Portia (moon)

Portia is an inner satellite of Uranus. It was discovered from the images taken by Voyager 2 on 3 January 1986, and was given the temporary designation S/1986 U 1.^[8] The moon is named after Portia, the heroine of William Shakespeare's play The Merchant of Venice. It is also designated Uranus XII.^[9]

Portia

Discovery
Discovered by	Stephen P. Synnott / Voyager 2
Discovery date	January 3, 1986
Designations
Designation	Uranus XII
Pronunciation	/ˈpɔːrʃə/[1]
Adjectives	Portian /ˈpɔːrʃən/[1]
Orbital characteristics[2]
Semi-major axis	66,097.265 ± 0.050 km
Eccentricity	0.00005 ± 0.00008
Orbital period (sidereal)	0.5131959201 ± 0.0000000093 d
Average orbital speed	9.37 km/s[a]
Inclination	0.05908 ± 0.039° (to Uranus' equator)
Satellite of	Uranus
Group	ring shepherd
Physical characteristics
Dimensions	156 × 126 × 126 km[3][note 1]
Surface area	~58 000 km2[a]
Volume	1296800±14.8% km3[4]
Mass	(1.1671±0.1730)×1018 kg[4]
Mean density	≥0.57 g/cm3[4] ~0.90 g/cm3 (assumed)
Surface gravity	~0.013–0.02 m/s2[a]
Escape velocity	~0.045–0.05 km/s[a]
Synodic rotation period	synchronous[3]
Axial tilt	zero[3]
Albedo	0.08 ± 0.01 [5] 0.07[6][7]
Temperature	~64 K[a]
Only two dimensions are known; the third dimension has been assumed to equal the smaller known dimension.

Portia is the second-largest inner satellite of Uranus after Puck. The Portian orbit, which lies inside Uranus's synchronous orbital radius, is slowly decaying due to tidal deceleration. The moon will one day either break up into a planetary ring or hit Uranus.^{[citation needed]}

It is the namesake of a group of satellites called the Portia group, which includes Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda, and Perdita.^[5] These satellites have similar orbits and photometric properties.^[5]

Little is known about Portia beyond its dimensions of about 156 × 126 km,^[3] orbit,^[2] and geometric albedo of about 0.08.^[5]

In Voyager 2 imagery, Portia appears as an elongated object whose major axis points towards Uranus. The ratio of axes of the Portia's prolate spheroid is 0.8 ± 0.1.^[3] Its surface is grey in color.^[3] Observations with Hubble Space Telescope and large terrestrial telescopes found water ice absorption features in the spectrum of Portia.^[5][10] Observations with James Webb Space Telescope have also found evidence for water ice or hydrated minerals on Portia's surface, and have also confirmed that Portia group members Juliet, Belinda, and Cressida have roughly similar spectra to Portia.^[11]

Gallery

• 
  Portia seen on January 23, 1986.

See also

• Moons of Uranus

Notes

1. Calculated on the basis of other parameters.