OUTLINE
• Basic Properties of Uranus, Neptune, and Pluto
• Discovery of the Outer Planets
• Space Probes to the Outer Planets
• Common Properties of Uranus and Neptune
  • Atmospheres of Uranus and Neptune
  • Rings of Uranus and Neptune
  • Magnetosphere and Internal Structure of Uranus and Neptune
  • Moons of Uranus and Neptune
• Pluto
• Solar System Derbis
  • Asteroids
  • Comets
  • Meteoroids

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Basic Properties of Uranus, Neptune, and Pluto

Planet order . from Sun Orbital period Semi-major axis Mass Radius Density Surface Gravity Escape speed Rotational Period Axial Tilt . (Inclination) Surface Mag. Field Surface Temp. Known Moons

Uranus 7 84 years 19.2 AU 14.5 x Earth 4.0 x Earth 1270 kg/m3 0.9 x Earth 21 km/sec -0.72 days 98o 0.74 x Earth 58 K 27

Neptune 8 164 years 30.1 AU 17.1 x Earth 3.9 x Earth 1640 kg/m3 1.2 x Earth 24 km/sec 0.67 days 30o 0.43 x Earth 59 K 13

Pluto 9 248 years 39.5 AU 0.002 x Earth 0.2 x Earth 2060 kg/m3 0.07 x Earth 1.2 km/sec -6.4 days 118o ? 50 K 1

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Discovery of the Outer Planets

Uranus (1781) discovered by William Herschel, British Astronomer, by chance while viewing sky with telescope

Neptune (1846) discovered when deviations (or perturbations) in orbit of Uranus led to prediction of Neptune's presence by two astronomers: (Adams and Leverrier ) and later observed (where predicted) by Galle

Pluto was discovered in 1930 by American Clyde Tombaugh

• Percival Lowell had estimated position of a ninth planet from perceived deviations in orbits of Uranus and Neptune
• Discovered 14 years after Lowell's death very close to Lowell's predicted location
  • Pluto is far too small to have detectable influence on Neptune or Uranus, and therefore Lowell's prediction was an accident

• Voyager 2 - (launched by USA, August 20, 1977)
  Voyager 2 flew by Uranus on Jan 30, 1986
  • flew by Neptune on Aug 15, 1989
• Pluto has not yet been visited
  • NASA plans to launch a mission to Pluto-Charon and beyond in 2006
    • New Horizons
    • would arrive at Pluto-Charon in 2015

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Uranus and Neptune
Outer Worlds of the Solar System

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lecture on Uranus and Neptune

Rotation and Tilt of Uranus

• Uranus' rotation axis tilted 98^o relative to orbital plane
  • north pole points directly at the Sun at one point in orbit
    • south total darkness
  • south pole points directly at the Sun 1/2 uranian year later
    • north total darkness
• Very strange seasons on Uranus
  • poles have summers with Sun directly overhead
  • winters with Sun never appearing
  • Origin of Tilt?
    • Catastrophic event? Grazing collison with another planet sized body?

Largest moon of Neptune (Triton) has Nitrogen atmosphere and active geysers

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Common Properties of Uranus and Neptune the Outer Jovian Planets

• Atmospheres of Uranus and Neptune
• Rings of Uranus and Neptune
• Magnetosphere and Internal Structure of Uranus and Neptune
• Moons of Uranus and Neptune

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Atmospheres of Uranus and Neptune

Comparison of the Elemental Makeup of the Jovian Atmospheres

	Jupiter	Saturn	Uranus	Neptune
hydrogen	86%	92%	84%	84%
helium	14%	7%	14%	14%
methane	trace%	0.2%	2%	3%
ammonia	trace%	0.02%	0%	0%

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Atmospheres of Uranus and Neptune

Elements
• 84% molecular hydrogen
• 14% helium
• methane (2% on Uranus and 3% on Neptune)
• (very little ammonia)
  • frozen out (much colder than Jupiter and Saturn)

Color

• Uranus and Neptune are a bluish color (Uranus blue-green)
  • due to methane in the atmosphere
  • methane absorbs out the red

Weather

• Uranus
  • Feature-less appearance
    • no internal heat source
  • Flows below surface create very efficient energy transfer
• Neptune
  • Cloud and band structure more easily seen
    • internal heat source -> convection
  • Cloud streaks
  • Upper atmosphere slightly warmer than Uranus
  • three features studied by Voyager 2:
    • Great Dark Spot, scooter, and small dark spot
  • Great Dark Spot
    • discovered by Voyager 2 (1989)

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Rings of Uranus and Neptune

All of the Jovian Planets have rings

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Rings of Uranus

• 11 Rings are known
  • Nine rings discovered from Earth (1977)
    • stellar occultation
• dark, narrow, widely spaced
• shepherd satellites
• all within the Roche Limit

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Rings of Neptune

• 5 known rings
• Outer ring (Adams) clumps into arcs
• all within the Roche Limit

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General Theory of Rings of the Jovian Planets

• All jovian worlds have rings
• Rings are relatively short-lived (millions of years)
• Therefore: formation is a common event
  • otherwise we would not see rings on all 4 planets
• Local environment important:
  • Moons, catastrophic events, etc.

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Magnetospheres and Internal Structure of Uranus and Neptune

Internal Structures of Uranus and Neptune

• Rocky cores
  • about 10 times the Earth's mass
• no metallic hydrogen (found in Jupiter and Saturn)

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Magnetospheres of Uranus and Neptune

• internal magnetic fields 100 x's Earth
• Uranus and Neptune magnetic fields are tilted to large angles relative to the rotation axis
• Uranus and Neptune magnetic fields are not centered on center of planet

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Moons of Uranus and Neptune

Moons of Uranus

• 27 known moons - major moons
• 3 classes
  • 11 small, inner moons, mostly discovered by Voyager
  • 5 medium sized moons
  • 11 newly discovered outer moons
• Many of the moons (the earliest discovered) revolve in Uranus skewed equatorial plane
  • the inner moons are in circular, tidally locked orbits(very regular orbits, excepting for tilt)
• No large moons
  • Only the 5 medium sized moons

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Moons of Neptune

• 13 known moons
• not a very regular moon system
  • no moons on roughly circular, equatorial, prograde orbits
    • unlike the regular moon system of Uranus
• Outer moon - Nereid
  • Elongated orbit
  • 1.4 million km -> 9.7 million km
• Triton
  • 7th largest moon in Solar System
  • Retrograde orbit (going around the "wrong" way)
  • 37K surface temperature
  • One of three objects in Solar System know to have a nitrogen dominated atmosphere
    • The other two are Earth, and Titan (Saturn)
  • Geysers
    • vaporized nitrogen erupting from below the surface
  • Surface covered by few meters of clear, frozen nitrogen
  • Origin of Triton?
  • Future of Triton
    • Spiraling toward Neptune (retrograde orbit)
    • Tidal gravitation of Neptune will rip Triton apart
      • Perhaps in 100,000,000 years
      • Spectacular Rings?

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Pluto
Outer World of the Solar System

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lecture on Pluto

Orbital Properties of Pluto

• Semi-major axis of 39.5 AU
• very faint object in Earth's sky
  • highly elongated and tilted orbit
• Passes inside Neptune's orbit at perihelion
  • Inside Neptune from 1979 to 1999
• Locked in 3:2 resonance with Neptune
  • Will not ever come close enough to Neptune to collide

Pluto's moon, Charon

• Discovered in 1978
• HST images much more distinct than ground-based observations
• Inclined orbit (118^o)
• by chance, a series of several eclipses (1985-1991) occurred
  • provided detailed info on size and orbits of Pluto and Charon
  • about one-half the radius of Pluto
    600 km vs. 1137 km (Pluto)
    medium sized moon
  • about one-sixth the mass of Pluto
    M_PLUTO = 0.0021 M_EARTH

Pluto's other properties

• Radius = 1137 km (more like a moon than any other planet)
• Density (2060 kg/m³) midway between Jovian and Terrestrial
  • Density similar to Triton (Neptune's moon)
  • Probably mostly water-ice compostion
  • Surface temperature of about 50K (solid methane)
• Neither terrestrial nor jovian
  • more like icy moons of Jupiter, and thought to be related to icy asteroids in the outer solar system

Pluto's origin

• Escaped moon?
• Icy asteroid?
  • and Charon?
    • more icy asteroids beyond Pluto?
• Do Triton and Pluto have a similar origin?
  • They are similar
    • density, mass, radius
    • Pluto: 2060 kg/m³, 0.002 M_E, 1137 km
    • Triton: 2100 kg/m³, 0.0035 M_E, 1350 km
  • Did both once move in independent orbits around the Sun?
  • Charon?

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Solar System Debris
Keys to Our Origin

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lecture on Solar System Debris

Asteroids

Asteroid belt

• Over a hundred-thousand rocky objects
• largest: Ceres
  • 940 km diameter
• All asteroids have mass of about 10% M_Moon
  • small fraction of the Solar System
• Prograde orbits, except 1
• Jupiter's gravity continuously disturbs these chuncks of matter, preventing them from accumulating (favored theory for existence)
  • unlikely that these are fragments of a planet that broke up long ago

Types of Asteroids

• C-type
  • carbonaceous(largely carbon, dark)
  • 75% of the asteroids
  • outer orbits
  • very old (4.6 billion years)
• S-type
  • silicate(largely silcon, light)
  • 15% of the asteroids
  • inner orbits
• M-type
  • nickel-iron
  • about 10% of the asteroids

Earth and Mars Crossing Asteroids

• Apollo asteroids
  • Earth crossing with semi-major axis greater than 1 AU
• Aten asteroids
  • Earth crossing with semi-major axis less than 1 AU
• Amor asteroids
  • Mars crossing
• all the Earth crossing asteroids will eventually collide with the Earth
  • this occurs about 3 times every million years

Orbital Resonances of Asteroids

• Trojan asteroids
  • orbit with Jupiter
    • 60^o ahead or behind at the Lagrange points
• Kirkwood gaps
  • caused by resonance with Jupiter

Close-up Views of Asteroids

• Gaspra
  • S-type, maximum diameter of 20 km, about 200 million years old, fragment of larger object
  • visited by Galileo (on journey to Jupiter)
• Ida
  • S-type, maximum diameter of 60 km, about 1 billion years old, fragment of larger object
  • tiny moon: Dactyl (1.5 km across)
  • visited by Galileo (on journey to Jupiter)
• Mathilde
  • C-type, low density of 1400 kg/m³, porous interior
  • visited by NEAR-Shoemaker on way to Eros on June 27,1997
• Eros
  • S-type, (33 km x 13 km x 13 km), 2700 kg/m³, appears homogeneous, not porous as Matilde
  • NEAR-Shoemaker (Near Earth Asteroid Rendezvous) - official web site
    • spacecraft misson to rendezvous with asteroid Eros
    • launched February 17, 1996
      • planned Eros rendezvous in early 1999
    • orbited Eros beginning February 14, 2000
      • orbited Eros for a year to do detailed study
      • crater densities indicated Eros is old, like Ida and Matilde, but unlike Gaspra
    • landed on Eros, February 12, 2001
• Collisions with Earth
  • about 3 asteroids collide with the Earth per a million years
  • 65,000,000 years ago - ???

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Comets

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• Icy matter ("dirty snowballs") in highly elliptical orbit
• unchanged since the formation of the Solar System
• heat up, emit radiation, and develop tail as they near the Sun

Comet Structure

• Nucleus - few km in diameter
  • Density : 100 kg/m³
  • Evaporation Rate: 10 tons/ second near the Sun
  • Lifetime: about 5000 orbits or about 400,000 years for Halley's Comet (in other words - short on cosmological scale)
• Coma- evaporated gas and dust forms halo around nucleus as comet approaches the Sun
• Hydrogen envelope
• Two tails
  • ion tail (straight out, blown by solar wind)
  • dust tail (broad, diffuse, curved, less deflected by solar wind)

Two sets of orbits

• short-period comets:
  • from Kuiper Belt (KBOs - Kuiper Belt Objects)
    • orbits of 30 - 100 AU in diameter
    • flat (near ecliptic plane) set of orbits
    • over 800 have been discovered
    • sizes range from diameters of 50km to more than 1000 km
    • related to Pluto, Charon, and Triton?
  
  
• long-period comets:
  • from Oort Cloud
    • orbits of 100,000 AU in diameter
    • surrounds Sun in all directions

Halley's Comet

• period of 76 years
• retrograde orbit
• Observation recorded every 76 years since 240 BC
• In 1705, Edmund Halley realized the repeated appearances
  and predicted that this comet would reappear in 1758
  • comet appeared right on schedule in 1758 (after Halley's death in 1742)

Comet Shoemaker-Levy Impact on Jupiter in 1994

• July, 1992 passed within Roche Limit of Jupiter -> broke apart
• Discovered in March, 1993
  • composed of several pieces
  • "string of pearls"
• Comet Shoemaker-Levy crashed into the surface of Jupiter in July, 1994
• Series of impacts
  • Huge fireballs (hundreds of km)
    • (like a billion nuclear explosions)
  • Jupiter disturbed over thousands of kilometers

Comets of 1996/1997

• Hyakutake
  • in January of 1996, an amateur comet astronomer in Japan discovered this comet with a pair of binoculars
  • the orbit was predicted to pass 0.1 AU from Earth
  • became a very bright, naked eye image, brightest comet in 20 years
• Hale-Bopp
  • discovered in July, 1995
    • simultaneously by Alan Hale in New Mexico and Thomas Bopp in Arizona
  • visible to naked eye in the 1997, one of the brightest comets of the 20th century
    • period is about 4000 years

Stardust Mission

• Mission launched in February, 1999 to collect cometary material from tail of Comet Wild-2 in January, 2004
• Will return to Earth in January, 2006

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Meteoroids

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• Meteoroid - chunk of debris before entering Earth's atmosphere
• Meteor - bright streak in the sky caused by piece of debris
  • "shooting star"
• Meteorite - a piece of a meteoroid that survives and lands on Earth
• Meteoroid swarm - fragments from comet that follow same orbit
• Meteor shower - event where frequent meteors can be seen

Cometary Fragments and Meteor Showers

• smaller meteoroids are rocky remains of broken up comets
• certain meteor showers are regular and (fairly) predictable events

Stray Asteroids

• larger meteoroids (more than a few centimeters) are generally from the asteroid belt
  • responsible for most craters in the inner Solar System

Earth is scarred with nearly 100 craters larger than 0.1 km in diameter

• Barringer Crater (1 km)
• Quebec's Manicouagan Reservoir - 70 km diameter
• Tunguska event of 1908 (in Siberia)
  • exploded in Earth's atmosphere before reaching surface - massive devastation

Major collisions of large meteoroids with the Earth occur every few hundred-thousand years

• Demise of the dinosaurs 65,000,000 years ago?

Meteorite Properties

• Fairly dense: up to 5000 kg/m³ (like asteroids)
• Most have rocky composition
• Some are composed mostly of iron and nickel
• Some show geologic history
  • indicating strong heating sometime in their past, perhaps when they were liberated from a larger body
• Most are very old (4.4 - 4.6 billion yrs)

Lunar and Martian Meteorites on Earth

• Over 30 meteorites have been established as having come to Earth from Mars or the Moon
  • typically billion year old
• Several are from the Moon

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