Titania (moon)

• 1. Introduction

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  Titania is the largest moon of Uranus and the eighth-largest moon in the entire Solar System, measuring 1,578 kilometres across. William Herschel spotted it on the 11th of January 1787, the very same night he found Oberon, Uranus's second-largest moon. What did Herschel unknowingly uncover that winter night? A world of ice and rock, scored by canyons stretching over a thousand kilometres, possibly hiding an ocean beneath its frozen skin. How did this remote body come to carry the name of Shakespeare's fairy queen? And what does its scarred, split surface tell us about the violent birth of the outer Solar System?

• 2. A Name from Shakespeare

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  All of Uranus's moons take their names from characters created by William Shakespeare or Alexander Pope, and Titania borrows hers from the Queen of the Fairies in A Midsummer Night's Dream. The suggestion came not from the discoverer himself but from his son, John Herschel, in 1852. John was responding to a request from astronomer William Lassell, who had discovered two other Uranian moons, Ariel and Umbriel, the year before. Whether the elder Herschel devised the names or Lassell did so and then sought John's blessing remains uncertain to this day.

  For nearly 50 years after the 1787 discovery, Titania and Oberon were observed by no instrument other than William Herschel's own telescope, though the moon is visible from Earth today with a high-end amateur setup. Lassell gave the moon the designation Uranus I in 1848, then reshuffled the numbering in 1851 when he ranked all four known satellites by their distance from the planet. Since then Titania has been Uranus III.

  Pronouncing the name turns out to be its own small puzzle. Shakespeare's character is spoken as tᵻˈteɪnjə, but the moon is commonly said taɪˈteɪniə, by analogy with the familiar element titanium. The name's ancient Greek root means "Daughter of the Titans", a lineage that sits oddly with a fairy queen but fits rather well for a frozen world at the edge of the Solar System. The surface features mapped on Titania are named after female characters and locations from Shakespeare's works, from Gertrude of Hamlet to Messina from Much Ado About Nothing.

• 3. Orbit and Seasons

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  Titania orbits Uranus at roughly 436,000 kilometres, placing it second farthest among the five major moons after Oberon. Its orbital period is about 8.7 days, matching its rotation exactly: Titania is tidally locked, keeping one face permanently turned toward Uranus.

  Uranus itself orbits the Sun almost on its side, and its moons travel in the planet's equatorial plane. The result is one of the most extreme seasonal cycles known anywhere. Each pole of Titania spends 42 years in total darkness, then 42 years in continuous sunlight, with the Sun climbing nearly to the zenith over whichever pole is tilted toward it at solstice. When Voyager 2 swept through in January 1986, it was southern hemisphere summer on Titania, and nearly the entire southern face was bathed in sunlight.

  Once every 42 years, when Uranus reaches its equinox, its equatorial plane lines up with Earth and the moons can pass in front of one another as seen from our planet. During the 2007-2008 equinox season, observers recorded two such mutual events: Umbriel occulted Titania on the 15th of August 2007 and again on the 8th of December 2007. These occultations offered a rare chance to refine orbital measurements from the ground.

  Titania's orbit also lies entirely within Uranus's magnetosphere. Plasma from that magnetosphere constantly strikes the trailing hemisphere of the moon, the side facing backward along its orbit. That bombardment darkens the trailing face, a pattern seen across almost all of Uranus's major moons.

• 4. Ice, Rock, and a Hidden Ocean

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  With a density of 1.68 grams per cubic centimetre, Titania is made of roughly equal parts water ice and denser non-ice material, likely rock and carbonaceous matter laced with heavy organic compounds. Infrared spectroscopy carried out between 2001 and 2005 confirmed crystalline water ice on the surface; the ice absorption is slightly stronger on the leading hemisphere than on the trailing one, a pattern opposite to what astronomers find on Oberon, and the cause remains unexplained.

  Carbon dioxide is the only other compound detected on the surface, concentrated mainly on the trailing hemisphere. It may be produced locally by solar ultraviolet radiation or magnetospheric particles acting on carbonates and organics in the surface layer, or it could be outgassing from CO2 trapped within the icy interior since the moon's formation. Either explanation ties the moon's surface chemistry to forces still active today.

  Beneath the surface, Titania is thought to be differentiated: a rocky core with a radius of about 520 kilometres, surrounded by an icy mantle. That core accounts for roughly 58 percent of the moon's total mass. At the boundary between core and mantle, a subsurface ocean may exist if the ice contains enough ammonia or other antifreeze to keep water liquid. If it does, the ocean could be up to 50 kilometres thick, sitting at a temperature of around 190 K. Pressure at Titania's center reaches about 0.58 GPa. More recent analysis suggests that larger Uranian moons, Titania among them, are in fact presumed to harbour active subterranean oceans rather than frozen interiors.

• 5. Canyons, Craters, and Scarps

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  Gertrude, named after the queen in Hamlet, is the largest confirmed crater on Titania, stretching 326 kilometres across. A second degraded basin nearby may reach about 330 kilometres, though it is difficult to measure precisely. All of the largest craters share flat floors and central peaks; the single exception is Ursula, named from Much Ado About Nothing, which has a pit at its center rather than a peak.

  Massina Chasma, the most prominent canyon on Titania, runs roughly 1,500 kilometres from near the equator almost to the south pole. The grabens that form such canyons range from 20 to 50 kilometres wide and drop 2 to 5 kilometres in depth. A second major canyon, Belmont Chasma, named after the Italian city in The Merchant of Venice, stretches 238 kilometres. These enormous fractures are thought to be the youngest geological features on Titania, cutting across both craters and smooth plains alike.

  The scarp Rousillon Rupes, named for Roussillon, France and drawn from All's Well That Ends Well, runs for 402 kilometres near the Ursula crater. Regions along some scarps appear smooth in Voyager 2 images, suggesting they were resurfaced after most of the craters formed, possibly by cryovolcanic eruptions of fluid material from the interior or by blankets of ejecta thrown out by nearby large impacts.

  The scarring and rifting trace back to a single underlying cause: Titania's interior expanded during and after its formation, stretching the crust until it cracked. The global expansion amounted to about 0.7 percent of the moon's total size. That fraction sounds small, but applied to a body 1,578 kilometres across, it was enough to tear open chasms that dwarf any canyon on Earth.

• 6. A Thin Breath of Carbon Dioxide

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  On the 8th of September 2001, Titania passed in front of the star HIP 106829, which shines at a visible magnitude of 7.2. Astronomers used the event to search for an atmosphere by watching how the star's light faded as Titania's edge swept across it. The observations set an upper limit on surface pressure of 1-2 mPa, or 10-20 nbar. That ceiling is still several times higher than the maximum vapor pressure carbon dioxide could achieve even at the warmest temperatures Titania ever reaches, so the measurements leave the question of an actual atmosphere unresolved.

  The peculiar geometry of the Uranian system means that Titania's poles receive more solar energy than its equatorial belt. Because CO2 vapor pressure rises steeply with temperature, carbon dioxide sublimates from whichever pole is in summer sunlight and migrates toward the opposite pole and toward the equator, accumulating as ice in shaded regions and on high-albedo patches. During southern hemisphere summers, polar temperatures can climb to 85-90 K before the cycle reverses.

  Magnetospheric particles complicate the picture further. They sputter carbon dioxide directly off the surface, gradually removing what the cold traps accumulate. Titania is thought to have lost a significant portion of the CO2 it was born with over the 4.6 billion years since its formation. Whether what remains amounts to even a wispy seasonal atmosphere akin to that of Callisto, or simply frost on a bare vacuum, is something Voyager 2's instruments were not precise enough to settle.

• 7. Voyager 2 and Future Missions

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  Voyager 2 remains the only spacecraft ever to have visited the Uranian system. Its closest approach to Titania brought it within 365,200 kilometres of the moon in January 1986, close enough to achieve an image resolution of about 3.4 kilometres per pixel. The images covered roughly 40 percent of the surface, but only 24 percent was photographed at the precision needed for geological mapping. Because the southern hemisphere was in midsummer during the flyby, the northern hemisphere lay entirely in darkness and was never imaged.

  One extension mission proposed for the Cassini spacecraft, after its Saturn tour, would have sent it on to Uranus; that idea was eventually set aside. A concept called the Uranus orbiter and probe was evaluated around 2010 but not funded at that time. The Innovative Interstellar Explorer precursor probe also considered a Uranian trajectory, though that mission concept remained exploratory.

  The picture changed with the 2023-2032 Planetary Science Decadal Survey, which ranked the Uranus Orbiter and Probe as the highest-priority large mission for NASA. Among the science questions driving that ranking are the bulk properties, internal structure, and geological history of the Uranian satellites, Titania included. A Uranus orbiter had already appeared on the previous Decadal Survey's list, ranked third for the 2013-2022 period. If a mission does fly, it would be the first to image Titania's northern hemisphere, a half of a world that no human instrument has ever seen.