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October 30, 2015November 18, 2017 by Matt Williams

Uranus’ “Sprightly” Moon Ariel

Mosaic of the four highest-resolution images of Ariel taken by the Voyager 2 space probe during its 1986 flyby of Uranus. Credit: NASA/JPL

The outer Solar System has enough mysteries and potential discoveries to keep scientists busy for decades. Case in point, Uranus and it’s system of moons. Since the beginning of the Space Age, only one space probe has ever passed by this planet and its system of moons. And yet, that which has been gleaned from this one mission, and over a century and a half of Earth- (and space-) based observation, has been enough to pique the interest of many generations of scientists.

For instance, just about all detailed knowledge of Uranus’ 27 known moons – including the “sprightly” moon Ariel – has been derived from information obtained by the Voyager 2 probe. Nevertheless, this single flyby revealed that Ariel is composed of equal parts ice and rock, a cratered and geologically active surface, and a seasonal cycle that is both extreme and very unusual (at least by our standards!)

Discovery and Naming:

Ariel was discovered on October 24th, 1851, by English astronomer William Lassel, who also discovered the larger moon of Umbriel on the same day. While William Herschel, who discovered Uranus’ two largest moons of Oberon and Titania in 1787, claimed to have observed four other moons in Uranus’ orbit, those claims have since been concluded to be erroneous.

A montage of Uranus's moons. Image credit: NASA — *A montage of Uranus’s major moons. Image credit: NASA*

As with all of Uranus’ moons, Ariel was named after a character from Alexander Pope’s The Rape of the Lock and Shakespeare’s The Tempest. In this case, Ariel refers to a spirit of the air who initiates the great storm in The Tempest and a sylph who protects the female protagonist in The Rape of the Lock. The names of all four then-known satellites of Uranus were suggested by John Herschel in 1852 at the request of Lassell.

Size, Mass and Orbit:

With a mean radius of 578.9 ± 0.6 km and a mass of 1.353 ± 0.120 × 10²¹ kg, Ariel is equivalent in size to 0.0908 Earths and 0.000226 times as massive. Ariel’s orbit of Uranus is almost circular, with an average distance (semi-major axis) of 191,020 km – making it the second closest of Uranus’ five major moons (behind Miranda). It has a very small orbital eccentricity (0.0012) and is inclined very little relative to Uranus’ equator (0.260°).

With an average orbital velocity of 5.51 km/s, Ariel takes 2.52 days to complete a single orbit of Uranus. Like most moons in the outer Solar System, Ariel’s rotation is synchronous with its orbit. This means that the moon is tidally locked with Uranus, with one face constantly pointed towards the planet.

Ariel orbits and rotates within Uranus’ equatorial plane, which means it rotates perpendicular to the Sun. This means that its northern and southern hemispheres face either directly towards the Sun or away from it at the solstices, which results in an extreme seasonal cycle of permanent day or night for a period of 42 years.

Size comparison between Earth, the Moon, and Ariel. Credit: NASA/JPL/USGS/Tom Reding

Ariel’s orbit lies completely inside the Uranian magnetosphere, which means that its trailing hemisphere is regularly struck by magnetospheric plasma co-rotating with the planet. This bombardment is believed to be the cause of the darkening of the trailing hemispheres (see below), which has been observed for all Uranian moons (with the exception of Oberon).

Currently Ariel is not involved in any orbital resonance with other Uranian satellites. In the past, however, it may have been in a 5:3 resonance with Miranda, which could have been partially responsible for the heating of that moon, and 4:1 resonance with Titania, from which it later escaped.

Composition and Surface Features:

Ariel is the fourth largest of Uranus’ moons, but is believed to be the third most-massive. Its average density of 1.66 g/cm³ indicates that it is roughly composed of equal parts water ice and rock/carbonaceous material, including heavy organic compounds. Based on spectrographic analysis of the surface, the leading hemisphere of Ariel has been revealed to be richer in water ice than its trailing hemisphere.

The cause of this is currently unknown, but it may be related to bombardment by charged particles from Uranus’s magnetosphere, which is stronger on the trailing hemisphere. The interaction of energetic particles and water ice causes sublimation and the decomposition of methane trapped in the ice (as clathrate hydrate), darkening the methanogenic and other organic molecules and leaving behind a dark, carbon-rich residue (aka. tholins).

The highest-resolution Voyager 2 color image of Ariel. Canyons with floors covered by smooth plains are visible at lower right. The bright crater Laica is at lower left. Credit: NASA/JPL — *The highest-resolution Voyager 2 color image of Ariel, showing canyons with floors covered by smooth plains (lower right) and the bright Laica crater (lower left). Credit: NASA/JPL*

Based on its size, estimates of its ice/rock distribution, and the possibility of salt or ammonia in its interior, Ariel’s interior is thought to be differentiated between a rocky core and an icy mantle. If true, the radius of the core would account for 64% of the moon’s radius (372 km) and 52% of its mass. And while the presence of water ice and ammonia could mean Ariel harbors an interior ocean at it’s core-mantle boundary, the existence of such an ocean is considered unlikely.

Infrared spectroscopy has also identified concentrations of carbon dioxide (CO²) on Ariel’s surface, particularly on its trailing hemisphere. In fact, Ariel shows the highest concentrations of CO² on of any Uranian satellite, and was the first moon to have this compound discovered on its surface.

Though the precise reason for this is unknown, it is possible that it is produced from carbonates or organic material that have been exposed to Uranus’ magnetosphere or solar ultraviolet radiation – due to the asymmetry between the leading and trailing hemispheres. Another explanation is outgassing, where primordial CO² trapped in Ariel’s interior ice escaped thanks to past geological activity.

The observed surface of Ariel can be divided into three terrain types: cratered terrain, ridged terrain and plains. Other features include chasmata (canyons), fault scarps (cliffs), dorsa (ridges) and graben (troughs or trenches). Impact craters are the most common feature on Ariel, particularly in the south pole, which is the moon’s oldest and most geographically extensive region.

False-color map of Ariel. The prominent noncircular crater below and left of center is Yangoor. Part of it was erased during formation of ridged terrain via extensional tectonics. Credit: NASA/JPL/USGS — *False-color map of Ariel, showing the prominent Yangoor crater (left of center) and patches of ridged terrain (far left). Credit: USGS*

Compared to the other moons of Uranus, Ariel appears to be fairly evenly-cratered. The surface density of the craters, which is significantly lower than those of Oberon and Umbriel, suggest that they do not date to the early history of the Solar System. This means that Ariel must have been completely resurfaced at some point in its history, most likely in the past when the planet had a more eccentric orbit and was therefore more geologically active.

The largest crater observed on Ariel, Yangoor, is only 78 km across, and shows signs of subsequent deformation. All large craters on Ariel have flat floors and central peaks, and few are surrounded by bright ejecta deposits. Many craters are polygonal, indicating that their appearance was influenced by the crust’s preexisting structure. In the cratered plains there are a few large (about 100 km in diameter) light patches that may be degraded impact craters.

The cratered terrain is intersected by a network of scarps, canyons and narrow ridges, most of which occur in Ariel’s mid-southern latitudes. Known as chasmata, these canyons were probably graben that formed due to extensional faulting triggered by global tension stresses – which in turn are believed to have been caused by water and/or liquid ammonia freezing in the interior.

These chasmata are typically 15–50 km wide and are mainly oriented in an east- or northeasterly direction. The widest graben have grooves running along the crests of their convex floors (known as valles). The longest canyon is Kachina Chasma, which is over 620 km long.

was taken Jan. 24, 1986, from a distance of 130,000 km (80,000 mi). The complexity of Ariel's surface indicates that a variety of geologic processes have occurred. Credit: NASA/JPL — *Image of Ariel, taken on Jan. 24, 1986, from a distance of 130,000 km (80,000 mi) showing the complexity of Ariel’s surface. Credit: NASA/JPL*

The ridged terrain on Ariel, which is the second most-common type, consists of bands of ridges and troughs hundreds of kilometers long. These ridges are found bordering cratered terrain and cutting it into polygons. Within each band (25-70 km wide) individual ridges and troughs have been observed that are up to 200 km long and 10-35 km apart. Here too, these features are believed to be a modified form of graben or the result of geological stresses.

The youngest type of terrain observed on Ariel are its plains, which consists of relatively low-lying smooth areas. Due to the varying levels of cratering found in these areas, the plains are believed to have formed over a long period of time. They are found on the floors of canyons and in a few irregular depressions in the middle of the cratered terrain.

The most likely origin for the plains is through cryovolcanism, since their geometry resembles that of shield volcanoes on Earth, and their topographic margins suggests the eruption of viscous liquid – possibly liquid ammonia. The canyons must therefore have formed at a time when endogenic resurfacing was still taking place on Ariel.

Uranus and Ariel — Ariel’s transit of Uranus, which was captured by the Hubble Space Telescope on July 26th, 2008. Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)

Ariel is the most reflective of Uranus’s moons, with a Bond albedo of about 23%. The surface of Ariel is generally neutral in color, but there appears to be an asymmetry where the trailing hemisphere is slightly redder. The cause of this, is believed to be interaction between Ariel’s trailing hemisphere and radiation from Uranus’ magnetosphere and Solar ultraviolet radiation, which converts organic compounds in the ice into tholins.

Like all of Uranus’ major moons, Ariel is thought to have formed in the Uranunian accretion disc; which existed around Uranus for some time after its formation, or resulted from a large impact suffered by Uranus early in its history.

Exploration:

Due to its proximity to Uranus’ glare, Ariel is difficult to view by amateur astronomers. However, since the 19th century, Ariel has been observed many times by ground-based on space-based instruments. For example, on July 26th, 2006, the Hubble Space Telescope captured a rare transit made by Ariel of Uranus, which cast a shadow that could be seen on the Uranian cloud tops. Another transit, in 2008, was recorded by the European Southern Observatory.

It was not until the 1980s that images were obtained by the first and only orbiter to ever pass through the Uranus’ system. This was the Voyager 2 space probe, which photographed the moon during its January 1986 flyby. The probe’s closest approach was at a distance of 127,000 km (79,000 mi) – significantly less than the distances to all other Uranian moons except Miranda.

Voyager 2. Credit: NASA — *Artist’s impression of the Voyager 2 space probe. Credit: NASA*

The images acquired covered only about 40% of the surface, but only 35% was captured with the quality required for geological mapping and crater counting. This was partly due to the fact that the flyby coincided with the southern summer solstice, where the southern hemisphere was pointed towards the Sun and the northern hemisphere was completely concealed by darkness.

No missions have taken place to study Uranus’ system of moons since and none are currently planned. However, the possibility of sending the Cassini spacecraft to Uranus was evaluated during its mission extension planning phase in April of 2008. It was determined that it would take about twenty years for Cassini to get to the Uranian system after departing Saturn. However, this proposal and the ultimate fate of the mission remain undecided at this time.

All in all, Uranus’ moon Ariel is in good company. Like it’s fellow Uranians, its axial tilt is almost the exact same as Uranus’, it is composed of almost equal parts ice and rock, it is geologically active, and its orbit leads to an extreme seasonal cycle. However, Ariel stands alone when its to its brightness and its youthful surface. Unfortunately, this bright and youthful appearance has not made it an easier to observe.

Alas, as with all Uranian moons, exploration of this moon is still in its infancy and there is much we do not know about it. One can only hope another deep-space mission, like a modified Cassini flyby, takes place in the coming years and finishes the job started by Voyager 2!

We have many interesting articles on Ariel and Uranus’ moons here at Universe Today. Here’s one about Ariel’s 2006 transit of Uranus, its 2008 transit, and one which answers the all-important question How Many Moons Does Uranus Have?

For more information, check out NASA’s Solar System Exploration page on Ariel, and The Planetary Society’s Voyager 2 Ariel image catalog.

Sources:

July 3, 2015November 18, 2017 by Matt Williams

Uranus’ Moon Titania

Voyager 2's highest-resolution image of Titania shows moderately cratered plains, enormous rifts and long scarps. Near the bottom, a region of smoother plains including the crater Ursula is split by the graben Belmont Chasma. Credit: NASA

Thanks to the Voyager missions, which passed through the outer Solar system in the late 1970s and early 1980s, scientists were able to get the first close look at Uranus and its system of moons. Like all of the Solar Systems’ gas giants, Uranus has many fascinating satellites. In fact, astronomers can now account for 27 moons in orbit around the teal-colored giant.

Of these, none are greater in size, mass, or surface area than Titania, which was appropriately named. As one of the first moon’s to be discovered around Uranus, this heavily cratered and scarred moon takes it name from the fictional Queen of the Fairies in Shakespeare’s A Midsummer Night’s Dream.

Discovery and Naming:

Titania was discovered by William Herschel on January 11th, 1787, the English astronomer who had discovered Uranus in 1781. The discovery was also made on the same day that he discovered Oberon, Uranus’ second-largest moon. Although Herschel reported observing four other moons at the time, the Royal Astronomical Society would later determine that this claim was spurious.

It would be almost five decades after Titania and Oberon was discovered that an astronomer other than Herschel would observe them. In addition, Titania would be referred to as “the first satellite of Uranus” for many years – or by the designation Uranus I, which was given to it by William Lassell in 1848.

By 1851, Lassell began to number all four known satellites in order of their distance from the planet by Roman numerals, at which point Titania’s designation became Uranus III. By 1852, Herschel’s son John, and at the behest of Lassell himself, suggested the moon’s name be changed to Titania, the Queen of the Fairies in A Midsummer Night’s Dream. This was consistent with all of Uranus’ satellites, which were given names from the works of William Shakespeare and Alexander Pope.

Size, Mass and Orbit:

With a diameter of 1,578 kilometers, a surface area of 7,820,000 km² and a mass of 3.527±0.09 × 10²¹ kg, Titania is the largest of Uranus’ moons and the eighth largest moon in the Solar System. At a distance of about 436,000 km (271,000 mi), Titania is also the second farthest from the planet of the five major moons.

Titania’s moon also has a small eccentricity and is inclined very little relative to the equator of Uranus. It’s orbital period, which is 8.7 days, is also coincident with it’s rotational period. This means that Titania is a synchronous (or tidally-locked) satellite, with one face always pointing towards Uranus at all times.

Because Uranus orbits the Sun on its side, and its moons orbit the planet’s equatorial plane, they are all subject to an extreme seasonal cycle, where the northern and southern poles experience 42 years of either complete darkness or complete sunlight.

*Uranus and its five major moons, with Titania being the farthest left. Credit: space.com*

Composition:

Scientists believe Titania is composed of equal parts rock (which may include carbonaceous materials and organic compounds) and ice. This is supported by examinations that indicate that Titania has an unusually high-density for a Uranian satellite (1.71 g/cm³). The presence of water ice is supported by infrared spectroscopic observations made in 2001–2005, which have revealed crystalline water ice on the surface of the moon.

It is also believed that Titania is differentiated into a rocky core surrounded by an icy mantle. If true, this would mean that the core’s radius is approx. 520 km (320 mi), which would mean the core accounts for 66% of the radius of the moon, and 58% of its mass.

As with Uranus’ other major moons, the current state of the icy mantle is unknown. However, if the ice contains enough ammonia or other antifreeze, Titania may have a liquid ocean layer at the core-mantle boundary. The thickness of this ocean, if it exists, is up to 50 km (31 mi) and its temperature is around 190 K.

Naturally, it is unlikely that such an ocean could support life. But assuming this ocean supports hydrothermal vents on its floor, it is possible life could exist in small patches close to the core. However, the internal structure of Oberon depends heavily on its thermal history, which is poorly known at present.

Voyager 2:

The only direct observations made of Titania were conducted by the Voyager 2 space probe, which photographed the moon during its flyby of Uranus in January 1986. These images covered about 40% of the surface, but only 24% was photographed with the precision required for geological mapping.

Voyager’s flyby of Titania coincided with the southern hemisphere’s summer solstice, when nearly the entire northern hemisphere was unilluminated. As with the other major moon’s of Uranus, this prevented the surface from being mapped in any detail. No other spacecraft has visited the Uranian system or Titania before or since, and no mission is planned in the foreseeable future.

Interesting Facts:

Titania is intermediate in terms of brightness, occupying a middle spot between the dark moons of Oberon and Umbriel and the bright moons of Ariel and Miranda. It’s surface is generally red in color (less so than Oberon), except where fresh impact have taken place, which have left the surface blue in color. The surface of Titania is less heavily cratered than the surface of either Oberon or Umbriel, suggesting that its surface is much younger.

Like all of Uranus’ major moons, it’s geology is influenced by a combination of impact craters and endogenic resurfacing. Whereas the former acted over the moon’s entire history and influenced all its surfaces, the latter processes were mainly active following the moon’s formation and resulted in a smoothing out of its features – hence the low number of present-day impact craters.

Overall, scientists have recognized three classes of geological feature on Titania. These include craters, faults (or scarps) and what are known as grabens (sometimes called canyons). Titania’s craters range in diameter from a few kilometers to 326 kilometers – in the case of the largest known crater, Gertrude. Titania’s surface is also intersected by a system of enormous faults (scarps); and in some places, two parallel scarps mark depressions in the satellite’s crust, forming grabens (aka. canyons).

The grabens on Titania range in diameter from 20 to 50 kilometers (12–31 mi) and in a relief (i.e. depth) from 2 to 5 km. The most prominent graben on Titania is the Messina Chasma, which runs for about 1,500 kilometers (930 mi) from the equator almost to the south pole. The grabens are probably the youngest geological features on Titania, since they cut through all craters and even the smooth plains.

Like Oberon, the surface features on Titania have been named after characters in works by Shakespeare, with all of the physical features are named after female characters. For instance, the crater Gertrude is named after Hamlet’s mother, while other craters – Ursula, Jessica, and Imogen – are named after characters from Much Ado About Nothing, The Merchant of Venice, and Cymebline, respectively.

Interestingly, the presence of carbon dioxide on the surface suggests that Titania may also have a tenuous seasonal atmosphere of CO², much like that of the Jovian moon Callisto. Other gases, like nitrogen or methane, are unlikely to be present, because Titania’s weak gravity could not prevent them from escaping into space.

Like all of Uranus’ moons, much remains to be discovered about this most-massive of her satellites. In the coming years, one can only hope that NASA, the ESA, or other space agencies decide that another Voyager-like mission is need to the outer Solar System. Until such time, Uranus and the many moons that orbit it will continue to keep secrets from us.

We have written many articles on Titania here at Universe Today. Here’s How Many Moons Does Uranus Have?, Uranus’ Moon Oberon and Uranus’ Moon Umbriel.

For more information, check out Nine Planets page on Titania and NASA’s Solar System Exploration page on Titania.

Astronomy Cast has an episode on the subject. Here’s Episode 172: William Herschel

Sources:

June 21, 2015May 6, 2016 by Matt Williams

How Many Moons Does Uranus Have?

Uranus and Moons — Uranus and its system of Moons. Credit: NASA/JPL

In the outer Solar System, there are many worlds that are so large and impressive to behold that they will probably take your breath away. Not only are these gas/ice giants magnificent to look at, they are also staggering in size, have their own system a rings, and many, many moons. Typically, when one speaks of gas (and/or ice) giants and their moons, one tends to think about Jupiter (which has the most, at 67 and counting!).

But have you ever wondered how many moons Uranus has? Like all of the giant planets, it’s got rather a lot! In fact, astronomers can now account for 27 moons that are described as “Uranian”. Just like the other gas and ice giants, these moons are motley bunch that tell us much about the history of the Solar System. And, just like Jupiter and Saturn, the process of discovering these moons has been long and involved on multiple astronomers.

Continue reading “How Many Moons Does Uranus Have?”

October 7, 2008January 11, 2016 by Matt Williams

Ten Interesting Facts About Uranus

Uranus as seen through the automated eyes of Voyager 2 in 1986. (Credit: NASA/JPL).

The gas (and ice) giant known as Uranus is a fascinating place. The seventh planet from out Sun, Uranus is the third-largest in terms of size, the fourth-largest in terms of mass, and one of the least dense objects in our Solar System. And interestingly enough, it is the only planet in the Solar System that takes it name from Greek (rather than Roman) mythology.

But these basic facts really only begin to scratch the surface. When you get right down to it, Uranus is chock full of interesting and surprising details – from its many moons, to its ring system, and the composition of its aqua atmosphere. Here are just ten things about this gas/ice giant, and we guarantee that at least one of them will surprise you.

Continue reading “Ten Interesting Facts About Uranus”