedSpaceflight.com. And if you haven't seen it already, you'll want to catch up with the sight of a crescent Phobos in Mars' dusky sky. 

Shine on, Harvest Moon
Our own moon is definitely worth watching over the next few days: Saturday brings a "Harvest Moon" — that is, the full moon that's closest to the September equinox. That's traditionally a good moon to bring in the harvest by, since it lights up the whole night for late-working farmers.

The Harvest Moon also can serve as a guidepost for finding the planet Uranus in the night sky, although the moon's glare interferes with the view this weekend. If you'd like some extra help, the Slooh Space Camera is planning a couple of online viewing parties over the weekend — with Uranus as the guest of honor. Video feeds will be coming in to the Slooh website from a variety of observatories, and a panel of experts will provide commentary. The first show begins at 7 p.m. ET on Saturday, with an encore performance at 10.

Next week, the moon continues to act as a guide, as Sky & Telescope's Alan M. MacRobert explains. On Oct. 3, the moon lingers near the Pleiades star cluster. The next night, it sits near the bright red star Aldebaran in the constellation Taurus. And on Oct. 5, the waning moon hangs out with Jupiter, starting around 10 p.m.

This weekend is also a good time to look for the International Space Station as well as the European Space Agency's Automated Transfer Vehicle, which undocked from the station today. To find out when and where to look, check out NASA's satellite sighting database.

Where in the Cosmos
Cassini's picture of Saturn and Enceladus served as today's "Where in the Cosmos" picture puzzle on the Cosmic Log Facebook page. It took just a few minutes for Ian Slota to solve the riddle and report that the speck in the picture was Enceladus. As a reward, I'm sending Ian a pair of big, beautiful, cardboard 3-D glasses, courtesy of Microsoft Research's WorldWide Telescope project. Those glasses will come in handy for seeing 3-D pictures of Saturn's moons. Click the "like" button for the Cosmic Log Facebook page, and you too may be a winner in next week's "Where in the Cosmos" game.

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+ circles. 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. You can also check out "The Case for Pluto," my book about the dwarf planet and the search for new worlds.

NASA / JPL / GSFC / SWRI / SSI

A false-color temperature map indicates hot spots along fissures in the "tiger stripes" on Enceladus' surface. These are the "split ends" of the stripes known as Alexandria Sulcus and Cairo Sulcus.

Temperature readings from the Cassini orbiter support the view that warmth is welling up through cracks in the icy surface of Enceladus, one of Saturn's most intriguing moons.

The readings were taken by the 6.4-ton spacecraft's infrared spectrometer and high-resolution camera during an August flyby, and discussed today in a series of news releases and advisories. In an e-mailed alert, Cassini imaging team leader Carolyn Porco said a "phenomenal amount of heat is emerging" through the south polar fractures known as tiger stripes.

The hot spots might not sound all that hot: The warmest areas registered surface temperatures of 120 degrees below zero Fahrenheit, or 190 Kelvin. But Porco said that's staggeringly higher than the coldest temperatures in the south polar terrain, which dip as low as 365 degrees below zero F (52 Kelvin). She called particular attention to a warm fissure known as Damascus Sulcus.

NASA / JPL / GSFC / SWRI / SSI

This map of the Damascus fissure on Enceladus is color-coded to reflect temperature readings, with blue, purple, red, orange and yellow denoting progressively more intense thermal radiation.

The readings indicate that the relatively warm material cools off quickly as you look farther away from Damascus' central trench. The heat also varies dramatically within just a few miles running along the trench. An associate on the imaging team, Cornell University's Paul Helfenstein, was quoted as saying that the warm section of Damascus Sulcus "is among the most structurally complex and tectonically dynamic of the tiger stripes."

So what's behind the heating? Porco said it's "undoubtedly the result of the tidal flexing of Enceladus brought about by its orbital resonance with Dione," another one of Saturn's more than 60 moons. "However, details of this heating process are still unclear and are being studied at this very moment," she added.

The temperature-coded picture of the Alexandria and Cairo fissures reveals another intriguing feature: an isolated warm spot toward upper left, just beyond the fissures' "split ends."

"The ends of the tiger stripes may be the places where the activity is just getting started, or is winding down, so the complex patterns of heat we see there may give us clues to the life cycle of tiger stripes," said John Spencer, a Cassini team member based at the Southwest Research Institute in Boulder, Colo.

Previous observations from Cassini have confirmed that geysers of water ice are welling up from the tiger stripes. The latest views add to the evidence suggesting that water or slush is pushing up through the fissures. And if there's a hidden ocean of water beneath the surface ice, could there be life as well?

A definitive answer to that question will have to be left to follow-up space missions. The August flyby served as Cassini's last chance to observe the active south polar region in sunlight. NASA says it was also Cassini's last chance to do remote thermal sensing at Enceladus until 2015. "The geometry of the many flybys between now and 2015 will not allow Cassini to do thermal scans like theses, because the spacecraft will be too close to scan the surface and will not view the south pole," NASA said in today's advisory.

NASA / JPL / SSI

Small water ice particles fly from fissures in the south poar region of the Saturnian moon Enceladus in this image, taken by the Cassini spacecraft during an August flyby. This view looks toward the night side of Saturn, which is in the lower left of the image. Sunlight scatters through the planet's atmosphere, forming the bright diagonal line.

A particularly close flyby took place today, when Cassini came within 30 miles (48 kilometers) of Enceladus' surface. Images from that flyby should be coming down over the next few days.

Enceladus isn't the only Saturnian moon in Cassini's spotlight: Today the spacecraft's science team also released images of Tethys, another moon that was observed during an August flyby. Just last week, Rhea and its thin, oxygen-rich atmosphere were in the news. And over the past couple of days, Cassini has gotten some good looks at Hyperion, a moon that's 165 miles wide (266 kilometers wide) and shaped like a potato.

For much, much more about the recent revelations, check out NASA's Cassini mission webpage as well as the online home for Cassini's imaging team. And be sure to check out our "Best of Cassini" slideshow.

Connect with the Cosmic Log community by "liking" the log's Facebook page or following @b0yle on Twitter.