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The Outer Worlds: Uranus, Neptune, and Pluto PTYS/ASTR 206 Outer Worlds 4/19/07 Announcements • Reading Assignment – Finish Chapter 17 • 6th homework due on Tuesday, April 24 • Quiz on Tuesday, April 24 (will cover all lectures since the last exam and Chapters 14-17) • Final Activity next Thursday – Come prepared with 2 multiple-choice final exam questions • Next study-group session is Monday, April 23, from 10:30AM12:00Noon – in room 330. • Public lecture – Tuesday, April 24 7:30PM (this room) •PTYS/ASTR Movie206Night – Monday, April 23, 6:30PM (this room) Outer Worlds 4/19/07 • “Discovered” by William Herschel in 1781 (discovered by chance) – Herschel was an English musician and amateur astronomer – He used a 6.5” homemade reflector (it took him over 200 attempts to make it!) – Prior to its discovery, Uranus had been seen, but not recognized as a planet (it had been designated 34 Tauri) • Herschel made many contributions to astronomy, including the discovery of binary star systems, and the motion of PTYS/ASTR 206 Outer Worlds the solar system in space 4/19/07 Discovery of Uranus Discovery of Neptune • The problem: – Uranus’s orbit didn’t match Newton’s Laws, with the forces from all known planets at that time • Proposed solution – there must be another planet ! – Bode’s Law gives the distance to be 39 AU – Its mass must be about the same as Uranus PTYS/ASTR 206 Outer Worlds 4/19/07 Neptune’s Discovery • John Couch Adams (English student) – Predicted its location in 1843 (using Newton’s Laws including the its force on Uranus to explain Uranus’s orbit) – George Airy (Astronomer Royal) wouldn’t look! • Wanted a few more details worked out PTYS/ASTR 206 Outer Worlds 4/19/07 Neptune’s Discovery • Urbain Le Verrier (French Professor) – Predicted its location in 1846 – published it – Airy (now interested) did not have good enough star charts – Johann Galle (German astronomer) did have good star charts – LeVerrier convinced Galle to look for it PTYS/ASTR 206 Outer Worlds 4/19/07 Neptune’s Discovery • Galle found it in 30 minutes ! • Galle is often credited for being the discoverer, although Adams and Le Verrier were the first to predict its presence – Instructors note: Why did Adams get credit? He never published it ! PTYS/ASTR 206 Outer Worlds 4/19/07 Uranus and Neptune from Earth • Uranus is visible with the naked eye under good (dark sky) conditions • Neptune is not visible to the naked eye • Both can be seen as planets with moderate-sized telescopes (rather than a point-like star) • No surface features are visible without a filter – Only recently were a groundbased telescope able to discern surface features on Uranus using an IR filter and adaptive optics PTYS/ASTR 206 Outer Worlds 4/19/07 Uranus Neptune • Equatorial diameter of 51,800 kilometers – about 4 Earth diameters Uranus: Basic Facts • Mass is about 14.5 Earth masses • Density =1.3 g/cm3 • Orbital period is 84.01 years – About the average lifespan of a human • average distance from the Sun is about 19 AU • Retrograde rotation • Rotational period is 17 hours 14 minutes. • It has 27 moons – The two largest are Titania and Oberon – Both are about ½ the size of our moon PTYS/ASTR 206 Outer Worlds 4/19/07 • Equatorial diameter of 49,500 kilometers – Nearly 4 Earth diameters – A little bit smaller than Uranus • Mass is about 17 Earth masses – A little bit more massive than Uranus • Density = 1.7 g/cm3 • Orbital period is about 165 years • It is about 29.8 AU from the Sun • Rotational period is about 16 hours • Has 13 moons – The206 largest PTYS/ASTR is Triton which isOuter about Worlds 4/19/07 3/4 the size of our moon Neptune: Basic Facts Uranus’s Composition and Appearance • The atmosphere of Uranus is composed of – 83% hydrogen, – 15% helium, – 2% methane • Methane absorbs red light, giving Uranus its blue-green color. – Methane on Uranus is 5 times more abundant than on Jupiter and Saturn • It lacks an appreciable internal heat source, thus there are no visible PTYS/ASTR Outer Worlds surface 206 features 4/19/07 Uranus’s Atmosphere • Visibly, Uranus appears as a virtually featureless hazy blue ball. – Lack of internal heat. – Clouds on Uranus are cold and don't billow up above the top haze layer. • Seen in false color (at the right), Uranus reveals atmospheric features • The atmosphere is arranged into clouds running at constant latitudes (like Jupiter and Saturn) PTYS/ASTR 206 Outer Worlds 4/19/07 Uranus has an unusual tilt • Herschel discovered that moons were orbiting in a plane perpendicular to its orbit – Moons orbit about their parent body’s equator • Uranus’s rotation axis is tilted 98o with respect to the plane of its orbit about the Sun • Thought to be the result of a collision with a planet sized body early in the solar system formation PTYS/ASTR 206 Outer Worlds 4/19/07 PTYS/ASTR 206 Outer Worlds 4/19/07 Neptune’s Atmosphere • Neptune's atmosphere is made up of hydrogen, helium, and methane. • Neptune's has clouds and storm systems that revolve around the planet, but with wind speeds of 300 m/sec (700 mi/hr • These winds are driven by an internal heat source (unlike Uranus) PTYS/ASTR 206 Outer Worlds 4/19/07 Neptune’s Clouds • The clouds seen on Neptune are frozen methane in the atmosphere • Much more cloud activity is seen on Neptune than on Uranus • This is because Uranus lacks a substantial internal heat source while Neptune does – Neptune radiates more than twice the energy it receives from the Sun PTYS/ASTR 206 Outer Worlds 4/19/07 The Giant Dark Spot on Neptune • Similar to Jupiter’s Giant Red Spot (GRS) • However, the giant dark spot on Neptune is not as long-lived as the GRS on Jupiter – Disappeared in 1994 – Another re-appeared in the northern hemisphere in 1995 • Rotates counterclockwise, as does the GRS on Jupiter PTYS/ASTR 206 Outer Worlds 4/19/07 Uranus and Neptune contain a higher proportion of heavy elements than Jupiter and Saturn • Both Uranus and Neptune may have a rocky core (about the size of Earth) surrounded by a mantle of water and ammonia (like Windex!) • Electric currents in the mantles may generate the magnetic fields of the planets PTYS/ASTR 206 Outer Worlds 4/19/07 The Origin of Magnetic Fields on Uranus and Neptune • Not massive enough to possess a liquid metallic hydrogen layer like Jupiter • H2O in the interior is under enough pressure to induce ionization – Ionization increases its electrical conductivity – An ionized-water dynamo! PTYS/ASTR 206 Outer Worlds 4/19/07 • The magnetic axes of both Uranus and Neptune are steeply inclined from their axes of rotation • The magnetic and rotational axes of all the other planets are more nearly parallel • The magnetic fields of Uranus and Neptune are also offset from the centers of the planets PTYS/ASTR 206 Outer Worlds 4/19/07 Uranus and Neptune each have a system of thin, dark rings PTYS/ASTR 206 Outer Worlds 4/19/07 PTYS/ASTR 206 Outer Worlds 4/19/07 PTYS/ASTR 206 Outer Worlds 4/19/07 • Discovered by William Lassell only a month after Neptune was discovered • Triton (Moon of Neptune) Orbits Neptune in a retrograde orbit ! – opposite to the rotation direction of Neptune – The only large moon to do this • The orbit is also highly inclined relative to Neptune’s equator • Density = 2066 kg/m3 – More rock than in Saturn’s moons PTYS/ASTR 206 Probably a captured asteroid! Outer Worlds 4/19/07 • Shows evidence of geological activity – Lacks large craters – it is a very young surface – Probably tidal heating from Neptune – Perhaps extreme tidal heating in the past Triton (Moon of Neptune) • It also has a tenuous nitrogen (N2) atmosphere – 70,000 times less atmospheric pressure than Earth – about 500 times thinner than the Martian atmosphere PTYS/ASTR 206 Outer Worlds 4/19/07 Pluto and Charon • How was Pluto Discovered? • What is the nature of the planet and its moon? • Is Pluto a planet? PTYS/ASTR 206 Outer Worlds 4/19/07 Planet “X” • Percival Lowell repeated Adams & Le Verrier calculations for planet “X” – He looked, but never found it • After Lowell Died, there was a tedious search PTYS/ASTR 206 Outer Worlds 4/19/07 • Lowell hired Clyde Tombaugh who discovered Pluto in 1930 • Pluto was named after the lord of Hades (in Greek mythology), “PL” are the initials of Percival Lowell PTYS/ASTR 206 Outer Worlds 4/19/07 Pluto’s Discovery • Only “planet” found by an American, and only planet found in the 20th century • Too small to effect the orbits of other planets – The calculations of Lowell didn’t matter PTYS/ASTR 206 Outer Worlds 4/19/07 PTYS/ASTR 206 Outer Worlds 4/19/07