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== Orbital characteristics ==

== Orbital characteristics ==

Dione is in a somewhat circular orbit around Saturn, at an average distance of 377,400 km. Its sidereal month is about 2.74 Earth days.

Dione is in a somewhat circular orbit around Saturn, at an average distance of 377,400 km. Its sidereal month is about 2.74 Earth days.

Dione appears to be in a 2:1 orbital resonance with [[Enceladus]], an inner moon. Some astronomers suggest that this causes significant [[tidal heating]] on Enceladus.<ref name=Britannica>Hubbard, William B. "[http://www.britannica.com/eb/article-9030539 Dione]." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 12 June 2008.</ref>

Dione appears to be in a 2:1 orbital resonance with [[Enceladus]], an inner moon. Some astronomers suggest that this causes significant [[tidal heating]] on Enceladus.<ref name=Britannica>Hubbard, William B. "[http://www.britannica.com/eb/article-9030539 Dione]." Encyclopædia Britannica. 2008. Encyclopædia Britannica Online. 12 June 2008.</ref>

Surprisingly, most of Dione's craters are found on the trailing hemisphere, not the leading. Shoemaker and Wolfe have suggested, from their studies of the [[Galilean moons]], that the leading hemisphere of any moon in [[tidal lock]] ought to have more craters than the trailing hemisphere.<ref name=Shoemaker>Shoemaker, E. M., and Wolfe, R. F. "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1982stjp.conf..277S&db_key=AST&data_type=HTML&format=&high=45eb6e10af25897 Cratering time scales for the Galilean satellites]." ''Satellites of Jupiter'', Tucson, [[Arizona]]: University of Arizona Press, 1982, pp. 277-339. Accessed June 12, 2008.</ref>

Surprisingly, most of Dione's craters are found on the trailing hemisphere, not the leading. Shoemaker and Wolfe have suggested, from their studies of the [[Galilean moons]], that the leading hemisphere of any moon in [[tidal lock]] ought to have more craters than the trailing hemisphere.<ref name=Shoemaker>Shoemaker, E. M., and Wolfe, R. F. "[http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1982stjp.conf..277S&db_key=AST&data_type=HTML&format=&high=45eb6e10af25897 Cratering time scales for the Galilean satellites]." ''Satellites of Jupiter'', Tucson, [[Arizona]]: University of Arizona Press, 1982, pp. 277-339. Accessed June 12, 2008.</ref>

This has led some astronomers to propose that, in the time when many bodies in the solar system underwent heavy bombardment, Dione was in tidal lock in an orientation opposite to that which Dione now has with respect to Saturn. Hamilton suggests that any object large enough to create an impact crater 35 km or wider in diameter would have given Dione sufficient energy to turn itself 180° about and come to a new tidal lock. He also points out that Dione has a number of craters more than 100 km in diameter.<ref name=Hamilton/>

This has led some astronomers to propose that, in the time when many bodies in the solar system underwent heavy bombardment, Dione was in tidal lock in an orientation opposite to that which Dione now has with respect to Saturn. Hamilton suggests that any object large enough to create an impact crater 35&nbsp;km or wider in diameter would have given Dione sufficient energy to turn itself 180° about and come to a new tidal lock. He also points out that Dione has a number of craters more than 100&nbsp;km in diameter.<ref name=Hamilton/>

Neukum ''et al.'' estimated that the surface of Dione, like those of several other moons of Saturn, must be geologically old on account of the number of craters found, and their distribution.<ref name=Neukum>Neukum, G., Wagner, R., Denk, T., ''et al.'' "[http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2034.pdf The Cratering Record of the Saturnian Satellites Phoebe, Tethys, Dione, and Iapetus in Comparison: First Results from Analysis of the Cassini ISS Imaging Data]." ''36th Annual Lunar and Planetary Science Conference'', 2005. Accessed June 12, 2008.</ref>

Neukum ''et al.'' estimated that the surface of Dione, like those of several other moons of Saturn, must be geologically old on account of the number of craters found, and their distribution.<ref name=Neukum>Neukum, G., Wagner, R., Denk, T., ''et al.'' "[http://www.lpi.usra.edu/meetings/lpsc2005/pdf/2034.pdf The Cratering Record of the Saturnian Satellites Phoebe, Tethys, Dione, and Iapetus in Comparison: First Results from Analysis of the Cassini ISS Imaging Data]." ''36th Annual Lunar and Planetary Science Conference'', 2005. Accessed June 12, 2008.</ref>