Diamond In The Sky

by Peter Jekel
No pressure, no diamonds.
--Thomas Carlyle

Look up into the clear night sky. How far into our Solar System do you think you can see unaided by a telescope? If you said Saturn, you would be wrong. In fact, it is the planet Uranus that is the furthest object we can see in the Solar System. Why then don’t any of the ancient cultures mention this? That is due to its extreme dimness and slow relative movement around the night sky; it was never recognized as one of the “wandering stars,” Mercury, Venus, Mars, Jupiter and Saturn. To the ancients, it appeared as just another bright star.
Uranus was only officially discovered with a telescope by English astronomer Sir William Herschel when he announced its discovery on March 13, 1781. Even then it was not recognized by its discoverer as a planet, but as a comet. In spite of his observations of the highly circular orbit as well as the lack of a tail common in comets, he stubbornly continued to describe it as a comet; it was other astronomers who concluded otherwise. It was Finnish-Swedish astronomer Anders Johan Lexell who analyzed the orbital data of Herschel’s “comet,” which led him to the conclusion that Uranus was a planet rather than a comet. Though Lexell did his calculations in the same year of Herschel’s discovery, he did not publish until 1783. Lexell found an ally in his “planet” conclusion of Uranus in German astronomer Johann Bode, who looked at the new “comet” as a planet as well due to its nearly circular orbit. Soon Uranus was universally accepted as a new planet.
History tells us that Herschel discovered Uranus, but others actually saw the planet before without knowing what they were seeing. Uranus was apparently observed at least six times in 1690 by English astronomer John Flamsteed, who mistakenly named it 34 Tauri as part of the Taurus constellation, which is over thirty light years distant. French astronomer Pierre Lemonnier is also reported as having observed the planet twelve times between 1750 and 1769.
Naming of the new planet proved to be less than simple. Herschel called it "Georgian Sidus" (George’s Star) in honour of King George II. However, his proposal was not popular outside of Great Britain. French astronomer Jerome Lalande felt that it should be named after its discoverer. Swedish astronomer Erik Properin proposed the name “Neptune,” which was supported by other astronomers, but Johann Bode came up with the name of “"Uranus.” His logic was that Saturn, which was beyond Jupiter, was Jupiter’s father in Roman mythology. Saturn’s father, in turn, was Uranus. Though a popular idea, it did not officially gain recognition until 1850, when His Majesty’s Nautical Almanac Officer switched the name from “Georgium Sidus” to Uranus.
The planet revolves around the Sun only once every 84 Earth years at a distance of about three billion miles. Unlike its lengthy revolution around the Sun, the interior rotational period of Uranus is a relatively fast seventeen hours and fourteen minutes in a clockwise fashion. Perhaps the most interesting observation of Uranus is its axial tilt; it has an axial tilt almost parallel to the plane of the solar system at 97.77 degrees compared with Earth’s at 23.5 degrees. A possible reason for the unusual orientation of Uranus is that during its formation, an Earth-sized object collided with the infant Uranus, skewing its orientation.
This extreme axial rotation allows the polar regions to receive greater solar energy when facing the Sun (calculated at about 1/400 of that received by the surface of the Earth) than at the equatorial regions, unlike other planets where the equatorial regions receive more sunlight. However, contrary to what might be expected, the equator of Uranus is hotter than the poles. Why this is happening is  unknown to science.
Uranus is the least massive of the gas giants, only 14.5 times the mass of Earth. Its diameter is just four times that of Earth and its density so low that only Saturn is less dense. The prevailing model of Uranus shows that it has a silicate/iron-nickel core, an icy mantle and an outer hydrogen-helium atmosphere. The core is relatively small at about 20% of the total planet size. The ice mantle is not composed of ice, as what one might envision, but is actually best described as a hot dense fluid consisting of water, ammonia and other volatiles including methane.
Research from the University of California, Berkeley, and Harvard suggests that the methane within the mantle may actually condense into an ocean of liquid diamond complete with diamond bergs due to the extreme heat and pressure found at that depth. Such an ocean would account for the unusual magnetic field orientation of Uranus, whose magnetotail appears as a corkscrew that projects hundreds of kilometers into space. Other planetary magnetotails appear as two lobes, a southern one and a northern one that project well into space beyond the planet itself.
Another interesting fact about Uranus is its temperature. Uranus’ internal heat is lower than that of the other gas giants. Why it is so low is not understood since Neptune, an apparent twin of Uranus and further from the Sun, is actually warmer at an average temperature of -214 degrees Celsius. The planet Uranus, on the other hand, has a minimum atmospheric temperature of -224 degrees Celsius. It is the coldest place in the Solar System.
Why so cold? One reason might be that Uranus may have been hit by a large object, causing it to expel most of its interior heat; this impact also explains the planet’s curious axial tilt. Another theory is that there is a barrier in the upper layers of Uranus’ atmosphere that prevents the core heat from reaching the surface. The nature of this barrier is also unknown to science.
There was a time when it was thought that the atmosphere of Uranus was relatively stable, with relatively few storms compared with the other gas giants. Its atmosphere is made up predominately of molecular hydrogen and helium with some methane, which gives the planet its blue-green (cyan) colour. The apparent stability of Uranus’ atmosphere all changed in the 1990s when the number of cloud features grew markedly due to improvements in high resolution imaging techniques. It was also found that there are differences in the atmosphere between the northern and southern hemispheres, with the northern clouds being smaller, sharper and brighter, lying at greater altitudes and having relatively short lives. In the south the clouds appear to be longer lived; one southern cloud appears to have survived since the time of the Voyager 2 flyby in 1986. Winds can blow at speeds of over 900 kilometers per hour.
If these wind speeds are not enough to raise eyebrows, there are also intense storm systems on Uranus. Like Jupiter’s famous and enormous storm system, the Red Spot, a similar storm system, dubbed the Uranus’ Great Dark Spot, was discovered by Voyager 2 during its flyby in 1986. It also detected a small dark spot, another less intense storm system.
It was William Herschel who described the first of fifteen known rings around Uranus in 1789. Herschel described the ring’s size, its angle relative to Earth and its apparent changes as Uranus travelled around the Sun. Due to its faintness, Herschel’s ring was not seen again for another two centuries.
The rings were officially discovered in 1977 by observers at the Kuiper Airborne Observatory (a mobile specialized Lockheed aircraft) and Perth Observatory in Australia. They were actually looking at the occultation of the star SAO 158687 to study Uranus’ atmosphere. The Kuiper group described five rings and the Perth group, six rings. With further analysis of the data and clear observations by Voyager 2 in 1986, scientists discovered more rings, bringing the total to thirteen. Only with the observations of the Hubble Space Telescope was the number of rings increased to fifteen in 2005.
The rings are composed of dark particles of extremely small size, unlike the bright ice particles of Saturn’s rings. Data shows that the rings did not form with Uranus but are more likely the result of an impact or impacts with a larger object such as a moon, quite probably the same impact that caused Uranus’ unusual axial orientation and rotation.
Not many science fiction authors have used Uranus as a setting for their stories. This is unfortunate, as it is a truly mysterious planet. In 1935, Stanley Weinbaum’s short story “The Planet of Doubt” describes the planet’s north pole being shrouded in a perpetual fog that absorbs all radio waves and visible light. In the fog lurks a large segmented predator—a little far-fetched with what we know of Uranus today, but an entertaining read nonetheless. In a more recent story by NASA engineer Geoffrey Landis, “Into the Blue Abyss,” an expedition to Uranus to search for life is described.
The rings are but one similarity that Uranus has with its gas giant neighbours, it also possesses a large family of moons. Though Uranus has 27 moons, all named after characters, there are just five major ones that orbit the planet. The smallest of the major moons is also the closest of the major satellites to the parent planet. It was discovered in 1948 by Dutch-born American astronomer Gerard Kuiper, who named it after the character Miranda from the Shakespearean play The Tempest. The only good data that we have on the moon comes from the Voyager 2 probe when it flew by in January 1986.
Based on density readings, it appears that the moon is made up of water ice with some silicate rock and organics. Voyager 2 data showed a pattern of broken terrain made up of huge canyons and grooved structures called coronae. It has been speculated that the coronae could be the result of a warmer unwelling of ice from below the surface. Some astronomers and planetary scientists speculate that cryovolcanism may also have played a role, suggesting a possible subterranean water reservoir.
Ariel, Uranus’ fourth largest moon, orbits at the equatorial plane of Uranus. It was discovered in 1851 by William Lassell, who named it for a character again from Shakespeare’s The Tempest. Like Miranda, it probably has an inner core of rock surrounded by a mantle of ice. The surface shows a lot of different geological features such as scarps, canyons and ridges theorized to have arisen from the past orbital resonance with its neighbouring moons.
Ariel serves as the setting for Paul McAuley’s short story "Dead Men Walking," which is about a robot assassin that has invaded the colony on the moon, complete with cities, penal colony and prison farm. Ariel also serves as the source of a mystery when scientists discover a mysterious alien radio wave emanating from the moon in Hugh Walters’ First Contact.
The darkest of the Uranian moons, Umbriel, was also discovered in 1851 by Lassell and named after another character in The Tempest. It is very similar in its structure to the other major moons.
Oberon and Titania were both discovered by Herschel in 1787. The name Oberon comes from the King of the Fairies from Shakespeare’s A Midsummer Night’s Dream. It is covered by numerous craters, many surrounded by bright icy material. It is believed that there may be a liquid ocean at the mantle-core boundary of Oberon, suggesting the possibility of alien life.
Titania, also named after yet another character in A Midsummer Night’s Dream, is the second largest and most massive of the Uranian moons. It is made up of equal amounts of ice and rocky material differentiated into a rocky core and icy mantle. Like Oberon, it is covered by numerous craters, scarps and canyons and may even possess a liquid ocean at the mantle-core boundary. Unique amongst the Uranian moons, Titania may even possess a tenuous seasonal atmosphere made up mainly of carbon dioxide. The classic Mars trilogy by Kim Stanley Robinson envisions in Blue Mars colonies on both Titania and Miranda.
Uranus when first looked at appears to be plain and commonplace; however, at closer examination it is a strange world, unique in the Solar System. What secrets lie beneath its cold cloudy exterior remain elusive and need further exploration. Is it possible that a liquid diamond sea complete with diamond bergs exists? Why does the planet have such an unusual tilt? Is there life on its moons? Why is it so cold? The list of unanswered questions is endless, and only with further exploration will we ever find the answers.
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