Astro 101 Fall 2013 Lecture 4 T. Howard

1. Astro101 Fall 2013 Lecture 4 T. Howard

2. The Solar System • Planets • Their Moons, Rings • Comets • Asteroids • Meteoroids • The Sun • A lot of nearly empty space Ingredients?

3. Solar System Perspective

4. Orbits of Planets All orbit in same direction. Most orbit in same plane. Elliptical orbits, but low eccentricity for most, so nearly circular.

5. Exceptions: MercuryPluto (no longer a planet) orbital tilt 7o orbital tilt 17.2o eccentricity 0.21 eccentricity 0.25

6. Sun, Planets and Moon to scale (Jupiter’s faint rings not shown)

7. Two Kinds of Planets "Terrestrial" Mercury, Venus, Earth, Mars "Jovian" Jupiter, Saturn, Uranus, Neptune Far from the Sun Large Close to the Sun Small Mostly Rocky High Density (3.9 -5.3 g/cm3) Mostly Gaseous Low Density (0.7 -1.6 g/cm3) Slow Rotation (1 - 243 days) Fast Rotation (0.41 - 0.72 days) Few Moons No Rings Main Elements Fe, Si, C, O, N Many Moons Rings Main Elements H, He

8. Asteroids -- • most between Mars and Jupiter • asteroid “belt” • some groups of asteroids with unusual locations or orbits • “Trojan” asteroids • Other “families” -- “Apollo”, etc. • Earth-crossing orbits --> perihelia closer to Sun than Earth’s orbit

9. Asteroids Rocky fragments ranging from 940 km across (Ceres) to < 0.1 km. 100,000 known. Most in Asteroid Belt, at about 2-3 AU, between Mars and Jupiter. The Trojan asteroids orbit 60 oahead of and behind Jupiter. Some asteroids cross Earth's orbit. Their orbits were probably disrupted by Jupiter's gravity.

11. 39.4 30.1 5.2 9.5 19.2 1 1.5 .07 4.0 5.3 2.5 .58 1.2 THE OUTER SOLAR SYSTEM Orbital radius (AU) Jupiter (5.2 AU) zodiacal cloud Light time from earth (hrs) 34.8 min. Kuiper Belt Oort Cloud Asteroid Belt Earth (1 AU) Solar wind Mars (1.5 AU) 4.2 minutes

12. CATALOGUED ASTEROIDS (C. 12/98) (VIEW ALONG ECLIPTIC PLANE) ecliptic

13. CATALOGUED ASTEROIDS (C. 12/98) (VIEW FROM ABOVE ECLIPTIC) Jupiter Trojan groups Main belt Source: Guide 7.0

15. asteroid Mathilde (253) Asteroid Eros (closeup) (from the NEAR mission)

16. Ida and Dactyl Gaspra Total mass of Asteroid Belt only 0.0008 MEarthor 0.07 Mmoon. So it is not debris of a planet. Probably a planet was trying to form there, but almost all of the planetesimals were ejected from Solar System due to encounters with Jupiter. Giant planets may be effective vacuum cleaners for Solar Systems.

17. Comets • Generally have highly elliptical orbits • Perihelion distance close to Sun • Aphelion distance in the outer Solar System • “Tails” -- two components • Dust tail • Ion (ionized gas) tail • Both directed away from Sun by Solar wind • Fuzzy appearance in camera/telescope images • Nucleus (solid body) • Coma (gaseous cloud surrounding nucleus) • Comets aren’t just “rocks” • Have volatile chemicals in the form of ices

18. Solar System Debris Comets Comet Hale-Bopp (1997) Comet Halley (1986) Long Period Comets Short Period Comets 50-200 year orbits Orbits prograde, close to plane of Solar System Originate in Kuiper Belt Few times 105 or 106 year orbits Orbits have random orientations and ellipticities Originate in Oort Cloud

19. Comet Structure Coma and tail due to gas and dust removed from nucleus by Solar radiation and wind. Far from Sun, comet is a nucleus only. Nucleus: ~10 km ball of ice, dust Coma: cloud of gas and dust around nucleus (~106 km across) Tail: can have both gas (blue) and dust tails (~108 km long). Always points away from Sun.

20. Comets Comet Halley (c. 1986) Comet Hale-Bopp (c. 1997)

21. Comets Comet Halley (c. 1986) Comet Hale-Bopp (c. 1997) Tail Coma Nucleus

22. Comet Hale-Bopp (1997) dust tail = white, ion tail = blue

23. Closeup -- nucleus of comet Halley seen by Giotto spacecraft

24. Meteor Showers Comets slowly break up when near Sun, due to Solar radiation, wind and tidal force. e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years. Fragmentation ofComet LINEAR  Debris spreads out along comet orbit. IF Earth's orbit crosses comet orbit, get meteor shower, as fragments burn up in atmosphere.

25. Other stuff ... • Kuiper Belt Objects • Group of icy, small (asteroid-size) objects beyond orbit of Neptune • Pluto now officially considered a Kuiper Belt Object • Origin of Short-period comets • Oort Cloud • “reservoir” of icy, inactive comets in far outer system • most don’t orbit in or near the ecliptic • origin of Long-period comets

26. Kuiper Belt Objects (KBOs) • From Neptune’s orbit (30 AU) • to about 50 AU • Discovered 1992 • Small bodies • Mostly frozen “volatiles” • water, CO2, CH4, etc. • About 200 x mass of main • asteroid belt • Known objects: ~1000 • Estimated: ~ 70,000 • Several larger objects known Green dots = KBOs  Scale is in AU

27. Oort Cloud is a postulated huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects? Ejected planetesimals. A passing star may dislodge Oort cloud objects, plunging them into Solar System, where they become long-period comets. If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its orbit may be changed and it may plunge towards the inner Solar System and become a short-period comet.

28. Meteors • Meteor = streak of light • Meteoroid = body (in space) that causes it • Meteorite = fragment that makes it to the ground • Solar system debris • Some meteor showers associated with comets • Swarm of debris results in repeated meteor shower • Dust grains and very small solids • Larger ones are probably from asteroids • (possibly debris from broken-up asteroids / collisions) • Meteoroid types: rocky or metallic (iron-nickel)

29. Meteor Showers Comets slowly break up when near Sun, due to Solar radiation, wind and tidal force. e.g. Halley loses 10 tons/sec when near Sun. Will be destroyed in 40,000 years. Fragmentation ofComet LINEAR  Debris spreads out along comet orbit. IF Earth's orbit crosses comet orbit, get meteor shower, as fragments burn up in atmosphere.

30. Russian Meteor 2/15/13 • How big? About 50 – 55 ft diameter, preliminary est. • How massive? Using density of known meteorites found on the ground (stony meteorites) • mass would have been ~ 10000 tons • How fast? Estimated based on images from spacecraft, • and consistent with speed it must have had to be in • heliocentric, but not Earth orbit: ~ 40,000 MPH • Kinetic energy = ½ mass x velocity2 • = about 1015 Joules = 300+ ktons/TNT • = equivalent to ~ 20 – 30 “Fat Man” atomic bombs

31. Russian Meteor 2/15/13

32. Russian Meteor 2/15/13

33. Impact Craters Chicxulub crater Yucatan

34. Chicxulub crater Yucatan Image: NASA

35. Comet Shoemaker-Levy, 1994 Image: NASA, HST, May 1994

36. Image: NASA – Galileo spacecraft, 7/22/94

37. Crater chain on Europa (from a similar event)

39. Artist’s conception: one fragment (G) if it had hit Earth Image: John Spencer's Astronomical Visualizations.

40. Astroblemes – some known impact sites Image: Steven Dutch, U. Wisconsin – Green Bay

41. Panspermia Did life originate in space?

42. Asteroid Mining ? -- maybe Finding Prospecting Images: Planetary Resources http://www.planetaryresources.com Mining

43. How did the Solar System Form? We weren't there. We need a good theory. Check it against other forming solar systems. What must it explain? - Solar system is very flat. - Almost all moons and planets orbit and spin in the same direction. Orbits nearly circular. - Planets are isolated in space. - Terrestrial - Jovian planet distinction. - Leftover junk (comets and asteroids). Not the details and oddities – such as Venus’ and Uranus’ retrograde spin.

44. Early Ideas René Descartes (1596 -1650) nebular theory: Solar system formed out of a "whirlpool" in a "universal fluid". Planets formed out of eddies in the fluid. Sun formed at center. Planets in cooler regions. Cloud called "Solar Nebula". This is pre-Newton and modern science. But basic idea correct, and the theory evolved as science advanced, as we'll see.

45. A cloud of interstellar gas a few light-years, or about 1000 times bigger than Solar System The associated dust blocks starlight. Composition mostly H, He. Too cold for optical emission but some radio spectral lines from molecules. Doppler shifts of lines indicate clouds rotate at a few km/s. Some clumps within clouds collapse under their own weight to form stars or clusters of stars. Clumps spin at about 1 km/s.

46. But why is Solar System flat, and why do planets orbit faster than 1 km/s? Pierre Laplace (1749 - 1827): an important factor is "conservation of angular momentum": When a rotating object contracts, it speeds up. mass x velocity mass x velocity x "size" "momentum" "angular momentum" (a property of a spinning or orbiting object) Well demonstrated by ice skaters . . .

47. So, as nebula contracted it rotated faster. Could not remain spherical! Faster rotation tended to fling matter outwards, so it could only collapse along rotation axis => it became a flattened disk, like a pizza crust. Hubble is seeing these now!

48. Now to make the planets . . . Solar Nebula: 98% of mass was gas (H, He) 2% in dust grains (Fe, C, Si . . .) Condensation theory: 1) Dust grains act as "condensation nuclei": gas atoms stick to them => growth of first clumps of matter. 2) Accretion: Clumps collide and stick => larger clumps. Eventually, small-moon sized objects: "planetesimals". 3) Gravity-enhanced accretion: objects now have significant gravity. Mutual attraction accelerates accretion. Bigger objects grow faster => a few planet-sized objects.

49. initial gas and dust nebula dust grains grow by accreting gas, colliding and sticking continued growth of clumps of matter, producing planetesimals planetesimals collide and stick, enhanced by their gravity Hubble observation of disk around young star with ring structure. Unseen planet sweeping out gap? a few large planets result

50. Terrestrial - Jovian Distinction Outer parts of disk cooler: ices form (but still much gas), also ice "mantles" on dust grains => much solid material for accretion => larger planetesimals => more gravity => even more growth. Jovian solid cores ~ 10-15 MEarth . Strong gravity => swept up and retained large gas envelopes of mostly H, He. Inner parts hotter (due to forming Sun): mostly gas. Accretion of gas atoms onto dust grains relatively inefficient. Composition of Terrestrial planets reflects that of initial dust – not representative of Solar System, or Milky Way, or Universe.