Introduction to Astronomy

1. Introduction to Astronomy • Announcements • Midterm Exam on Thursday • Closed-book/notes/etc.

2. This is what happens when a massive star dies. About 1400 ly distant, it covers an area of sky = 5 full Moons!

3. The Jovian PlanetsJupiter, Saturn, Uranus, Neptune

4. The Jovian planets to correct scale Also known as the outer planets or the gas giants

5. 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)

6. Jupiter The King of the Planets

8. Jupiter • Name from Roman God of Gods • Largest of the planets • 11x diameter of Earth, 318x mass of Earth • Dense, brightly-colored atmospheric bands • Fast-moving H, He, NH3, CH4, H2O • Fast rotation period (10 Earth hours) • Results in significant equatorial bulge

9. Interior • Avg. density ~ 1.3 g/cm3 (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

10. 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 N2 to enter blood stream and dissolve into brain tissue, which is highly toxic.

12. 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

13. On Jupiter, those same high- and low-pressure systems are stretched into globe-circling bands Here on Earth, we have localized high- and low-pressure systems

15. 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

17. Just in case you don’t believe me, here’s an example of the Coriolis deflection • Note I did not say Coriolis “force” • Movie

18. Speed of cloud bands varies widely from place to place

19. 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

20. HUGE “Jet Streams” Smaller storms

21. The Kelvin-Helmholtz Instability

23. 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

25. Jupiter has Aurorae just like on Earth! Image of Jupiter & aurorae in UV wavelengths (Hubble/STIS)

27. March 2007: Auroral observations by Chandra X-ray Telescope overlaid w/ HST image

28. Jovian Moon “footprints”

29. Rings • Thin rings confirmed by Voyager I fly-by in 1979 Cassini confirmed rings are small, irregular “chips” from nearby moons This is a false-color image from Voyager 2 looking back as it passed Jupiter Why is the planet’s surface dark?

30. 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

31. 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

32. 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

34. 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…

35. IR view: simultaneous, surface-wide eruptions

36. 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 H2O, red areas from mineral-rich water that oozed through surface cracks • Perhaps organic? Red algae?

39. Size comparison of Europa’s ice fields

40. Tidal heating by Jupiter’s gravity keeps subsurface H2O 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 temps, high acidity, low light levels, etc…

42. Saturn

43. Saturn • Avg. density ~ 0.7 g/cm3 • 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 (NH3) freezes into tiny cloud particles • Dense cloud cover masks details of surface

45. Saturn’s weather bands as imaged by Cassini

48. Rings • Originally thought to be solid • Closer examination shows inner rings rotate faster than outer rings • Kepler’s 3rd 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 H2O ice, but see signs of carbon compounds

49. Substructure of Saturn’s rings: composition & speed differences, many gaps

50. 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…