Portia (moon)

Voyager 2 Stephen P. Synnott

Portia (also Uranus XII) is the seventh and one of the medium of the 27 known moons of the planet Uranus. It is the largest moon of the Portia Group and the eighth largest moon in the Uranus system.

Discovery and designation

Portia was discovered on January 3, 1986 by the astronomer Stephen P. Synnott together with Juliet on photographic images of the Voyager 2 spacecraft. The discovery was announced on 16 January 1986 by the International Astronomical Union ( IAU); the moon first received the provisional designation S/1986 U 1

Portia is a female character from William Shakespeare's The Merchant of Venice. Bassanio who wants to marry the rich nobles Portia, earn the required money received from his friend Antonio, the Merchant of Venice, which is indebted for it at Shylock. As a condition Shylock demands instead of interest " a pound of flesh " from the body of Antonio, this should not pay back the money on time. Meanwhile, Bassanio must consist late father in Belmont an examination of Portia. As it looks like that Antonio can not repay the debt on time, Portia emerged as a lawyer Balthasar on dressed up and solves the situation at the last minute.

All the moons of Uranus are named after characters from Shakespeare or Alexander Pope. The first four moons discovered Uranus ( Oberon, Titania, Ariel, Umbriel ) were proposed by John Herschel, the son of Uranus discoverer William Herschel, named. Later, the tradition of naming was retained.

Web properties

Orbit

Portia Uranus orbits on a prograde, nearly perfectly circular orbit at an average distance of about 66,097 kilometers (about 2,586 Uranus radii ) from its center, ie 40,538 km above the cloud tops. The orbital eccentricity of 0.00005, the orbit is inclined 0.05908 ° relative to the equator of Uranus.

Portia is the erstentdeckte, fünftinnerste and largest moon of the Portia Group, which also includes Bianca, Cressida, Desdemona, Juliet, Rosalind, Cupid, Belinda and Perdita belong. These moons have similar orbits and similar spectral properties.

The orbit of the next inner moon Juliet is on average 1739 kilometers away from Portia's orbit, the moon of the next outer Rosalind 3830 km.

Portia is in the midst of two Uranus rings; the inward current ε ( epsilon) - ring is on average about 14,948 km from Portia orbit is removed, and the inner edge of the outer ν ( Ny ) dust ring lies almost exactly on the orbit of Portia.

Portia Uranus rotates in 12 hours, 19 minutes and 0.13 seconds. Since this is faster than the rotation of Uranus, is Portia seen from Uranus from the west, and set in the east.

She moves in a descending path within the synchronous orbit around the planet and will eventually be torn apart as a result of tidal forces into a ring or plunge into Uranus ' atmosphere or burn up.

Rotation

It is believed that Portia synchronously rotates and its axis having an inclination of 0 °.

Physical Properties

Portia has a mean diameter of 135.2 kilometers. In the shots of the Voyager 2 probe Portia appeared as an elongated object with dimensions of 156 × 126 × 126 km, the longitudinal axis is aligned with Uranus.

Their average density is much lower than the density of the Earth at 1.3 g/cm3 and points out that the moon is composed mainly of water ice. Spectral analysis by the Hubble Space Telescope and ground-based telescopes to confirm this assumption. Portia has a very low albedo of 0.08, i.e., 8 % of the incident solar light is reflected from the surface. It is thus a very dark celestial bodies. On its surface, the gravitational acceleration is 0.0246 m/s2, this corresponds to about 2 ‰ of the earth. The average surface temperature of Portia is on between -184 ° and -209 ° C ( 89-64 K ) is estimated. In the spectrum of the surface of Portia appears gray.

Otherwise, not much is known about this moon since images of the probe came at a great distance and therefore have a low resolution.

Research

Since the flyby of the Voyager 2 spacecraft, the Uranus system of ground-based observations as well as the Hubble Space Telescope has been studied intensively. The orbital parameters of Portia could be clarified.