1. Lecture 9: Small Bodies of the Solar System
SCI238 W08
Lecture 9: Small Bodies of
the Solar System
the inner SS is filled with asteroids and other small bodies
L9 – Feb 5/08
Small SS Bodies

2. This week’s events: the Moon: New Moon Feb 7
Venus: visible low in east before sunrise; brightest “morning” star
Mars: is visible all night, rises at sunset
Jupiter: low in east before sunrise
Saturn: rises at 8pm
Annular Solar Eclipse visible in far South (Antarctica)
NOTE: lunar eclipse on 21 Feb. in the evening
L9 – Feb 5/08
Small SS Bodies

3. Today’s Lecture “left-overs”
Moons of Giant/Jovian planets
rings
today’s topic: small bodies of the Solar System
asteroids
meteorites
comets
L9 – Feb 5/08
Small SS Bodies

4. Saturn’s Moons
many moons, but probably a large number still to be discovered
Titan: largest of Saturn’s moon’s – has a thick atmosphere, extending far out into space, probably has frozen water (ice)
innermost moons tidally locked to planet
moons interact with rings
L9 – Feb 5/08
Small SS Bodies

5. Titan: Saturn’s largest moon; has a thick atmosphere
methane,
nitrogen…
smoggy
L9 – Feb 5/08
Small SS Bodies

6. Huygens probe shows some surface features on Titan
Titan: icy “rocks”, “river” valleys…
L9 – Feb 5/08
Small SS Bodies

7. Saturn’s Moons are varied, enigmatic
deathstar Mimas
icy Enceladus
cratered Tethys
old Dione
streaky Rhea
two-faced Iapetus
L9 – Feb 5/08
Small SS Bodies

8. Uranus’ moons
more than 20 small ones, some irregular in shape, some retrograde
five midsized: Miranda, Ariel, Umbriel, Titania, Oberon
All icy, varied geological history
Miranda is strange – very fractured surface, some cataclysmic event?
L9 – Feb 5/08
Small SS Bodies

9. near IR image shows Neptune’s rings and seven of its moons
L9 – Feb 5/08
Small SS Bodies

10. Urabus’ Miranda shows a fractured surface, obvious violence, but from what?
messed-up
Miranda
L9 – Feb 5/08
Small SS Bodies

11. Neptune’s moons five small inner moons near ring system
two larger, icy…
five small outer, retrograde
Triton
very much larger than any other Neptune moon
has thin nitrogen atmosphere
surface has nitrogen geysers
retrograde (backwards) orbit
spiraling in towards Neptune
L9 – Feb 5/08
Small SS Bodies

12. Triton: moon of Neptune
Nitrogen
atmosphere
surface has been
reshaped by
geological activity
L9 – Feb 5/08
Small SS Bodies

13. Saturn’s Rings: the brightest, best known
within “Roche Limit” of Saturn where any large rocks (moons) break up due to tidal effects
major rings (“A”, “B”, and “C”) with gaps between them “Cassini’s Division”
composed of icy particles with sizes between 1 micron and 10 meters
Voyager discoveries:
several other rings
fine rings within each ring, like record grooves: “spiral density waves”
short-lived spokes
braided rings
“shepherd” satellites
rings are very thin – at most a few tens of meters
L9 – Feb 5/08
Small SS Bodies

14. Saturn’s rings are wide, thin, and complex
Radio image
colours ->
particle size
purple: mainly >5cm
green: mainly <1cm
white: particle
density too high
L9 – Feb 5/08
Small SS Bodies

15. Shepherd moons control the width of rings…
L9 – Feb 5/08
Small SS Bodies

16. …and also make gaps
L9 – Feb 5/08
Small SS Bodies

17. “spokes” in Saturn’s rings: charged particles briefly suspended above the plane?
L9 – Feb 5/08
Small SS Bodies

18. Galileo spacecraft shows rings around Jupiter
ring particles are small and dark: debris from Metis..
L9 – Feb 5/08
Small SS Bodies

19. Near IR image shows rings around Uranus and some of its moons
L9 – Feb 5/08
Small SS Bodies

20. Neptune’s rings are narrow and bright with dusty regions
L9 – Feb 5/08
Small SS Bodies

21. Asteroids small rocky bodies, no atmosphere, orbit the Sun
first asteroid, Ceres, found in 1801: dwarf planet…
a=2.77 A.U., P=4.6 years, diameter is 940 km
second asteroid, Pallas, found in 1802: a=2.77AU
30,000 asteroids now catalogued – most in “Asteroid Belt” at a=2-3.3 A.U. mostly low eccentricities
a few have “Earth-crossing” orbits, most have D<1 km
Trojan asteroids are in Jupiter orbit: i.e. a=5.2AU
orbital resonances with Jupiter alter asteroid orbits
three types are roughly: carbonaceous (very dark, carbon, 75% of all asteroids), siliceous (silicates, 15%), and metallic (iron, rarest type)
L9 – Feb 1/07
Small SS bodies

22. asteroids often found as “star trails”
asteroids are seen as streaks in images tracking stars as Earth rotates
L9 – Feb 1/07
Small SS bodies

23. Distribution of asteroids: semimajor axes mainly 2-3.5AU
Trojan asteroids: orbital resonance at ±60o
Some asteroids have more eccentric orbits bringing them inside Earth’s orbit or beyond Jupiter’s
L9 – Feb 1/07
Small SS bodies

24. distribution of asteroid orbits has “peaks” and “gaps” caused by orbital resonance with Jupiter
L9 – Feb 1/07
Small SS bodies

25. asteroid sizes determined in many ways
asteroid sizes determined in many ways... spacecraft closeups confirm ground-based results
Gaspra:
16km
Ida:
53km
Eros:
40km
Mathilde:
59km
L9 – Feb 1/07
Small SS bodies

26. masses determined from binary asteroids => densities too...
Ida and Dactyl
Eugneia + moon
Densities => range of composition and structure: rubble, ice, rock
L9 – Feb 1/07
Small SS bodies

27. Meteors and Meteorites
a meteor is interplanetary debris that falls toward Earth
meteorite is meteor that survives to hit surface of Earth
“shooting stars”, occasionally “meteor showers”, rarely “meteor storm”
three main meteor types: evidence shows meteors mostly same as asteroid types
largest meteorite ever found is ~2000 kg
organic materials found in some meteorites
L9 – Feb 1/07
Small SS bodies

28. meteor: a bright streak produced when a piece of interplanetary debris heats the atmosphere
L9 – Feb 1/07
Small SS bodies

29. this car was hit by a 12kg meteorite
analysis of the meteor’s path through the atmosphere shows it originated in the asteroid belt
L9 – Feb 1/07
Small SS bodies

30. Iron meteorite Stony meteorite
dark, smooth crust created by surface melting
characteristic crystalline patterns => slow cooling inside larger body
L9 – Feb 1/07
Small SS bodies

31. primitive meteorites: unchanged since formation; ages ≤4.6x109yr
L9 – Feb 1/07
Small SS bodies

32. Processed meteorites: show evidence of change since original formation
L9 – Feb 1/07
Small SS bodies

33. meteors radiate from same part of the sky occur at same time each year
Meteor shower:
meteors radiate from same part of the sky
occur at same time each year
L9 – Feb 1/07
Small SS bodies

34. meteor showers: due to Earth passing through debris left behind by a passing comet
L9 – Feb 1/07
Small SS bodies

35. L9 – Feb 1/07
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36. some meteors (and impacts) are from cometary nuclei…and some of these break up
Shoemaker Levy before it hit Jupiter
Crater chain on Ganymede: multiple impacts from broken comet?
L9 – Feb 1/07
Small SS bodies

37. impact of Shoemaker-Levy on Jupiter
L9 – Feb 1/07
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38. Manicouagan Lake: large impact basin formed ~2x108 yr ago
Earth has impact craters – they’re just often hard to find!
L9 – Feb 1/07
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39. Meteor crater – Arizona…formed ~40,000 yr ago
L9 – Feb 1/07
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40. large (2ookm) and mostly under water
Yucatan crater:
large (2ookm) and mostly under water
L9 – Feb 1/07
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41. major impact event ~65x106 yr ago…
Yucatan crater:
major impact event ~65x106 yr ago…
L9 – Feb 1/07
Small SS bodies

42. Crater size correlates with impactor size…
L9 – Feb 1/07
Small SS bodies

43. Gravity field of jovian planets alters paths of outer SS objects
more inner SS impacts without Jupiter…
L9 – Feb 1/07
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44. Comets distinguished by their appearance: coma, tail…
bright ones are not very common
virtually all have very eccentric orbits, but there appear to be two populations
short period from “Kuiper Belt”: P < 200yr
long period from “Oort Cloud”: P > 200yr
complicated structure that changes with time
relatively low mass objects
most of mass is in form of ices (H2O, CO2 )
L9 – Feb 1/07
Small SS bodies

45. Comet Hale-Bopp over Arizona
bright coma and two tails…
L9 – Feb 1/07
Small SS bodies

46. Comets: visible coma and tails; hidden nucleus
L9 – Feb 1/07
Small SS bodies

47. but normally the nucleus is hidden from view by the surrounding “coma”
the “nucleus” of a comet is an active place, often with jets of material shooting out on the “sunward” side
but normally the nucleus is hidden from view by the surrounding “coma”
L9 – Feb 1/07
Small SS bodies

48. A comet’s appearance changes with distance from the Sun
an “ordinary” SS object at large distance
Cometary “signature” inside ~3-5AU
L9 – Feb 1/07
Small SS bodies

49. comets “come from” Kuiper Belt… Oort Cloud...
Oort Cloud: extends to ~50,000AU
main Kuiper Belt:
~30-50AU
scattered disk objects beyond Kuiper Belt
L9 – Feb 1/07
Small SS bodies

50. Oort Cloud >> scale than Kuiper Belt >> inner SS
Oort Cloud orbits random in inclination
Kuiper Belt more disk-like; smaller range of orbital inclination
L9 – Feb 1/07
Small SS bodies