Rhea (moon)
Rhea is the second-largest moon of Saturn after Titan. It has a diameter of approximately 1,527 km and a surface covered with numerous craters and bright fractures. Rhea is less than half the diameter of the Moon, but it is one of Saturn's largest icy satellites.[1]
The Cassini spacecraft detected an extremely tenuous gas envelope around Rhea containing oxygen and carbon dioxide. The gas is linked to processes on the icy surface and its interaction with Saturn's magnetosphere.[4]
Size and physical characteristics
Rhea has a mean radius of 763.5 km and a mass of 2.31 × 1021 kg. Its mean density is only 1.24 g/cm³, consistent with a body containing a large proportion of water ice and a smaller rocky component.[1]
Surface gravity is approximately 0.264 m/s², almost 37 times weaker than Earth's. Rhea is larger than Tethys and Enceladus, but its mass is approximately 32 times lower than that of the Moon.[1]
The cold outer ice shell preserves ancient impact terrain. Bright areas lie beside darker material, and clean ice walls along faults are especially conspicuous against the cratered plains.[6]
Orbit and rotation
Rhea follows an almost circular orbit with a semi-major axis of approximately 527,000 km. One revolution takes 4.52 Earth days, or about 108 hours. Its orbit lies farther from Saturn than those of Tethys and Dione.[5]
Rotation is synchronous, so the same hemisphere always faces Saturn. The leading hemisphere points forward along the orbit and the trailing hemisphere faces backwards. Their surface properties differ because they experience different particle environments.[2][6]
Impact craters and basins
Craters cover almost the entire surface. Ancient structures overlap one another, while ejecta from younger impacts form bright patches and rays. Voyager 1 images already showed a densely cratered icy landscape resembling parts of the Moon and Mercury.[7]
One of the largest impact structures, Tirawa, is approximately 360 km across, nearly one-quarter of Rhea's diameter. Estimates based on Voyager images place the basin's depth at about 5 km in some areas.[8]
Bright ejecta from younger craters are visible near Tirawa. The combination of large ancient basins and smaller later impacts preserves the sequence of collisions that gradually reshaped the moon.[8]
Tectonic fractures
Voyager discovered long bright streaks on Rhea's trailing hemisphere. They were initially attributed to deposits produced by icy volcanism, but Cassini's higher-resolution observations revealed complex systems of fractures and scarps.[6]
Bright walls appear where dark loose material has fallen away and exposed cleaner water ice. Some faults cross cratered plains and cut through individual craters, recording tectonic deformation after the impact terrain had formed.[2]
Enhanced-colour images also reveal variations around the fractures. These may represent differences in composition or in the grain size of the surface ice.[6]
Exosphere
In 2010, Cassini measurements revealed molecular oxygen, O₂, and carbon dioxide, CO₂, around Rhea. The spacecraft sampled the gas directly during flybys. Peak concentrations were approximately 5 × 1010 oxygen molecules and 2 × 1010 carbon dioxide molecules per cubic metre.[4]
Oxygen is produced when energetic particles trapped by Saturn's magnetic field strike water ice. The irradiation breaks water molecules apart and allows oxygen to escape from the surface. The origin of the CO₂ is less certain; possible sources include carbon-bearing impurities and material delivered by micrometeorites.[4]
A later analysis of plasma measurements confirmed the detection of CO₂⁺ ions escaping from Rhea. Several instruments therefore trace an exchange between the surface, the neutral gas envelope and the surrounding plasma.[9]
Temperature and surface layer
Rhea's temperature changes strongly with illumination and season. Sunlit terrain can reach approximately 99 K, or −174 °C, while polar regions become substantially colder during the long winter.[2]
Measurements by Cassini's Composite Infrared Spectrometer (CIRS) produced estimates of 25.4 ± 7.4 K at the southern winter pole in 2013 and 24.7 ± 6.8 K in 2015. The northern summer pole reached 66.6 ± 0.6 K during the 2013 observations. These values apply to the specific polar regions and observing conditions.[3]
The loose surface material absorbs and releases heat slowly. Its thermal properties are consistent with a highly porous layer of fine ice particles that may remain unconsolidated to a depth far greater than the uppermost millimetres.[3]
Bluish equatorial features
Enhanced-colour images show a chain of comparatively bluish patches in a narrow band along the equator. The band is approximately 10 km wide, and individual patches are associated with small craters and parts of their slopes.[10]
The bluish appearance describes a relative difference in reflectance through the spectral filters used for the images. The region would not appear vividly blue to the human eye. One possible explanation is the exposure of comparatively fresh ice.[10]
Ring hypothesis
In 2008, an unusual decrease in the flux of electrons near Rhea was interpreted as a possible sign of material orbiting the moon, including narrow rings. The proposal attracted attention because it implied the first known ring system around a natural satellite.[11]
A dedicated search in Cassini images found none of the expected material. The 2010 analysis showed that rings could not explain both the electron measurements and the optical observations. Rhea is therefore not regarded as a moon with confirmed rings, and the original electron absorptions require a different explanation.[11]
Internal structure
The low mean density establishes that Rhea contains a large proportion of ice, but the distribution of rocky material is less certain. Cassini gravity measurements do not identify either a uniform interior or a large compact rocky core as the sole established model.[12]
The 2016 analysis examined interiors with different degrees of separation between ice and rock. Departures of the moon's shape and gravity field from a simple equilibrium state have a substantial effect on the result. The dimensions and properties of any core therefore remain model-dependent.[12]
Discovery and exploration
Giovanni Domenico Cassini discovered Rhea on 23 December 1672. The moon is named after Rhea, the Titaness who was the mother of Zeus in Greek mythology. The historical designation Saturn V remains in astronomical catalogues.[2]
Voyager 1 and Voyager 2 obtained the first close images during their encounters with Saturn in 1980–1981. Cassini later observed Rhea repeatedly, collecting images, spectra, gravity measurements and data on the surrounding plasma.[13][7]
During the flyby on 2 March 2010, Cassini passed approximately 100 km above the surface. The close trajectory allowed its instruments to detect the extremely tenuous gas and study its interaction with Saturn's magnetosphere.[4][14]
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. Rhea: 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 Rhea // NASA Science.
- ↑ 3.0 3.1 3.2 Howett C. J. A. et al. Thermal Properties of Rhea's Poles: Evidence for a Meter-Deep Unconsolidated Subsurface Layer // Icarus. 2016. Vol. 272.
- ↑ 4.0 4.1 4.2 4.3 4.4 Thin Air — Cassini Finds Ethereal Atmosphere at Rhea // NASA/JPL-Caltech. 29 November 2010.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 Planetary Satellite Mean Elements // NASA JPL. Rhea: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 6.0 6.1 6.2 6.3 Rhea's Wisps in Color // NASA/JPL/Space Science Institute. 23 February 2006.
- ↑ 7.0 7.1 Rhea — Icy Cratered Surface // NASA/JPL. Voyager 1 image from 12 November 1980.
- ↑ 8.0 8.1 Tirawa at Twilight // NASA/JPL/Space Science Institute. 2005.
- ↑ Desai R. T. et al. Cassini CAPS Identification of Pickup Ion Compositions at Rhea // Geophysical Research Letters. 2018.
- ↑ 10.0 10.1 Rhea's Blue Streaks — Looking West // NASA/JPL/Space Science Institute/USRA/LPI. 2010.
- ↑ 11.0 11.1 Tiscareno M. S. et al. Cassini Imaging Search Rules Out Rings around Rhea // Geophysical Research Letters. 2010. DOI: 10.1029/2010GL043663.
- ↑ 12.0 12.1 Tortora P. et al. Rhea Gravity Field and Interior Modeling from Cassini Data Analysis // Icarus. 2016. Vol. 264. P. 264–273.
- ↑ Cassini-Huygens // NASA Science.
- ↑ Rhea Flyby R-2 — March 2, 2010 // NASA Science.