THE OUTER PLANETS

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FIRST, LET'S FINISH SATURN'S MOONS

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SIX Middling Moons

  • 400 < D < 1530 km; 1.0 < density < 1.4 g cm^{-3}

    Most of them have high albedo's; along w/ low density -> icy exteriors

  • Mimas: smallest of this lot
    -> Cassini division in the rings, via a 2:1 orbital resonance.
  • Enceladus: very shiny albedo nearly 1 -- water volcanos?
  • Tethys: cratered and cracked surface -- also, two Lagrangian moons
    off by 60 degrees in orbit: Telesto and Calypso.
  • Dione: bright streaks and younger ``maria'' -- one Lagrangian moon: Helene
  • Rhea: biggest of this group -- very hard ice surface with lots of craters
  • Iapetus: very dark leading side; very bright trailing side

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    Some Weird Little Moons

    200 < D < 370 km

  • Just outside outermost ring are Janus & Epimetheus:
    co-orbital satellites, which lap each other every 4 years and swap nearly identical orbits.
  • Hyperion: Titan's gravity prevents it from having a circular orbit,
    so not synchronous: apparently tumbles fully CHAOTICALLY.
  • Phoebe is much farther than any other satellite, at 13,000,000 km away;
    its retrograde orbit implies capture.

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    URANUS and NEPTUNE:

    The Most Distant Real Planets

    NEARLY TWINS

  • M_U = 14.54 M_E ; M_N = 17.15 M_E .
  • R_U = 25,559 km or 4.01 times Earth's.
  • R_N = 24,766 km or 3.88 times Earth's.
  • Density_U = 1.27 g cm^-3;
  • Density_N = 1.64 g cm^-3 .

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    As seen from Earth, URANUS:

  • Was discovered by Wm. Herschel in 1781
    (barely visible to naked eye, greenish)
  • Period of 83.75 years.
  • Semi-major axis of 19.19 AU, e = 0.047.
  • 5 moons discovered from the ground pre-space;
    10 by Voyager,
    2 more by 200-inch.
  • Tilted by 98 deg --- > LONG seasons
  • Magnetic (interior) spin period: 17.2 h (retrograde)

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    As seen from Earth, NEPTUNE:

  • Was predicted by John Adams (1845) and Urbain Leverrier (1846)
    as producing the growing error in Uranus's position.
  • Found in the right place by Johann Galle in Sept. 1846 -- bluish
  • Period of 163.7 years.
  • Semi-major axis of 30.07 AU, e = 0.009 .
  • 2 moons discovered from the ground; 6 from space
  • Magnetic (interior) spin period: 16.1 h; surface markings: 17.3 h

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    ATMOSPHERES

  • Temperatures: Uranus, 58 K; Neptune 59 K
  • Both Uranus and Neptune are about 84% H _2 and 14% He.
  • Methane (CH_4 ) is most of the remainder
    Methane absorbs red light, so planet looks blueish/green
    More CH_4 in Neptune, so it's bluer
  • Ammonia is frozen out in these very cold atmospheres
  • No internal heat source for Uranus, so washed out
    bands and zones covered by a haze.
  • Fairly strong winds distribute Sun's heat so sunny side isn't
    much hotter than shady side.
  • Neptune has an internal heat source: probably extra methane
    has helped to trap internal heat of formation
    --- > more noticeable bands and some storms

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    INTERIORS

  • Below atmospheres, each about 1/3 of R:
  • MOLECULAR HYDROGEN (unlikely to \break convert to metal)
  • below that, SLUSH, including water ice and dissolved ammonia
  • ROCKY CORE (maybe 10 M _E )

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    MAGNETIC FIELDS

  • Despite not having (much) metallic hydrogen both have
    substantial internal fields (100 times earth's, 1/10th Saturn)
  • But, not like bar magnet:
  • Off-center and misaligned with spin axis (60 deg for Uranus, 46 deg
    for Neptune, vs. 10 deg for Jupiter and well-aligned for Saturn).
  • Perhaps: reversing; reflecting a blow; ionic NH _3 ?

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    URANUS'S ICY MOONS

  • 5 moderate sized ones ( 480 < D < 1580 km} )
  • 1.1 < Density < 1.7 g cm}^{-3} , so ice/rock mixtures
  • All very dark: organic molecules formed from solar wind and UV bombardment?

  • MIRANDA: a = 130,000 km; smallest, weirdest: cratered part; ridges; oval faults.
    Blown apart and reassembled?
  • ARIEL: a = 191, 000 km and
  • UMBRIEL: a = 266,000 km are very similar in size but Ariel
    shows some evidence for ancient activity, which Umbriel lacks.
  • TITANIA: a = 436,000 km is the biggest,
  • but OBERON, a = 583,000 km is only slightly smaller.
  • 10 smaller moons are mostly within the ring system
    and perform sheparding duties.
  • 2 very distant moons have inclined, retrograde orbits; captured asteroids.

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    NEPTUNE'S MOONS: ODD ORBITS

    TRITON was discoverd in 1846 by Wm. Lassell

  • a = 355,000 km; D = 2710 km;
  • M = 0.292 M _Moon ; Density = 2.1 g cm^-3
  • Retrograde period of 5.88 days; inclined about 20 degrees to
    plane of Neptune's equator.
  • Pretty big: about 1/2 of Europa's mass
  • Has a thin nitrogen atmosphere; freezes out at T = 37 K.
  • Few craters, implies activity: nitrogen geysers.
  • Spiralling TOWARDS Neptune; inside Roche limit in 10^8 y
    So Neptune will then lose a moon and (very probably) gain a spectacular ring system!

    NEREID: discovered by G. Kuiper in 1949:

  • a = 5,510,000 km; D = 340 km; e = 0.76
  • Some powerful interactions; perhaps Pluto??

    5 little moons known inside the Roche limit; affect rings.

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    ALL OUTER PLANETS HAVE RINGS

  • URANUS has 11 sets of rings
  • Between 1.45 and 2.00 planetary radii
  • Five densest discovered by stellar occultation in 1977
  • 4 more seen from Earth, 2 more only by Voyager in 1986
  • Much narrower, darker, with wider gaps than Saturn's
  • Kept in line by shepard satellites

    NEPTUNE has 5 sets of rings

  • Between 1.65 and 2.54 planetary radii
  • They are incomplete -- more arcs than rings
  • All discovered by Voyager 2 in 1989
  • Since ring systems are believed to be transitory, they must be crated fairly easily
    and often as we have found them around all 4 Jovian planets!

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    The PLUTO - CHARON SYSTEM

  • PLUTO was predicted by Percival Lowell, and located finally by Clyde Tombaugh
    in 1930 (but not the cause of unreal ``discrepancies'' in Uranus's orbit).
  • M = 0.0021 of Earth's
  • R = 1137 km (0.18 of Earth)
  • a = 39.48 AU; e = 0.249 (closer than Neptune from 1979--1999)
  • Inclination of 17.2 degrees
  • In a 3:2 orbital resonance with Neptune --- > they will never come close --- > saved from explusion too
  • Frozen methane on its surface; T < 50 K.
  • Density of 2.1 g/cm^3 --- > Pluto basically similar to icy moons of
    Jupiter through Neptune; nearly a twin to Triton
  • Largest of the PLUTINOS -- large asteroids in 3:2 resonance with Neptune,
    Pluto's really the KING of the KUIPER BELT OBJECTS

    CHARON is about 1/2 radius of Pluto and 1/6 of its mass

  • There were many mutual eclipses visible from 1985 to 1991 In retrograde orbit of 6.2 days;
  • Charon tilted by 118 degrees to plane of Pluto's orbit
  • Co-formation or capture?