Thursday, March 3, 2011

Our Solar System Part 9: Uranus

When Voyager 2 approached Uranus  in 1986, it captured a planet that looks more like a cyan blue billiard ball than a celestial body.


This odd planet, 15 times more massive than Earth and orbiting the extreme far reaches of the solar system between Saturn and Neptune, rotates with an axial tilt of 98 degrees, meaning that it is nearly perfectly tilted on its side. Why this planet, and only this planet, is tilted so extremely is one of the great mysteries of our solar system.











Its faint rings, a southern collar of white cloud and a bright cloud (white splotch) in the northern hemisphere can be made out in this  image taken by the Hubble telescope in 2005.







Uranus' Dynamic Atmosphere

In 1986, Voyager 2's rather boring billiard ball image (at the top of this article) gave Uranus a bad rap as a planet where nothing happens. But now, as long-awaited spring comes to its northern hemisphere, bright bands and clumps of convective clouds are bubbling up across the planet. Currently a northern band of clouds seems to be organizing in addition to the earlier southern band of clouds seen in the Hubble image above. You can see the cloud features in excellent detail in the 2004 Keck II Telescope image below, of the two sides of Uranus at near infrared wavelengths. Uranus is oriented about the same, with its north pole at the 4 o'clock position in this image. It is at the 3 o'clock position in the image above.

Photo credit: courtesy Lawrence Sromovsky, UW-Madison Space Science and Engineering Center.

The following time-lapse video made up of Hubble images taken between 1994 and 1998 dramatically shows seasonal changes on the planet, and a fragile wobbling ring system.



Some clouds seem to be as long-lived as 10 years while others pop up and disappear over a matter of days. Scientists are busy observing the planet from the Keck II observatory, which is outfitted with an adaptive optics system  that dynamically changes the shape of the observatory's main mirror in order to reduce the blurring effect from Earth's changeable atmosphere. As of yet, they have no theories to explain these rapid changes in Uranus' atmosphere.

Rings

Studies of Uranus' faint rings suggest that they are relatively young, having formed long after Uranus formed, and are likely the remnants of a moon that broke up from an impact or from gravitational stresses.











This schematic diagram of its rings hints at a close relationship between its rings and its moons.








Our discussion of Uranus' ring system will pick back up when we explore the moons.

Uranus' Name Could Have Been Worse

The discovery and naming of Uranus went through an awkward period. Even though it is visible to the naked eye, it moves so slowly in its orbit, that it was long considered to be a star. Not until Russian astronomer, Johan Lexell calculated the orbit of Uranus in 1781, during the Age of Reason, did it become clear to the scientific community that they were dealing with a planet. After much debate, and going through possible names such as Georgium Sidus, Herschel, and even Neptune Great Britain (which didn't go over well with the scientific community outside Britain) the name Uranus was settled on. It is the Latinized version of the Greek god of the sky, Ouranos.

The Interior of Uranus

Uranus is an ice giant like its sister, Neptune, and like Neptune, it is composed primarily of water, ammonia and methane ices. Unlike Jupiter or Saturn, both of which are gas giants, hydrogen and helium comprise only a small fraction of Uranus' makeup. Uranus is believed to be composed of three basic layers: an insubstantial silicate/iron-nickel core surrounded by an icy mantle that, rather than being ice in the conventional sense, is a hot highly conductive and highly pressurized fluid composed of water, ammonia and other volatiles that is sometimes called a water-ammonia ocean. This fluid layer gradually transitions into a gaseous atmosphere, composed mostly of hydrogen and helium, with decreasing enrichment by volatiles such as water, ammonia and methane as one reaches the outer surface, so that the outermost atmosphere is almost purely hydrogen and helium gas at an extremely cold 49 K, above which a tenuous photochemical haze blankets the planet. This haze, detected around the sunlit pole of Uranus, radiates significant ultraviolet light, a phenomenon called "electroglow." It is difficult to measure wind speeds on Uranus because its clouds are rarely defined, but they have been clocked as high as 860 km/h in the direction of its rotation (as on all the giant planets). Small amounts of methane in the upper atmosphere give Uranus its distinctive blue colour.


Magnetic field

Like Uranus's strange tilt, its magnetic field is peculiar. It doesn't come from the planet's center, but rather its magnetic center is shifted to its south rotational pole and titled 59 degrees from its rotational axis, as illustrated below.

This shift makes its magnetic field highly asymmetrical and about ten times more powerful on its northern hemispheric surface than on its southern surface. Earth's magnetic field is about the same strength at each of its poles, and Earth's magnetic center is about the same as its geographical center. Interestingly, Neptune also has a highly tilted magnetic field, at about 47 degrees, and it is even more severely offset from the planet's center, in this case by at least half a radius. This data points to something these planets share in common that is different from the magnetic fields of either the gas giants or the terrestrial planets. One idea suggests that ice giant magnetic fields are generated by relatively shallow motion within their (electrically convective) water/ammonia oceans, rather than by molten metallic core convection in the case of the rocky planets and liquid metallic hydrogen convection in the case of the gas giants.

Orbit and Discovery of Neptune

Uranus orbits around the Sun once every 84 Earth years, at an average distance from the Sun of 3 billion km. Because it is about twenty times further from the Sun than Earth is, it receives 1/400 of the sunlight Earth does. Once Uranus was discovered, discrepancies in its orbit calculated using Newton's law of gravitation led astronomers to wonder if another unknown body was tugging on the planet. This led astronomers to accurately predict the location of a new planet, Neptune, which orbits much further out at about 4.5 billion km from the Sun, in 1846.

Thanks to Uranus' extreme axial tilt, each pole receives 42 years of continuous sunlight followed by 42 years of complete darkness. Even though the polar regions of Uranus receive more average energy from the Sun over one year, it is hotter at its equator than at its poles. The reason for this is unknown. Uranus total internal heat is significantly lower than that of the other giant planets. Neptune, very close to Uranus both in size and composition (and yet much farther out from the Sun) radiates 2.6 times more heat than it receives from the Sun. Uranus hardly radiates any excess heat at all, making it the coldest planet in the solar system.

Two Unusual Characteristics of Uranus and Two Models Proposed to Explain Them

These two factors, much lower than expected internal heat and a highly tilted axial spin, may point to a massive impact sometime in Uranus' past. One model suggests that an object with a mass of between one and two Earths could have upended the planet and stirred its contents vigorously enough to bring hot material from deep within its interior up to its surface, allowing significantly more heat to dissipate than would have occurred without any such impact. This impact would have occurred fairly early in Uranus' history because its moons, which we will soon explore, orbit around its equator, meaning that they must have formed after the impact.

Another model can accommodate Uranus' unique axial tilt through what is called the Nice model. This widely accepted model of early planetary formation and migration is discussed in detail with regard to Neptune in my Neptune article, in the section "Formation and Early Jostling Around." In this scenario, Uranus was tilted during this early period of planetary migration. For this model to work, Uranus would have needed an additional moon and a temporary large inclination, which itself could have come about from an instability phase acting on all the giant planets at the time, as outlined in the Nice model. That extra moon could have been ejected from its orbit by a close encounter with another body near the end of the planetary migration, during a period called the Late Heavy Bombardment, during which unstable resonances between newly formed giant planets turned a great many asteroids, some of which were enormous (up to several kilometers across) into ultra-fast projectiles. This NASA artist's rendition of our young forming solar system may give you some idea of the chaotic environment in which all the planets formed.


This latter model alone cannot explain Uranus' coldness, but its unusual coldness could be the result of some mechanism through which ordinary convection is inhibited. In this case Uranus does indeed have a hot interior but its heat is somehow prevented from escaping upward and, as a result, the surface is extremely cold and the inner heat eludes our detection. Yet others believe an opposite mechanism explains Uranus' coldness. In this case, something about its interior allows it to release heat more easily than other planets. Why Uranus is so cold remains an open question as no thermodynamic models of these proposed processes have yet been worked out.

Exploration of Uranus

Everything we know about Uranus has come solely through the data from one mission, NASA's Voyager 2 mission in the 1980s. This is what its trajectory looked like:


Voyager 2 is still transmitting data as of August 2, 2011. It is traveling at a mind-boggling 55 thousand km/h and is now about 14.3 billion km away from the Sun. It is now about as far away as the most distant orbiting comets, yet it is still within our theoretical solar system boundary, the heliopause, where solar wind is stopped by the interstellar medium.


Several American NASA-led and European ESA-led missions designed to study the Uranus system have been proposed and have reached the conceptual stage, with launch dates at around 2020, and arrival at Uranus between 12 and 17 years later. Earlier this year a Uranus orbiter and probe mission was recommended by NASA after a mission study was completed but there are currently no plans to carry out any future missions to Uranus. However, as we have seen, new telescope technologies are making it possible to gather a great deal of information about the Uranus system right here from home (or from close Earth orbit).

Uranus' Moons

Unlike the pretty planet itself, Uranus' moons and its rings, are all drab dark grey in colour. Highly energetic ions within Uranus' magnetosphere, probably sloughed off from the planet's upper atmospheric haze, strike the moons as they sweep through them. This bombardment causes relatively rapid darkening of their surfaces, a process called space weathering. This is the same phenomenon that makes ancient impact craters on our moon appear much darker than recent impacts. This NASA image of our moon taken by Apollo 15 shows a young bright crater to at the lower right, in contrast to the darker older craters around it.


The montage image below shows the relative sizes and positions of Uranus' moons (from left to right) Puck, Miranda, Ariel, Umbriel, Titania and Oberon, in comparison to the planet itself.


Uranus has 27 moons in total, all named after characters in the works of William Shakespeare and Alexander Pope.

Its moons are divided into 3 types: 13 inner moons, 5 major moons and 9 irregular moons. The Uranian inner moon system looks like this:


Astronomers, scrutinizing Hubble images of Uranus in 2003, discovered Mab (after Queen Mab, a sprite in Shakespeare's Romeo and Juliet) and Cupid (seemed to the astronomers naming her to be an appropriate name for this tiny body, just 18 km across, orbiting among the great lovers of Shakespeare's literature).

All 13 inner moons orbit within Miranda's orbit (itself a major moon, shown at the left in the diagram above). All of the inner moons are very closely connected to the ring system, which is believed to have come from the fragmentation of one or more inner moons. These moons shepherd, and are a source of, the rings. They constantly perturb one another, creating a chaotic system in which some of these moons will probably eventually collide with each other.

Uranus has 5 major moons, Miranda, Ariel, Umbriel, Titania and Oberon, ranging in diameter from just 472 km (Miranda) to 1578 km (Titania). Titania is about 1/20 of the mass of our moon. These larger moons either arose from the accretion disc that remained just after Uranus' formation or as debris from the impact of a large object with Uranus early in its history. The larger moons may have come from a combination of both processes. Models suggest that past resonances between these moons created significant tidal heating as evidenced by canyons and striations across their surfaces. Most of these features are old, however, meaning that they are not currently undergoing as much flexing. They are also heavily cratered - they have undergone many impacts over the eons that have not been erased by any internally driven resurfacing process, such as volcanism. The largest of these major moons are likely to be internally differentiated into rocky cores surrounded by icy mantles. Titania and Oberon may even have liquid water oceans at their core/mantle boundaries. None of these major moons has any appreciable atmosphere. This image shows their orbits in perspective:


From the center out, they are Miranda, Ariel, Umbriel, Titania and Oberon. The red line indicates Uranus' orbit around the Sun.

Each of Uranus' 9 irregular moons orbits much further out than Oberon. They are probably all objects captured by Uranus' gravity soon after the planet formed. They all have retrograde orbits except for one, and all are very small, no more than 150 km in diameter.

Sunrise on Titania

Because Uranus is tilted on its side, the Sun would follow a small circular orbit around either the north or south pole of the planet, depending on the season. The Sun's trajectory across the sky of one of the major moons would look very similar as well because the moons all have axial tilts very similar to that of Uranus. This artist's conception gives you an idea of what the summer Sun would look on one of the major moons such as Titania.


Next, we will explore our fascinating solar system as a whole

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