 The high resolution Voyager 2 image of Titania taken on January 24, 1986 | |||||||
| Discovery | |||||||
|---|---|---|---|---|---|---|---|
| Discovered by | William Herschel | ||||||
| Discovery date | January 11, 1787[1] | ||||||
| Designations | |||||||
| Alternate name | Uranus III | ||||||
| Adjective | Titanian | ||||||
| Semi-major axis | 435 910 km[2] | ||||||
| Eccentricity | 0.0011[2] | ||||||
| Orbital period | 8.706 234 d[2] | ||||||
| Inclination | 0.340° (to Uranus's equator)[2] | ||||||
| Satellite of | Uranus | ||||||
| Physical characteristics | |||||||
| Mean radius | 788.4 ± 0.6 km (0.1235 Earths)[3] | ||||||
| Surface area | 7 820 000 km²[note 1] | ||||||
| Volume | 2 065 000 000 km³[note 2] | ||||||
| Mass | 3.527 ± 0.09 × 1021 kg (5.908 × 10−4 Earths)[4] | ||||||
| Mean density | 1.711 ± 0.005 g/cm³[3] | ||||||
| Equatorial surface gravity | 0.38 m/s2[note 3] | ||||||
| Escape velocity | 0.773 km/s[note 4] | ||||||
| Rotation period | presumed synchronous[5] | ||||||
| Albedo | 0.35 (geometrical), 0.17 (bond)[6] | ||||||
| Surface temp. solstice[3] |
| ||||||
| Apparent magnitude | 13.9[7] | ||||||
Titania (pronounced /tɨˈtɑːnjə/, also /taɪˈteɪniə/[note 5]) is the largest moon of Uranus and the eighth largest moon in the Solar System.
Discovery and naming
Titania was discovered by William Herschel on January 11, 1787; on the same day he discovered Uranus's second largest moon, Oberon.[1][8] He later reported the discoveries of four more satellites,[9] although they were subsequently revealed as spurious.[10] For nearly fifty years following their discovery, Titania and Oberon would not be observed by any instrument other than William Herschel's,[11] although the moon can be seen from Earth with a present-day high end amateur telescope.[7]
All of the moons of Uranus are named after characters created by William Shakespeare or Alexander Pope. The name Titania was derived from Titania, the Queen of the Fairies in A Midsummer Night's Dream.[12] The names of all four satellites of Uranus then known were suggested by Herschel's son John in 1852, at the request of William Lassell,[13] who had discovered the other two moons, Ariel and Umbriel, the year before.[14]
Titania was initially referred to as "the first satellite of Uranus", and in 1848 was given the designation Uranus I by William Lassell,[15] although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV).[16] In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet by Roman numerals, and since then Titania has been designated Uranus III.[17]
Shakespeare's character's name is pronounced [tɨˈtɑːnjə], but the moon is often /taɪˈteɪniə/, by analogy with the familiar chemical element titanium. The adjectival form, Titanian, is homonymous with that of Saturn's moon Titan.
Orbit
Titania orbits Uranus at the distance of about 436,000 km, being the second furthest from the planet among its five major moons.[note 6] Titania's orbit has a small orbital eccentricity and inclination (relative to the equator of Uranus).[2] Its orbital period is around 8.7 days, coincident with its rotational period. In other words, Titania is a synchronous satellite, tidally locked, with one face always pointing toward the planet.[5] Titania's orbit lies completely inside the Uranian magnetosphere.[18] This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by the magnetospheric plasma, which co–rotates with the planet.[19] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).[18] Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Oberon) are subject to an extreme seasonal cycle. Both northern and southern hemispheres spend 42 years in a complete darkness, and another 42 years in continuous sunlight.[18] Once every 42 years, when Uranus has an equinox and its equatorial plane intersects the Earth, mutual occultations of Uranus's moons become possible. In 2007–2008 a number of such events was observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007.[20]
Composition and internal structure
Titania is the largest and most massive of Uranian moons, and the eighth most massive moon in the Solar System.[note 7] Titania's density of 1.71 g/cm3,[4] which is much higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and a dense non-ice component.[22] The latter could include rocks and heavy organic compounds.[5] The presence of water ice is supported by spectroscopic observations, which have revealed crystalline water ice on the surface of the moon.[18] Water ice absorption bands are stronger on Titania's leading hemisphere than on the trailing hemisphere. This is the opposite of what is observed on Oberon, where the trailing hemisphere exhibits stronger water ice signatures.[18] The cause of this asymmetry is not known, but it may be related to the bombardment by charged particle from the magnetosphere of Uranus, which is stronger on the trailing hemisphere (due to co–rotation of the plasma).[18] They tend to sputter water ice and decompose organic leaving a dark carbon rich material behind.[18]
Except for water the only other compound identified on the surface of Titania is carbon dioxide, which is concentrated mainly on the trailing hemisphere.[18] Other plausible candidates for the dark surface materials include rocks, various salts and organic compounds.[5] The origin of the carbon dioxide is not completely clear. It can be produced locally from carbonates or organic compounds by the energetic charged particles coming from the magnetosphere of Uranus or by the solar ultraviolet radiation. Another possible source is the outgassing of the primordial CO2 trapped by water ice in the Titania's interior. The escape of CO2 may be related to the past geological activity on this moon.[18]
Titania may be differentiated into a rocky core surrounded by an icy mantle.[22] If this is the case, the radius of the core (520 km) is about 66% of the radius of the moon, and its mass is around 58% of the moon’s mass—the parameters are dictated by moon's composition. The pressure in the center of Titania is about 0.58 GPa (5 kbar).[22] The current state of the icy mantle is unclear. If the ice contains enough ammonia or other antifreeze, Titania may possess a liquid ocean layer at the core-mantle boundary. The thickness of this ocean, if it exists, is up to 50 km and its temperature is around 190 K.[22] However the internal structure of Titania depends heavily on its thermal history, which is poorly known at present.
Surface features
Scientists recognise the following geological features on Titania: Chasmata (chasms), Craters, Rupes (scarps).[23]
A major surface feature is a huge canyon that dwarfs the scale of the Grand Canyon on Earth and is in the same class as the Valles Marineris on Mars or Ithaca Chasma on Saturn's moon Tethys.
The largest crater Gertrude is 326 km in diameter.[24]
Atmosphere
The presence of carbon dioxide on the surface of Titania gives rise to a hypothesis that it may have a thin CO2 atmosphere much like that of the jovian moon Callisto.[note 8] Other gases like nitrogen or methane are unlikely to be present, because the weak gravity of the moon is unable to keep them from escaping into the space. At the maximum temperature attainable during a solstice on Titania—89 K the vapor pressure of the carbon dioxide is about 3 nBar.[3]
On September 8, 2001, Titania occulted a bright star HIP106829 (visible magnitude of 7.2); this was an opportunity to both refine its diameter and ephemeris, and to detect any extant atmosphere. The data revealed no atmosphere to a surface pressure of 10–20 nanobars; if it exists, it would have to be far thinner than that of Triton or Pluto.[3] This upper limit is still several times higher that the maximum possible surface pressure of the carbon dioxide, meaning that the measurements place essentially no constraints on parameters of the atmosphere.[25][26]
Origin and evolution
Titania is thought to have formed from an accretion disc or subnebula; a disc of gas and dust that either existed around Uranus for some time after its formation or was created by the giant impact that most likely gave Uranus its large obliquity.[27] The precise composition of the subnebula is not known; however, the relatively high density of Titania and other Uranian moons compared to the moons of Saturn indicates that it may have been relatively water-poor.[note 9][5] Significant amounts of nitrogen and carbon may have been present in the form of carbon monoxide and N2 instead of ammonia and methane.[27] The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as clathrate) and more rock, explaining the higher density.[5]
Titania's accretion probably lasted for several thousand years.[27] The impacts that accompanied accretion caused heating of the moon's outer layer.[28] The maximum temperature of around 250 K was reached at the depth of about 60 km.[28] After the end of formation, the subsurface layer cooled, while the interior of Titania heated due to decay of radioactive elements present in its rocks.[5] The cooling near-surface layer contracted, while the interior expanded. This caused strong extensional stresses in the moon's crust leading to cracking. The present-day system of canyons may be a result of this process, which lasted for about 200 million years,[29] implying that any endogenous activity ceased billions years ago.[5]
The initial accretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice if some antifreeze like ammonia (in the form of ammonia hydrate) was present.[28] Further melting may have led to the separation of ice from rocks and formation of a rocky core surrounded by an icy mantle. A layer of liquid water (ocean) rich in dissolved ammonia may have formed at the core–mantle boundary.[22] The eutectic temperature of this mixture is 176 K.[22] If the temperature dropped below this value the ocean would have frozen by now. The freezing of the water led to the expansion of the interior, which may have also been responsible for the formation of canyons.[30] Still, the present knowledge of the evolution of Titania is quite limited.
Exploration
So far the only close-up images of Titania have been from the Voyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest distance between Voyager 2 and Titania was only 365,200 km,[31] the best images of this moon have spatial resolution of about 3.4 km (only Miranda and Ariel were imaged with a better resolution).[30] At the time of the flyby the southern hemisphere of Titania (like other moons) was pointed towards the Sun, so the northern (dark) hemisphere could not be studied.[5] No other spacecraft ever visited Uranus (and Titania), and no mission to this planet is planned in the foreseeable future.
See also
Notes
- ^ Surface area derived from the radius r: 4πr2.
- ^ Volume v derived from the radius r: 4πr3 / 3.
- ^ Surface gravity derived from the mass m, the gravitational constant G and the radius r: Gm / r2.
- ^ Escape velocity derived from the mass m, the gravitational constant G and the radius r: √2Gm/r.
- ^ In US dictionary transcription, us dict: t
ı·tâ′·nyə, tī·tā′·nē·ə. - ^ The five major moons are Miranda, Ariel, Umbriel, Titania and Oberon.
- ^ The seven moons more massive than Titania are Ganymede, Titan, Callisto, Io, Earth's Moon, Europa, and Triton.[21]
- ^ The partial pressure of CO2 on the surface of Callisto is about 10 pBar.
- ^ For instance, Tethys, a Saturnian moon, has the density of 0.97 g/cm3, which means that it contains more than 90% of water.[18]
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