Dione
Who discovered Dione and when?
What are Dione’s surface features?
What suggests geologic activity on Dione?
Dione, fourth nearest of the major regular moons of Saturn. It was discovered by the Italian-born French astronomer Gian Domenico Cassini on March 21, 1684, and named for a daughter of the Titan Oceanus in Greek mythology.
Orbit
Dione has a diameter of 1,118 km (695 miles) and revolves around Saturn in a prograde (in the same direction as Saturn’s rotation, counterclockwise as viewed from above the planet’s north pole), nearly circular orbit at a mean distance of 377,415 km (234,515 miles), which is within the outer part of Saturn’s tenuous E ring, a vast band of material extending from 180,000 to 482,000 km (111,850 to 300,000 miles) away from the surface of Saturn. Dione is accompanied in its orbit by two much smaller moons, Helene and Polydeuces (also named for Greek mythological figures). Helene, which has a diameter of about 30 km (20 miles), maintains a gravitationally stable position 60° ahead of Dione. Polydeuces has less than half the diameter of Helene and follows Dione by 60°, but with large deviations from its mean position. The orbits of these tiny companion moons can be compared to those of Jupiter’s Trojan asteroids.
Tidal interactions with Saturn have slowed Dione’s rotation, so that it now turns synchronously with its orbital motion, always keeping the same hemisphere toward the planet and always leading with the same hemisphere in orbit.
Like many objects in orbit around Saturn, Dione is involved in an orbital resonance; that is, its 66-hour trip around Saturn is twice that of the nearer moon Enceladus. This relationship has been proposed as a source of the dramatic tidal heating seen in Enceladus, but the details of this mechanism have not been worked out.
- Year discovered: 1684
- Discoverer: Gian Domenico Cassini
- Distance from Saturn: 377,415 km (234,515 miles)
- Orbital period: 2.74 Earth days
- Diameter: 1,118 km (695 miles)
- Notable feature: One day and one year are the same length on Dione.
Surface
Dione’s surface shows substantial brightness contrasts, the trailing hemisphere being generally darker than the leading one. On average, however, Dione is highly reflective, which indicates a surface composed of large amounts of relatively uncontaminated water ice, some of which may come from Saturn’s distended E ring. Besides water ice, the surface is composed of trace amounts of carbon dioxide ice and small particles of iron. The moon’s low density, which is 1.5 times that of water, is consistent with a bulk composition of about equal amounts of ice and rock.
In some places, Dione is heavily covered with impact craters; their density and distribution of sizes suggest a geologic age of close to four billion years. In other areas, however, the crater density is lower, which suggests that the moon may have experienced substantial ice melting and resurfacing. It is also possible that Dione’s surface is continually coated by new ice particles deposited from the E ring.
Most of Dione’s craters are found on the bright, leading hemisphere. The opposite hemisphere is broken by many bright linear features that form a polygonal network. Some of these bright features are associated with linear troughs and ridges and may have been caused by episodic tectonic activity. At least one feature near the south pole is similar to the “tiger stripes” at the southern polar region of Enceladus, from which the active plumes of Enceladus originate.
The longest of the bright streaks that score Dione’s darker hemisphere extend more than 1,000 km (600 miles) across the surface, such as the 1,100-km (684-mile) Palatine Chasmata. Scientists once believed that the streaks could be supercooled underground material spilled onto the moon’s surface by cold volcanoes, but 21st-century scans of the moon confirmed that these are systems of long, narrow canyons with reflective sides.
A low level of geologic activity on Dione is suggested by the effect the moon has on the charged particles associated with Saturn’s magnetic field. Wispy features seen in Voyager spacecraft images had been thought to be deposits of recondensed volatile material that erupted from Dione’s interior along linear fractures. Higher-resolution images from the Cassini spacecraft, however, show no evidence of such activity, though large cliffs appear at the same location as the wispy features. The brighter appearance of these features is most likely caused by differences in particle sizes of the ice and the effects of illumination. The asymmetry of the moon’s surface is not understood, although there is evidence of a major impact near the center of the linear network on the trailing hemisphere.