Titania consists of approximately equal amounts of ice androck, and is probably differentiated into a rockycore and an icymantle. A layer of liquid water may be present at thecore–mantle boundary. Its surface, which is relatively dark and slightly red in color, appears to have been shaped by bothimpacts andendogenic processes. Although Titania is covered with numerousimpact craters reaching up to 326 kilometres (203 mi) in diameter, it is less heavily cratered thanOberon, the outermost of Uranus's five large moons.
It may have undergone an early endogenic resurfacing event which obliterated its older, heavily cratered surface. The surface is cut by a system of enormouscanyons andscarps, the result of the expansion of its interior during the later stages of its evolution. Like all major moons of Uranus, Titania probably formed from anaccretion disk which surrounded the planet just after its formation. Uranus and Neptune (and thus their moons) may both appear apparent retrograde orbits when viewed from Earth.
Infrared spectroscopy conducted from 2001 to 2005 revealed the presence of waterice as well as frozencarbon dioxide on Titania's surface, suggesting it may have a tenuous carbon dioxideatmosphere with a surface pressure of about 10 nanopascals (10−13 bar). Measurements during Titania's occultation of astar put an upper limit on the surface pressure of any possible atmosphere at 1–2 mPa (10–20 nbar). The Uranian system has been studied up close only once, by the spacecraftVoyager 2 in January 1986. It took several images of Titania, which allowed mapping of about 40% of its surface.
Titania was discovered by William Herschel on January 11, 1787, the same day he discovered Uranus's second largest moon,Oberon.[1][10] He later reported the discoveries of four more satellites,[11] although they were subsequently revealed as spurious.[12] For nearly the next 50 years, Titania and Oberon would not be observed by any instrument other than William Herschel's,[13] although the moon can be seen fromEarth with a present-day high-end amateur telescope.[9]
Titania was initially referred to as "the first satellite of Uranus", and in 1848 was given the designationUranus I byWilliam Lassell,[18] although he sometimes used William Herschel's numbering (where Titania and Oberon are II and IV).[19] In 1851 Lassell eventually numbered all four known satellites in order of their distance from the planet byRoman numerals, and since then Titania has been designatedUranus III.[20]
Shakespeare's character's name is pronounced/tɪˈteɪnjə/, but the moon is often pronounced/taɪˈteɪniə/, by analogy with the familiar chemical elementtitanium.[21] The adjectival form,Titanian, is homonymous with that of Saturn's moonTitan. The nameTitania is ancient Greek for "Daughter of the Titans".
Planetary moons other than Earth's were never given symbols in the astronomical literature. Denis Moskowitz, a software engineer who designed most of thedwarf planet symbols, proposed a T (the initial of Titania) combined with the low globe ofJérôme Lalande's Uranus symbol as the symbol of Titania (). This symbol is not widely used.[22]
Titania orbits Uranus at the distance of about 436,000 kilometres (271,000 mi), being the second farthest from the planet among its five major moons after Oberon.[g] Titania's orbit has a smalleccentricity and isinclined very little relative to theequator of Uranus.[4] Itsorbital period is around 8.7 days, coincident with itsrotational period. In other words, Titania is asynchronous ortidally locked satellite, with one face always pointing toward the planet.[7]
Titania's orbit lies completely inside the Uranianmagnetosphere.[23] This is important, because the trailing hemispheres of satellites orbiting inside a magnetosphere are struck by magnetospheric plasma, which co-rotates with the planet.[24] This bombardment may lead to the darkening of the trailing hemispheres, which is actually observed for all Uranian moons except Oberon (see below).[23]
Because Uranus orbits the Sun almost on its side, and its moons orbit in the planet's equatorial plane, they (including Titania) are subject to an extreme seasonal cycle. Both northern and southernpoles spend 42 years in a complete darkness, and another 42 years in continuous sunlight, with the sun rising close to thezenith over one of the poles at eachsolstice.[23] TheVoyager 2 flyby coincided with the southern hemisphere's 1986 summer solstice, when nearly the entire southern hemisphere was illuminated. Once every 42 years, when Uranus has anequinox and its equatorial plane intersects the Earth, mutualoccultations of Uranus's moons become possible. In 2007–2008 a number of such events were observed including two occultations of Titania by Umbriel on August 15 and December 8, 2007.[25][26]
Voyager 2's highest-resolution image of Titania shows moderately cratered plains, enormousrifts and longscarps. Near the bottom, a region of smoother plains including the craterUrsula is split by the graben Belmont Chasma.
Titania is the largest and most massive Uranian moon, the eighth most massive moon in the Solar System, and the 20th largest object in the Solar System.[h] Its density of 1.68 g/cm3,[28] which is much higher than the typical density of Saturn's satellites, indicates that it consists of roughly equal proportions of water ice and dense non-ice components;[29] the latter could be made ofrock andcarbonaceous material including heavyorganic compounds.[7] The presence of water ice is supported byinfraredspectroscopic observations made in 2001–2005, which have revealedcrystalline water ice on the surface of the moon.[23] Water iceabsorption bands are slightly 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.[23] The cause of this asymmetry is not known, but it may be related to the bombardment by charged particles from themagnetosphere of Uranus, which is stronger on the trailing hemisphere (due to the plasma's co-rotation).[23] The energetic particles tend tosputter water ice, decomposemethane trapped in ice asclathrate hydrate and darken other organics, leaving a dark, carbon-richresidue behind.[23]
Except for water, the only other compound identified on the surface of Titania by infrared spectroscopy iscarbon dioxide, which is concentrated mainly on the trailing hemisphere.[23] The origin of the carbon dioxide is not completely clear. It might be produced locally fromcarbonates or organic materials under the influence of the solarultraviolet radiation or energetic charged particles coming from the magnetosphere of Uranus. The latter process would explain the asymmetry in its distribution, because the trailing hemisphere is subject to a more intense magnetospheric influence than the leading hemisphere. Another possible source is theoutgassing of theprimordial CO2 trapped by water ice in Titania's interior. The escape of CO2 from the interior may be related to the past geological activity on this moon.[23]
Titania may be differentiated into a rockycore surrounded by an icymantle.[29] If this is the case, the radius of the core 520 kilometres (320 mi) is about 66% of the radius of the moon, and its mass is around 58% of the moon's mass—the proportions are dictated by moon's composition. The pressure in the center of Titania is about 0.58 GPa (5.8 kbar).[29] The current state of the icy mantle is unclear. If the ice contains enough ammonia or otherantifreeze, Titania may have asubsurface ocean at the core–mantle boundary. The thickness of this ocean, if it exists, is up to 50 kilometres (31 mi) and its temperature is around 190 K (close to the water–ammoniaeutectic temperature of 176 K).[29] However the present internal structure of Titania depends heavily on its thermal history, which is poorly known. Recent studies suggest, contrary to earlier theories, that Uranus largest moons like Titania in fact could have active subsurface oceans.[30][31]
Titania with some surface features labeled. The south pole is situated close to the unlabeled bright crater below and left of the crater Jessica.
Among Uranus's moons, Titania is intermediate in brightness between the dark Oberon and Umbriel and the bright Ariel andMiranda.[8] Its surface shows a strongopposition surge: its reflectivity decreases from 35% at a phase angle of 0° (geometrical albedo) to 25% at an angle of about 1°. Titania has a relatively lowBond albedo of about 17%.[8] Its surface is generally slightly red in color, but less red than that ofOberon.[32] However, fresh impact deposits are bluer, while the smooth plains situated on the leading hemisphere nearUrsula crater and along some grabens are somewhat redder.[32][33] There may be an asymmetry between the leading and trailing hemispheres;[34] the former appears to be redder than the latter by 8%.[i] However, this difference is related to the smooth plains and may be accidental.[32] The reddening of the surfaces probably results fromspace weathering caused by bombardment by charged particles andmicrometeorites over the age of theSolar System.[32] However, the color asymmetry of Titania is more likely related to accretion of a reddish material coming from outer parts of the Uranian system, possibly, fromirregular satellites, which would be deposited predominately on the leading hemisphere.[34]
Scientists have recognized three classes of geological feature on Titania:craters,chasmata (canyons) andrupes (scarps).[35] The surface of Titania is less heavily cratered than the surfaces of either Oberon or Umbriel, which means that the surface is much younger.[33] The crater diameters reach 326 kilometers for the largest known crater,Gertrude[36] (there can be also a degraded basin of approximately the same size).[33] Some craters (for instance,Ursula andJessica) are surrounded by bright impact ejecta (rays) consisting of relatively fresh ice.[7] All large craters on Titania have flat floors and central peaks. The only exception is Ursula, which has a pit in the center.[33] To the west of Gertrude there is an area with irregular topography, the so-called "unnamed basin", which may be another highly degraded impact basin with the diameter of about 330 kilometres (210 mi).[33]
Titania's surface is intersected by a system of enormousfaults, or scarps. In some places, two parallel scarps mark depressions in the satellite's crust,[7] forminggrabens, which are sometimes called canyons.[37] The most prominent among Titania's canyons isMessina Chasma, which runs for about 1,500 kilometres (930 mi) from the equator almost to the south pole.[35] The grabens on Titania are 20–50 kilometres (12–31 mi) wide and have a relief of about 2–5 km.[7] The scarps that are not related to canyons are called rupes, such asRousillon Rupes near Ursula crater.[35] The regions along some scarps and near Ursula appear smooth atVoyager's image resolution. These smooth plains were probably resurfaced later in Titania's geological history, after the majority of craters formed. The resurfacing may have been either endogenic in nature, involving the eruption of fluid material from the interior (cryovolcanism), or, alternatively it may be due to blanking by the impact ejecta from nearby large craters.[33] The grabens are probably the youngest geological features on Titania—they cut all craters and even smooth plains.[37]
The geology of Titania was influenced by two competing forces:impact crater formation and endogenic resurfacing.[37] The former acted over the moon's entire history and influenced all surfaces. The latter processes were also global in nature, but active mainly for a period following the moon's formation.[33] They obliterated the original heavily cratered terrain, explaining the relatively low number of impact craters on the moon's present-day surface.[7] Additional episodes of resurfacing may have occurred later and led to the formation of smooth plains.[7] Alternatively smooth plains may be ejecta blankets of the nearby impact craters.[37] The most recent endogenous processes were mainlytectonic in nature and caused the formation of the canyons, which are actually giant cracks in the ice crust.[37] The cracking of the crust was caused by the global expansion of Titania by about 0.7%.[37]
The presence of carbon dioxide on the surface suggests that Titania may have a tenuous seasonal atmosphere of CO2, much like that of the Jovian moonCallisto.[j][5] Other gases, likenitrogen ormethane, are unlikely to be present, because Titania's weak gravity could not prevent them from escaping into space. At the maximum temperature attainable during Titania'ssummer solstice (89 K), thevapor pressure of carbon dioxide is about 300 μPa (3 nbar).[5]
On September 8, 2001, Titaniaocculted a bright star (HIP 106829) with avisible magnitude of 7.2; this was an opportunity to both refine Titania's diameter andephemeris, and to detect any extant atmosphere. The data revealed no atmosphere to a surface pressure of 1–2 mPa (10–20 nbar); if it exists, it would have to be far thinner than that ofTriton orPluto.[5] This upper limit is still several times higher than the maximum possible surface pressure of the carbon dioxide, meaning that the measurements place essentially no constraints on parameters of the atmosphere.[5]
The peculiar geometry of the Uranian system causes the moons' poles to receive moresolar energy than their equatorial regions.[23] Because the vapor pressure of CO2 is a steep function of temperature,[5] this may lead to the accumulation of carbon dioxide in the low-latitude regions of Titania, where it can stably exist on high albedo patches and shaded regions of the surface in the form of ice. During the summer, when the polar temperatures reach as high as 85–90 K,[5][23] carbon dioxidesublimates and migrates to the opposite pole and to the equatorial regions, giving rise to a type ofcarbon cycle. The accumulated carbon dioxide ice can be removed from cold traps by magnetospheric particles, which sputter it from the surface. Titania is thought to have lost a significant amount of carbon dioxide since its formation 4.6 billion years ago.[23]
Titania is thought to have formed from anaccretion 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 largeobliquity.[39] The precise composition of the subnebula is not known; however, the relatively high density of Titania and other Uranian moons compared to themoons of Saturn indicates that it may have been relatively water-poor.[k][7] Significant amounts ofnitrogen andcarbon may have been present in the form ofcarbon monoxide and N2 instead ofammonia and methane.[39] The moons that formed in such a subnebula would contain less water ice (with CO and N2 trapped as a clathrate) and more rock, explaining their higher density.[7]
Titania's accretion probably lasted for several thousand years.[39] The impacts that accompanied accretion caused heating of the moon's outer layer.[40] The maximum temperature of around 250 K (−23 °C) was reached at a depth of about 60 kilometres (37 mi).[40] After the end of formation, the subsurface layer cooled, while the interior of Titania heated due to decay ofradioactive elements present in its rocks.[7] The cooling near-surface layer contracted, while the interior expanded. This caused strongextensional stresses in the moon's crust leading to cracking. Some of the present-day canyons may be a result of this. The process lasted for about 200 million years,[41] implying that any endogenous activity ceased billions of years ago.[7]
The initialaccretional heating together with continued decay of radioactive elements were probably strong enough to melt the ice if some antifreeze like ammonia (in the form ofammonia hydrate) orsalt was present.[40] 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.[29] Theeutectic temperature of this mixture is 176 K (−97 °C).[29] If the temperature dropped below this value, the ocean would have subsequently frozen. The freezing of the water would have caused the interior to expand, which may have been responsible for the formation of the majority of the canyons.[33] However, the present knowledge of Titania's geological evolution is quite limited. Whereas more up to date analysis suggest that larger moons of Uranus are not only capable of having active subsurface oceans; but in fact; presumed to have subterranean oceans beneath them.[42][43]
So far, the only close-up images of Titania have been from theVoyager 2 probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest distance betweenVoyager 2 and Titania was only 365,200 km (226,900 mi),[44] the best images of this moon have a spatial resolution of about 3.4 km (only Miranda and Ariel were imaged with a better resolution).[33] The images cover about 40% of the surface, but only 24% was photographed with the precision required forgeological mapping. At the time of the flyby, the southern hemisphere of Titania (like those of the other moons) was pointed towards theSun, so the northern (dark) hemisphere could not be studied.[7]
No other spacecraft has ever visited the Uranian system or Titania.One possibility, now discarded, was to sendCassini on from Saturn to Uranus in an extended mission. Another mission concept proposed was theUranus orbiter and probe concept, evaluated around 2010. Uranus was also examined as part of one trajectory for a precursor interstellar probe concept,Innovative Interstellar Explorer.
^"Titania".Lexico UK English Dictionary.Oxford University Press. Archived fromthe original on March 2, 2020. Lexico/OED. Only the first pronunciation is used inA Midsummer Night's Dream, e.g. Shakespeare Recording Society (1995)The Tempest (audio CD). The second is used by interviewees in a podcast by the Folger Shakespeare Library, but not by the narrator:Brave New Worlds: The Shakespearean Moons of Uranus
^Jacobson, R. A.; Campbell, J. K.; Taylor, A. H.; Synnott, S. P. (June 1992). "The masses of Uranus and its major satellites from Voyager tracking data and earth-based Uranian satellite data".The Astronomical Journal.103 (6):2068–2078.Bibcode:1992AJ....103.2068J.doi:10.1086/116211.
^abcdeBell III, J.F.; McCord, T. B. (1991).A search for spectral units on the Uranian satellites using color ratio images. Lunar and Planetary Science Conference, 21st, March 12–16, 1990. Houston, TX, United States: Lunar and Planetary Sciences Institute. pp. 473–489.Bibcode:1991LPSC...21..473B.
^USGS/IAU (October 1, 2006)."Gertrude on Titania".Gazetteer of Planetary Nomenclature. USGS Astrogeology. Archived fromthe original on 2012-05-27. Retrieved2012-02-23.
^abcdefCroft, S. K. (1989).New geological maps of Uranian satellites Titania, Oberon, Umbriel and Miranda.Proceeding of Lunar and Planetary Sciences. Vol. 20. Lunar and Planetary Sciences Institute, Houston. p. 205C.Bibcode:1989LPI....20..205C.
^Strobell, M.E.; Masursky, H. (1987). "New Features Named on the Moon and Uranian Satellites".Abstracts of the Lunar and Planetary Science Conference.18:964–65.Bibcode:1987LPI....18..964S.
^Hillier, John; Squyres, Steven W. (August 1991). "Thermal stress tectonics on the satellites of Saturn and Uranus".Journal of Geophysical Research.96 (E1): 15,665–15, 674.Bibcode:1991JGR....9615665H.doi:10.1029/91JE01401.
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