Density and Composition
Uranus is a gaseous, icy planet with a mass about 14 times that of Earth, but only about one twentieth that of the largest planet Jupiter. The average density of Uranus is about 1.3 g/cm3, which is about one quarter that of Earth. Thus the material Uranus is made out of must be light and icy.
In contrast to the other gas planets (Jupiter and Saturn), the composition of Uranus is not dominated by hydrogen and helium. Hydrogen accounts for only 15 % of the planets mass. Most of the planet is made up of methane, ammonia and water. There are three layers or regions inside the planet. The dense core (30 %) contains silicate/iron-nickel rock and various ices, but no liquid metallic hydrogen. The mantle (40 %) is probably highly compressed water ice with some methane and ammonia. The outer layer (30 %) lies at the base of the atmosphere, is considered to be ocean, and composed of mostly gaseous or liquid hydrogen, helium and methane (see Fig. 11.2).
Fig. 11.2Interior structure of Uranus.
The strength of gravity on Uranus is actually less than Earth’s gravity (8.2 N/kg compared to Earth’s 9.8 N/kg). This means that a 75 kg person, who weighs 735 N on Earth, would weigh only 615 N on Uranus.
The Surface
Being a gaseous planet, there is no solid surface layer on Uranus. The outer layer of the planet is made up of icy molecules of water, methane and ammonia. The surface we see from Earth is actually Uranus’s atmosphere. Clouds are also visible in the atmosphere. The planet radiates about the same energy as it receives from the Sun and has little internal heat.
Uranus’s internal heat appears markedly lower than that of the other giant planets. The lowest temperature recorded in Uranus’s tropopause is −220 °C, making Uranus the coldest planet in the solar system. One of the hypotheses for this coldness suggests that a super massive impactor that caused it to expel most of its primordial heat hit Uranus. This impact left the planet with a depleted core temperature. Another hypothesis is that some form of barrier exists in Uranus’s upper layers that prevent the core’s heat from reaching the surface.
The Atmosphere
The atmosphere of Uranus is composed of mainly of hydrogen (83 %), and helium (15 %), and ices (such as water, ammonia, and methane), along with traces of hydrocarbons. The planet has the coldest atmosphere in the solar system, with a minimum temperature of −220 °C. The methane that is trapped high in the atmosphere absorbs red light from the visible spectrum, and this makes the planet appear blue-green in colour.
Along with methane, trace amounts of various hydrocarbons are found in the stratosphere of Uranus, which are thought to be produced from methane by photolysis induced by the solar ultraviolet radiation. The hydrocarbons include ethane, acetylene, methyl acetylene, and diacetylene. Spectroscopy has also uncovered traces of water vapor, carbon monoxide and carbon dioxide in the upper atmosphere, which can only originate from an external source such as infalling dust and comets.
Voyager 2 data showed the atmosphere contains three distinct cloud layers. The top layer contains ammonia, the next layer ammonium hydrosulfide, and the third or lower layer contains water ice. These layers are found deep in Uranus’s atmosphere where temperature and pressures are higher. The atmospheric pressure beneath the cloud layer is about 1.3 times that at the Earth’s surface (see Fig 11.3).
Fig. 11.3This infrared image taken by the Hubble ST allows astronomers to probe the structure of Uranus’ atmosphere. The red around the planet’s edge represents a very thin haze at a high altitude. The yellow is another hazy layer. The deepest layer, the blue on the right, shows a clearer atmosphere. The rings have been brightened in this image; in reality, the rings are as dark as charcoal. The bright spots are violent storms (Credit: HST/NASA).
Like the other gas planets, Uranus has bands of clouds that blow around rapidly parallel to the equator. These bands are very faint and can only be seen with image enhancement of the Voyager 2 photographs. Winds at mid-latitudes are propelled in the rotational direction of the planet. Winds at equatorial latitudes blow in the opposite direction.
Recent observations with the Hubble Space Telescope show larger and more pronounced streaks and some spots. The spots are probably violent swirling storms like a hurricane.
Astronomers have also been able to chart the wind speeds on Uranus, and have found they can travel at 250 m/s.
In August 2014 scientists using the Keck telescope in Hawaii spotted huge storms on the planet Uranus. One image, taken in infrared light on 5 August, shows a few storms as bright spots in photos taken of the planet. A second photo of Uranus, taken on 6 August, reveals many more bright spots. One very large storm seen by the telescope has particularly interested researchers analyzing the views because it reaches into the high altitudes of the planet’s atmosphere. The swirling clouds and violent winds are being driven by massive bands of jet streams that can surround the entire planet. The planet’s strange tilt also contributes to such bizarre weather systems (see Fig. 11.4).
Fig. 11.4Scientists using the 10 m Keck Telescope in Hawaii spotted huge bright storms on Uranus in August 2014 (Credit: Imke de Pater (UC Berkeley)/Keck Observatory).
The Rings
Uranus has a number of thin rings surrounding it. These were discovered by chance in 1977 when Uranus appeared to pass in front of the faint star SAO158687 in the constellation of Libra, as seen from Earth. The rings temporarily interrupted light from the star and pulses of starlight were seen each side of the planet, suggesting there was something around the planet. In 1986, Voyager 2 confirmed the existence of a ring system. The ring system lies in the Uranian equatorial plane, circling Uranus between 38,000 km and 52,000 km from its centre.
The rings are faint and composed of particles ranging in size from fine dust to several metres in diameter. Voyager 2 found that the gaps between the rings were not empty but contained fine dust that may have originated from collisions between the larger particles forming the main rings or from the surrounding moons. The matter in the rings may once have been part of a moon (or moons) that was shattered by high-speed impacts.
The outer ring is the most massive, and its particles are kept in orbit by the gravitational influence of two moons, Cordelia and Ophelia. The outermost ring is about 100 km wide but only 10–100 m thick. The ring material is probably composed of chunks of ice, covered by a layer of carbon (see Fig. 11.5).
Fig. 11.5Voyager 2 image of the rings of Uranus showing slight colour differences. The bright white ring at the bottom is the furtherest ring from Uranus—called Epsilon (Credit: NASA).
In December 2005, the Hubble Space Telescope detected a new pair of rings located twice as far from Uranus as the previously known rings. These new rings are so far from Uranus that they are called the “outer” ring system. Hubble also spotted two small satellites, one of which, Mab, shares its orbit with the outermost newly discovered ring. The new rings bring the total number of Uranian rings to 13. In April 2006, images of the new rings from the Keck Observatory showed the outermost is blue and the other one red. One hypothesis concerning the outer ring’s blue color is that it is composed of minute particles of water ice from the surface of Mab that are small enough to scatter blue light. In contrast, Uranus’s inner rings appear grey.
The Uranian rings were the first to be discovered after Saturn’s. This was an important finding since we now know that rings are a common feature of large planets, not a peculiarity of Saturn alone. Uranus’s rings are much darker than those of Saturn and harder to see from Earth. In 2014, astronomers reported that the rings of Uranus are probably very young, forming relatively recently, and not when the planet itself formed.
Temperature and Seasons
The temperature in the upper atmosphere of Uranus is about −200 °C. At this low temperature methane and water condense to form clouds of ice crystals. Because methane freezes at a lower temperature than water, it forms higher clouds over Uranus. Methane absorbs red light, giving Uranus its blue-green colour.
In the interior, the tempera
ture rises rapidly to about 2300 °C in the mantle and about 7000 °C in the rocky core. The pressure in the core is about 20 million times that of the atmosphere at the Earth’s surface.
The planet radiates as much heat as it receives from the Sun, back into space. Because its axis is titled at 98°, its poles receive more sunlight during a Uranian year than does its equator. However, the weather system seems to distribute heat fairly evenly over the planet.
As the planet orbits the Sun, its north and south poles alternately point directly toward or directly away from the Sun, resulting in exaggerated seasons. During summer near the north pole, the Sun is almost directly overhead for many Earth years. At the same time southern latitudes are subjected to a continuous frigid winter night. Forty-two years later, the situation is reversed.
In August 2006, the Hubble Space Telescope captured images of a huge dark cloud on Uranus. The cloud measured about 1700 km by 3000 km. Scientists are not certain about the origin of the cloud.
Magnetic Field
Uranus has a magnetic field about 50 times stronger than Earth’s. The axis of the field (an imaginary line joining its north and south poles) is tilted 59° from the planet’s axis of rotation. The centre of the magnetic field does not coincide with the centre of the planet—it is offset by almost one third of Uranus’s radius or nearly 7700 km. Because of the large angle between the magnetic field and its rotation axis, the magnetosphere of Uranus wobbles considerably as the planet rotates. One hypothesis is that, unlike the magnetic fields of the terrestrial and gas giants, which are generated within their cores, the ice giants’ magnetic fields are generated by motion at relatively shallow depths, for instance, in the water–ammonia ocean. Another possible explanation for the magnetosphere’s alignment is that there are oceans of liquid diamond in Uranus’s interior that would deter the magnetic field.
Voyager 2 passed through Uranus’s magnetosphere as it flew by Uranus. The magnetic field traps high energy, electrically charged particles, mostly electrons and protons, in radiation belts that circle the planet. As these particles travel back and forth between the magnetic poles, they emit radio waves. Voyager 2 detected these waves, but they are weak and cannot be detected from Earth. Uranus also has relatively well developed auroras, which are seen as bright arcs around both magnetic poles (Fig. 11.6).
Fig. 11.6Uranus’ strange magnetic field.
The magneto tail of Uranus was measured by Voyager 2 to extend 10 million km out into space. Unlike other magneto tails, the magnetic field lines in the tail are cylindrical and appear wound around each other—similar to a corkscrew. This is probably due to the strange axial tilt of the planet.
Moons
Before the visit of Voyager 2 to Uranus in January 1986, only five moons were known to orbit Uranus. These moons were discovered between 1787 and 1948 and they range in size from 480 km to 1550 km in diameter. Voyager discovered ten more moons around Uranus, all less than 50 km in diameter. Several of these tiny, irregularly shaped moons are shepherd satellites whose gravitational pull confines them within the rings of Uranus. Astronomers used Earth-based telescopes to find two more moons in 1997 and three more in 1999.
Unlike the other bodies in solar system that have names from classical mythology, the names of Uranus’s moons are derived from the writings of Shakespeare and the English writer Alexander Pope.
Uranus has at least 27 moons, arranged in three groups: 13 small dark inner ones, five large moons, and nine more distant ones recently discovered by telescopes. Most of these moons have nearly circular orbits in the plane of Uranus’s equator. The outer moons have elliptical orbits and many have retrograde motion. It may be that some of the smaller moons, especially the outer ones, are captured asteroid-like bodies (see Table 11.3).Table 11.3The eight largest moons of Uranus
Name of moon
Distance (km)
Period (days)
Diameter (km)
Discovered (year)
Portia
66,000
0.51
135
1986
Puck
86,000
0.76
162
1986
Miranda
130,000
1.41
471
1948
Ariel
191,000
2.52
1157
1851
Umbriel
266,000
4.14
1170
1851
Titania
436,000
8.71
1578
1787
Oberon
583,000
13.46
1522
1787
Sycorax
12,180,000
1288
190
1997
Most of the moons of Uranus are quite dark. This may be due to radiation darkening of methane on their surfaces. The Uranian satellites have relatively low albedos; ranging from 0.20 for Umbriel to 0.35 for Ariel. They are ice–rock conglomerates composed of roughly 50 % ice and 50 % rock. The ice may include ammonia and carbon dioxide.
The five largest moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon) have higher densities than expected (1.4–1.7 g/cm3). This suggests that they may contain more than 50 % rock or silicates, with smaller amounts of water ice than Saturn’s similar-sized moons.
Miranda is one of the most unusual moons because it has a surface like no other in the solar system. The older areas are relatively smooth, cratered plains. But other areas look as if they have been clawed and gouged by something. Miranda possesses fault canyons 20 km deep, with terraced layers, and a chaotic variation in surface ages and features. Astronomers think Miranda’s core originally consisted of dense rock while its outer layers were mostly ice; this structure has at some time in the past been dramatically changed. Surface variations suggest an asteroid or tectonic activity may have shattered the moon, breaking its surface into several pieces—the pieces have since been reassembled by gravity, leaving behind great scars. Miranda is just one-seventh the size of Earth’s moon and is not quite spherical. It takes only 1.41 days to orbit the planet (see Fig. 11.7).
Fig. 11.7A Voyager 2 photo of the surface of Miranda (Credit: NASA).
Ariel is a young moon with few craters, most being less than 50 km in diameter. It takes 2.52 days to orbit Uranus. The surface suggests there has been a lot of geological activity with many faults, fractures and valleys visible. Photographs taken by Voyager 2, suggest many of the features are volcanic in origin and that some form of viscous fluid once flowed across the surface. Many of the fissures have been partially filled in with frozen deposits of an unknown material (see Fig. 11.8).
Fig. 11.8A Voyager 2 photo of the surface of Ariel (Credit: NASA).
Umbriel is the darkest of the five larger moons orbiting Uranus. It takes 4.14 days to orbit Uranus. The surface of this moon is almost uniformly covered with craters. Many large craters suggest the surface is fairly old, although the covering of dark material appears to hide many features.
Titania is the largest of the Uranian moons, with a diameter of 1578 km. It takes 8.71 days to orbit Uranus. The surface of this moon contains numerous impact craters, with some young valleys, faults and fractures. One heavily fractured region is thought to have been caused by the crust fracturing as water froze below the surface (see Fig. 11.9).
Fig. 11.9A Voyager 2 photo of the surface of Titania (the largest moon of Uranus) (Credit: NASA).
Oberon has a diameter of 1522 km, making it slightly smaller than Titania. The moon takes 13.46 days to orbit Uranus. Its surface contains many impact craters some of which are surrounded by bright rays. One mountain towers above the surrounding terrain to a height of 20 km (see Fig. 11.10).
Fig. 11.10The moon Oberon is Uranus’s second largest moon (Credit: NASA).
Further Information
For fact sheets on any of the planets including
Uranus check out
http://nssdc.gsfc.nasa.gov/planetary/planetfact.html
www.space.com/uranus/
https://solarsystem.nasa.gov/planets/ (click on Saturn)
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_12
12. Neptune: Another Cold World
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
The Voyager 2 probe found Neptune to be a large blue planet, with many markings, cloud bands and a system of faint rings.
The planet Neptune has a profound impact on a region directly beyond it, known as the Kuiper belt.
Neptune’s atmosphere is very active with rapid changes in weather occurring regularly, and westward moving winds reaching speeds up to 2000 km/h (the fastest of all the planets).
Neptune is a source of both continuous emission and irregular bursts of radio waves.
Neptune has 14 moons; the most recent was discovered in 2013.
Neptune is the smallest of the gas giants and the eighth planet from the Sun. This planet is also the fourth largest in the solar system with a diameter of 49,532 km. Neptune is smaller in diameter than Uranus but it has more mass than Uranus. Although four times the size of Earth, Neptune has 17 times the mass of Earth but it is less dense than Earth.
The Solar System in Close-Up Page 20
