1. Astro 101
Fall 2013
Lecture 4
T. Howard

2. Ingredients? 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. orbital tilt 7o orbital tilt 17.2o eccentricity 0.21 eccentricity 0.25
Exceptions:
Mercury Pluto
(no longer a planet)
orbital tilt 7o orbital tilt o
eccentricity eccentricity 0.25

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

7. Jupiter, Saturn, Uranus, Neptune
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 ( g/cm3)
Mostly Gaseous
Low Density ( g/cm3)
Slow Rotation ( days)
Fast Rotation ( 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 o ahead of and behind Jupiter. Some asteroids cross Earth's orbit. Their orbits were probably disrupted by Jupiter's gravity.

11. THE OUTER SOLAR SYSTEM Asteroid Belt Jupiter (5.2 AU) zodiacal cloud1
1.5
5.2
9.5
19.2
30.1
39.4
.07
.58
1.2
4.0
2.5
5.3
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)

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

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

16. Total mass of Asteroid Belt only 0.0008 MEarth or 0.07 Mmoon.
Gaspra
Ida and Dactyl
Total mass of Asteroid Belt only MEarth or 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 Short Period Comets Long Period Comets
Comet Halley (1986)
Comet Hale-Bopp (1997)
Short Period Comets
Long Period Comets
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 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.
Coma and tail due to gas and dust removed from nucleus by Solar radiation and wind.
Far from Sun, comet is a nucleus only.

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

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

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

23. nucleus of comet Halley seen by Giotto
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 of Comet 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 of Comet 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 ~ 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. Did life originate in space?
Panspermia
Did life originate in space?

42. Asteroid Mining ? -- maybe
Finding
Prospecting
Images: Planetary Resources
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. René Descartes (1596 -1650) nebular theory:
Early Ideas
René Descartes ( ) 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. Well demonstrated by ice skaters . . .
But why is Solar System flat, and why do planets orbit faster than 1 km/s?
Pierre Laplace ( ): an important factor is "conservation of angular momentum":
When a rotating object contracts, it speeds up.
"momentum"
"angular momentum" (a property of a spinning or orbiting object)
mass x velocity
mass x velocity x "size"
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 ~ 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.

51. Remaining gas swept out by Solar Wind.
Asteroid Belt
Perhaps a planet was going to form there. But Jupiter's strong gravity disrupted the planetesimals' orbits, ejecting them out of Solar System. The Belt is the few left behind.
And Finally . . .
Remaining gas swept out by Solar Wind.

52. Result from computer simulation of planet growth
Shows growth of terrestrial planets. If Jupiter's gravity not included, fifth terrestrial planet forms in Asteroid Belt. If Jupiter included, orbits of planetesimals there are disrupted. Almost all ejected from Solar System.
Simulations also suggest a few Mars-size objects formed in Asteroid Belt. Their gravity modified orbits of other planetesimals, before they too were ejected by Jupiter's gravity.
Asteroid Ida