ASTRONOMY 161. Introduction to Solar System Astronomy. Class 26

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1 ASTRONOMY 161 Introduction to Solar System Astronomy Class 26

2 Asteroids Friday, March 9

3 and Comets

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9 Ceres: Basic characteristics Mass = kg Diameter = 920 km Density = 2,080 kg/m³ Sidereal rotation period = d Albedo = 0.11 Average distance from Sun = 2.77 A.U.

10 Asteroids: Key Concepts (1) Asteroids are mostly in the asteroid belt between Mars and Jupiter. (2) Asteroids are strongly influenced by the gravity of Jupiter. (3) Asteroids are made of rock, metal, or a combination of both. (4) Meteorites are usually asteroid fragments that have struck Earth.

11 (1) Most asteroids are in the asteroid belt Late 18 th century: Astronomers were puzzled by the gap between Mars (1.5 A.U.) and Jupiter (5.2 A.U.). They searched for a planet there. between Mars and Jupiter.

12 January 1, 1801: Discovery of the object Ceres Diameter = 920 kilometers. Orbit semimajor axis = 2.8 A.U. When additional bodies were found between Mars and Jupiter, they were collectively named ASTEROIDS.

13 Hundreds of thousands of asteroids have been discovered within the Solar System. 136,000 have orbits known well enough to be given official numbers. Of these, some 13,500 have official names.

14 About 90% of asteroids lie in the asteroid belt between Mars and Jupiter. The asteroid belt contains about 100,000 asteroids more than one kilometer across. Distance between asteroids is much larger than the size of an asteroid.

15 Ceres: largest asteroid/smallest dwarf planet. Internal structure similar to Pluto: Contains 1/3 of the belt's total mass.

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18 Asteroids are not uniformly distributed through the asteroid belt. For certain orbit size, there are very few asteroids. (These gaps in the distribution are called Kirkwood gaps.)

19 (2) Asteroids are strongly influenced by the gravity of Jupiter. Kirkwood gaps are caused by an orbital resonance with Jupiter. (Similar to the orbital resonance that creates gaps in Saturn s rings.)

20 Jupiter has a = 5.2 A.U. and thus P = years. If an asteroid had a = 3.28 A.U., it would have P = 5.93 years No asteroids have a = 3.28 A.U. The 2:1 orbital resonance would tug them out of the gap. Similar gaps at 3:1 resonance, 5:2 resonance, etc

21 Two groups of asteroids precede and follow Jupiter on its orbit. They are called the Trojan asteroids (named after Homeric heroes).

22 Trojan asteroids are kept from straying by the combined gravity of Sun and Jupiter. Jupiter has some 1800 known Trojan asteroids (1100 Greeks and 700 Trojans).

23 Designation (and name) Prov. Des. Ln q Q H Epoch M Peri. Node Incl. e a Opps. Ref. (588) Achilles 1906 TG L MPC (617) Patroclus 1906 VY L MPC (624) Hektor 1907 XM L MPO (659) Nestor 1908 CS L MPO (884) Priamus 1917 CQ L MPC (911) Agamemnon 1919 FD L MPO (1143) Odysseus 1930 BH L MPC 22574

24 The total mass of all asteroids is estimated to be 1/15 th mass of the Moon. Q: Why didn t asteroids accumulate to form a fifth terrestrial planet? A. Because they are stirred up by Jupiter s gravit Some astronomers postulate that such a planet ( Artemis ) actually did form, but then was ejected by Jupiter 700 million years after the Solar System was formed. Could explain fairly low mass of Mars.

25 Asteroids occasionally collide. The asteroid Ida (as seen by Galileo spacecraft) has a moon, named Dactyl. Dactyl is probably a chip off the old block.

26 Near-Earth Objects (NEOs)

27 (3) Asteroids are made of rock, metal, or a combination of both. What asteroids are made of can be deduced from their spectra. Some are rich in iron and nickel. Other (such as the asteroid Eros) are rich in rocks of various types:

28 Density of asteroids: 1 Ceres 2.05 ± Pallas 4.2 ± Vesta 4.3 ± Psyche 1.8 ± Massalia 2.7 ± Eugenia 1.2 ± Hermione1.8 ± Ida 2.7 ± Mathilde 1.3 ± Eros 2.67 ± 0.0

29 Jargon Alert! ASTEROID: Object of rock or metal, smaller than a planet, orbiting the Sun. METEOROID: Object of rock or metal, smaller than an asteroid, orbiting the Sun. (Objects less than a few hundred meters across qualify as meteoroids.)

30 Meteoroids occasionally collide with Earth. METEOR: Streak of light produced as a meteoroid enters the Earth s atmosphere. METEORITE: Remnant of a meteoroid that survives the passage through the Earth s atmosphere.

31 The Earth sweeps up 300 tons of meteoroids every day. Most meteoroids are vaporized by friction in the atmosphere. Only meteoroids over an inch across survive the plunge and become meteorites.

32 (4) Meteorites are usually asteroid fragments that have struck the Earth. 95% of all meteorites are stony meteorites (with spectra similar to rocky asteroids). It is sometimes hard to tell stony meteorites from plain Earth rocks. New Concord, OH, ordinary chondrite. May 1, 1860

33 4% of all meteorites are iron meteorites (with spectra similar to metal-rich asteroids). Iron meteorites can be found with a metal detector. The remaining 1% of meteorites are stony iron meteorites.

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35 Special Meteorites Some stony meteorites are from the Moon or Mars. Other (~5%) stony meteorites are carbonaceous chondrites, containing complex carbon compounds. Oldest carbonaceous chondrites: 4.56 billion years old.

36 The orbits of some meteoroids (such as the Tagish Lake meteoroid) have been traced back to the asteroid belt. Large meteoroids are chips of asteroids. Small meteoroids are dust shed by comets.

37 Comets are big, dusty snowballs. If a comet comes close to the Sun, the ice is vaporized, and the dust is freed. Thus, comets in the inner Solar System are surrounded by dust clouds. If the Earth passes through the dust, a meteor shower results.

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43 Comets: Key Concepts (1) Comets are dirty snowballs : ice mixed with dust & carbon compounds. (2) When a comet is close to the Sun, it grows an ion tail and a dust tail. (3) Most comets are in the Kuiper belt or the Oor cloud, far from the Sun. (4) A comet or asteroid impact may have caused the extinction of dinosaurs.

44 Naked-eye comets are spectacular: for much of history they were unpredictable. Adoration of the Magi, Giotto, AD 1304

45 18 th Century: Edmund Halley found that a comet in AD 1607 had the same orbit as a comet in AD 1682 It is the same comet! Now called comet Halley; its most recent return was in 1986.

46 Comet Halley has an orbit which is highly eccentric highly inclined, and retrograde Orbital period of 76 years

47 (1) Comets are dirty snowballs : ice mixed with dust and carbon compounds. Strip away their tails, and comets are just snowballs several km across. A comet contains: frozen water, frozen carbon dioxide, ammonia, dust & rocks, carbon, complex carbon compounds

48 Dirty snowball model for the comets was first proposed in 1950 by Fred Lawrence Whipple ( ). During World War II, Whipple invented a device for cutting tinfoil into chaff to confuse enemy radar tracking Allied aircraft. From 1955 director of the Smithsonian Astrophysical Observatory, remaining in this post until Whipple Observatory on Mount Hopkins in Arizona is named after him.

49 The nucleus (central snowball) of comet Halley Irregular in shape, very dark, and very low in density. Emitting jets from the sunlit side.

50 Comet nuclei are fragile. When comet Shoemaker-Levy 9 went inside the Roche lobe of Jupiter, it was immediately ton apart:

51 (2) When a comet is close to the Sun, it grows an ion tail and a dust tail. When a comet comes close to the Sun: 1) Nucleus: snowball 10 km across. 2) Coma: gas cloud 1 million km across. 3) Tails: 100 million km long!

52 A tale of two tails ION TAIL (top, blue): ionized gas pushed away from the Sun by the solar wind. DUST TAIL (bottom, white): fine dust pushed away from the Sun by radiation pressure.

53 A tale of two tails

54 Some comets, like Comet Halley, have orbital periods of less than 200 years. However, other comets, like Comet Hale- Bopp, have orbital periods of millions of years.

55 (3) Most comets are in the Kuiper belt or the Oort cloud, far from the Sun. Comets with short orbital periods come from the Kuiper belt, A.U. from the Sun. We know the Kuiper belt is full of icy objects we have seen them!

56 Comets with long orbital periods come from the Oort cloud, ,000 A.U. from the Sun. Oort cloud is a swarm of comets that stretches one-fifth the way to the nearest neighboring star.

57 It is estimated there are several trillion comets in the Oort cloud. Total mass of Oort cloud = 100 times mass of Earth. We only see those rare comets on extremely eccentric orbits.

58 A comet or asteroid packs quite a wallop. An object 10 kilometers across, traveling at 10 km/sec, will upon impact release as much energy as a million 100 megaton bombs. It will blast out a crater at least 100 kilometers across.

59 (4) A comet or asteroid impact may have caused the extinction of the dinosaurs. In the Cretaceous mass extinction, 65 million years ago, 70% of all species were killed off including all dinosaurs. The extinction was rapid, geologically speaking.

60 Current favorite explanation: COLLISION HYPOTHESIS A comet or asteroid struck the Earth 65 million years ago, triggering the mass extinction. Testing the Collision Hypothesis: Look for a crater 65 million years old, as least 100 km across.

61 Recent discovery: The Chicxulub impact crater, 180 km across, buried under thick sediment. Estimated age: million years.

62 Radar topography reveals the 180 kilometer (112 mile) wide ring of the crater.

63 Did the impact cause the extinction, or was it just a coincidence? Few dinosaurs were hit on the head by the impact. More were killed by the blastwave. To kill all the dinosaurs, you need an indirect method.

64 Killing Method: Colliding object ejects dust into atmosphere Months of darkness and cold Death of plants Starvation of herbivores Starvation of carnivores (Not to mention the tsunamis, forest fires, acid rain, and other unpleasant events.)

65 Few closing questions: 1) Why is the ion tail blue? 2) Why is the dust tail white? 3) Why is the ion tail straight? 4) Why is the dust tail curved? 5) The Tunguska event: June 30, ) Why do we suspect the presence of the Oort cloud?

66 Few closing questions: 1) How do we know the mass of Ceres? 2) Can asteroids have atmosphere? 3) Do asteroids have volcanoes? 4) Do asteroids have craters? 5) Do asteroids have satellites? 6) How do we know the density of asteroids?