Dione (moon)
Giovanni Domenico Cassini set up a large aerial telescope on the grounds of the Paris Observatory in 1684. He found Dione that year while searching for moons around Saturn. Cassini named this moon and three others Sidera Lodoicea to honor King Louis XIV. The name meant the stars of Louis. No one called these satellites by their current names until 1847. John Herschel published Results of Astronomical Observations made at the Cape of Good Hope that year. He suggested using names from Greek mythology for all Saturnian moons. William Lassell reported this naming convention in Monthly Notices of the Royal Astronomical Society on the 14th of January 1848. Denis Moskowitz later designed a symbol combining a Greek delta with the crook of the Saturn symbol. This symbol remains rarely used today.
Dione orbits Saturn with a semimajor axis about 2% less than Earth's Moon. Its orbital period is one tenth that of Earth's Moon due to Saturn's greater mass. The moon completes one orbit for every two orbits completed by Enceladus. This 1:2 mean-motion orbital resonance maintains Enceladus's orbital eccentricity at 0.0047. That eccentricity provides heat for Enceladus's cryovolcanic geyser-like jets. Dione itself experiences tidal heating because its orbit has an eccentricity of 0.0022. Two co-orbital trojan moons named Helene and Polydeuces share Dione's path. They sit within Dione's Lagrangian points 60 degrees ahead and behind it. Stephen P. Synnott reported a leading co-orbital moon twelve degrees ahead of Helene in 1982. The Guinness Book of Astronomy published details about these trojans in its second edition in 1983.
Scientists believe Dione contains a rocky core roughly 400 km in radius. A water ice envelope surrounds this core with a thickness of about 160 km. Models suggest the lowermost part of this layer could be liquid salt water. Downward bending of the surface associated with the Janiculum Dorsa ridge supports this theory. The ridge stands 1.5 km high and causes the crust to pucker beneath it. This deformation suggests the icy shell was warm when the ridge formed. Tidal flexing from a subsurface ocean would increase that warmth. Cassini gravity observations indicate deviations from hydrostatic equilibrium maintained by isostasy. Dione's ice shell varies in thickness by less than 5%. Thinnest areas occur at the poles where tidal heating is greatest.
Voyager 1 photographed Dione in 1980 showing wispy features covering its trailing hemisphere. These bright streaks were mysterious because they did not obscure underlying surface features. Scientists initially thought cryovolcanism resurfaced much of the moon long ago. Later eruptions along cracks fell back as snow or ash. Cassini flybys on the 13th of December 2004 proved these wisps were actually ice cliffs. Tectonic fractures called chasmata created these bright ice cliffs instead. Some cliffs reach several hundred meters in height according to oblique images taken on the 11th of October 2005. Linear virgae run parallel to the equator up to hundreds of kilometers long. These lines are brighter than surrounding terrain and overlay ridges and craters. They appear only at latitudes below 45 degrees north or south. Much heavily cratered terrain exists on the trailing hemisphere while plains dominate the leading side.
Instruments aboard Cassini detected molecular oxygen ions around Dione on the 7th of April 2010. The density ranged from 0.01 to 0.09 per cubic centimeter. Scientists prefer calling this layer an exosphere rather than a tenuous atmosphere. Highly charged particles from Saturn's radiation belts split water molecules in the ice. This process releases hydrogen and oxygen into space. Cassini plasma spectrometer data confirmed the presence of these ions. Water detection remained impossible due to high background levels. The interaction between surface ice and planetary radiation creates this thin atmospheric layer. Molecular oxygen ions form the primary component of this exosphere.
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Common questions
When was the moon Dione discovered by Giovanni Domenico Cassini?
Giovanni Domenico Cassini discovered the moon Dione in 1684 while searching for moons around Saturn. He set up a large aerial telescope on the grounds of the Paris Observatory to make this finding.
Who named the moon Dione and when did that naming convention begin?
John Herschel suggested using names from Greek mythology for all Saturnian moons in 1847. William Lassell reported this naming convention in Monthly Notices of the Royal Astronomical Society on the 14th of January 1848.
What is the orbital relationship between Dione and Enceladus?
Dione completes one orbit for every two orbits completed by Enceladus. This 1:2 mean-motion orbital resonance maintains Enceladus's orbital eccentricity at 0.0047.
How thick is the water ice envelope surrounding the rocky core of Dione?
A water ice envelope surrounds the rocky core with a thickness of about 160 km. Models suggest the lowermost part of this layer could be liquid salt water.
When did Cassini flybys prove that wispy features on Dione were ice cliffs?
Cassini flybys on the 13th of December 2004 proved these wisps were actually ice cliffs created by tectonic fractures called chasmata. Some cliffs reach several hundred meters in height according to oblique images taken on the 11th of October 2005.
What molecular component forms the primary part of Dione's exosphere detected by Cassini?
Molecular oxygen ions form the primary component of this exosphere around Dione. Instruments aboard Cassini detected these ions on the 7th of April 2010 with a density ranging from 0.01 to 0.09 per cubic centimeter.