PTYS/ASTR 206Outer Worlds 4/19/07 The Outer Worlds: Uranus, Neptune, and Pluto

PTYS/ASTR 206Outer Worlds 4/19/07 Announcements Reading Assignment –Finish Chapter 17 6 th 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 10:30AM- 12:00Noon – in room 330. Public lecture – Tuesday, April 24 7:30PM (this room) Movie Night – Monday, April 23, 6:30PM (this room)

PTYS/ASTR 206Outer Worlds 4/19/07 Discovery of Uranus “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 the solar system in space

PTYS/ASTR 206Outer Worlds 4/19/07 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 206Outer 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 206Outer 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 206Outer 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 206Outer 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 ground- based telescope able to discern surface features on Uranus using an IR filter and adaptive optics Uranus Neptune

PTYS/ASTR 206Outer Worlds 4/19/07 Uranus: Basic Facts Equatorial diameter of 51,800 kilometers –about 4 Earth diameters Mass is about 14.5 Earth masses Density =1.3 g/cm 3 Orbital period is 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 206Outer Worlds 4/19/07 Neptune: Basic Facts 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/cm 3 Orbital period is about 165 years It is about 29.8 AU from the Sun Rotational period is about 16 hours Has 13 moons –The largest is Triton which is about 3/4 the size of our moon

PTYS/ASTR 206Outer Worlds 4/19/07 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 surface features

PTYS/ASTR 206Outer Worlds 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 206Outer 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 98 o 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 206Outer Worlds 4/19/07

PTYS/ASTR 206Outer 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 206Outer 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 206Outer 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 206Outer 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 206Outer 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 H 2 O in the interior is under enough pressure to induce ionization –Ionization increases its electrical conductivity –An ionized-water dynamo!

PTYS/ASTR 206Outer 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 206Outer Worlds 4/19/07 Uranus and Neptune each have a system of thin, dark rings

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PTYS/ASTR 206Outer Worlds 4/19/07 Triton (Moon of Neptune) Discovered by William Lassell only a month after Neptune was discovered 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/m 3 –More rock than in Saturn’s moons Probably a captured asteroid!

PTYS/ASTR 206Outer Worlds 4/19/07 Triton (Moon of Neptune) 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 It also has a tenuous nitrogen (N 2 ) atmosphere –70,000 times less atmospheric pressure than Earth –about 500 times thinner than the Martian atmosphere

PTYS/ASTR 206Outer 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 206Outer 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 206Outer 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 206Outer Worlds 4/19/07 Pluto’s Discovery Only “planet” found by an American, and only planet found in the 20 th century Too small to effect the orbits of other planets –The calculations of Lowell didn’t matter

PTYS/ASTR 206Outer Worlds 4/19/07