Solar-System Debris

1. Solar-System Debris Haley’s Comet Asteroid 243 Ida The Planets and the Solar System VOCABULARY The solar system also includes debris such as comets, asteroids, and meteoroids. comet asteroid meteor meteorite meteor shower

2. Comets

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

4. Appearance of Comets • Observed since antiquity • Typical comets appear as rather faint, diffuse spot of light – smaller than the Moon, and many times less brilliant. • Small chunk of icy material that develop an atmosphere as they get closer to the Sun. • As they get “very close” they may develop a faint, nebulous tail extending far from the main body of the comet. • Appearance seemingly unpredictable • Typically remain visible for periods from a few days to a few months.

5. Comet structure • Theory of comet structure first proposed by Fred Whipple, Harvard, 1950. • Nucleus is • Solid object a few kilometers across • Composed of mainly water ice, with traces of other ices, mixed with silicate grains and dust. • Model known as the “dirty snowball” model. • Water vapor + other volatiles escape from the nucleus when heated by sunlight. • No large fragments of solid matter from a comet ever survived passage through Earth’s atmosphere – full composition of the nuclei - not known.

6. Comet Orbits • Scientific study of comets dates back to Newton who first recognized their orbit as elongated ellipse. • Edmund Halley (a contemporary of Newton) calculated/published 24 cometary orbits (1705). • Noted that the orbits of bright comets seen in 1531, 1607, 1682 were quite similar – and could be the same comet – returning to the perihelion every 76 years. He predicted a return of the comet in 1758. • When the comet did appear in 1758, it was given the name Comet Halley.

7. Comet Halley • Observed/Recorded on every passage at intervals from 74 to 79 years since 239 B.C. • Period variations caused by Jovian planets • 1910, Earth was brushed by the comet tail. – causing much public concern… • Last appearance in our skies – 1986. • Met by several spacecrafts • Return in 2061. • Nucleus approximately 16x8x8 kilometers.

8. Comet Components • dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the unaided eye; • ion tail:as much as several hundred million km long composed of plasma and laced with rays and streamers caused by interactions with the solar wind. • nucleus: relatively solid and stable, mostly ice and gas with a small amount of dust and other solids; • coma: dense cloud of water, carbon dioxide and other neutral gases sublimed off of the nucleus; • hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen;

9. 0 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

11. 0 Gas and Dust Tails of Comet Mrkos in 1957

12. 0 Comet Hale-Bopp in 1997

13. Discovery of C/Hale-Bopp • Discovered in 1995 by Alan Hale (professional astronomer in New Mexico, and Thomas Bopp (amateur astronomer in Arizona) • Both were observing at their home locations on the evening of July 22nd-23rd, 1995 with their amateur telescopes • Hale-Bopp located at 7.15 AU, just outside the Jupiter orbit! Alan Hale

14. “C” for Long Period • ~ 4200 yrs ago since last appearance…~2380 yrs for next appearance • Closest approach: • Earth: March 27, 1997 @ 1.315 AU • Sun: April 1, 1997 @ 0.914 AU

15. Some Properties of the Nucleus of Hale-Bopp • Known from measurements: • R  30 km (2nd largest comet!) • Spin Period  11.5 • Obliquity  86 degrees • Unknown, but guess: • Density  700 kg m-3 • Specific heat  1400 J kg-1 K-1 • Bond Albedo  0.04 • Emissivity  0.9 Top: Blue filter image. Bottom: false color version. Image credit: http://www2.jpl.nasa.gov/comet/ampo145.html

16. 0 The Geology of Comet Nuclei Comet nuclei contain ices of water, carbon dioxide, methane, ammonia, 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.

17. 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.

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

19. 0 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. Interactions with planets may perturb orbits further, capturing comets in short-period orbits. 10,000 – 100,000 AU Oort Cloud

20. Oort Cloud • Estimated 1012 comets in the Oort cloud. • 10 times this number of comets could be orbiting the Sun between the planets and the Oort cloud. • Such objects undiscovered because to small, to reflect sufficient light to be detectable at large distances, and because their stable orbit do not bring them closer to the Sun. • Total number of comets in the sphere of influence of our Sun could be of the order of 1013! • Represents a mass the order of 1000 Earths.

21. The Kuiper Belt Second source of small, icy bodies in the outer solar system: Kuiper Belt, at ~ 30 – 100 AU from the sun. Pluto and Charon may be captured Kuiper-Belt objects.

22. Kuiper Belt • First object discovered in 1992. • Diameter ~ 200 km. • Period ~ 300 years. • 60 objects found since then. • Share orbital resonance with Neptune – two orbits completed for three by Neptune. • Nicknamed Plutinos for this reason. • Speculated that Pluto is the largest example of this group.

23. Fate of Comets • Comets spent nearly all their existence in the Oort cloud or Kuiper belt • At a temperature near absolute zero. • As comet enter the Solar System, their “life” changes altogether! • If they survive the initial passage near the Sun, they return towards the cold aphelia – and may follow a quasi-stable orbit for a “while”. • May impact the Sun • May be completely vaporized as they fly by the Sun • May interact with a planet • Final impact • Speed up and ejection • Perturbed into an orbit of shorter period. • Each flyby the Sun reduces the size and mass of the nucleus of the comets. • Few comets end their life catastrophically by breaking apart. • Shoemaker-Levy 9 broke into ~20 pieces when it passed close to Jupiter in July 1992. • Fragments of Shoemaker-Levy captured into a very elongated 2 year around Jupiter – In 1994 the comet fragments crashed into Jupiter.

24. Conclusion • New examples are detected using spectra analysis at an average rate of 5- 10 per year. • Comparison of abundances to interstellar sources showed similarities between comets • There is a strong link between comets and interstellar ices. • Comets give clues about the origins of life, despite their historical role as omens of death and destruction.

25. Meteorites • Meteoroid= small body in space • Meteor= meteoroid colliding with Earth and producing a visible light trace in the sky (shooting star) Distinguish between: • Meteorite= meteor that survives the plunge through the atmosphere to strike the ground

26. Meteorites 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).

27. The Origins of Meteorites 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 → Remains of planetesimals should still exist. → Asteroids

28. Meteorite Types • Iron: primarily iron and nickel; similar to type M asteroids • Stony Iron: mixtures of iron and stony material like type S asteroids • Chondrite: by far the largest number of meteorites fall into this class; similar in composition to the mantles and crusts of the terrestrial planets

29. Meteorite Types • Carbonaceous Chondritevery: similar in composition to the Sun less volatiles; similar to type C asteroids • Achondrite: similar to terrestrial basalts; the meteorites believed to have originated on the Moon and Mars are achondrites

30. 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

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

32. The Leonid Meteor Shower in 2002

33. 0 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 few sporadic meteors are not associated with comet orbits.

34. Radiants of Meteor Showers Tracing the tracks of meteors in a shower backwards, they appear to come from a common origin, theradiant. ↔ Common direction of motion through space. The Perseid Meteor Shower

35. Asteroids

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

37. Asteroids • Asteroids are also categorized by their position in the solar system: • Main Belt: located between Mars and Jupiter roughly 2 - 4 AU from the Sun; further divided into subgroups: Hungarias, Floras, Phocaea, Koronis, Eos, Themis, Cybeles and Hildas (which are named after the main asteroid in the group). • Near-Earth Asteroids (NEAs): ones that closely approach the Earth

38. Asteroids • Atens: semimajor axes less than 1.0 AU and aphelion distances greater than 0.983 AU; • Apollos: semimajor axes greater than 1.0 AU and perihelion distances less than 1.017 AU • Amors: perihelion distances between 1.017 and 1.3 AU;

39. Where do we find most asteroids in the solar system? • In a belt between the Earth and Mars. • In a belt between Mars and Jupiter. • In a belt far outside the orbits of the planets. • On highly elliptical orbits, coming as close to the sun as Mercury’s orbit, and reaching as far out as Pluto’s orbit or beyond. • In elliptical orbits around Jupiter.

40. 0 The Asteroid Belt Most asteroids orbit the sun in a wide zone between the orbits of Mars and Jupiter. Mars Jupiter Pluto Uranus Saturn Neptune (Distances and times reproduced to scale)

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

42. 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. Some potentially colliding with Mars or Earth.

43. Non-Belt Asteroids • Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit). Several hundred such asteroids are now known; it is estimated that there may be a thousand or more altogether. Curiously, there are many more in the leading Lagrange point (L4) than in the trailing one (L5). (There may also be a few small asteroids in the Lagrange points of Venus and Earth (see Earth's Second Moon) that are also sometimes known as Trojans; 5261 Eureka is a "Mars Trojan".)

44. Asteroid Types Asteroids are classified into a number of types according to their spectra (and hence their chemical composition and albedo: • C-type, includes more than 75% of known asteroids: extremely dark (albedo 0.03); similar to carbonaceous chondrite meteorites; approximately the same chemical composition as the Sun minus hydrogen, helium and other volatiles; Image of 253 Mathilde, a 66 by 48 by 46 km C-type asteroid.

45. Asteroid Types On October 29, 1991, the Galileo spacecraft flew past 951 Gaspra, an S-type asteroid situated in the inner asteroid belt. Gaspra measures 19 by 11 kilometers, and its faceted shape suggests that it is a fragment from a larger object that was shattered by collision roughly 500 million years ago. • S-type, 17%: relatively bright (albedo .10-.22); metallic nickel-iron mixed with iron- and magnesium-silicates;

46. Asteroid Types Shape model rendering from radar data of the M-type asteroid 216 Kleopatra (NASA/JPL).The refelectance characteristics of M-type asteroids like Kleopatra suggestthat they may be composed of iron-nickel which hints at a possible source for iron meteorites. • M-type, most of the rest: bright (albedo .10-.18); pure nickel-iron. • There are also a dozen or so other rare types.

47. 0 Beyond the Solar System