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Uranus
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Uranus
Uranus presented a featureless disk to Voyager 2 in 1986. Its appearance reflects the presence of a high-altitude hydrocarbon photochemical haze overlying clouds of methane, which in turn overlie clouds of hydrogen sulfide and/or ammonia (below these are additional unseen cloud decks of different compositions). The blue-green coloration results from the absorption bands of methane.
Discovery
Discovered by
William Herschel
Discovery date
March 13, 1781
Designations
Pronunciation
i/'j??r?n?s/ or i/j?'re?n?s/
Adjective
Uranian
Orbital characteristics
Epoch J2000
Aphelion
3,004,419,704 km* 20.083 305 26 AU
Perihelion
2,748,938,461 km* 18.375 518 63 AU
Semi-major axis
2,876,679,082 km* 19.229 411 95 AU
Eccentricity
0.044 405 586
Orbital period
30,799.095 days* 84.323 326 yr* 42,718 Uranus solar days
Synodic period
369.66 days
Average orbital speed
6.81 km/s
Mean anomaly
142.955 717°
Inclination
0.772 556° to Ecliptic
6.48° to Sun's equator
1.02° to Invariable plane
Longitude of ascending node
73.989 821°
Argument of perihelion
96.541 318°
Satellites
27
Physical characteristics
Equatorial radius
25,559 ± 4 km
4.007 Earths
Polar radius
24,973 ± 20 km
3.929 Earths
Flattening
0.022 9 ± 0.000 8
Circumference
159,354.1 km
Surface area
8.115 6×109 km2
15.91 Earths
Volume
6.833×1013 km3
63.086 Earths
Mass
(8.6810 ± 0.0013)×1025 kg
14.536 Earths
GM=5 793 939 ± 13 km3/s2
Mean density
1.27 g/cm3
Equatorial surface gravity
8.69 m/s2
0.886 g
Escape velocity
21.3 km/s
Sidereal rotation period
0.718 33 day (Retrograde)
17 h 14 min 24 s
Equatorial rotation velocity
2.59 km/s
9,320 km/h
Axial tilt
97.77°
North pole right ascension
17 h 9 min 15 s
257.311°
North pole declination
-15.175°
Albedo
0.300 (Bond)
0.51 (geom.)
Surface temp.
min
mean
max
1 bar level
76 K
0.1 bar
(tropopause)
49 K
53 K
57 K
Apparent magnitude
5.9 to 5.32
Angular diameter
3.3"–4.1"
Atmosphere
Scale height
27.7 km
Composition
(Below 1.3 bar)
83 ± 3%
hydrogen (H2)
15 ± 3%
helium (He)
2.3%
methane (CH4)
0.009%
(0.007–0.015%)
hydrogen deuteride (HD)
Ices:* ammonia (NH3)* water (H2O)* ammonium hydrosulfide (NH4SH)* methane ice (?) (CH4•5.75H2O)
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Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both are of different chemical composition than the larger gas giants Jupiter and Saturn. For this reason, astronomers sometimes place them in a separate category called "ice giants". Uranus's atmosphere, although similar to Jupiter's and Saturn's in its primary composition of hydrogen and helium, contains more "ices" such as water, ammonia, and methane, along with traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (-224 °C). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds. In contrast, the interior of Uranus is mainly composed of ices and rock.Like the other gas giants, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. Its north and south poles therefore lie where most other planets have their equators. In 1986, images from Voyager 2 showed Uranus as a virtually featureless planet in visible light without the cloud bands or storms associated with the other giants. Terrestrial observers have seen signs of seasonal change and increased weather activity in recent years as Uranus approached its equinox. The wind speeds on Uranus can reach 250 meters per second (900 km/h, 560 mph).
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