1. Meteorites, Asteroids, and Comets

2. Guidepost
In Chapter 19, we began our study of planetary astronomy by asking how our solar system formed. In the five chapters that followed, we surveyed the planets, but we gained only limited insight into the origin of the solar system. The planets are big, and they have evolved as heat has flowed out of their interiors. In this chapter, we have our best look at unevolved matter left over from the solar nebula. These small bodies are, in fact, the last remains of the nebula that gave birth to the planets.
This chapter is unique in that it covers small bodies. In past chapters, we have used the principles of comparative planetology to study large objects— the planets. In this chapter, we see that the same principles apply to smaller bodies, but we also see that we need some new tools in order to think about the tiniest worlds in the solar system.

3. Outline I. Meteorites A. Meteoroid Orbits
B. Meteorite Impacts on Earth
C. An Analysis of Meteorites
D. The Origins of Meteorites
II. Asteroids
A. The Asteroid Belt
B. Nonbelt Asteroids
C. Composition and Origin
III. Comets
A. Properties of Comets
B. The Geology of Comet Nuclei
C. The Origin of Comets

4. Outline (continued) IV. Impacts on Earth A. Impacts and Dinosaurs
B. The Tunguska Event

5. Comets of History
Throughout history, comets have been considered as portents of doom, even very recently:
Appearances of comet Kohoutek (1973), Halley (1986), and Hale-Bopp (1997) caused great concern among superstitious.
Comet Hyakutake in 1996

6. Meteorites Distinguish between: Meteoroid = small body in space
Meteor = meteoroid colliding with Earth and producing a visible light trace in the sky
Meteorite = meteor that survives the plunge through the atmosphere to strike the ground...
Sizes from microscopic dust to a few centimeters.
About 2 meteorites large enough to produce visible impacts strike the Earth every day.
Statistically, one meteorite is expected to strike a building somewhere on Earth every 16 months.
Typically impact onto the atmosphere with 10 – 30 km/s (≈ 30 times faster than a rifle bullet). .

7. Meteor Showers
Most meteors appear in showers, peaking periodically at specific dates of the year.

8. Radiants of Meteor Showers
Tracing the tracks of meteors in a shower backwards, they appear to come from a common origin, the radiant.
 Common direction of motion through space.

9. Meteoroid Orbits
Meteoroids contributing to a meteor shower are debris particles, orbiting in the path of a comet.
Spread out all along the orbit of the comet.
Comet may still exist or have been destroyed.
Only a few sporadic meteors are not associated with comet orbits.

10. Meteorite Impacts on Earth
Over 150 impact craters found on Earth.
Famous example: Barringer Crater near Flagstaff, AZ:
Formed ~ 50,000 years ago by a meteorite of ~ 80 – 100 m diameter

11. Impact Craters on Earth
Barringer Crater: ~ 1.2 km diameter; 200 m deep
Much larger impact features exist on Earth:
Impact of a large body formed a crater ~ 180 – 300 km in diameter in the Yucatán peninsula, ~ 65 million years ago.
Drastic influence on climate on Earth; possibly responsible for extinction of dinosaurs.

12. Finding Meteorites Good place to find meteorites: Antarctica!
Most meteorites are small and do not produce significant craters.
Good place to find meteorites: Antarctica!
Distinguish between:
Falls = meteorites which have been observed to fall (fall time known).
Finds = meteorites with unknown fall time.

13. Analysis of Meteorites
3 broad categories:
Iron meteorites
Stony meteorites
Stony-Iron meteorites

14. What Does a “Meteorite” Look Like?
Selection bias:
Iron meteorites are easy to recognize as meteorites (heavy, dense lumps of iron-nickel steel) – thus, more likely to be found and collected.

15. The Allende Meteorite
Carbonaceous chondrite, fell in 1969 near Pueblito de Allende, Mexico
Showered an area about 50 km x 10 km with over 4 tons of fragments.
Fragments containing calcium-aluminum-rich inclusions (CAIs)
Extremely temperature-resistant materials.
Allende meteorite is a very old sample of solar-nebula material!

16. The Origins of Meteorites
Probably formed in the solar nebula, ~ 4.6 billion years ago.
Almost certainly not from comets (in contrast to meteors in meteor showers!).
Probably fragments of stony-iron planetesimals
Some melted by heat produced by 26Al decay (half-life ~ 715,000 yr).
26Al possibly provided by a nearby supernova, just a few 100,000 years before formation of the solar system (triggering formation of our sun?)

17. The Origins of Meteorites (2)
Planetesimals cool and differentiate
Collisions eject material from different depths with different compositions and temperatures.
Meteorites can not have been broken up from planetesimals very long ago
 so remains of planetesimals should still exist.
 Asteroids

18. Asteroids
Last remains of planetesimals that built the planets 4.6 billion years ago!

19. Sizes and shapes of the largest asteroids, compared to the moon
The Asteroid Belt
Small, irregular objects, mostly in the apparent gap between the orbits of Mars and Jupiter.
Thousands of asteroids with accurately determined orbits known today.
Sizes and shapes of the largest asteroids, compared to the moon

20. Kirkwood’s Gaps
The asteroid orbits are not evenly distributed throughout the asteroid belt between Mars and Jupiter.
There are several gaps where no asteroids are found:
Kirkwood’s gaps (purple bars below)
These correspond to resonances of the orbits with the orbit of Jupiter.
Example:
2:3 resonance

21. Non-Belt Asteroids Not all asteroids orbit within the asteroid belt.
Apollo-Amor Objects:
Trojans: Sharing stable orbits along the orbit of Jupiter:
Asteroids with elliptical orbits, reaching into the inner solar system.
Trapped in the Lagrangian points of Jupiter.
Some potentially colliding with Mars or Earth.

22. Colors of Asteroids
M-type: Brighter, less reddish asteroids, probably made out of metal rich materials; probably iron cores of fragmented asteroids
S-type: Brighter, redder asteroids, probably made out of rocky materials; very common in the inner asteroid belt
C-type: Dark asteroids, probably made out of carbon-rich materials (carbonaceous chondrites); common in the outer asteroid belt
“Colors” to be interpreted as albedo (reflectivity) at different wavelengths.

23. The Origin of Asteroids
Distribution: S-type asteroids in the outer asteroid belt; C-type asteroids in inner asteroid belt  may reflect temperatures during the formation process.
However, more complex features found:
Vesta shows evidence for impact crater and lava flows.
Images of the Asteroid Vesta show a complex surface, including a large impact crater.
Heat for existence of lava flows probably from radioactive decay of 26Al.
Meteorite probably fragmented from Vesta

24. Comet Ikeya-Seki in the dawn sky in 1965
Comets
Comet Ikeya-Seki in the dawn sky in 1965

25. Two Types of Tails
Ion tail: Ionized gas pushed away from the comet by the solar wind. Pointing straight away from the sun.
Dust tail: Dust set free from vaporizing ice in the comet; carried away from the comet by the sun’s radiation pressure. Lagging behind the comet along its trajectory

26. Gas and Dust Tails of Comet Mrkos in 1957

27. Build A Comet
(SLIDESHOW MODE ONLY)

28. Comet Hale Bopp (1997)

29. Dust Jets from Comet Nuclei
Jets of dust are ejected radially from the nuclei of comets.
Comet Hale-Bopp, with uniform corona digitally removed from the image.
Comet dust material can be collected by spacecraft above Earth’s atmosphere.

30. Only small rocky fragments remained.
Fragmenting Comets
Comet Linear apparently completely vaporized during its sun passage in 2000.
Only small rocky fragments remained.

31. The Geology of Comet Nuclei
Comet nuclei contain ices of water, carbon dioxide, methane, ammonia, etc.:
Materials that should have condensed from the outer solar nebula.
Those compounds sublime (transition from solid directly to gas phase) as comets approach the sun.
Densities of comet nuclei: ~ 0.1 – 0.25 g/cm3
Not solid ice balls, but fluffy material with significant amounts of empty space.

32. Fragmentation of Comet Nuclei
Comet nuclei are very fragile and are easily fragmented.
Comet Shoemaker-Levy was disrupted by tidal forces of Jupiter
Two chains of impact craters on Earth’s moon and on Jupiter’s moon Callisto may have been caused by fragments of a comet.

33. The Origin of Comets
Comets are believed to originate in the Oort cloud:
Spherical cloud of several trillion icy bodies, ~ 10,000 – 100,000 AU from the sun.
Gravitational influence of occasional passing stars may perturb some orbits and draw them towards the inner solar system.
10,000 – 100,000 AU
Interactions with planets may perturb orbits further, capturing comets in short-period orbits.
Oort Cloud

34. The Kuiper Belt Kuiper belt, at ~ 30 – 100 AU from the sun.
Second source of small, icy bodies in the outer solar system:
Kuiper belt, at ~ 30 – 100 AU from the sun.
Few Kuiper belt objects could be observed directly by Hubble Space Telescope.
Pluto and Charon may be captured Kuiper belt objects.

35. Impacts on Earth
Comet nucleus impact producing the Chicxulub crater ~ 65 million years ago may have caused major climate change, leading to the extinction of many species, including dinosaurs.
Gravity map shows the extent of the crater hidden below limestone deposited since the impact.

36. The Tunguska Event
The Tunguska event in Siberia in 1908 destroyed an area the size of a large city!
Explosion of a large object, probably an Apollo asteroid of 90 – 190 m in diameter, a few km above the ground.
Energy release comparable to a 12-megaton nuclear weapon!
Area of destruction from the Tunguska event superimposed on a map of Washington, D.C. and surrounding beltway.

37. Impacts on Earth
(SLIDESHOW MODE ONLY)

38. New Terms radiant sporadic meteor fall find iron meteorite
selection effect
Widmanstätten pattern
stony meteorite
chondrite
chondrule
carbonaceous chondrite
CAI
achondrite
stony-iron meteorite
Kirkwood’s gaps
Apollo–Amor objects
Trojan asteroids
Hirayama families
gas (type I) tail
dust (type II) tail
coma
Oort cloud
Kuiper belt

39. Discussion Questions
1. Futurists suggest that we may someday mine the asteroids for materials to build and supply space colonies. What kinds of materials could we get from asteroids? (Hint: What are S-, M-, and C-type asteroids made of?)
2. If cometary nuclei were heated by internal radioactive decay rather than by solar heat, how would comets differ from what we observe?
3. From what you know now, do you think the government should spend money to locate near-Earth asteroids? How serious is the risk?

40. Quiz Questions
1. What type of meteorite is the most common, at about 80% of all falls?
a. Irons.
b. Stony-irons.
c. Chondrites.
d. Achondrites.
e. Carbonaceous chondrites.

41. Quiz Questions
2. If most falls are stony meteorites, why are most finds iron meteorites?
a. Stony meteorites are less weather-resistant.
b. Stony meteorites look more like Earth rocks.
c. Stony meteorites penetrate the ground more deeply.
d. Both a and b above.
e. All of the above.

42. Quiz Questions
3. How do observations of meteor showers reveal one of the sources of meteoroids?
a. The radiants of meteor showers are at locations where Earth crosses the debris trail of comets.
b. The meteorites that result from meteor showers contain icy materials that we match to comets.
c. Many shower meteors can be traced back to our moon.
d. Meteor showers are usually best viewed after midnight.
e. Both a and b above.

43. Quiz Questions
4. What evidence do we have that some meteorites have originated inside large bodies?
a. Some meteorites are very large.
b. Chondrules can only form inside a large body that cools slowly.
c. We can track their orbits back to the asteroid belt.
d. The Widmanstätten patterns in iron meteorites indicate very slow cooling.
e. Both a and c above.

44. Quiz Questions
5. Which type of meteorite is rich in volatiles, and are thus the best samples of the solar nebula?
a. Irons.
b. Stony-irons.
c. Chondrites.
d. Achondrites.
e. Carbonaceous chondrites.

45. Quiz Questions
6. Of the following, which type of meteorites comes from undifferentiated bodies?
a. Irons.
b. Stony-irons.
c. Chondrites.
d. Achondrites.
e. Both a and b above.

46. Quiz Questions
7. What do we suspect was the heat source that melted planetesimals that were as small as 20 km in diameter?
a. Impact energy of the planetesimals’ constituent particles.
b. Long-lived radioactive isotopes such as Uranium 238.
c. Short-lived radioactive isotopes such as Aluminum 26.
d. Gravitational energy released by differentiation.
e. Electrical discharges in the solar nebula.

47. Quiz Questions
8. Why do we think that a supernova event may be the source of the shockwave that triggered the gravitational collapse that formed the solar system?
a. Supernova events produce Aluminum 26.
d. The half-life of aluminum 26 is 715,000 years.
b. Some iron meteorites cooled in planetesimals as small as 20 km in diameter.
c. Magnesium 26 is found in meteorite minerals that usually contain aluminum.
e. All of the above.

48. Quiz Questions
9. What evidence do we have that meteorites are pieces of recently broken planetesimals?
a. The cosmic-ray-exposure ages of meteorites are typically less than 100 million years.
b. Some meteorites are pieces of differentiated larger bodies.
c. Some meteorites are breccias.
d. Both a and b above.
e. All of the above.

49. Quiz Questions
10. How can most meteors be cometary if most, perhaps all, meteorites are asteroidal?
a. Asteroids are rocky and metallic in composition.
b. Comet debris particles are small and full of volatiles.
c. In Earth’s vicinity the meteoroids are mostly cometary particles.
d. Both a and b above.
e. All of the above.

50. Quiz Questions 11. What causes the Kirkwood gaps of the asteroid belt?
a. Orbital resonances with Earth.
b. Orbital resonances with Mars.
c. Orbital resonances with Jupiter.
d. Orbital resonances with Saturn.
e. Both a and b above.

51. Quiz Questions
12. Studies show that the orbits of Apollo and Amor objects are not stable; that is, these orbits cannot have existed since the beginning of the solar system. What is most likely source of the Apollo-Amor objects?
a. They are asteroids that were ejected from the Kirkwood gaps.
b. They are most likely impact fragments from the Moon.
c. They are most likely impact fragments from Mars.
d. They have been perturbed from the Kuiper Belt.
e. They have been perturbed from the Oort Cloud.

52. Quiz Questions
13. In 1928 Kiyotsugu Hirayama grouped some asteroids into families. What is similar for the asteroids of one Hirayama family?
a. The semimajor axes of their orbits.
b. The eccentricity of their orbits.
c. The inclination of their orbits.
d. Both a and b above.
e. All of the above.

53. Quiz Questions
14. The small asteroid Braille and the Eucrite meteorites are believed to be pieces of the large asteroid Vesta. What evidence is there for this connection?
a. The Hubble Space Telescope has detected a large impact crater on Vesta.
b. The reflectance spectrum of the two asteroids matches the reflectance spectrum of the Eucrite meteorite group.
c. Calculating the orbits of the two asteroids backward in time shows that they were at the same location 50,000 years ago.
d. Both a and b above.
e. All of the above.

54. Quiz Questions
15. The three main classes of asteroids, based on similarities in their infrared reflectance spectra, are C, S, and M. How do the different types of meteorites match up to the main asteroid classes?
a. C-type = stony, S-type = iron, M-type = carbonaceous chondrites.
b. C-type = carbonaceous chondrites, S-type = stony, M-type = iron.
c. C-type = iron, S-type = stony, M-type = carbonaceous chondrites.
d. C-type = carbonaceous chondrites, S-type = iron, M-type = stony.
e. C-type = stony, S-type = carbonaceous chondrites, M-type = iron.

55. Quiz Questions
16. Why do we believe that comets are loosely consolidated, fluffy mixtures of ice and rock?
a. A few comets are known to have broken apart due to close passage to the Sun and Jupiter.
b. We have measured comet densities to range from 0.1 to 0.25 grams per cubic centimeter.
c. The spectra of comet tails reveals ionized molecules and atoms consistent with sublimated ices.
d. Comets have dust tails that are most likely rocky bits of material like those collected at high altitude.
e. All of the above.

56. Quiz Questions
17. What are the characteristics of a type I comet tail?
a. Type I tails generally point outward, away from the Sun.
b. Type I tails have an emission line spectrum of ionized gases.
c. Type I tails have a reflected solar absorption line spectrum.
d. Both a and b above.
e. Both a and c above.

57. Quiz Questions
18. If you analyze the chemical composition of several typical long period comets and several typical short period comets, you are likely to find a greater abundance of low condensation temperature ices in one group relative to the other. Which group has the greater abundance, and why?
a. Long-period comets, because they come from the Oort Cloud, which is at a greater distance from the Sun.
b. Short-period comets, because they come from the Kuiper Belt, which is closer to the Sun.
c. Long-period comets, because these bodies originally formed among the Jovian planets.
d. Short-period comets, because these bodies originally formed beyond the orbit of Neptune.
e. None of the above reasons will prove true.

58. Quiz Questions
19. A key feature of the impact hypothesis for the mass extinction that occurred 65 million years ago is the presence of the metallic element iridium in high abundance in the clay layer at the Cretaceous-Tertiary boundary. If both the meteoroid and Earth formed from the solar nebula, how could the metallic element iridium be in low abundance at Earth’s surface and in high abundance in meteoritic material?
a. Earth’s iridium resides in its core.
b. The meteoroid was from outside the Solar System.
c. The meteoroid was undifferentiated or a piece of a differentiated iron core.
d. Both a and b above.
e. Both a and c above.

59. Quiz Questions
20. Of the following, which is a major flaw with the comet impact hypothesis for the Tunguska Event of 1908?
a. A comet would have been easily seen in the predawn skies the morning of the impact.
b. A small comet body would vaporize much higher in Earth’s dense atmosphere.
c. The impact crater was too large for a comet body impact.
d. No known comets went missing in 1908.
e. No ice was found at the impact site.

60. Answers
1. c
2. d
3. a
4. d
5. e
6. c
7. c
8. e
9. e
10. e
11. c
12. a
13. e
14. d
15. b
16. e
17. d
18. d
19. e
20. b