Dione: Saturn’s Active Little Moon-World

Saturn is perhaps the most beautiful planet in our Solar System, with its famous system of enchanting, gossamer rings, and its 62 known moons and myriad of tiny somersaulting moonlets that are mostly composed of glittering ice. The moons and moonlets do their dance both within and outside of Saturn’s rings. Saturn, the smaller of the two gas giant planets dwelling in our Solar System (the largest gas giant is Jupiter), is certainly able to bewitch us. In May 2013, astronomers studying the very heavily populated Saturn-system reported that Dione, a mid-sized moon, was likely an active little icy world in the past–and may still be active today!

Saturn dwells in the outer region of our Solar System far from the golden light and friendly warmth of our Star, the Sun. Some scientists think that both gas giants–Saturn and Jupiter–do not sport solid surfaces hidden deep beneath their massive, immense gaseous atmospheres. However, other scientists think that they probably do harbor relatively small solid cores composed of rocky-icy stuff. The other two large inhabitants of our Solar System’s outer fringes, Uranus and Neptune, are ice-giants, harboring much larger icy cores buried beneath their heavy atmospheres which, though very massive, are not nearly as heavy as the gaseous envelopes carried by the two gas giants,

Until 2004, no spacecraft had visited Saturn for more than twenty years. Pioneer 11 had taken the very first close-up picture of this beautiful giant planet when it zipped past it in 1979. Voyager 1 made its historic visit about a year later, and in August 1981 Voyager 2 had its short but extremely valuable encounter. At long last, on July 1, 2004, NASA’s Cassini spacecraft entered into Saturn orbit, and started snapping fabulous images.

The majority of Saturn’s natural satellites are tiny, icy jitter-bugging moonlets. However, the larger, mostly icy moons twirl around their enormous parent-planet in a bewitching ballet.

From a great distance, Saturn’s mid-size moon Dione appears to be a featureless, bland, lifeless icy ball. Looks can be deceiving.

Dione

Dione has a diameter of about 698 miles, and it is Saturn’s fourth largest moon. Indeed, small world that it is, Dione is the 15th largest moon in our Sun’s lovely family. Dione circles Saturn approximately once every 2.7 Earth-days, at a distance of 234,000 miles–about the same distance between our own planet and its large Moon. Dione is a mysterious icy moon-world, pockmarked by strange cratering and wispy terrain.

Dione was the fourth of Saturn’s moons to be detected, and the third to be discovered by the great Italian astronomer Giovanni Cassini. Cassini spotted the bewitching little moon in 1684 while observing Saturn from the Paris Observatory. Cassini called it one of the Sidera Lodoicea, which means the “Stars of Louis,” in honor of King Louis XIV. Later astronomers went on to name Saturn’s moons numerically, according to their distance from their giant parent-planet. This system dryly designated Dione as Saturn IV.

The enchanting little ball of ice was not honored with a formal name until 1847. In that year, the British astronomer John Herschel suggested that Saturn’s moons be named for the Titans. In ancient Greek mythology, the Titans were the siblings of the Greek god Cronus, who was known to the ancient Romans as Saturn. In Greek mythology, Dione (pronounced die-OH-nee”) was the daughter of Oceanus and Tethys, and she bore a great resemblance to the Earth goddess known as Gaia. Dione was also the mother of Aphrodite, the goddess of love, according to Homer’s Iliad. In fact, Dione was the mother of many.

The moon Dione is speckled with craters in a variety of different sizes. Some of the craters are as large as 62 miles. However, the location of the cratered areas presents an intriguing mystery. Usually, the leading hemisphere of an object moving through Space plunges head-on through a mess of debris, and therefore the leading hemispheres are usually the more heavily cratered. In the case of Dione, however, the reverse is true, and it is more heavily cratered on its trailing hemisphere than on the side that takes the lead. Astronomers have postulated that, perhaps, a smash-up with another object could have twirled the unlucky moon around. In this case, the impacting object would have been the cause of one of the 22-mile craters seen on Dione, and could have had the blasting power to whirl the moon around like a ballerina. However, the fact that Dione was apparently spun precisely 180 degrees is somewhat bizarre.

When Voyager I visited the little moon back in 1980, it detected strange wispy features on its trailing side. NASA’s Cassini took up-close and personal pictures more than two decades later, showing the puzzling wisps to be cliffs composed of ice. As dark stuff tumbled down from the cliffs, brightly shining ice was shown to exist underneath. These fractured areas, created by tectonic activity, crisscross the little moon-world, and extend tens to hundreds of miles in length.

Dione is approximately 1.48 times as dense as water. This suggests that the little moon sports a dense core that is encased by ice. The temperature on Dione averages -302 degrees Fahrenheit. This extremely frigid temperature causes the ice to behave more like rock than the ice we are familiar with on Earth.

Dione is tidally locked, with the same side always facing Saturn. This is not unusual–many moons in our Solar System are tidally locked to their parent-planets, including our own Moon. Dione’s gravitational grip also keeps two smaller moons, Polydeuces and Helene, locked into the same orbital position, as the sisterly icy trio circles their planet.

Cassini also spotted a tenuous oxygen atmosphere on Dione. This very thin atmosphere was approximately the same as conditions about 300 miles above Earth.

A “Fossil of Wondrous Activity”

In May 2013, astronomers announced that they had found evidence that Dione was likely an active little icy world in the past–and may still be today.

“A picture is emerging that suggests Dione could be a fossil of the wondrous activity Cassini discovered spraying from Saturn’s geyser moon Enceladus or perhaps a weaker copycat of Enceladus. There may turn out to be many more active worlds with water out there than we previously thought,” noted Dr. Bonnie Buratti in a May 29, 2013 NASA Jet Propulsion Laboratory (JPL) Press Release. The JPL is located in Pasadena, California.

Enceladus, another mid-sized icy moon of Saturn, is thought to sport a subsurface ocean beneath its frozen surface. Other bodies, in addition to Enceladus, are also believed to harbor subsurface global oceans, secreted beneath their icy shells–including Europa of Jupiter, and the very intriguing large, misty, hydrocarbon-tortured moon, Titan, also of Saturn. These bizarre moons, that frolic so mysteriously around the two gas giant planets of the outer Solar System, are among the most geologically jumping worlds inhabiting our Sun’s remarkable family. They have been beckoning targets for astrobiologists searching for the stuff of life elsewhere in our Solar System, and the presence of still another subsurface ocean at Dione would make this icy little world yet another target. Dione, once considered a lifeless little ball of boring ice, is now thought to be a geologically active sphere that could potentially host delicate living tidbits.

Alluring clues that Dione is an active little moon came from the Cassini mission. The highly prolific spacecraft’s magnetometer spotted a faint stream of particles emanating from the moon, and images revealed hints of a slushy, or possibly liquid, layer hidden beneath Dione’s coating of rock-hard ice. Still other images from Cassini have revealed inactive, ancient cracks on the surface of Dione, which resemble those observed at Enceladus, that geyser-like spray out water ice and organic particles.

A mountain that was examined in a paper published in the journal Icarus in March 2013, called Janiculum Dorsa, ranges in height from approximately 0.6 to 1.2 miles. Dione’s icy crust seems to pucker beneath this mountain as much as 0.3 mile.

“The bending of the crust under Janiculum Dorsa suggests the icy crust was warm, and the best way to get that heat is if Dione had a subsurface ocean when the ridge formed,” Dr. Noah Hammond explained in the May 29, 2013 JPL Press Release. Dr. Hammond, that paper’s lead author, is of Brown University in Providence, Rhode Island.

Dione gets nicely warmed up because it repeatedly gets stretched and squeezed as it travels closer to and farther from its parent-planet in its orbit. With an icy coating that can slip around independently of the little moon’s interior, the gravitational tugs of Saturn get exaggerated and create about 10 times more heat, Dr. Hammond continued to explain.

It is still a puzzle as to why Enceladus became so geologically active while Dione seems to be considerably less so. Tidal heating was, perhaps, stronger on Enceladus, or perhaps the larger component of rock in the core of Enceladus provided more radioactive heating from heavy elements.