Titan, hidden under an atmospheric veil that was once impenetrable to the gaze of spacecraft, has enticed scientists for decades. Now, Cassini, and its probe Huygens, have revealed a craggy surface carved by liquid methane and pocked with methane lakes like the one pictured here, shining in the sun.

This moon's surface temperature (minus 290 F) is close to the triple point of methane, the temperature at it which can exist as a liquid, a solid and a gas. On Titan, methane behaves much like water behaves on Earth: It evaporates into the atmosphere, forms clouds and rains down on the surface, cutting riverbeds and collecting in basins as lakes.

This drawing of Titan's interior is based on evidence from Cassini and Huygens. Radio data and electric-field measurements suggest that the moon is a mixture of water ice and rock dusted with a sandy, and in places sodden, veneer of hydrocarbons (organic compounds consisting entirely of hydrogen and carbon) and pockets of water ice. The drawing shows the surface in cloudy ocher over a light gray layer of ice. That layer may be 300 miles thick and composed of nearly pure water ice. The blue layer underneath is, speculatively, a subterranean ocean of liquid water and ammonia. Beneath is a dark gray core of unstratified ice and rock.

These images are clues to the composition of Titan's surface. Cassini snapped this radar image of the Sinlap impact crater (top of image) during a flyby of Titan in February 2005. Researchers overlaid the original black and white image with a color map of radiometric-brightness temperature readings from the spacecraft's synthetic-aperture radar.

The bottom half of this image shows part of a dielectric constant map, a measure of a material's ability to carry an electric current. This one is made from polarized radiometric data. The red square corresponds to the area of the image above. The dielectric constant for water ice is larger than three, therefore it appears red on this map. Titan's surface is blue and light blue, indicating that there is no water ice on the surface. The yellow areas in Sinlap show that the dielectric constant is higher there, closer to that of water. The suggestion is that whatever made the crater unearthed a pocket of water ice below the surface. And this is the only evidence for water discovered on Titan's surface so far.

On Cassini's flyby of Titan in April 2006, it shot the images that form this composite with its Visual and Infrared Mapping Spectrometer. The dark blue areas may contain water ice, though hydrocarbons are the dominant compounds on the surface. "Sinlap is the only place on [Titan's surface] where we have indications that there may be water ice," says Christophe Sotin, a senior research scientist at NASA's JPL in an interview. Findings like this begin to paint a picture of Titan as a ball of water ice covered entirely by organic chemicals.

In January 2005, Huygens parachuted to Titan's surface to measure the electric field there. It found an expansive, conductive layer underground, which researchers believe is a mixture of liquid water and ammonia.

The figures above show electrical-field measurements at different altitudes as the probe descended. Galactic and cosmic rays (GCR in the figures) account for the electrical readings until the probe reached the surface. The discrepancy could indicate an ocean buried about 45 kilometers below the surface.

Enceladus appears here as a crescent dangled beside the broad curve of Saturn's surface. Sharp eyes will detect the haze of water-ice plumes erupting into space from the moon's bottom edge. The ice blows out of sinuous fissures, which NASA's researchers call "tiger stripes," near the moon's south pole. With images like this and other data, Cassini has answered questions that have puzzled researchers for decades, such as the moon's relationship to the ring it travels in, and raised new ones, such as the possibility of a liquid ocean under its surface.

Cassini revealed that Enceladus's ice plumes pile water into Saturn's magnetosphere and change the rotation of the planet's magnetic field. Cassini has detected water and organic chemicals in those plumes, prompting speculation about life in the moon's surface vents.

Cassini snapped this image of Saturn's moon Enceladus in visible light with its wide-angle camera at a distance of 10,000 miles (17,000 km) from the moon on Nov. 21, 2009.

These are the best visible-light images that Cassini will take of the erupting fractures before a years-long winter darkens the pole of Enceladus. The fractures are each named after Middle Eastern cites. Here, the Baghdad Sulcus is pictured with others nearby.

"We always knew that Enceladus was unusual," Buratti says. Voyager showed that it is bright as fresh snow and it is nestled in the densest part of Saturn's E Ring. "The theory was that the ring came from Enceladus," she says. "But we didn't see anything, just all these hints; intriguing hints." Now they can see that the moon likely spews out the material that makes up the ring, and they are beginning to understand how it does that.

The spacecraft took this wide-angle image of the south pole of Enceladus at a distance of 1200 miles (2000 km) during a flyby on Nov. 21, 2009.

Cassini photographed more than 30 jets of water ice spewing from Enceladus's south pole in this composite of two narrow-angle images taken on Nov. 21, 2009.

In July 2005, the spacecraft yielded its first closeup views of the moon and these fractures. "What we saw was basically a boiling cauldron," Buratti says. "We saw a hotspot at the south pole, 70 kelvin [126 F] above what it should be."

This is a composite image of the erupting fissure called the Damascus Sulcus. It is a blend of images that Cassini took laid over a new topographic map Paul Schenk made at the Lunar and Planetary Institute in Houston, Texas.

The temperature in the fissures may be high enough for liquid water and the moon may hide an ocean of water underground. "There are a lot of arguments," Buratti says. "But I would say that there is consensus that there is liquid water, that there's a liquid ocean under the surface." If so, that liquid water may also erupt from the fissures.

This infrared image visualizes temperatures in the Baghdad Sulcus. It is the most detailed image to date of the heat emanating from the moon's interior through the fissures. The heat is squeezed into a strip about half a mile wide and reaches temperatures of minus 180 Kelvin (140 F). The narrow fissure in the center is probably warmer, possibly warm enough to melt water ice. Baghdad Sulcus is about 1600 feet deep and 109 miles long.

Two forces may drive the eruptions on Enceladus. Twice in its orbit around Saturn, it passes beside another moon, Dione, pictured here behind Enceladus. The gravitational pull from the planet and the other moon may propel powerful tides inside Enceladus and generate the heat required for eruptions.

There are other theories to explain the heat coming from Encaladus. Some suggest there are nuclear elements, such as uranium, in the rocks of the moon's interior that release heat as they decay. Others—the minority of researchers—say that chemical reactions could spark explosions that blow ice out of the interior.

Life on Earth may have first arisen from oceanic vents, Buratti says, and "the same conditions are there on Enceladus." This artist's rendering shows Cassini flying by the geysers at the moon's south pole. They contain the basic elements of life and may be warm enough for liquid water.

"We know that there are prebiotic elements like light hydrocarbons—simple molecules. The conditions for life are there," Buratti says. But, "the thing about Enceladus is that it really is an alien world. It's so cold out there."