
March 2006: Liquid Water Possible on Enceladus

The geysers erupting on Enceladus are backlit by the sun. Photo: NASA/JPL

Cassini flybys of Saturn’s bright little moon Enceladus have revealed geysers spewing water vapor and ice crystals high above the moon’s south pole. It’s quite remarkable that this small moon, only 300 miles (less than 500 km) in diameter, is geologically active. But the real surprise is the probable source of the eruptions: pockets of liquid water close to the satellite’s surface.

With the possibility of liquid water comes the question, could life arise? According to Dr. Carolyn Porco, head of the Cassini imaging team, “If we are right [that there is evidence of liquid water], we have significantly broadened the diversity of solar system environments where we might possibly have conditions suitable for living organisms.”

The parallel fault lines near the south pole of Enceladus—dubbed “tiger stripes”—are part of the geyser activity. According to the model developed by Cassini scientists, pockets of pressurized liquid water reach the surface through vents in the tiger stripes. Photo: NASA/JPL


Recent explorations of Mars indicate that liquid water once flowed on its surface, and scientists suspect that a huge sea underlies the frozen surface of Jupiter’s moon Europa. If life has developed in the solar system anywhere other than on Earth, Mars and Europa are considered likely places. Now Enceladus has been added to this very exclusive list, making this unusual moon one of the most exciting places in the solar system.

October 2005: Moon Count Climbs
While the Cassini spacecraft hunts for moons amid Saturn’s rings, earth-based telescopes search the more remote regions of the Saturnian sky. On May 3, 2005, astronomers from the University of Hawaii revealed the discovery of 12 outlying moons. Temporarily dubbed S/2004 S7 through S/2004 S18, the moons were first seen in December 2004, but the astronomers observed them for the first few months of 2005 with a variety of powerful telescopes before announcing their find.

These moons have elongated, inclined orbits, which puts them in a class called irregular satellites. And all but one travel around Saturn in the opposite direction to Saturn’s rotation. Both the irregularity and the direction of their orbits indicate that they were captured by Saturn from their original paths around the sun. Under the current conditions of the solar system, it wouldn’t be possible for a planet to kidnap, say, a passing asteroid, so irregular satellites must have been captured long ago. Studying these moons will tell us about an earlier time, possibly a time shortly after the planets were formed.

Moons Make Waves
A new moon spotted by Cassini in May 2005 gratified scientists who had predicted its existence. The little moon, known as S/2005 S1 for now, was found hiding in the Keeler gap in Saturn’s A ring. The tip-off came when scientists noticed that the uneven edges of the Keeler gap were similar to those of the Encke gap, also in the A ring, which is home to the moon Pan. The wavy edges of the gaps indicate that the embedded moons have an effect on the material that makes up the rings.

Moonlet S/2005 S1, 4 miles (7 km) in diameter, sails through the middle of the Keeler gap in Saturn’s A ring.
Photo credit: NASA/JPL/Space Science Center

The photo (above) of the Keeler gap provides a compelling lesson in orbital mechanics. The top of the image is closest to Saturn and the moon is moving leftward. The ring particles closer to Saturn (above the moon) orbit faster than the moon so that they carry the wavy perturbations ahead of the moon, to the left. The ring particles below the moon orbit slower than the moon and so the ripples stretch behind the moon to the right.

An important property of the ripples is how fast they die out. Notice that they are larger near the moon and smaller farther away. The ring particles orbit in the vacuum of space so they move without friction in the classical earthbound sense. But something is causing the ripples to die out. Finding out the exact origin of the damping of the waves will help us understand the size distribution and density of particles that make up the rings and also how they interact with each other in the presence of Saturn’s gravity.

Moons Receive New Names
The first moons discovered by Cassini in the summer of 2004 have traded in their temporary appellations for more melodious mythological names. No longer plain S/2004 S1 and S/2004 S2, these tiny inner moons are now Methone and Pallene, named for two of seven sisters known as the Alkyonides. They orbit between two of Saturn’s major moons, Mimas and Enceladus.

A third moon, S2004 S5, is now called Polydeuces. In Greek mythology, Polydeuces (or Pollux) is the son of Zeus, which makes him the grandson of Cronus—who the Romans called Saturn. Polydeuces is interesting because it’s a Trojan moon, a moon that travels in the same orbit as a larger moon, either 60 degrees ahead of it or behind it. Polydeuces travels behind Dione, which has a second companion, Helene, that travels in front of it.

January 2005 : Huygens Probe Huge Success
The European Space Agency’s Huygens probe parachuted through the smoggy atmosphere of Saturn’s mysterious moon Titan on January 14, accomplishing all the tasks set for it—and more.

To hear the sounds that the Huygens probe recorded,
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After a seven-year journey aboard the spaceship Cassini, Huygens separated from its mother ship on December 25 and coasted a million miles toward Titan. It reached its target exactly as planned, then spent two-and-a-half hours descending 700 miles through Titan’s atmosphere. Along the way, it sampled and analyzed gases in the atmosphere, recorded sounds, took images of Titan’s surface, and collected a wealth of other data.

A soft landing
Scientist couldn’t anticipate whether Huygens would land on a hard or soft surface, or possibly splash down into a liquid lake, but it turned out that they had picked an ideal location. Approaching Titan at 15 mph, the probe settled onto a spongy, forgiving surface.

Undamaged by the landing, Huygens survived on Titan for about five hours—far exceeding the “best case” expectations of thirty minutes—and was able to transmit about two hours of useful data.

Like the earth—only different
The images Huygens provided of Titan seem startlingly familiar. There are plains strewn with rocks, for example, and meandering, branching riverbeds.

Surface of Titan
The tangerine sky is due to Titan’s smoggy atmosphere. Sunlight shining through the atmosphere onto rocks of white water ice would make the rocks look orange, too.
Photo credit: ESA/NASA

The main visual difference between Titan’s landscape and that of the earth is that there’s no vegetation or other sign of life. But there are major chemical differences as well. First of all, those rocks are likely to consist of water ice. With a surface temperature hovering at around -300°F (-180°C), rocks of water ice would be as hard as the silicate rocks on earth and would be in no danger of melting.

The rocks show signs of weathering, however, and clouds over Titan indicate that the moon does experience weather. In place of the liquid water that rains onto the earth, though, it appears that Titan is showered with methane. At Titan’s temperatures, methane can exist as a solid, liquid, or gas, so it’s very probable that Titan has a “methane cycle” similar to the earth’s water cycle.

The land in between the rocks in the image is like the soil Huygens landed on. It appears that it’s a granular water ice mixed with hydrocarbons and saturated with liquid methane. Heat generated by Huygens caused bursts of methane gas to rise from the soil, where the gas was detected by two of the Huygens instruments.

Another feature of Titan’s soil is that there are dark spots, particularly in riverbeds and other low-lying areas. It seems that hydrocarbons from the atmosphere settle on the land but are washed off high surfaces by rain.

A young surface
The Huygens images, along with images taken by Cassini, show that Titan has an extremely varied topography. Conspicuous by their absence, though, are large craters such as those that cover our own moon and many of Saturn’s other satellites. Titan’s surface is “geologically young,” having been renewed by precipitation, erosion, and other processes that also shape the surface of the earth.

We have, then, a place that shares many of earth’s geophysical and meteorological processes, but processes that work on entirely different substances and at temperatures that are so cold it’s hard to imagine them.

Titan is nothing short of astonishing.

December 2004: Peering through Titan’s Haze
The Cassini spacecraft has recently had two close encounters with Saturn’s moon Titan. On October 26, 2004, it flew within 750 miles (1,200 km) of this enigmatic satellite. It returned for a second flyby on December 13, this time approaching within about 1,500 miles (2,400 km). Outfitted with cameras, infrared sensors, and radar, Cassini set about penetrating Titan’s smoggy atmosphere to investigate the lunar surface. Data beamed back to earth have delighted scientists—but have raised many questions as well.

Titan is surrounded by layers of haze hundreds of miles above its surface.

Titan’s intriguing surface
Although Titan has undoubtedly been bombarded by comets and asteroids during its history, Cassini did not find a cratered surface such as exists on our own moon. In fact, in the area surveyed by radar, approximately 1 percent of the surface, there were only slight variations in elevation. A strong possibility is that the surface of the moon is being fashioned by geologic processes that make it “young.”

What the Cassini radar did see is that there’s considerable variation in the terrain. Basically, bright areas are interpreted as rough terrain and dark areas as smooth. But what makes up the smooth areas? Ice is likely, but liquid would have a smooth appearance as well. Whether or not lakes exist is still a major question. With a surface temperature of -290° Fahrenheit (-180° C), though, we know that any liquid bodies can't consist of pure water. Methane and ethane, or a combination of the two, would be possible, however.

Other data gathered by Cassini indicate the chemical composition of surface materials. Interpretation of these data seem to confirm the supposition that Titan is covered with hydrocarbons—organic compounds that may be similar to those that existed on earth before life evolved. By studying Titan, scientists may gain insight into how basic hydrocarbons develop into complex molecules that can lead to life.

Titan has weather
During the October flyby, Cassini observed a field of clouds, thought to consist of methane, near the moon’s south pole. Other than that, the Titan skies were curiously clear. Images taken during the December 13 flyby, however, show patches of clouds nearer to the moon’s equator, which is the first direct evidence that Titan has changing weather patterns. This new information will help scientists understand wind speeds and atmospheric circulation.

An upcoming exploration of Titan’s atmosphere

Data gathered in these two recent flybys will also aid the scientists responsible for the Huygens probe that will descend through Titan’s atmosphere in January 2005. Huygens, which hitched a ride aboard Cassini, has been dormant for the seven years long years of its journey. But it’s about to awake.

Huygens separated from the Cassini orbiter in late December and began coasting towards Titan. On January 14 it will parachute through Titan’s atmosphere for more than two hours, collecting images, temperature readings, wind measurements, and pressure profiles.

It will also analyze the chemical composition of the gases it passes through and any particulate matter the gases contain. This is of particular interest because the December 15 flyby revealed that the high haze, rather than being homogenous, is made up of many discrete layers.

In addition, Huygens will be listening to sounds in Titan’s atmosphere. Should the probe pass through a storm, it might record audible emissions that indicate the presence of lightning, or the sounds of rain striking its surface.

When Huygens reaches Titan’s surface, it could crash into ice, sink into a hydrocarbon snow, or possibly splash down into a methane lake. It’s unlikely that the probe will survive the landing, but scientists hope that Huygens will be able to transmit for a few minutes from the surface of this distant world.

October 2004: New Saturnian Moons Discovered
Images taken by the Cassini spacecraft in the summer of 2004 may show the presence of as many as four previously undetected moons. If confirmed, this will bring the number of Saturn’s known lunar attendants to thirty-five.

The first two sighted, which are known for now as S/2004 S1 and S/2004 S2, are only 2 miles (3 km) and 2.5 miles (4 km) in diameter, so it’s not surprising that they would have previously escaped notice. They were found between the orbits of two medium-sized inner moons, Mimas and Enceladus, in imaging sequences designed to look for moons in this inner region. S/2004 S1 was observed for about six hours, and S/2004 S2 was observed for more than nine hours.

Scientists think that other small moons will be found in gaps between the rings, and they plan, in particular, for Cassini to go moon hunting in the area between the A Ring and the F Ring. Already, they’ve seen what may be two moons, about 2.5 or 3 miles (4–5 km) each in diameter, orbiting on either side of the F Ring. Scientists aren’t sure if they’ve spotted moons or temporary clumps. It could happen, for example, that a number of rocks orbiting near the F Ring temporarily traveled together—the way cars briefly form clusters on a highway—giving the appearance of a solid object. Nor can scientists say for certain if there are really two distinct objects or one object that crosses the ring. Nonetheless, the objects sighted are being treated as two potential moons, and they’ve provisionally been named S/2004 S3 and S/2004 S4.

The discovery of the moon
S/2004 S2 was announced by NASA on August 16, 2004.
Image courtesy of NASA.

Before Cassini arrived on the scene, no Saturnian moon smaller than about 12 miles (20 km) in diameter had been discovered. Cassini’s improved imaging technology and its initial discoveries show that we have an exciting new capability for learning about tiny planetary satellites. If Cassini finds many new moons, it will test the theories scientists currently have about these small bodies and will expand our understanding of how small moons form and evolve.

Cassini will be in orbit around Saturn for four years. It’s a good bet that the number of known Saturnian moons will have increased considerably by the end of that time, along with our knowledge about them.

July 2004: Cassini Enters Saturn's Orbit
The first spacecraft ever to enter Saturn's orbit, Cassini sent back this image of a portion of the planet's rings. It was taken by the spacecraft's narrow angle camera and shows the dark, or unlit, side of the rings.

Courtesy NASA/JPL/Space Science Education Forum

+ View the latest raw data sent back from Cassini

+ See a computer rendering of Cassini's current position in the rings of Saturn