Interplanetary Vagabonds

Transcription

1 Interplanetary Vagabonds Lecture19-1

2 The Vagabonds Asteroids Rocky objects primarily between Mars and Jupiter Comets Objects in highly eccentric orbits Meteoroids Bright objects streaking across the sky Beyond Neptune Lecture19-2

3 Asteroids Primarily between the orbits of Mars and Jupiter Upwards of 200,000 asteroids have been discovered and the numbers of charted ones grow by 150 to 200 per year Measured sizes range from hundreds of kilometers down to kilometer size objects Smaller ones exist The large asteroids tend to be spherical, while the smaller ones are jagged in shape Are in the plane of ecliptic and most have eccentric orbits Lecture19-3

4 Asteroids Vary in composition and reflectivity Composition varies from metal free to those containing metallic compounds Three-fourths of the larger ones are carbonaceous, while most of the rest are silicon bearing Lecture19-4

5 Asteroids Lecture19-5

6 Asteroids Are classified in types: C-type (common in outer asteroid belt) Extremely dark low reflectivity 75% of all asteroids No evidence of high mineral content Carbon rich S-type (silicates inner belt) Spectral evidence for olivine a silicate mineral M-type (rare) Metallic - iron and nickel Lecture19-6

7 Asteroids Two small S-type asteroids, Gaspra and Ida, were visited by the Galileo probe Gaspra (left) is in false color; it is really gray Ida (right) has a small moon, Dactyl. Lecture19-7

8 Asteroids Mathilde, a C-Type asteroid, like many other asteroids, has a very low density and is probably not solid Lecture19-8

9 Asteroids Eros does seem to be solid Lecture19-9

10 Apollo Asteroids Some asteroids have very eccentric orbits so that they cross Earth s orbit They are called Apollo asteroids and raise the concern of a possible collision with the Earth 4400 such asteroids have been discovered so far, of which about 800 have been designated as potentially hazardous, due to their size Another group of asteroids that only intersect the orbit of Mars Amor Asteroids Lecture19-10

11 Asteroid Collisions Asteroids also collide with each other breaking into fragments Fragments then travel together following the original orbits The groups of fragments are known as Hirayama Families If the relative velocity is high enough then the resulting fragments can be kicked out of asteroid area These fragments are known as meteoroids when traveling through the atmosphere If they survive to hit the ground they are known as meteorites Lecture19-11

12 Trojan Asteroids Some asteroids, called Trojan asteroids, orbit at the L 4 and L 5 Lagrange points of Jupiter s orbit Lecture19-12

13 Distribution of Asteroids Not uniformly distributed between Mars and Jupiter Gaps in the radial positions of the asteroids The gaps are not random Gaps were explained by Kirkwood They are due to orbital resonance effects with Jupiter Orbital period of asteroid is an integer multiple of Jupiter's period Lecture19-13

14 Meteoroids, Meteors and Meteorites A meteoroid is a small asteroid, which becomes a meteor or shooting star as it penetrates the Earth s atmosphere While there is no precise definition for the size of an meteoroid, objects smaller than about 100 m across are typically termed meteoroids Most vaporize completely before striking the ground A meteorite is a portion of a meteor reaching the ground intact, possibly producing an impact crater A meteor shower occurs when the Earth moves through the debris left behind by a comet Carbon dating shows that the oldest meteorites are about 4½ billion years old; i.e. the same age as the solar system Lecture19-14

15 Meteoroids On an average dark night, you can see a few meteors every hour The flash is caused by heating Most meteors do not survive to reach the ground Lecture19-15

16 Types of Meteorites Stony meteorites (94% of all falls) ordinary chondrites (~80%) chondrules - silicate balls other inclusions - Al, Mg, S achondrites (~10%) stones with no inclusions carbonaceous chondrites (~ 4 %) carbon-rich inclusions complex organic molecules (amino acids) water-rich chondrules - never melted! Iron meteorites (5% of all falls) nearly pure iron/nickel alloy large crystals - very slow cooling Stony- Iron meteorites (1% of all falls) mixture of iron and slicates Lecture19-16

17 Meteoroids Are defined as being less than 100 m in diameter Most of the smaller ones are the remnants of comets that have broken up If the Earth s orbit intersects the comet s, meteor showers will occur every year on the same date, until the meteoroids have burned out Lecture19-17

18 80 -Ton Asteroid Hits the Nubian Desert In October 2008, an asteroid (which could also be called a meteorite) about 12 ft across with a mass of about 80,000 kg (80 metric tons) exploded at an altitude of about 37 km, the parts scattering in the Nubian desert of Sudan By a lucky accident, asteroid 2008 TC3 became the first such object to be observed before impact Fifteen fragments were later recovered over a region of length 29 km The black jagged rocks collected contained metals, such as iron and nickel, graphite, and nanodiamonds Lecture19-18

19 Stony Meteorite Found in Texas Stony meteorites look like ordinary rocks, often covered with a dark crust, caused by the melting of the outer surface during its descent through the atmosphere Lecture19-19

20 Iron Meteorite Found in Australia Iron meteorites contain iron-nickel compounds Lecture19-20

21 Impact Crater Formed by a 12 kg Meteorite Lecture19-21

22 Meteor Crater, Arizona Caused some 50,000 years ago by a meteoroid about 50 m in diameter Meteor crater is about 1.2 km across and 200 m deep Lecture19-22

23 Meteoroids Larger meteoroids are usually loners from the asteroid belt and have produced most of the visible craters in the Solar System The Earth has about 100 craters more than 0.1 km in diameter Erosion has made most of them hard to discern One of the largest is in Canada: Lecture19-23

24 Meteoroids Here are the major meteor showers Lecture19-24

25 Meteoroids Meteoroids that burn up in the Earth s atmosphere have densities of 500 to 1000 kg/m 3 and are probably comet-like in composition Meteoroids that reach the surface have densities around 5000 kg/m 3 and are similar to asteroids Lecture19-25

26 Meteoroids Most meteorites are rocky (left) Some are iron (right): Lecture19-26

27 Comets Comets differ from asteroids in several ways Asteroids are in the plane of the ecliptic Comets are randomly oriented with respect to the ecliptic plane Asteroids orbits largely confined to be between Mars and Jupiter Comets have highly elliptical orbits, coming close to the Sun Comets contain ices and dust particles When they do come close to the Sun, they begin to glow Lecture19-27

28 Comets Comets that come close enough to the sun to be detectable from Earth have very eccentric orbits Lecture19-28

29 Nucleus Comet Structure Solid part of the comet From here come the ices and dust that vaporize Size ranges from a few km to 40 km Composition variable, though H 2 0, CO, CO 2 are seen Coma The glowing ball that is seen Caused by the vaporization of the outer layers of the comet s nucleus Interaction with solar radiation and transform chemically Anywhere from 10 5 to 10 6 km in size Also surrounded by a cloud of hydrogen gas This hydrogen cloud could be ten times larger Lecture19-29

30 Comet s Tail Two tails to a comet 1. Tail that is pushed by the solar wind is pointed directly away from the Sun and it is ionized 2. Tail that is pushed by the radiation pressure is curved and contains both dust particles and ions Comet Hale-Bopp Lecture19-30

31 Comets As the comet approaches the Sun The incident sunlight heats the surface of the comet The heat energy warms the nucleus The surface becomes warm enough that ices sublimate into gases A dusty crust forms around the nucleus, insulating the nucleus and regulating the sublimation process Lecture19-31

32 Halley s Comet One of the most famous It has a period of 76 years and has been observed since antiquity Its most recent visit, in 1986, was not spectacular Left: The comet in 1910, as seen with the naked eye Right: The comet in 1986, as seen through a telescope Lecture19-32

33 Halley s Comets Has a shorter period than most comets, but its orbit is not in the plane of the solar system, probably due to an encounter with a larger object Lecture19-33

34 Mark Twain I came in with Halley's Comet in It is coming again next year (1910), and I expect to go out with it. Lecture19-34

35 Comets Typical cometary mass: to kg Each trip close to the Sun removes some material Halley s comet, for example, is expected to last about another 40,000 years Sometimes a comet s nucleus can disintegrate violently, as comet LINEAR did Lecture19-35

36 Stardust Mission Flew through the tail of comet Wild-2, gathering dust particles in detectors and returning them to Earth for analysis Findings indicate the formation of at least some comets may have included materials ejected from the inner solar system to the far and cold outer edge of the solar nebula Found very high-temperature minerals Comets are not composed entirely of volatile rich materials but rather are a mixture of materials formed at all temperature ranges, at places very near the early sun and at places very remote from it Lecture19-36

37 Deep Impact Mission Mission slammed a projectile into comet Tempel 1 Studied the material expelled in order to analyze the composition of the comet Material excavated by the impact Contained more dust and less ice than had been expected Finer than expected Included clays, carbonates, sodium, and crystalline silicates Comet was about 75% empty space Lecture19-37

38 Comets Most comets that enter the inner solar system reside in the Kuiper belt outside the orbit of Neptune Occasionally a comet from the far larger Oort cloud wanders into the inner solar system as well Lecture19-38

39 Comet Knowns and Unknowns Comet Facts Comets have the most primitive, accessible material in the solar system. Comets must become dormant. There must be many dormant comets masquerading as asteroids. We know more chemical and physical details than for other small bodies. Abundance of gases in the coma is widely used to infer the ices in the protoplanetary disk. Comets break apart under small stresses. What We Still Don't Know We do not know what is hidden below the evolved surface layers. We do not know whether ice is exhausted or sublimation is inhibited. We do not know how to identify these bodies. We do not know how to use these details to constrain our models for comets. We do not know the relationship between coma abundances and those in the nucleus. Nothing is known about variation of material strength with scale. Lecture19-39

40 Pluto Pluto was discovered in 1930 It was thought to be needed to explain irregularities in the orbits of Uranus and Neptune, but it turned out that there were no such irregularities Lecture19-40

41 Pluto Data Average distance from Sun: 39.5 AU Mass: 0.2% of Earth (0.002 M E ) Diameter: 2370 km (19% of Earth, 49% of Mercury) Average density: 36% Earth density Orbital eccentricity: 0.25 Siderial revolution period: 248 Earth years Rotation period: 6.4 Earth days (retrograde) For 20 years of its 248 year orbit, Pluto is closer to the Sun than Neptune, to which it is locked in a 3:2 orbital resonance Lecture19-41

42 Pluto s Orbit There is an angle of 17 o between the orbits of Pluto and Neptune Orbit also crosses the orbit of Neptune Lecture19-42

43 About Pluto and Charon Pluto s orbit is significantly more elliptical and more tilted with respect to the ecliptic than is any other planet Pluto s satellite Charon has a diameter of 1190 km (just half that of Pluto), and its distance from Pluto is less than 5% of the Moon-Earth distance Pluto s surface contains frozen nitrogen, methane and carbon monoxide (CO), while Charon s may be covered with water ice Pluto and Charon move in a unique form of synchronous rotation, in which each has a face locked to the other body Thus, observed from Pluto, Charon, would appear to hover in the sky, and vice-versa Lecture19-43

44 Pluto Observations of eclipses of Pluto and Charon allowed measurement of orbital details Lecture19-44

45 Kuiper Belt The solar system appears to contain two comet reservoirs The Kuiper Belt, estimated to contain over a hundred million comets, begins near Neptune in the plane of the ecliptic, extending from 30 AU to at least 55 AU from the Sun For reasons not yet known, Kuiper Belt objects show a wide range of color, from slightly blue to very red The dwarf planets, Pluto, Eris, Makemake and Haumea are Kuiper Belt objects Eris, which is about 50% larger than Pluto, is further from the Sun, with an elliptical orbit, of period about 650 years Other large objects in the Kuiper Belt are Sedna and Quaoarh Lecture19-45

46 Kuiper Belt Comparison of several trans-neptunian objects with Earth and its moon Lecture19-46

47 Kuiper Belt Pluto is the second largest known object in the Kuiper belt 47 Lecture19-47

48 Oort Cloud The Oort cloud is a spherical distribution of icy bodies orbiting the Sun, which extends from about 10,000 AU to about 100,000 AU 100,000 AU is about 9.3 trillion miles (or 1.6 ly), which is almost 40% of the way to the nearest star, Proxima Centauri The periods of most comets from the Oort cloud may be millions of years, compared to about 100 years for the Kuiper belt comets The Oort cloud is estimated to contain trillions of comets, only very few of which get gravitational pushes into the inner solar system, where there become short-period comets with altered orbits, such as Halley s Comet 48 Lecture19-48

49 The Oort Cloud 49 Lecture19-49