Uranus

1. Uranus Roman God Father of Saturn The Bland Planet William Hershel “discovered” tried to name after King George III Moons named after Shakespeare’s plays & from Pope’s Rape of the Lock

2. General Information • Seventh planet from the Sun • 3rd largest in SS • Larger but less dense than Neptune • Discovered in 1781 even though it had been observed previously and thought to be a fixed star • Unique in our SS for it’s almost 90 degree spin axis • Displaced magnetic field • As of July 2004 – 22 named and 5 un-named moons • 11 dark rings • Emits a significant amount of heat

3. General InformationMagnetic Field • Distinctive magnetic field • Weak by Earth standards • Magnetic poles are tilted ~50 degrees relative to rotational axis • Center of field is displaced from center of planet ~10,000km toward south pole • Caught in a magnetic polar reversal?

4. Planetary Statistics • Mean distance from Sun: 2.87 billion km • 19.2 A.U. • Orbital Period = 84.1 Earth yrs • Rotational Period = 17.2 hours • Refined by Voyager measurements • Axis Tilt = 97.8 degrees (23.5) • Debate about which is actually N/S pole • Prograde vs retrograde rotation • Diameter = 51,118km (12,756) • Mass = 8.686 x 1025 (5.97 x 1024) • Density = 1.29g/cm3 (5.515gm/cm3)

5. Planetary Statistics (cont’d) • Equatorial surface gravity = 7.77m/sec2 (9.78m/sec2) • Visual albedo = 0.51 (.37) • Moons = 22 • 10 of which were found by Voyager 2 • Atmospheric composition • Hydrogen 83% • Helium 15% • Methane 2% (at depth) • Mean surface temperature = -193C • Rings • 11 major divisions or rings • Very dark in color • Large particle size (ice boulders) • Thin distribution of dust throughout system

6. Internal Composition • Our knowledge of the internal structure of Uranus is inferred from the planet's radius, mass, period of rotation, the shape of its gravitational field and the behavior of hydrogen, helium, and water at high pressure • Its internal structure is similar to that of Neptune except for the fact that it is less active in terms of atmospheric dynamics and interior heat flow • Uranus is composed of an outer envelope of molecular hydrogen, helium and methane • Below this region Uranus appears to be composed of a mantle rich in water, methane, ammonia, and other elements • These elements are under high temperatures and pressures deep within the planet • Uranus's core is composed of rock and ice

7. Uranus’ Atmosphere • Atmospheric segregation is invisible w/out dramatic image enhancements • Blue-green color results from methane • At time of Voyager flyby its S pole was pointing at Sun • Polar areas receive more direct sunlight than equatorial zones • Even so equatorial zones are still hotter than poles • Unknown why • Lacks metallic H zone like J & S • Not enough mass • Latitudinal circulation in spite of severe axial tilt (700kph) • Interesting result of how axial tilt often doesn’t significantly alter general atmospheric patterns (Mars)

8. Uranus’ Internal Makeup • Half the total mass is water ice • <50% is silicate rock • ~10% is H, He and other gases • Possible that ice and rock are segregated into discrete layers • Small magnetic filed • 10x’s smaller than Earths • Tilted 60 degrees to axis of rotation • Offset from planet’s center • From convective movement of water • Possibly in magnetic reversal?

9. Uranus Orientation Today • Referenced by Northern Hemisphere it is early Spring on Uranus • 2006 – Vernal Equinox • 2027 – Summer Solstice reached with N Pole facing Sun • Each Pole receives ~42yrs of sunlight and darkness

10. Telescopic images made using near-infrared camera & Keck Adaptive Optics system • High, white cloud features mostly in the northern (right hand) hemisphere, with medium level cloud bands in green and lower level clouds in blue • The severe tilt of its rotational axis, seasons on Uranus are extreme and last nearly 21 Earth yrs Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory

11. Keck Telescope IR Images, Hawaii • More recent infrared images of Uranus • Note complex weather patterns in clouds at North Pole (right) • Strange scalloped cloud band along equatoral • Smaller, white storm systems appear and disappear at varying latitudes • Infrared wavelengths provide greater detail or upper atmospheric structure

12. Uranus’ Ring System • Detected from Earth-based telescopes in 1977 • Particles are some of the darkest material in the SS • Narrow in extent and thin • Limited by shepherd satellites • Large diameter particles (boulder sized) • Possibly rich in carbon residues made as methane breaks down after exposure to solar radiation • Possibly organic compounds and ice • Rings are equatorial • Formed after axial tilting

13. Uranus’ Ring System • This false-color view of the rings of Uranus was made from images taken by Voyager 2 • Nine known rings are visible here; the somewhat fainter, pastel lines seen between them are contributed by the computer enhancement • The image shows that the brightest, or epsilon, ring at top is neutral in color, with the fainter eight other rings showing color differences between them

14. Shepherd Satellites • Voyager 2 discovered two 'shepherd' satellites associated with the rings of Uranus • The two moons -- designated 1986U7 and 1986U8 -- are seen here on either side of the bright epsilon ring • Nine of the known Uranian rings are visible • Based on likely surface brightness properties, the moons are of roughly 2O- and 3O-km diameter, respectively

15. Uranus’ Satellites • Five major satellites discovered from Earth-based observations • Miranda, Ariel, Umbriel, Titania, and Oberon • Tidally locked orbits • Formed from circum-Uranus nebula after tilting • No atmospheres • Water ice but no methane or ammonia detected at surfaces • Mixtures of low density ice and denser silicates and perhaps iron • Denser than Saturn’s satellites

16. Uranus’ Satellites • Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope • Three distinct classes • 11 small very dark inner ones discovered by Voyager 2 • 5 large ones • Newly discovered much more distant ones • Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic) • Outer 4 are much more elliptical

17. Miranda • Inner most satellite and small in diameter • <470km • High resolution images from Voyager 2 • Striking features not expected in such a small icy satellite • Barely enough gravitational pull to create a sphere • Two very different terrains • Heavily cratered surface • Old surface (4 by) • Similar to lunar highlands • Coronae • Ovoid complexes • Light/dark banding • Topographic structural relief

18. Miranda • Coronae • Unique features in our SS • Sharply fault-bounded • Graben-like structures ring outer extent • Relief is quite large (km’s) • Formed by upwelling ice bands interspersed with denser silicate bands that were sinking • Few craters • Developed after early bombardment period

19. Miranda • Chevron Structure • Strange angular V-shaped structure • Racetrack lineations similar to Ganymede surround the core of the coronae • Many sharp boundaries and faults cutting older terrain • Grooves and ridges • Tensional/compressional features

20. Miranda – Verona Rupes • Fault System • Older and younger than coronae complexes • Long lateral extent • Dramatic and sheer relief (5-7km offsets) • Deeper than Valles Marineris • Due to? • Global expansion • Large impacts • Torque caused by rotation of Uranus off vertical • Verona Rupes shows grooves probably made by the contact of the fault blocks as they slid past one another (slickensides)

21. Ariel • Much of surface is densely pitted with craters 5 to 1Okms across • Valleys and fault scarps crisscross the highly pitted terrain • Valleys may have formed in down-dropped fault blocks (grabens) • Apparently, extensive faulting has occurred as a result of expansion and stretching of crust • Largest fault valleys, as well as a smooth region near the center of this image, have been partly filled with deposits that are younger and less heavily cratered than the pitted terrain

22. Voyager 1 & 2

23. Voyager 1 & 2