- Slides: 86
Introduction to Astronomy • Announcements – Midterm Exam on Thursday • Closed-book/notes/etc.
This is what happens when a massive star dies. About 1400 ly distant, it covers an area of sky = 5 full Moons!
The Jovian Planets Jupiter, Saturn, Uranus, Neptune
The Jovian planets to correct scale Also known as the outer planets or the gas giants
Commonalities • All are much larger than Earth • Densities are all lower than the terrestrial planets • All have small ice/rock cores surrounded by liquid layer surrounded by thick gas atmosphere (which contains optical surface of planet) • Rings • Belts and Zones (high & low pressure)
Jupiter The King of the Planets
Jupiter • Name from Roman God of Gods • Largest of the planets – 11 x diameter of Earth, 318 x mass of Earth • Dense, brightly-colored atmospheric bands – Fast-moving H, He, NH 3, CH 4, H 2 O • Fast rotation period (10 Earth hours) – Results in significant equatorial bulge
Interior • Avg. density ~ 1. 3 g/cm 3 (slightly higher than water) – Much less than Earth density, so must be made of lighter elements • 1/6 of the way to the core, gaseous H turns to liquid H – Compression by overlying layers • Even deeper, liquid turns to liquid metallic H • Rocky, Iron core
Divers know this phenomenon well… As depth increases, pressure increases. Need for decompression stops to avoid Nitrogen Narcosis (“The Bends”)…pressure on body drops too rapidly, allows bubbles of N 2 to enter blood stream and dissolve into brain tissue, which is highly toxic.
Atmosphere • Coriolis effect & cloud bands • Heat in interior drives convection currents upward – Stirs up atmosphere, brings hot gases to surface while cooler gases fall back down below surface – Coriolis effect • Gases moving toward equator get pushed to the west, gases moving away from equator get pushed to the east
Here on Earth, we have localized high- and low-pressure systems On Jupiter, those same high- and low-pressure systems are stretched into globe-circling bands
How to stretch a weather cell If no rotation, hot gases in the interior convectively rise. In pot of boiling water, convection carries bubbles of WATER VAPOR to surface, where it is released to condense into STEAM When these hot gases reach the surface, they spread out horizontally. Some travel toward the equator, some travel toward the poles
• Just in case you don’t believe me, here’s an example of the Coriolis deflection – Note I did not say Coriolis “force” • Movie
Speed of cloud bands varies widely from place to place
• As opposite-moving winds blow, other uprising material may get caught between them and get “twisted” up into Jovian storms – Like ball rolling between two conveyor belts – Some transient (short-lived) – GREAT RED SPOT permanent (as far as we know) – Brownish-red bands from Sulfur and/or Phosphorous tinting
HUGE “Jet Streams” Smaller storms
The Kelvin-Helmholtz Instability
• Convection & rotation in liquid Hydrogen sustains DYNAMO – BJupiter ~ 20 – 20, 000 x stronger than Earth’s magnetic field • Largest magnetosphere of all the planets • Intense auroral displays observed by Hubble and by Voyager fly-bys • Radiation belts • Thunderstorms & lightning
Jupiter has Aurorae just like on Earth! Image of Jupiter & aurorae in UV wavelengths (Hubble/STIS)
March 2007: Auroral observations by Chandra X-ray Telescope overlaid w/ HST image
Jovian Moon “footprints”
Rings • Thin rings confirmed by Voyager I fly-by in 1979 This is a false-color image from Voyager 2 looking back as it passed Jupiter Why is the planet’s surface dark? Cassini confirmed rings are small, irregular “chips” from nearby moons
Moons • Galilean moons – 4 largest of Jupiter’s moons – Mini model of solar system • In both appearance and formation • Denser, rockier moons closer in • Icier moons further out This is why objects cannot be classified as planets based only on size
Io, Europa, and Ganymede have a synchronous orbital resonance: because the moons are closely-spaced, they create a rhythmic gravitational pull on each other… For every orbit of Ganymede, Europa orbits twice and Io orbits four times
Io • Closest moon, tidally-locked – Gravitational disturbances prevent closed orbit…makes ROSETTE pattern • Most spectacular volcanic activity in solar system – Caused by gravitational/tidal forces that constantly stretch and pull on interior, heating it up (like a RUBBER BAND) – Sulfuric volcanoes: “lava” is not molten rock, but molten Sulfur • But occasionally see evidence of silicate lava
Visible image of Io, showing a HUGE volcanic eruption of Sulfur This plume rose about 65 miles above the surface of the moon! This puts any volcanic eruptions on Earth to shame… Molten sulfur lava flows sown volcanic peak before solidifying…
IR view: simultaneous, surface-wide eruptions
Europa • Perhaps most interesting moon in the solar system • Icy moon – Surface layer of ice ~ 6 miles thick – LIQUID WATER underneath! – White areas are frozen H 2 O, red areas from mineral-rich water that oozed through surface cracks • Perhaps organic? Red algae?
Size comparison of Europa’s ice fields
• Tidal heating by Jupiter’s gravity keeps subsurface H 2 O in LIQUID form – Explains lack of cratering…liquid water would quickly smooth over any surface depressions – Also has glacier-like surface flows of ice – LIFE? • Extremophiles: life-forms that survive in frigid temps, high acidity, low light levels, etc…
Saturn • Avg. density ~ 0. 7 g/cm 3 – This is less than density of water, so if dropped into our oceans, SATURN WOULD FLOAT! – Mostly H & He • Internal heat source: friction/drag on falling, condensed liquid Helium droplets • Similar to Jupiter, but colder, so that ammonia (NH 3) freezes into tiny cloud particles – Dense cloud cover masks details of surface
Saturn’s weather bands as imaged by Cassini
Rings • Originally thought to be solid • Closer examination shows inner rings rotate faster than outer rings – Kepler’s 3 rd law again! – Therefore, rings must be made of a swarm of small, separate bodies a few cm to a few meters in diameter – Mapped via radar echo techniques • Determined size and composition (Voyager I) • Primarily H 2 O ice, but see signs of carbon compounds
Substructure of Saturn’s rings: composition & speed differences, many gaps
• Ring gaps – Cassini’s Division: large gap caused by orbit of moon Mimas periodically pushing particles out of that particular area – Shepherding Satellites: small moon-like objects (larger than average) that direct ring material into narrow lanes, creating small gaps between them These are rings around Uranus, but principle is the same…
F-ring Potato-shaped moon, Prometheus
Distortion of Saturn’s F-ring by interacting moonlets (smaller than moons, but larger than ring particles)
Cassini Division Saturnian Shadow Filtered Sunlight
Cassini image from 9 Sep 2006 Shows new ringlets not seen in Voyager fly-bys 25 years ago
Mimas in white-light and IR a. k. a. The Death Star
Ring Origins • Destruction of moons & asteroids by tidal forces • Roche Limit (RRoche = 2. 44 Rplanet) – Gravity pulls harder on near-side of orbiting object than on far-side – If this differential gravitational force is stronger than the object’s self-gravitation (what holds the object together), it gets torn apart – Gravity pulls pieces into orbit – Side-by-side differences in rings suggest whole ring system formed from MANY destroyed bodies
Saturn’s B & C rings in the infrared, highlighting not only temperature differences, but compositional differences as well
Moons • Mostly ice, but less dense than Jupiter’s moons • Most interesting moon: – Titan • D = 3000 miles > DMercury • So cold, N 2 moves slowly, doesn’t reach escape velocity – Titan has Nitrogen atmosphere! • Spectroscopy indicates presence of liquid methane oceans & ethane-methane “rainclouds”
Introduction to Astronomy • Announcements – Midterm Exam tomorrow – HW #4 due Monday
Uranus • Named for Ouranos • Pretty much featureless except for faint cloud bands and faint ring system • Rich in H (in the form of H 2 O and CH 4) • Methane causes blue color – Absorbs red-end of solar spectrum, scatters blue-end off clouds of frozen methane • Avg. density ~ 1. 2 g/cm 3 – Mostly light elements probably with rocky/icy core
Rings & Moons • Rings similar to Jupiter & Saturn but darker and narrower – Probably more carbonbased molecules – Thin rings held by shepherding satellites • 5 large moons & 20 small ones As a side note, all Uranus’ moons are named for Shakespearean characters
• Miranda (large moon) – Very puzzling appearance – Large impact, destruction & reformation? – Rising & sinking motions in interior? NOT to scale…
Tilt of Uranus • Equator nearly perpendicular to orbit (spins on its side) • Rings & moons are similarly tilted – Offers clues about formation – Giant impact knocked the planet over, spewed out material that then formed rings & moons • otherwise, wouldn’t expect rings and moons to have tilted orbits… – One pole in perpetual day, one in perpetual night (for half of the Uranian year, 42 Earth-years) • odd rotation + odd heating = lack of atmospheric phenomena?
Which is really the north pole?
Neptune • Similar to Uranus – Cloud bands, deep blue color – Had a “Great Dark Spot” similar to Jupiter’s Great Red Spot • Discovered through orbital anomaly – Uranus’ position did not quite agree with predictions – The gravity of another planet located past Uranus explained disagreement • Leverrier & John Adams (1843) • Confirmed when Neptune first discovered (Johanne Galle, 1846)
• Discovered by Voyager 2 in August 1989 • Thought to be a “hole” in the methane atmosphere – Much like our ozone hole
18 hours from first to last frame Waves in overlying clouds
Atmosphere • Cloud bands generated by Coriolis effect – 1300 mph winds! • Methane tinting – Like Uranus • Great Dark Spot disappeared (sometime < 1997)…why? ! – not well-understood
Rings & Moons • Very narrow, tenuous (faint) rings • 6 moons close-in, 7 further out • Triton – Orbits opposite to Neptune’s rotation – Highly-tilted orbit • left-over or captured planetesimal? – Has retained an acidic atmosphere – “wrinkly”, volcanic soot-laden surface
Outer ring is more clumpy or braided
Triton is actually spiraling-in towards Neptune… …the Solar System will have a new ring system to rival Saturn’s!
Pluto • Roman god of the underworld • Is it a planet? – Highly-elliptical orbit out of the plane of the rest of the solar system • Avg. density ~ 2. 1 g/cm 3 • Composed of H 2 O, Methane, Nitrogen & CO 2 ices – Polar caps of frozen Methane • Thin atmosphere of N 2 & CO • Surface temperature a constant -380 °F
Discovered by Clyde Tombaugh in 1930, working at Lowell Observatory in Flagstaff, AZ Compared (by eye!) an enormous amount of pairs of photographic images, looking for faint objects whose positions slowly changed… …this is NO small task.
Even the smallest bodies in the solar system can have multiple moon systems In fact, Pluto and Charon are actually a binary system! The center-of-mass of the two objects is above the surface of Pluto Where did this name come from?
New Horizons, scheduled for Pluto-rendezvous in 2015 HST, 1996
NEXT TIME • Meteors, Asteroids, and Comets If Pluto were any closer to the Sun, it would be a comet!