Uranus and Neptune Uranus and Neptune are much smaller than Jupiter and Saturn, but still giants compared to Earth Both are worlds we know relatively little about Voyager 2 is the only spacecraft to visit these planets Uranus and Neptune Properties Semimajor axis Eccentricity Orbital Period Rotational Period Mass Radius Average Density Surface gravity Cloud Top Temp Atmosphere Uranus 19.2 A.U. 0.047 83.75 years -0.72 days 8.68 x 10 25 kg (14.5 x Earth) 25,559 km (4.0 x Earth) 1.27 g/cm 3 (0.23 x Earth) 8.87 m/s 2 (0.91 x Earth) 58 K hydrogen, helium Neptune 30.1 A.U. 0.009 163.7 years 0.67 days 1.02 x 10 26 kg (17.2 x Earth) 24,766 km (3.88 x Earth) 1.64 g/cm 3 (0.30 x Earth) 11.1 m/s 2 (1.14 x Earth) 59 K hydrogen, helium Discovery of Uranus Unlike the other planet's we've discussed, Uranus is not easily visible in the night sky It's brightness is barely detectable by the human eye, and it was discovered by telescope William Herschel noticed its motion against the background stars in 1781 Image of Uranus from Earth (arrows indicate moons) 1
Discovery of Neptune Neptune's further distance makes it impossible to see with the naked eye After watching the orbit of Uranus, astronomers noticed the planet was being pulled by some unseen body After much searching, Neptune was finally found in 1846 Image of Neptune from Earth (arrows indicate moons) Visiting Uranus and Neptune Uranus For each of these planets, it provided the first detailed images of the planets and their moons While Uranus was relatively featureless, Neptune showed cloud bands and storms much like Jupiter Neptune Voyager 2 continues to travel out of the Solar System and is now twice the distance from the Sun as Pluto Despite their smaller size, Uranus and Neptune share many characteristic with Jupiter and Saturn The Jovian Family The slightly higher densities of Uranus and Neptune reveal that their rocky cores probably make up a significant portion of the planet 2
Uranus - A Planet on Its Side Uranus is the only planet in the Solar System to have a rotational axis almost perpendicular to its revolution axis Astronomer can't definitively say what caused this tilt, but it may have been a catastrophic collision early in Neptune's formation Atmospheres of Uranus and Neptune Like all the Jovian planets, both Uranus and Neptune's atmospheres are mainly hydrogen (84%) and helium (13-14%) The main difference in color between the two worlds results from different concentrations of methane _ _ Methane is very good at absorbing red and yellow light, leaving only the blues and greens to reflect off the cloud tops Weather on Uranus While no major activity is visible to the naked eye, computer enhanced images do show some storms in Uranus' atmosphere The last image is in infrared, which shows much more details in the cloud bands and Uranus' rings Uranus' colder temperatures keep the majority of the weather at much deeper layers which are hidden under a layer of haze 3
Weather on Neptune Neptune's weather is much more prominent and active, despite its greater distance from the Sun Unlike Uranus, Neptune does appear to have an internal heat source, though its origins are unknown _ Voyager 2 witnessed a large storm, named the Great Dark Spot, but this storm has since disappeared Hubble images revealed the weather systems are still constantly active in the atmosphere Magnetospheres for Uranus and Neptune The size of the bars indicates strength of the magnetic field The location of the bars shows the origin of the magnetic field Notice that the magnetic axis is not always aligned with the rotation axis Uranus and Neptune Interiors The fact that both Uranus and Neptune's magnetic fields are strange suggests that their interiors are much different Unlike Jupiter and Saturn, pressure and temperatures outside the cores are not high enough to change hydrogen into the liquid metallic state Current theories suggest that Uranus and Neptune have a large slushy layer of water and water ice 4
The Moons of Uranus Following the trend for gas giants, Uranus has many moons, the number continually increasing The five largest, are all much smaller than our Moon and have diameters around 1000 kilometers Moons of Uranus All of the moons are quite dark in color, suggesting a dirtier, older surface Recent theories suggest that ultraviolet sunlight and high energy particles cause radiation darkening on these outer worlds The high energy photons break up material on the surface and react to form a dark hydrocarbon soot Umbriel Ariel Miranda takes the prize for weirdest moon of Uranus Miranda Voyager 2 images of Miranda showed a very dramatic surface of varying terrain A large impact may have severely fractured the moon into several pieces which later reformed 5
Moons of Neptune Like Uranus, Neptune has many moons, though the number is smaller (around a dozen) Most of these moons are small (10's of kilometers) Proteus Triton Triton is the largest moon at 2710 km and a mass that is 0.29 times the mass of Earth's Moon Triton Geysers of nitrogen gas were seen erupting from the surface and help maintain a very thin atmosphere of nitrogen There is also evidence of past heating which created pools of frozen water on the surface Triton is the only large moon in the Solar System which has a retrograde orbit around its planet The Fate of Triton Remember that tidal forces between the Earth and Moon gradually 'push' the Moon away The opposite situation happens with Triton, and it will eventually drift inside the Roche limit of Neptune 6
The Rings of Uranus They were discovered in 1977 by a technique called stellar occultation Astronomers measure the light from a background star as Neptune passes in front of it Dips in the light correspond to the star being blocked by Uranus' rings The Rings of Uranus The naming scheme for Uranus' rings is a mess, with a combination of Greek letters and numbers All the rings are much more narrow and confined than Saturn's The average size of particles is also much larger, with few chunks less than a centimeter in size Ophelia Cordelia The Rings of Neptune Neptune also displays a thin, dim ring system Three of the rings are relatively narrow, while two other are much wider in size The outermost ring also appears 'clumpy', probably from interactions with moons 7