Uranus and Neptune Uranus: The god of the heavens—The son and husband of Gaia—he hated his sons which angered Gaia. She fashioned a sickle which Saturn, his son, used to castrate him. Neptune: The god of the sea—brother of Jupiter and Pluto
The Solar System CHAPTER 14 Uranus, Neptune, Pluto & TNO’s
What We Learn About Uranus, Neptune, Pluto & TNO’s • How Uranus and Neptune were discovered • The unusual properties of the axis, orbit and atmosphere of Uranus • What gives Neptune’s clouds and atmosphere their distinctive appearance • The internal structures of Uranus and Neptune • The unique orientations of the magnetic fields of Uranus and Neptune • Why the rings of Uranus and Neptune are hard to see
What We’ve Learned About Uranus, Neptune, Pluto & TNO’s • What could have powered geologic activity on Uranus’s moderate-sized moons • Why Neptune’s satellite Triton is destined to be torn apart • How Pluto came to be discovered • What shapes the orbits of Pluto and the thousands of other objects that orbit beyond Neptune • What TNO’s are and why Pluto, Charon, and Triton are part of this population.
Uranus—Voyager 2, 1986 False Color True Color South Pole
Hubble Image 2004 Ultraviolet, Visible, infrared composite Springtime on Uranus
Uranus’ Structure Atmosphere: H2 , He, CH4 Outer Mantle: Liquid H2, He ≈ 4 ME Inner Mantle: Liquid H2O, NH3, CH4 ≈ 11 ME Core: Fe, Ni and Mg silicates ≈ 1 ME
Uranus Rings • Rings are thin and dark • Typically 0.1–10 m size • larger particles than Saturn’s rings • Methane ice—undergoes radiation darkening
Hubble Image of Rings • Short exposure to reveal planet and inner rings • Long exposure to reveal outer rings
How were the rings of Uranus discovered? By Galileo who originally saw them but thought the looked like ‘ears’ sticking out on the sides of Uranus, which he mistakenly thought were moons. By a Cornell astronomer who hoped to measure the size of Uranus during an anticipated stellar occultation in 1977, when he surprisingly observed that light from the target star blinked on and off before and after it was actually occulted by Uranus. By Voyager I, when its electronics failed as it passed through Uranus’ rings. By Voyager II, during its 1986 flyby.
Surface of Miranda Miranda initially in resonance orbit with Umbriel Tidal heating melted Miranda causing differentiation Differentiation failed to complete when tidal heating ceased
Neptune Data • Discovered by Johann Gotfried Galle Sept 23, 1846 • Mass (Earth = 1) 17.135 • Equatorial radius (Earth = 1) 3.8799 • Mean density (gm/cm 3)1.64 • Mean distance from the Sun (Earth = 1) 30.0611 • Rotational period (hours) 16.11 • Orbital period (years) 164.79 • Orbital eccentricity 0.0097 • Tilt of axis (degrees)29.56 • Orbital inclination (degrees) 1.774 • Equatorial surface gravity (m/sec 2) 11.0 • Albedo 0.41 (Magnitude 7.84) • Mean cloud temperature -193 to -153°C • Atmospheric pressure (bars) 1-3 • Atmospheric composition Hydrogen 85%, Helium 13%, Methane 2%
When the English student, John Couch Adams, and the French astronomer, Urbain Le Verrier, independently predicted the existence of Neptune on the basis of measured irregularities of Uranus’ orbital motion, what factor ultimately led to Neptune’s discovery by Johann Galle at Berlin Observatory rather than by James Challis at Cambridge, England, even though Challis was told of the prediction much earlier than Galle? better eyesight Galle used photography, while Challis had to rely on eye observation The Berlin Observatory had better star charts and Galle took LeVerrier seriously while Challis and Sir George Biddell Airy failed to take Adams seriously less cloud cover
Neptune Structure Atmosphere: H2 , He, CH4 ≈ 2 ME Mantle: Liquid H2O, NH3, CH4 ≈ 14 ME Core: Fe, Ni and Mg silicates, ≈ 1 ME
Triton and Neptune • Triton in retrograde orbit • Orbit inclined 23o from Neptune’s equatorial plane • How does satellite co-form this way? • Diameter 2706 km • Largest captured satellite
Triton and Neptune Absence of large craters—why?
Triton Volcanic Surface Frozen ‘lakes’—calderas of extinct volcanoes? Triton initially in highly elliptical orbit Tidal flexing as on Io until orbit ‘circularized’
Triton’s Surface • Pink material—N2 frost?
What future awaits Triton, the largest satellite of Neptune? eventual escape from Neptune as it gradually spirals outward tidal breakup as it slowly spirals closer to Neptune and falls within its Roche limit probable destruction when it eventually collides with Pluto vaporization as it eventually falls into the Sun
Pluto and Charon Pluto: The god of the Underworld—brother of Neptune Charon:The ferryman who carried the doomed souls across the River Styx into Hell
Discovery of Pluto—1930 Pluto shifts about 1 arcmin/day relative to background stars
Pluto and Charon Charon discovered in 1978, James Christy
Pluto and Charon “He gave way to the queer, savage feeling that sometimes takes by the throat a husband twenty years’ married, when he sees, across the table, the same face of his wedded wife, and knows that, as he has sat facing it, so must he continue to sit until the day of its death or his own.” Rudyard Kipling
Pluto Data • Discovered by Clyde W. Tombaugh, Feb 18, 1930 • Mass (kg)1.27 x 10 22 (Earth = 1) 2.125 x 10 -3 • Radius (km) 1,137 (Earth = 1) 0.1783 • Mean density (gm/cm -3) 2.05 • Mean distance from the Sun (Earth = 1) 39.5294 • Rotational period (days) 6.3872 • Orbital period (years) 248.54 • Orbital eccentricity 0.2482 • Tilt of axis (degrees) 122.52 • Orbital inclination (degrees )17.148 • Equatorial surface gravity (m/sec2) 0.4 • Albedo 0.3 (Magnitude 15.12) • Atmospheric composition Nitrogen Methane
The orbital and rotational periods of the Pluto-Charon system bear what relationship (if any) to each other? Charon and Pluto orbit each other’s common center of mass with identical orbital and rotational periods. Charon orbits Pluto twice, while Pluto orbits Charon once. There is no relationship between rotation period of Pluto and orbital period of Charon. Charon orbits Pluto once each month.
Kuiper Belt and Oort Cloud • Kuiper Belt extends from about 30 to 60 AU • Oort cloud, a spherical distribution, extends to about 100,000 AU!
Known KBO’s • Green–main belt • Orange–scattered objects