The Uranus is the third largest planet in solar system and seventh planet from the Sun. The Uranus is the ice giant which has no solid surface. It has coldest atmosphere in solar system. It has unique axis of rotation which is tilted by sideways.
Properties of Uranus
- The Uranus was discovered by William Herschel in 13th march 1781.
- The Uranus has blue-green colour due to presence of methane in atmosphere.
- It is the ice giant with 13 rings.
- The Uranus is the coldest planet in solar system having temperature -224oC.
- It was named after ancient Greek deity of sky Uranus.
- Equatorial radius of the Uranus is 25,559 km which is four times that of the Earth.
- It has rocky core, ice mantle and outer gaseous layer.
- The mass of the Uranus is 8.68 X 1025kg (Fourteen times of Earth) and mean density is 1.27 g/cm3.
- Solid diamond rainfall is occurred on Uranus.
- Orbital speed is 6.80 km/s.
- The Uranus orbits around the sun in 84 years while spins on its axis in 17hours and 14 minutes.
- The distance of Uranus from the sun is 2.871 billion km.
- The axial tilt of Uranus is 97.77o.
- Maximum speed of wind on Uranus is 860 km/hr.
- The Uranus has 27 moons.
- The magnetic field of Uranus is tilted by 59o inclined to axis of rotation.
The Uranus was formed when rest of solar system was formed from solar nebula, a ball of gas and dust by gravitational collapse. At the time of formation Uranus was near to the Sun but after that it was moved to outer solar system.
The Uranus is made of ices of water, ammonia and methane. The mass of ice in Uranus may be 9.3 to 13.5 earth’s mass. Among them about 0.5 to 1.5 of Earth’s masses is hydrogen and helium while rest of it is rocky material.
The internal parts of the Uranus consist of three layers as rocky core, icy mantle and gaseous envelope. The Core of Uranus is made of silicate/iron nickel and about 20% of Uranus’s total radius with mass of 0.55 Earth’s masses. Density of core is 9 gm/cm3 and 8 million bar pressure in centre with temperature of about 4700oC. The Mantle is icy with around Earth’s masses. The mantle is composed of hot and dense fluid of water, ammonia and other volatiles which is known as water-ammonia Ocean. The outermost layer is of hydrogen/helium and about Earth’s masses. Extreme temperature and pressure of Uranus break up methane molecules and condense the carbon into diamond crystals. Solid diamonds rainfall is done on Uranus. Internal heat of planet is lower than other gas giant.
Orbit and Rotation
The Uranus completes one revolution around the Sun in 84 years. The average distance of Uranus from the sun is 2,870,972,200 km. The orbit of Uranus around the Sun is elliptical, so the farthest distance (Aphelion) from the sun is 3,006,390,000 km while closest distance (Perihelion) is 2,735,560,000 km. In 2033, The Uranus will complete its third revolution around the sun since being discovered in 1781.
The Uranus spins once around its axis in17hours and14 minutes. The rotational axis of Uranus is approximately parallel to the orbital plane around the sun. Axial tilt of Uranus is about 97.77o and due to this tilt, seasonal changes are occurred on the planet. One pole is continuously toward the sun while other pole is in darkness. Each pole gets constant sun light for average 42 years and other darkness for same time. Near the equator small strip faces rapid day-light cycle. Although due to axial inclination, poles are getting more energy from the Sun than on equator, the equator is hotter than the pole which is unknown mystery. The axial inclination may be the result of collision of earth sized proto planet to Uranus before 3 to 4 billion years ago.
The outermost portion of gaseous envelope of Uranus is called atmosphere. The atmosphere of the Uranus is composed of mainly molecules of hydrogen and helium and also methane. The aquamarine colour of Uranus is because of presence of methane in atmosphere. Along with methane trace amount of water, ammonia, hydrogen sulphide, ethane, acetylene, methylacetylene, diacetylene are also present in atmosphere of Uranus.
The atmosphere of Uranus is divided into three layers Troposphere, Stratosphere and Thermosphere.
Troposphere is the lowest layer of planet from -300 km to 50km. It is the densest layer in which temperature is decreased with attitude. The pressure is ranging from 100 bar to 1 bar with temperature is 47oC to -220oC. Strong winds, bright clouds and seasonal changes are experienced in troposphere. In troposphere various types of clouds like water clouds, ammonium hydrosulfide clouds, ammonia or hydrogen sulphide clouds and thin methane clouds are present at various pressure.
Stratosphere lies between 50 km to 4000 km where temperature is gradually increased from -220o C to 577o C. The temperature of stratosphere is caused by absorption of UV and IR radiation by methane and hydrocarbons and also heat conduction from hot thermosphere. Heavier hydrocarbons are lies between 160 km to 320 km with pressure 10 to 0.1 mbar and temperature is -173o C to -143o C. Stratosphere contains hydrocarbons like acetylene, ethane, methylacetylene, diacetylene and also carbon monoxide, water vapour and carbon dioxide.
Thermosphere is extended from 4000 km to 50000 km with constant temperature 527o C to 577o C. Why such temperature is sustained is not understood. The thermosphere and upper layer of stratosphere is composed of large number of ions and electrons which is called ionosphere which extends from 1000 km to 10000 km. Thermosphere is the source of far ultraviolet emissions which is called Electroglow or Dayglow.
The southern hemisphere is divided into two regions a bright polar cap and dark equatorial bands which are separated by – 45o latitude. Band between -45o to -50o latitude is brightest portion which is called Southern Collar. Zonal winds are blowing in upper troposphere. At the equator, the winds blow in reverse direction of planet rotation which is called Retrograde. The wind speed increases with distance from equator and reaches to zero at -20o latitude. Near the poles the winds are Prograde means blows in direction of rotation of planet. Wind speed at -40o latitude range from 540 km/hr to 720 km/hr. In northern hemisphere maximum wind speed 860 km/hr has observed.
The magnetic field of Uranus is abnormal because it is not generated from its geometric centre and also it is tilted by 59o from axis of rotation. Magnetic dipole is shifted from its centre of rotation by one third of radius of Uranus. The average field density on surface is 0.23 gauss while on North and South poles the density is 1.1 gauss and 0.1 gauss respectively. In ice giants, the magnetosphere is generated by motion of water-ammonia oceans. Protons and electrons are mainly charged particles of Uranus’ magnetosphere. Uranus has bright auroras which can be seen on both the poles.
Moons of Uranus
The Uranus has 27 moons which are named from the characters used in works of William Shakespeare and Alexander Pope. The Uranus’ moons are divided into three groups as thirteen inner moons, five major moons and nine irregular moons. The Uranian satellite system has combined mass of five major satellite is less than half that of Triton. Triton is the seventh largest moon in solar system.
The Uranus has 13 inner moons which are dark objects made from ice water with dark material. All inner moons are part of rings of Uranus. Cordelia and Ophelia are source of Uranus’ ε Ring while Mab, the small moon is the source of μ ring. Puck is the largest of inner moons. Puck and Mab are two outermost satellites of inner moons.
Miranda, Ariel, Umbriel, Titania and Oberon are the five major moons of Uranus. Major moons are made of dark objects. The Umbriel is the darkest moon while the Ariel is the brightest moon. The major moons range from 472 km in diameter (Miranda) to 1578 km in diameter (Titania). All major moons are composed of rock and ice except Miranda. The ice components are mainly ammonia and carbon dioxide.
Irregular moons were formed at the time of formation of Uranus. Irregular moons of Uranus are range from 120-200km (Sycorex) to 20 km (Trinculo) in size. Franscisco, Caliban, Stephano and Trinculo are the inner group moons having moderate eccentricity while Sycorex, Prospero, Setebos and Ferdinand are the outer group moons having high eccentricity. The Margaret is the irregular moon have highest orbital eccentricity about 0.80 which is largest to any other moons in entire solar system.
Rings of Uranus
The ring system around the Uranus was first observed by William Herschel in 1789. The rings are made of dark particles which are vary from micrometer to fraction of meter. The ring system was originated by collision of moons which were orbited around the Uranus before 600 million years ago. Total 13 rings of Uranus are discovered till date which are 1986U2R/ζ, 6, 5, 4, α, β, η, γ, δ, λ, ε, ν and μ. The inner and outer ring lies at the radius of 38000 km to 98000 km respectively. The rings are made of water ice and other radiation processed organics and dust and also contains large bodies of 20cm to 20 m in diameter.
The ring system is divided into three groups.
- Narrow main rings – Nine rings
- Two dusty rings – Two rings
- Two outer rings – Two rings
Narrow Main Rings :
The ε Ring is most eccentric and very less orbital inclined ring of Uranus’ ring system. It is the brightest and densest ring which generates about two-third of light reflected from whole ring system. This ring has highest brightness at apoapsis and lowest at periapsis. ε Ring is very thin about 150m but it has several layers of particles.
The δ Ring has main two part , one is narrow optically dense components and second is broad inward shoulder with low optical depth. Narrow components have width about 4.1-6.1 km while broad components have 10-12 km.
The γ Ring has orbital inclination is almost zero which is narrow, slight eccentric and dense. The width of the ring varies from 3.6 km to 4.7 km.
Like the γ ring, the η Ring has also zero orbital inclination and eccentricity. It has main two part, one is narrow optically dense components and second is broad inward shoulder with low optical depth. The narrow component’s width is 1.9 km to 2.7 km and depth is 0.42 km. The broad component’s width is 40 km and depth is 0.85 km.
α and β rings
α and β rings are brightest rings after ε Ring which have regular variation in brightness and width. Width of the rings is 4.8-10 km and 6.1-11.4 km respectively while optical depth is 0.3-0.7 km and 0.2-0.35 km respectively. These rings are brightest and widest at 30o from apoapsis and dimmest and narrow at 30o from periapsis.
Rings 6, 5 and 4
Rings 6, 5 and 4 are most inclined and narrowest rings of the Uranus. They are innermost and dimmest rings. The width of the rings is 1.6 to 2.2 km,1.9 to 4.9 km and 2.4 to 4.4 km respectively.
Dusty Rings :
The λ Ring is located inside the ε ring which is narrow about 1 km to 2 km and faint having optical depth is 0.1 km to 0.2 km. the ring composed of micrometer sized dust.
The 1986U2R/ζ ring is located between 37000km to 39500 km from the centre of Uranus and 1200 km upper the clouds.
Outer ring system :
The outer most rings are known as μ and ν rings. The μ ring is outer most from two rings and as bright as the η ring. The outer rings are broad than inner rings and about 17000km and 3800km wide respectively. Both the rings are much brighter and composed of micrometer sized dust which is made of ice particles. The μ ring is blue in colour while V ring is red in colour.
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Authored by BK Suthar.
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