Dione (moon)
Dione is an icy moon of Saturn with a diameter of approximately 1,123 km. It is slightly larger than Tethys and almost one and a half times smaller than Rhea. Its surface is covered with craters and extensive bright fractures, which are especially prominent on one hemisphere.[1][6]
The Cassini spacecraft detected molecular oxygen ions near Dione and measured its gravity field. These observations showed that the moon has a tenuous gas envelope and that liquid water may persist beneath its ice crust.[3][4]
Size and physical characteristics
Dione has a mean radius of 561.4 km and a mass of approximately 1.10 × 1021 kg. Its diameter is about 3.1 times smaller than the Moon's, while its mass is nearly 67 times lower. The mean density is 1.48 g/cm³, indicating a mixture of water ice and denser rocky material.[1][4]
Surface gravity is approximately 0.232 m/s², about 42 times weaker than Earth's. Dione is only slightly larger than Tethys but is approximately 1.8 times more massive, a consequence of its greater mean density.[1]
The typical surface temperature is approximately 87 K, or −186 °C. At this temperature, water ice forms a rigid outer shell that preserves the traces of impacts and tectonic deformation.[2][6]
Orbit and rotation
Dione follows an almost circular orbit with a semi-major axis of approximately 377,700 km. This is close to the mean distance between Earth and the Moon, but Saturn's much greater mass allows Dione to complete one revolution in only 2.74 Earth days.[5]
Rotation is synchronous: the same hemisphere always faces Saturn. The leading hemisphere points forward along the orbit, while the trailing hemisphere faces backwards. The network of bright fractures is particularly conspicuous on the trailing side.[2][6]
Dione is in a 2:1 orbital resonance with Enceladus: the inner moon completes approximately two revolutions during each orbit of Dione. Their gravitational interaction maintains the eccentricity of Enceladus's orbit and helps sustain tidal heating in its interior.[7]
Trojan moons
Two small moons share Dione's orbit: Helene and Polydeuces. They orbit Saturn near the stable Lagrange regions of the Saturn–Dione system. Helene travels approximately 60° ahead of Dione, while Polydeuces follows behind it.[8]
These bodies are known as Trojan moons. Their motion shares Dione's orbital period, but they are not satellites of Dione itself.[9]
Impact craters and plains
Heavily cratered regions lie beside plains with fewer impact structures. The contrast records different surface histories: some terrain accumulated impact scars for longer, while other areas were resurfaced.[2]
One of the largest impact structures is Evander, a multi-ring basin approximately 350 km across. Its diameter is nearly one-third that of Dione. Cassini obtained a detailed view during its final close flyby in August 2015.[10]
Most craters crossed by the studied faults are cut by those fractures. Much of the crustal deformation must therefore have occurred after the craters formed.[11]
Ice fractures and scarps
In Voyager images, Dione's bright streaks appeared as thin, wispy lines. Their nature was uncertain, and surface frost deposits were among the proposed explanations. Cassini's cameras revealed that many of the streaks are bright ice walls along tectonic faults.[6]
Dark loose material falls away from the slopes and exposes cleaner water ice. The scarps consequently stand out against the surrounding cratered plains. Fracture systems cross the surface over great distances and sometimes form complex networks.[2][6]
These features record the stretching and disruption of the ice shell in the past. Tidal stresses and changes in the orbit may have contributed to their formation. Their absolute ages are less certain than their sequence relative to craters because the estimate depends on models of impact frequency.[6][11]
Interior and possible ocean
The moon's density, shape and gravity field are consistent with a rocky inner region surrounded by a water-ice shell. The distribution of mass cannot, however, be reconstructed uniquely by treating Dione as a fully relaxed body in hydrostatic equilibrium: its measured gravity field departs significantly from that simple model.[4]
In 2016, Mikael Beuthe and colleagues proposed a model in which an ice shell 99 ± 23 km thick overlies a global ocean 65 ± 30 km deep. This structure reproduced the observed combination of shape and gravity data.[12]
A later analysis published in 2020 confirmed that the interpretation remains ambiguous. Liquid water beneath a thick ice crust is among the permitted structures, while the models that best matched the data favoured a thicker shell and a thinner ocean. The presence and dimensions of a present-day ocean on Dione are therefore less securely established than those of the ocean on Enceladus.[4]
Oxygen exosphere
During the flyby on 7 April 2010, Cassini's plasma spectrometer detected molecular oxygen ions, O₂⁺, near Dione. The result, announced in 2012, demonstrated the presence of an extremely tenuous oxygen exosphere.[3]
The measured ion concentration was approximately 90,000 per cubic metre. This value describes the ionised component in the region sampled by the spacecraft, not the total density of neutral gas at the surface.[3]
Solar radiation and energetic particles striking water ice can split water molecules and release oxygen through subsequent reactions. This process does not require biological activity.[3]
Discovery and exploration
Giovanni Domenico Cassini discovered Dione on 21 March 1684. It received the designation Saturn IV. The name refers to a figure from Greek mythology and belongs to the naming system proposed by John Herschel.[2]
Voyager returned the first images of the bright streaks, while Cassini resolved their structure at close range. During its mission at Saturn, Cassini made five close flybys of Dione. Radio tracking during three encounters near closest approach was used to determine the moon's gravity field.[6][4]
The closest flyby occurred in December 2011 at an altitude of approximately 100 km. The final close encounter took place on 17 August 2015, when the spacecraft passed about 474 km above the surface. Some details in the images were illuminated by sunlight reflected from Saturn into Dione's shadowed terrain.[13]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Planetary Satellite Physical Parameters // NASA JPL. Dione: gravitational parameter from the SAT441 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 Dione // NASA Science.
- ↑ 3.0 3.1 3.2 3.3 3.4 Cassini Detects Hint of Fresh Air at Dione // NASA/JPL. 2 March 2012.
- ↑ 4.0 4.1 4.2 4.3 4.4 4.5 Zannoni M. et al. The Gravity Field and Interior Structure of Dione // Icarus. 2020. DOI: 10.1016/j.icarus.2020.113713.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Planetary Satellite Mean Elements // NASA JPL. Dione: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 6.0 6.1 6.2 6.3 6.4 6.5 6.6 Chasms on Dione // NASA/JPL-Caltech/Space Science Institute. 17 August 2015.
- ↑ Ćuk M. et al. Long-Term Evolution of the Saturnian System // Space Science Reviews. 2024.
- ↑ Dione's Companion // NASA/JPL/Space Science Institute.
- ↑ Robutel P. et al. Influence of the Coorbital Resonance on the Rotation of the Trojan Satellites of Saturn // arXiv:1111.1076. 2011; updated 2012.
- ↑ Imminent Approach to Dione // NASA. 20 August 2015.
- ↑ 11.0 11.1 Hirata N. Timing of the Faulting on the Wispy Terrain of Dione Based on Stratigraphic Relationships with Impact Craters // Journal of Geophysical Research: Planets. 2016. Vol. 121. P. 2325–2334. DOI: 10.1002/2016JE005176.
- ↑ Beuthe M., Rivoldini A., Trinh A. Enceladus's and Dione's Floating Ice Shells Supported by Minimum Stress Isostasy // Geophysical Research Letters. 2016. Vol. 43. P. 10088–10096. DOI: 10.1002/2016GL070650.
- ↑ Cassini's Final Breathtaking Close Views of Dione // NASA/JPL. 20 August 2015.