Artist concept of a balloon-borne Titan probe. Credit: NASA
JPL/Corby Waste

Since 2004, when NASA’s Cassini spacecraft and the European Huygens lander started exploring Saturn’s largest satellite, Titan, layer after layer of the mysterious moon have been peeled away, revealing what has turned out to be an awesome alien world that stands apart from all other moons in the solar system–and even holds its own with the planets.

Cryovolcanoes: cool! Imagine a 3000 foot tall mountain gushing freezing water-ammonia slush, which pours down its slopes to become solid “rock ice” flows as it freezes, and spouting a plume of water vapor that quickly turns into snow. Is that really how a volcano of cold water, ice, and maybe ammonia and methane would behave? Maybe, maybe not—but now scientists have observational evidence of what might be such a process on Titan, and the similarities in patterns to Earthly lava flows is compelling. Take a look at the video….

The thought makes me want to go and explore Titan right now!

At the annual American Geophysical Union (AGU) conference in San Francisco last December, I voiced a concern to a presenter who was describing explorations of Titan by Cassini and Huygens. My concern was that, though the images returned by Huygens from the surface of that frigid smoggy world were unprecedented up-close views of a distant alien land, I fear that with the cost of space missions and budgets and economies and all, we might not return there for further exploration within my lifetime…and I’d have to make do with the images from Huygens. (Though very cool images, they were essentially from a spot on Titan that is little more than a plane of soil and pebbles; I want lakes and volcanoes!)

“Not so fast….” There may be more missions to Titan’s surface yet to come. There is certainly a lot of scientific interest in Titan because of its “hydrological” cycle—that is, the liquid hydrocarbons, not water, that rain from its atmosphere and flow and collect in rivers and lakes on its surface.

Some of the possibilities the presenter suggested for surface and atmosphere probes sounded pretty interesting. One idea is another “lander,” but deliberately targeted to splash down in one of Titan’s big methane lakes, and with a longer-life battery than what powered Huygens (Huygens’ battery lasted about 90 minutes after landing). What kind of battery? Nuclear, maybe, though there are restrictions to lobbing radioactive isotopes onto the surface of another world—especially one where the existence of indigenous life hasn’t been ruled out.

Such a “floater” could drift around the lake, transported by liquid currents, taking environmental readings, pictures, and maybe sounding the lake bottom with sonar. Eventually, lake currents might be expected to naturally deliver the floater probe to a shoreline, where it could then explore Titan where land meets lake.

Another possibility on Titan is a flying aircraft probe, either winged or buoyant. Due to the thickness of Titan’s nitrogen atmosphere (four times denser than Earth’s at the surface), and the relatively low gravity of Titan itself, flight there is a very different prospect from flight on Earth. Titan’s surface gravity is only 14% that of Earth’s, so what would be a 200-pound payload on Earth would weigh 28 pounds on Titan.

A balloon-borne probe, with greater buoyancy due to Titan’s thicker atmosphere, could drift around for a long time and cover a great deal of territory at very close range, perhaps even setting down on the surface temporarily. A winged-aircraft could glide at relatively low speed, again owing to the lower gravity and higher lift from the thick air.

Yeah, I want to go and see the volcano slushy! Please, can I?

NASA's Cassini spacecraft captured this image of sunlight
reflecting off of a lake on the surface of Saturn's moon Titan.
Credit: NASA/Cassini

The evidence for the existence of lakes on Titan has been building since 2004, when the Cassini spacecraft, and its Titan-landing probe Huygens, arrived in the Saturn system and began collecting data.

First it was imagery from the Huygens camera as it descended through Titan's thick, hazy atmosphere: what looked like dark, flat, featureless regions defining apparent coastlines along solid terrain, as well as dentritic patterns like river channels draining into them.

Then it was imagery from Cassini's smog-penetrating infrared cameras showing numerous systems that looked for all the moon like giant lakes—on the order of size of Earth's Great Lakes. Cassini radar bouncing off the surfaces suggested that they were exceptionally flat, as one expects a watery surface to be.

Then there was confirmation of surface liquid in Titan's Southern Hemisphere—but not in the north, where the lake-like shapes were by far more numerous…an apparent land o' lakes.

Finally, what scientists hoped to see appeared, in a flash: sunlight, reflecting off of a Titanian lake called Kraken Mare, came shining through the moon's haze and was captured by Cassini's infrared camera. It's the same kind of reflection you see when sunlight blazes off the surface of the ocean before sunset. Astronauts see the same thing from Earth orbit, looking down at the ocean-lined limb of the Earth when the Sun is at the right position over it.

It required the right conditions; the Sun had to be in just the right position relative to the lake for the reflection to be seen. Ever seen the light of the setting Sun reflecting off the window of a distant house, though only for a moment when the geometry is just right? And only recently did the Sun rise over Kraken Mare's extreme northern latitude, after the 15-year dark of a Titanian arctic winter.

Titan is the only Solar System object known to have surface liquid, other than Earth. Jupiter's moon Europa is thought to harbor a vast ocean of liquid water, but under its outer crust of ice. Another of Saturn's moon's, Enceladus, also appears to hold liquid water inside, but its surface is as dry as the Earth's Moon. And yes, the Moon was recently confirmed to have surface water—but it's all frozen and mixed in with the lunar soil in a sort of dry cryo-mud.

The difference with Titan, however, is that its surface liquid is not water at all, but methane. It's too cold on Titan's surface for liquid water to exist there—but Titan's atmospheric pressure and temperature are right for methane to exist in its liquid state. So while on Earth we know methane as a greenhouse gas emitted by decomposing plants and the guts of cows—to name a couple of sources—on Titan it is the stuff of cloud, of rain, of river, and of lake.

What a wild world!

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Artist concept of a geyser erupting on Enceladus.
Credit: David Seal.

A layer of the veil around Saturn’s moons was removed when Pioneer 11 and Voyagers 1 and 2 made flybys of Saturn in the '70s and '80s. The Saturnian moons, it appeared, were not the lumps of rock and dust that Earth's own Moon is made of, but objects containing no small amount of water ice. Not terribly surprising, considering the low temperatures of the outer solar system where ice-rich comets roam.

Visions of frozen alien landscapes, replete with icicles and ice cliffs and ice fields and ice ice ice! were conjured in my imagination, and in artist depictions of majestic ringed Saturn seen from moons like Rhea or Dione or Enceladus.

Four years ago, Saturn’s first permanent visitor from Earth–the Cassini spacecraft–arrived there, and since has been making extreme closeup examinations of Saturn, its rings, and its increasingly wondrous and beautiful moons. Cassini is almost literally ripping apart veil after veil of our ignorance of these little worlds.

Far from a contingent of enormous but simple snow cone balls, Cassini has shown us that some of Saturn’s moons are apparently alive with liquid motion. First, there were the surface “lakes” and “seas” on Titan, probably made of extremely cold liquid hydrocarbons like methane and ethane–the stuff that spouts out of the gas range in your kitchen. Lakes and seas and rolling waves of liquid natural gas are fine and dandy for an imagined shoreline scene–but take a dip in those "waters" and an actual water-based creature like you would freeze solid in seconds. Scenic, but not inviting for a swim…

But recent observations by Cassini have shown that Titan's frigid unearthly lakes and Enceladus' snowball exterior may just be additional veils that are now being lifted.

In March, Cassini flew within 30 miles of the surface of Enceladus and right through a plume of material venting into space from the moon’s interior—an enormous "geyser." Earlier observations had sensed the presence of water in the plume, giving rise to speculation that liquid water in some form might exist beneath Enceladus' surface—perhaps chambers of liquid heated by tidal stressing of the interior.

When Cassini flew through the plume, its chemical sensors "sniffed" more than just water in the stream, but a good deal of organic molecules as well…not unlike material found in comets, stuff left over from the formation of the Solar System that may have been the building blocks of life on Earth.

The other "water find" was that of a possible liquid ocean under the crust of Titan–similar perhaps to the deep liquid water ocean believed to exist under the surface of Jupiter's moon Europa. Unexpected "drift" in the locations of landmarks on Titan's surface is what suggests a liquid ocean–water with perhaps some ammonia–that the frozen crust may be floating on.

With all the liquid water and organic chemistry being revealed in the Saturn system (and elsewhere in the outer solar system), our imaginations can shift from the older standards of envisioning otherworldly landscapes of sculpted ice or even seascapes of liquid hydrocarbon lapping on shores of water ice sand, to something a little more, shall we say, "lively…"?

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.

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