1. Courtesy of NASA, ESA, and A. Feild (STScI) Pluto and Solar System Debris Chapter 10 10-5 thru 10-8

2. 10-5 The Oort Cloud and the Kuiper Belt 1. In 1950 Jan Oort proposed that a comet cloud exists in a spherical shell between 10,000 and 100,000 AU from the Sun. Billions of comet nuclei are thought to exist in this Oort cloud. 2. Long-period comets are believed to originate in the Oort cloud. Interactions between comets in the cloud or between a comet and a star passing-by could deflect some comets into the inner solar system.

3. Figure 10.17: Long-period comets are believed to originate in the Oort cloud.

4. 3.In 1951 Gerard Kuiper proposed that a second, smaller band of comets must exist inside the Oort cloud. The Kuiper belt is a disk-shaped region beyond Neptune’s orbit, 30–500 AU from the Sun, and presumed to be the source of short-period comets. 4. The first object in the Kuiper belt was observed in 1992. Eris, currently the largest dwarf planet, Pluto, Sedna and other trans-Neptunian objects are Kuiper-belt objects.

5. The Origin of Short-Period Comets 1. Long-period comets sometimes become short-period comets through the gravitational influence of Jupiter and the Sun. 2. Most comet orbits are either elliptical or parabolic (but not hyperbolic). Understanding how comets are distributed and what they are made of will put important constraints on models describing the formation and early evolution of our solar system. 3. The average distance between comets in the Oort cloud is greater than 16 AU! The Oort cloud and the Kuiper belt are far from crowded.

6. Question 1 Objects in the Oort Cloud can not be seen even with the best telescopes. Why then, do we believe it exists?

7. 10-6 Meteors and Meteor Showers 1. A meteor is the phenomenon of a streak in the sky caused partially by the burning of a rock or dust particle as it falls into our atmosphere. 2. A meteoroid is an interplanetary chunk of matter smaller than an asteroid. 3. A meteorite is an interplanetary chunk of matter after it has hit a planet or moon. 4. The first confirmation of a fall of rocks from an exploding meteor occurred in France in 1803.

8. Figure 10.20: The streak of light is a Perseid meteor. © Sven Kohle, AlltheSky.com

9. Meteors 1. A fireball is an extremely bright meteor. 2. A meteoroid’s typical speed is 50 km/s (100,000 mph), so when it hits the Earth’s atmosphere, it heats up and begins to vaporize. 3. A typical meteor begins to glow at a height of 130 km (80 mi) and burns out at about 80 km (50 mi).

10. Meteoroids 1. Most meteors are produced by meteoroids with masses ranging from a few milligrams (grain of sand) to a few grams (marble-size rock). 2. Since a meteor can be seen only if it is within 150– 200 km of the viewer, it is calculated that over the entire Earth there must be about 25 million meteors a day visible to the naked eye. 3. It is estimated that 1,000 tons of meteoritic material hit the Earth every day.

11. 5. Unlike most asteroids, meteoroids may orbit the Sun in any orientation. 6. It is thought that many small meteoroids are debris from asteroid collisions. 7. Many meteors come from material evaporated from a comet’s nucleus.

12. Meteor Showers 1. A meteor shower is the phenomenon of a large group of meteors seeming to come from a particular area of the celestial sphere. It is actually caused by the Earth passing through a swarm of small meteoroids. 2. Meteor showers are named after the constellation from which they seem to originate. 3. The radiant of a meteor shower is the point in the sky from which the meteors of a shower appear to radiate.

13. Figure 10.23a: Meteors in a meteor shower seem to come from a single point in the sky.

14. Figure 10.21: Leonid meteor shower Courtesy of Kitt Peak National Observatory/AURA/NOAO/NSF

15. 4. Most of the major meteor showers are associated with comets. 5. Some showers change in intensity from year to year because the particles that cause the shower clump together in one region of the comet’s orbit. 6. The best time to observe a meteor shower is in the early morning hours. This results from the Earth’s rotation and its motion through the swarm of particles left behind by the comet.

17. Question 2 What causes a meteor shower and why do the meteors appear to come from just one part of the sky?

18. 10-7 Meteorites and Craters 1. Meteorites are classified into 3 categories: (a) Irons—iron meteorites that are made up of 80%– 90% iron (with some nickel). (b) Stones—stony meteorites that can contain flakes of iron and nickel. (c) Stony irons—meteorites that are half stone and half iron. 2. About 95% of all meteorites are stones but if you find one it will likely be an iron.

19. Figure 10.24a: A part of an iron meteorite that fell in Canyon Diablo, Arizona. Courtesy of Theo Koupelis

20. Figure 10.24b: The sawed face of the stony Martian meteorite Courtesy of NASA/JPL-Caltech

21. 3. Chondrites comprise an important class of stony meteorites; they make up 91% of the about 24,000 known meteorites. They formed early in the history of the solar system and they are thought to be the building blocks of the planets. They consist mostly of chondrules, small rock spheres in a mix of other mineral or metal grains. 4. Carbonaceous chondrites are the most important subclass of chondrites. They contain high levels of water, organic compounds and minerals.

22. Figure 10.24c: A thin section from a C2 chondrite meteorite Courtesy of ANSMET/NASA

23. 3. The Hoba meteorite in Namibia weighs 65 tons and is the largest meteorite ever found. 4. The second largest (34 tons) is on display in New York City at the American Museum of Natural History. 5. One of the most prominent impact craters on Earth is Meteor Crater near Winslow, Arizona. It is nearly a mile across, 180 m (600 ft) deep and has a rim rising 45 m (150 ft) above the surrounding desert.

24. Figure 10.25: Meteor Crater Courtesy of Meteor Enterprises Inc., Flagstaff, AZ.

25. 6. The meteorite that formed Meteor Crater is estimated to have had a total mass of 300 million kg (300,000 tons) and to have been about 45 m across. It struck about 25,000 years ago at a speed of about 11 km/s (25,000 mph), releasing an energy equivalent to a 25-megaton bomb. 7. It is estimated that a meteorite larger than 1 km in diameter strikes the Earth on average once every few hundred thousand years.

26. 8. A hit by a 1-km meteorite would produce a crater 10- km in diameter and be equivalent to a 5000- megaton bomb. 9. There is compelling evidence that an asteroid some 10 km in diameter struck the Earth (near the Yucatan peninsula) 65 million years ago and led to the subsequent extinction of the dinosaurs. Figure 10.B03: Chicxalub Crater in Mexico

27. 10-8 The Importance of the Solar System Debris 1. We believe that life on Earth started about 3.8 billion years ago, at the end of the heavy bombardment period. There is evidence for biological activity at the end of this period and known fossils on Earth date as far back as 3.5 billion years ago. 2. The building blocks of life could have been delivered by asteroid and comet impacts.

28. 3. Understanding the chemical makeup of comets helps in understanding the composition and conditions of Earth’s formation 4.6 billion years ago. 4. Collisions supplied Earth with water, volatiles and carbon-based molecules. 5. Understanding the structure, composition and orbits of space debris will help protect against catastrophic collisions.

29. 6. Asteroids and comets may prove to be a rich supply of water, life-sustaining carbon-based molecules and structural raw materials to supply space exploration and construction

30. Question 3 Name the three main types of meteorites. Which is most common? Which is easiest to find? Why?