Chapter 13: Uranus, Neptune, and Pluto The Outer Worlds of the Solar System

1. Uranus Neptune Pluto and Charon Chapter 13: Uranus, Neptune, and PlutoThe Outer Worlds of the Solar System • Discovery • Comparisons • Physical Properties • Moons and Rings

2. Discovery • The three outermost planets were unknown to the ancients. • All were discovered by telescopic observations: • Uranus in 1781 • Neptune in 1846 • Pluto in 1930

3. Comparison of the Jovian Planets

4. Uranus

5. Seeing Uranus • Uranus is just barely visible to the naked eye. • The light coming from Uranus during a time span of 200 years would be equivalent to the light shining from a flashlight in just 1 second! • Uranus moves so slowly against the background stars, that it went unnoticed as a planet until Herschel's discovery in 1781.

6. William Herschel • Called the “greatest observational astronomer ever” and the “Father of Stellar Astronomy.” • Musician by training; earned his living for a great part of his life teaching, performing, and composing music. • Built his own 6” reflector. • Discovered the planet Uranus. • With help of his sister Caroline, completed first through study of stars and nebulae; discovered existence of double stars; first to correctly describe the form of our Galaxy, The Milky Way.

7. Caroline Herschel • Caroline was born 16th March,1750. Her brothers brought her to England as a singer, but she soon followed them into the field of astronomy, initially as William's assistant, noting measurements, grinding telescope mirrors, and carrying out calculations on astronomical data, on top of running the household. In 1782 William gave her a small telescope and she began 'comet hunting'. In her sweeps of the sky she found nebulae, new clusters and a total of eight comets. • One of the first two women elected to honorary membership in the Royal Astronomical Society.

8. Uranus: View from Earth • Discovered in 1781 by William Herschel. • Least massive of the Jovian planets • 15 x Earth’s mass (~1/20 mass of Jupiter) • 4 x Earth’s radius (~1/3 radius of Jupiter) • Small angular diameter ( ~4” at opposition) makes few features visible from Earth-based telescopes. • Discoveries from Earth-based observations: • rotation axis lies nearly in the ecliptic, • five satellites, • system of 9 rings found studying occultation of a star.

9. Uranus: View from Space • Voyager 2 arrived in 1986 and observed: • no surface markings, • little excess energy emitted from the planet, • a planetary magnetic field • ~100 x Earth’s, • tilted at 600 to rotation axis, and • offset from planet center by 1/3 of planet radius, • rotation period 17.2 hours w/ differential rotation of atmosphere • atmosphere at poles rotates faster than at equator • ten small moons inside orbit of Miranda, • all in circular, synchronous orbits in equatorial plane • many related to ring system • new views of the five known moons, • confirmed 9 rings and discovered 2 more rings.

10. Uranus • Mass: 8.68 x 1025 kg (14.5 x Earth’s) (1/20 x Jupiter’s) • Diameter: 51,118 km (4.0 x Earth’s) (0.36 x Jupiter’s) • Density: 1.27 g/cm3 • Average distance from Sun: 19.19 AU • Rotation period: -17.9 hours (retrograde) • Revolution period: 84 years • Tilt of axis: 97.9o • Orbit eccentricity: 0.047 • Atmospheric components: 83% hydrogen, 15% helium, 2% methane (at depth) • Rings: system of 11 narrow, faint rings; particles are dark, drab gray; may consist of rocky or carbonaceous material. • Magnetic field strength: 100 x Earth’s (surface strength: 0.7 x Earth’s) • Moons: 21 known

11. Orientation and Visibility • The tilt of Uranus' rotation axis of about 98o means that its rotation is retrograde. • Equatorial plane is nearly perpendicular to its orbital plane. • This causes it to have the greatest seasonal changes of any planet in the Solar System.

12. Uranus’ Unusual Seasons From N-pole summer solstice, see Sun move in circle every 17 hours; eventually begin daily cycle of setting and rising with longer nights until, at autumnal equinox,see 8.5 hour day and night; day length decreases until reach period of total darkness.

13. Atmosphere from HST True color image Color enhanced image

14. Atmosphere of Uranus • Upper atmosphere like Jupiter • 84% hydrogen • 14% helium • 2% methane • NO ammonia!! • Hazes of smog high in atmosphere. • Atmospheric temperature varies from 120 K to 55 K. • Methane clouds found low in atmosphere are only clouds observed on Uranus. • Clouds found at height where temperature is 70K, 1 bar pressure • ammonia gas condenses, falls to interior • little gaseous ammonia in atmosphere • Blue color from methane gas.

15. Atmosphere: Comparisons • Uranian atmosphere is similar to Jupiter's and Saturn's. • Composed mostly of hydrogen and helium. • Methane (CH4) is abundant in the upper clouds. • Ammonia (NH3) has frozen out of the atmosphere or perhaps dissolved in an internal water layer. • The blue-green color is caused by methane, which absorbs red light, so that light reflected from the upper layers of the atmosphere is deficient in red. • Jupiter and Saturn have other gases that overly the methane and produce other colors. • Acetylene and ethane formed in upper atmosphere • Very few cloud features are seen. • Energy to drive its cloud system must come from the Sun, because there is very little source of internal heat.

16. Clouds on Uranus HST recently found about 20 clouds - nearly as many clouds on Uranus as the previous total in the history of modern observations. Taken on August 8, 1998, with Hubble's Near Infrared Camera and Multi-Object Spectrometer.

17. Uranus: Hydrosphere • Uranus may have an extensive layer of liquid water under the cloud tops. • The motion of this super dense water might be the source for the planet's strong magnetic field.

18. Uranus: Biosphere • No life is expected either in the clouds of Uranus or any of its moons. The lack of liquid water at the necessary temperature and pressure would seem to exclude the formation of life here.

19. Interior of Uranus • H+ He atmosphere extends ~1/5 of way to planetary center. • Underlain by ocean of methane, water, ammonia. • Little difference between atmosphere and ocean. • Methane ice begins forming in atmosphere. • Continues to increase until slush is formed, and then solid ice. • The ice is warm (for Uranus) and can flow • like rocky mantle layers of Earth. • Center is a small, rocky core • 3.5 times the mass of the Earth • Probably well differentiated • but may be somewhat more mixed than either Jupiter or Saturn.

20. Compared to Jupiter and Saturn, Uranus has very little (or no) metallic hydrogen and much more ice. Core composed of heavier, rocky and metal elements. IF magnetosphere generated in metallic hydrogen dynamo, Uranus’ magnetosphere should be much smaller than Jupiter’s and aligned with rotation axis. Observed field is NOT aligned or centered. Conducting layer possibly ammonia dissolved in water/methane slush, produce observed magnetic field. Some current reports discount likelihood of ammonia source. No good theory to explain absence of excess heat. Magnetic Field & Interior Structure

21. Magnetic Field: Generation • One possible explanation for orientation of magnetic field is that Uranus had a gigantic impact while it was forming, knocking the planet on its "side" and disrupting its magnetic field. • Another idea is that magnetic field is not generated near core, but rather in large volume of liquid water surrounding the core.

22. Magnetic Fields of Jovian Planets

23. Rings from HST

24. Uranus: Rings • The rings are thinner and more widely spaced than Saturn’s. • e.g., if Uranus were the size of a golf ball, rings would be as thin as a spider web. • The ring particles are very dark and made of larger particles that Saturn's rings. • Asymmetric and partial rings have been detected. • Some shepherd moon were discovered by the Voyager spacecraft flyby mission, but most of the ring structure is still unexplained.

25. Uranus’ Rings • 9 main rings are visible (left) as horizontal lines. • Broad lanes of dust surround dark, narrow, widely-spaced main rings. • The brightest, or epsilon ring, at top is neutral in color. • Fainter 8 remaining rings show slight color differences.

26. Discovery of Uranus’ Rings Discovered in 1977 when stellaroccultation observed before and after occultation by planet: preceded discovery of rings for Jupiter (1979) and Neptune (1989).

27. Composition of Rings • Low reflectivity and lack of color suggest that Uranus’ rings may be mostly carbon. • Possible origin: • originally methane ice that decomposed to carbon when exposed to energetic particles in the magnetosphere, • rings formed from carbon-rich asteroid that broke up under Uranus’ tidal influence

28. Epsilon-ring and it’s Shepherd Moons Designated 1986U7 (Cordelia) and 1986U8 (Ophelia), the two shepherd moons seen here on either side of the bright epsilon ring confine it by their gravitational effects.

29. Rings and Moons from HST

30. The Moons of Uranus • 21 known moons • Divided into 3 classes • 11 very small, dark inner • 5 large • 4 small, distant • Most in nearly circular orbits about equator; outer 4 more elliptical. • Named for characters from Shakespeare, Pope.

31. Moons of Uranus Distance Radius Mass Satellite(x1000 km)(km)(kg) Cordelia 50 13 ? Ophelia 54 16 ? Bianca 59 22 ? Cressida 62 33 ? Desdemona 63 29 ? Juliet 64 42 ? Portia 66 55 ? Rosalind 70 27 ? Belinda 75 34 ? 1986U10 75 20 ? Puck 86 77 ? Miranda 130 236 6.30e19 Ariel 191 579 1.27e21 Umbriel 266 585 1.27e21 Titania 436 789 3.49e21 Oberon 583 761 3.03e21 Caliban 7100 30 ? 1999U1 10000 20 ? Sycorax 12200 60 ? 1999U2 25000 20 ?

32. The “Large” Moons of Uranus • The “large” moons of Uranus are actually moderate to small sized, icy objects. • They tend to show a surprising amount of geologic activity. • Seem to be more complex the closer they are to the planet, suggesting that tidal forces may be the cause of this activity. • They are generally quite similar to the small, icy moons of Saturn.

33. 5 Largest Moons of Uranus • Miranda Ariel Umbriel Titania Oberon • Believed similar to Saturn’s mid-sized moons, but with differences: • slightly higher density • darker • relatively further from Saturn • show more evidence of internal activity • Composed of 40-50% water ice, the remainder rocky.

34. Miranda • Innermost of large satellites. • Some of the most complex terrain of any moon or planet in the Solar System. • Surface composed mostly of rolling cratered plains. • Half of Miranda's surface is younger based on fewer number of craters. • consists of complex sets of parallel and intersecting scarps and ridges. • bright V-shaped feature in grooved area is Inverness Corona, nicknamed the "Chevron". • 5 km cliff on 485 km diameter satellite. • Huge, jagged canyon on right limb is in direction of Uranus.

35. Ariel • Complex terrain captured in this view of Ariel’s southern hemisphere. • Most of surface is intensely cratered terrain, transected by fault scarps and graben. • Some of largest graben (seen near the terminator) are partly filled with younger deposits and less heavily cratered. • Bright spots (near the limb and toward the left) are crater rims.

36. Umbriel • Umbriel is an enigma with an observed surface albedo of only 10-15% . • Uranus’ satellites found inside and outside Umbriel's orbit are much brighter. • The process by which Umbriel's ancient cratered surface was darkened, leaving only a few bright icy white rings, remains a mystery.

37. Titania • Obvious abundance of impact craters and prominent global tectonic features. • Large fault valleys, 1500 km (930 mi) long, 75 km (47 mi) wide, run approximately perpendicular to ecliptic plane. • Surface has recorded many types of geologic activity. • numerous impact scars, • large, trench-like feature near the terminator at middle right suggests at least one episode of tectonic activity.

38. Oberon • Several large impact craters, surrounded by bright rays, are visible. • Near the center of disk is a large crater with bright central peak and a floor partially covered with black material. • May be icy, carbon-rich material that spilled onto crater floor sometime after crater formed. • Large mountain, ~6 km high, peeking out on lower left limb.

39. Neptune

40. Comparison of the Jovian Planets

41. Neptune: View from Earth • Discovered in 1845/1846 • Adams and Leverrier independently applied perturbation theory to orbit of Uranus to predict location. • Too dim to be visible to the naked eye. Small angular size allows little detail from Earth. • Before the Voyager mission, Neptune thought to be very similar to Uranus, nearly featureless. But Neptune shows light and dark spots, white clouds, and a distinctly banded appearance. • Smallest size and largest density of jovian planets. • 17 x Earth’s mass • 3.9 x Earth’s radius • 1.64 gm/cm3 • Two satellites known from Earth-based observations • Triton and Neried • Hints of ring system from Earth-based observations.

42. Neptune: View from Space • Voyager 2 revealed atmospheric details: • various sizes of storms and cloud systems • high-level clouds moved rapidly • layered cloud structure detected • emits more excess energy than any other jovian planet • Magnetic field tilted at 47o to rotation axis and offset by half the planet radius. • Low mass indicates lack of a metallic hydrogen layer. • Rings are thin and dark; material not uniformly distributed. • Six new satellites discovered. • Images of large moon Triton revealed complex surface structure, possible ice geysers, and thin nitrogen atmosphere.

43. Neptune Vital Statistics • Mass: 1.02 x 1026 kg (17.1 x Earth’s) • Diameter: 49,528 km (3.9 x Earth’s) • Density: 1.64 g/cm3 • Ave. distance from Sun: 30.06 AU • Rotation period: 16.1 hours • Revolution period: 164.8 years • Tilt of axis: 29.6o • Orbit inclination: 1.77o • Orbit eccentricity: 0.009 • Mean temperature: 48K • Atmospheric components: 74% hydrogen 25% helium 1% methane (at depth) • Rings: narrow, dark, contain concentrations of particles called ring arcs. • Moons: 8, but they do not form regular system. • Magnetic field: ~100 x Earth’s, 46o tilt to rotational axis, displaced 0.55 radii, reverse polarity to Earth’s.

44. Neptune: Interior • Like the other gas giant planets, Neptune is dominated by the volatile elements, predominantly hydrogen and helium. • Descending from the cloud tops toward the core, the gases gradually change to liquid and then solid. • There is probably a substantial amount of liquid water at some level. • Neptune's dynamic weather patterns are probably caused by its great amount of internal heat. • Neptune emits more excess energy relative to its mass than any other planet: 2.7 times more energy into space than it receives from the Sun.

45. Neptune’s Interior • Smaller than Saturn, but more massive. • Voyager 2 data indicates water is a major constituent. • Presumed to contain • small, rocky core • with icy mantle topped by • deep layer of liquid hydrogen. • Pressure outside core too low to force hydrogen into metallic state, so hydrogen stays in molecular form.

46. Neptune: Magnetic Field • Neptune’s magnetic field is similar to Uranus’: • tilted to the rotation axis (by 55o) • offset from planet center • same strength • The irregularities in the magnetosphere of Uranus and Neptune are one of the outstanding questions left by the Voyager encounters with the outer planets.

47. Neptune’s Aurora • Neptune’s aurora is centered around the planet’s magnetic pole. • Type-B aurora has also been observed. • No radio signals have been detected which relate to aurora or auroral processes. • Scientists are still working to understand the currents which cause aurora.

48. Neptune: Atmosphere • Hydrogen and helium dominant gases. • Blue appearance (like Uranus) due to methane gas in upper atmosphere. • Upper level clouds are methane. • Thin, high white clouds are also of methane. • Look like Earth’s cirrus clouds. • Believed to have been carried 75 km higher in atmosphere by convection currents carrying excess heat from interior. • Belts, zones, storms similar to Jupiter & Saturn. • Highest E-W wind speeds at equator ( 2100 km/hr). • Twice speed of peak winds on Saturn .

49. Neptune: Hydrosphere • Like Uranus, Neptune probably has a large amount of liquid water in a layer beneath the upper atmosphere and clouds. This may be the source of Neptune's off-centered magnetic field.

50. Neptune: Biosphere • None is expected in the atmosphere of Neptune or the surface of Triton. • It is either too cold or, in the case of Neptune's atmosphere, the water is at too high a pressure.