Dione (moon)

• 1. Introduction

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  Dione is a moon of Saturn, and for most of human history it was nothing more than a pinprick of light glimpsed through a large telescope pointed at the night sky over Paris. Giovanni Domenico Cassini spotted it in 1684, gave it a name meant to flatter a king, and then the world largely moved on. For nearly three centuries, Dione remained a mystery wrapped in ice. Then spacecraft began to arrive, and what they found transformed a distant dot into a world of towering frozen cliffs, hidden oceans, and a whisper of oxygen hanging in the void. How did an astronomer's tribute to royalty become one of the Solar System's more geologically surprising objects? And what does it mean that this frozen moon may harbor liquid water beneath its surface right now?

• 2. A Name for a King

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  Giovanni Domenico Cassini discovered Dione on the 30th of March 1684, working with a large aerial telescope he had set up on the grounds of the Paris Observatory. He did not name the moon after the Greek Titaness Dione straight away. Instead, he grouped it with the three other moons he had found, Tethys, Rhea, and Iapetus, and called them collectively Sidera Lodoicea, meaning "the stars of Louis," in honor of King Louis XIV. The names that stuck today were not formally proposed until 1847, when John Herschel, son of astronomer William Herschel, published his Results of Astronomical Observations made at the Cape of Good Hope. Herschel suggested that the moons of Saturn should carry the names of the Titans, the sisters and brothers of the god Cronus in Greek mythology. In that tradition, Dione takes its name from the Titaness of Greek myth. Even in modern times, someone found it necessary to give Dione a symbol: Denis Moskowitz, a software engineer who designed most of the dwarf planet symbols, proposed a Greek delta combined with the crook of the Saturn symbol. The resulting emblem is not widely used, but it exists.

• 3. Ice, Rock, and Hidden Water

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  With a mean diameter of 1,123 km and a density of about 1.48 g/cm3, Dione sits at a precise midpoint between worlds of pure ice and worlds of pure rock. Rock and water ice are roughly equal in mass within it. Cassini spacecraft measurements suggested that beneath roughly 160 km of water ice lies a rocky core with a radius of about 400 km. Some models go further, proposing that the lowest part of that ice envelope could be a liquid salt water ocean, a situation similar to what exists inside neighboring moon Enceladus. The strongest evidence comes from a mountain ridge called Janiculum Dorsa, which stands between 1 and 2 km high. The crust beneath it appears to sag by about 0.5 km, a kind of downward bending that is most easily explained if an ocean underneath is increasing the tidal flexing of the shell. Dione's ice shell is thought to vary in thickness by less than 5%, with the thinnest areas sitting at the poles, where tidal heating operates most intensely.

• 4. The Cliffs Nobody Expected

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  When Voyager 1 photographed Dione in 1980, the images revealed pale, wispy markings trailing across the moon's dark hemisphere. Scientists debated their origin for more than two decades. One hypothesis held that Dione had been geologically active early in its history and that cryovolcanic eruptions had sent material raining back down as snow or ash, streaking the surface. It was a reasonable guess, but it was wrong. On the 13th of December 2004, the Cassini spacecraft completed a close flyby that shattered the old interpretation. What Voyager had photographed as soft, diffuse wisps were actually enormous ice cliffs, bright because fresh fractured ice reflects light powerfully and steep because they were carved by tectonic fracturing of the crust. A follow-up flyby on the 11th of October 2005 brought Cassini to within 500 km of the surface and captured oblique images confirming that some of those cliffs rise several hundred metres high. These features are now classified as chasmata, long and deep canyon-like depressions, and they dominate the trailing hemisphere of the moon.

• 5. A World Spun by Impacts

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  Dione's cratering pattern contains a puzzle that points back to its violent early history. The heavily cratered terrain is concentrated on the trailing hemisphere, while the less cratered plains occupy the leading hemisphere. That distribution is the reverse of what scientists had predicted. Shoemaker and Wolfe had proposed a model in which a tidally locked moon should accumulate the most craters on its leading side, the face it presents forward as it orbits. The reversed pattern on Dione suggests that during the period of heavy bombardment, the moon was locked to Saturn in the opposite orientation from today. Because Dione is relatively small, an impactor capable of producing a crater only 35 km across could have been energetic enough to spin the entire satellite. Given that many craters larger than 35 km are visible on the surface, Dione was likely spun multiple times during that era of intense bombardment. The current orientation appears stable; the bright albedo of the leading side suggests Dione has held its present position for several billion years.

• 6. An Exosphere in the Void

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  On the 7th of April 2010, instruments aboard Cassini detected something unexpected around Dione: a thin envelope of molecular oxygen ions. The density is vanishingly small, measured at between 0.01 and 0.09 ions per cubic centimeter, which is why scientists describe it as an exosphere rather than an atmosphere in any conventional sense. The leading hypothesis for its origin involves Saturn's powerful radiation belts. Charged particles from those belts bombard the icy surface, splitting water molecules into hydrogen and oxygen. The oxygen that does not escape drifts above the surface in that sparse halo. Cassini's instruments could not directly measure water in the exosphere because background interference was too high, but the oxygen signal was clear. Dione is not alone in having this kind of exosphere; its orbital resonance partner Enceladus hosts far more dramatic activity, including cryovolcanic jets that feed directly into Saturn's E ring.

• 7. The Resonance That Keeps Enceladus Alive

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  Dione completes one orbit of Saturn for every two orbits completed by Enceladus, a 1:2 mean-motion orbital resonance. That gravitational rhythm is not merely an astronomical curiosity. It maintains an orbital eccentricity of 0.0047 in Enceladus, and that eccentricity is what drives the tidal flexing that heats the interior of Enceladus and powers its famous cryovolcanic geyser-like jets. Without Dione holding that resonance in place, Enceladus would lose heat, its liquid water would freeze, and its jets would cease. The same resonance keeps a smaller eccentricity of 0.0022 in Dione's own orbit, providing a modest source of tidal heating to Dione as well. Dione also has two co-orbital companions, called trojan moons: Helene, with a diameter of about 36.2 km, discovered on the 1st of March 1980, and Polydeuces, roughly 3 km across, discovered on the 21st of October 2004. They sit at Dione's Lagrangian points, 60 degrees ahead and behind it in its orbit. A third possible leading co-orbital companion was reported by Stephen P. Synnott in 1982, located about 12 degrees ahead of Helene.