Phoebe (moon)
Phoebe is a distant moon of Saturn, approximately 213 km in diameter, that travels in the opposite direction to the planet's major regular satellites. Its orbit lies far beyond those of Titan and Iapetus, and one revolution takes approximately 550 Earth days.[1][3]
Phoebe's dark surface is covered with craters whose walls sometimes expose brighter ice-rich material. Its orbit and composition indicate that it probably formed outside the Saturnian satellite system and was later captured. Phoebe is also considered a principal source of Saturn's enormous outer dust ring.[6][7][8]
Size and physical characteristics
Phoebe has a volumetric mean radius of 106.5 km and a mass of approximately 8.31 × 1018 kg. Its diameter is about 16 times smaller than the Moon's. Phoebe is nevertheless approximately one and a half times more massive than Hyperion, despite having a considerably smaller volume.[1]
The difference results from its greater density: 1.64 g/cm³, compared with approximately 0.54 g/cm³ for Hyperion. Phoebe contains water ice and a substantial fraction of denser rocky material and is not thought to possess Hyperion's exceptionally high bulk porosity.[1][7]
Phoebe is roughly spherical, although large craters and other relief distort its shape. A spherical estimate gives a surface gravity of approximately 0.049 m/s², about 200 times weaker than Earth's, and an escape velocity of only 102 m/s.[1][7]
Orbit and rotation
Phoebe's orbital semi-major axis is approximately 12.93 million km. This is more than three and a half times farther from Saturn than the orbit of Iapetus and almost ten times farther than Titan's orbit. Its eccentricity of approximately 0.164 produces a substantial change in distance over each revolution.[3]
Phoebe is an irregular satellite with a retrograde orbit. It travels around Saturn in the opposite direction to Titan, Rhea and the other major regular moons. This motion is strong evidence that Phoebe was captured rather than formed in Saturn's original circumplanetary disc.[3][7]
One rotation takes 9.27365 hours, or about 9 hours 16 minutes. Phoebe therefore turns more than 1,400 times during each orbit of Saturn and does not keep one hemisphere permanently facing the planet.[4][3]
Craters and exposures of ice
Cassini images show numerous overlapping craters, scarps and grooves. Dark material has moved downslope on some crater walls, revealing brighter terrain, while small bright craters expose material beneath the dark surface covering.[6]
One of the largest impact structures, Jason, measures 101 km across, almost half Phoebe's mean diameter. It is named after the leader of the Argonauts in Greek mythology.[9]
A reanalysis of Cassini spectra presented by Wesley Fraser and Michael Brown in 2018 linked the largest impact basins with enhanced water-ice abundance. The most water-rich regions are associated with Jason and the south-polar basin.[10]
Impacts may have penetrated an ice-poor upper layer and exposed deeper material richer in water. None of the studied regions was completely devoid of spectral signatures of water.[10]
Surface composition and temperature
Cassini instruments confirmed water ice and detected carbon dioxide, signs of hydrated minerals and carbon-bearing compounds. The surface is compositionally heterogeneous, with different components distributed unevenly.[2]
Infrared measurements during the 2004 flyby produced temperatures of approximately 110 K (−163 °C) on sunlit terrain. The surface becomes still colder at night. Rapid cooling is consistent with a loose and porous surface layer, even though the body as a whole is considerably denser.[2]
Origin and early evolution
Phoebe is regarded as a surviving planetesimal, one of the small bodies from which the planets of the outer Solar System formed. Its material has been compared with icy objects from the trans-Neptunian region. Capture by Saturn explains why its orbit differs from those of moons formed in a disc around the planet.[7]
Early in its history, the decay of short-lived radioactive isotopes may have heated Phoebe. The porous body compacted and became more spherical, while rocky material moved towards the centre. Liquid water may have existed internally during this stage.[7]
This history is reconstructed from shape, density and thermal models. It does not imply that Phoebe has a confirmed liquid ocean today. Its exact formation region and the circumstances of its capture remain uncertain.[7]
Phoebe ring
In 2009, the Spitzer Space Telescope discovered an enormous and tenuous dust ring around Saturn whose region includes Phoebe's orbit. It lies far beyond the bright main rings and contains extremely sparse fine particles. Impacts ejecting dust from Phoebe make the moon a likely source of much of this material.[8]
Later observations examined the dust distribution at distances of approximately 80–260 Saturn radii, or about 4.8–15.7 million km from the planet's centre. This interval describes the measured radial profile rather than definitive boundaries of the entire ring.[11]
Phoebe need not be the sole supplier: other outer irregular moons may contribute dust. Fine particles are affected not only by gravity but also by solar radiation, which gradually changes their orbits.[11]
Connection with Iapetus
Dust from retrograde orbits can drift inward and collide with the leading hemisphere of prograde Iapetus. Calculations show that this deposition can account for the initial contamination of Cassini Regio, the dark part of its surface.[12]
The models include dust from other outer moons as well as Phoebe. This matters because the spectrum of Iapetus's dark material does not exactly match Phoebe's surface. The connection is a transfer of mixed material rather than a simple copy of one body's composition.[12]
Discovery and exploration
William Henry Pickering identified Phoebe on photographic plates taken by DeLisle Stewart at Arequipa, Peru. The earliest images date from 16 August 1898, and the discovery was announced on 17 March 1899. The moon received the designation Saturn IX and was named after the Titaness Phoebe.[5]
On 11 June 2004, Cassini passed approximately 2,068 km above Phoebe. It was the spacecraft's first targeted flyby of a Saturnian moon, occurring before it entered orbit around the planet. Images from different directions established the moon's shape, while tracking of the spacecraft's motion provided its mass.[13][2]
Ground-based observations continued after the encounter. On 6 July 2017, Phoebe occulted a star in the first observed stellar occultation by an irregular satellite. Combining later occultations and light curves with the Cassini shape model refined the rotation period to 9.27365 ± 0.00002 hours.[4]
See also
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Planetary Satellite Physical Parameters // NASA JPL. Phoebe: gravitational parameter from the SAT441 solution. Mass calculated from GM using G = 6.67430 × 10−11 m³/(kg·s²).
- ↑ 2.0 2.1 2.2 2.3 2.4 Cassini Opens a Cosmic Time Capsule // NASA/JPL. 23 June 2004.
- ↑ 3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 Planetary Satellite Mean Elements // NASA JPL. Phoebe: mean elements for 1 January 2000, 12:00 TDB.
- ↑ 4.0 4.1 4.2 Gomes-Júnior A. R. et al. The First Observed Stellar Occultations by the Irregular Satellite Phoebe (Saturn IX) and Improved Rotational Period // Monthly Notices of the Royal Astronomical Society. 2020. Vol. 492, issue 1. P. 770–781.
- ↑ 5.0 5.1 5.2 Denk T. Phoebe: Designations and Discovery Circumstances // Cassini ISS science team member website; with references to the original Harvard College Observatory announcements.
- ↑ 6.0 6.1 The Face of Phoebe // NASA/JPL/Space Science Institute. 13 June 2004.
- ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 Cassini Finds Saturn Moon Has Planet-Like Qualities // NASA/JPL. 26 April 2012.
- ↑ 8.0 8.1 NASA Space Telescope Discovers Largest Ring around Saturn // NASA/JPL. 6 October 2009.
- ↑ Jason // Gazetteer of Planetary Nomenclature. USGS/IAU.
- ↑ 10.0 10.1 Fraser W. C., Brown M. E. Phoebe: A Surface Dominated by Water // 2018. DOI: 10.3847/1538-3881/aac213.
- ↑ 11.0 11.1 Tamayo D. et al. Radial Profiles of the Phoebe Ring, a Vast Debris Disk around Saturn // Icarus. 2016. Vol. 275. P. 117–131.
- ↑ 12.0 12.1 Tamayo D. et al. Finding the Trigger to Iapetus' Odd Global Albedo Pattern: Dynamics of Dust from Saturn's Irregular Satellites // Icarus. 2011. DOI: 10.1016/j.icarus.2011.06.027.
- ↑ Arrival and Departure at Phoebe // NASA/JPL-Caltech/Space Science Institute.