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Transcript
Chapter 12 Asteroids Comets, Asteroids, Comets and Dwarf D arf Planets Their Nature, Orbits, and Impacts 12.1 Asteroids and Meteorites Our goals for learning: • What are asteroids like? • Why is there an asteroid belt? • Where do meteorites come from? Can Asteroids destroy the Earth ? Asteroid is messing up my picture! Vermon of the sky! y Galileo at launch from shuttle Great asteroid images on the way to Jupiter! Gaspra close up via Galileo HOW DID GALILEO GET SAFELY ACROSS THE ASTEROID BELT! Ida & moon Dactyl Galileo photo album Mathilde G Gaspra Id Ida NEAR –Shoemaker Spacecraft MISSION Fall 2000spring 2001 N.E.A.R. = NEAR EARTH ASTEROID RANDEVOUS Eros Eros close-up Asteroid Facts • Asteroids are rocky leftovers of planet formation. g is Ceres,, diameter ~1,000 , km • Largest • 150,000 in catalogs, and probably over a million with diameter >1 km. • Small asteroids are more common than large asteroids. • All the asteroids in the solar system wouldn’t add up to even a small ll terrestrial t t i l planet. l t Gaspra Matilde eros id ida Asteroids are cratered and not round Asteroids with ith Moons • Some large asteroids have their own moon • Asteroid Ida has a tiny moon named Dactyl Densit of Asteroids Density • Measuring orbit of asteroid’s moon tells us asteroid’s mass • Mass and size tell us asteroid’s density • Some asteroids are solid rock; others just piles of rubble Asteroid Orbits • Most asteroids orbit in a belt between Mars and Jupiter • Trojan asteroids follow Jupiter’s Jupiter s orbit • Orbits of near-Earth asteroids cross Earth’s orbit Thought Question Whyy are there very y few asteroids beyond y Jupiter’s p orbit? A. There was no rocky material beyond Jupiter’s orbit. orbit B. The heaviest rocks sank towards the center of the solar system. C. Ice could form in the outer solar system. D. A passing star probably stripped away all of those asteroids asteroids, even if they were there at one time. Thought Question Why are there very few asteroids beyond Jupiter’s orbit? A There A. Th was no rocky k material t i l beyond b d Jupiter’s J it ’ orbit. B The heaviest rocks sank towards the center of the B. solar system. C. Ice could form in the outer solar system. D. A passing star probably stripped away all of those asteroids, even if they were there at one time. ti Why is there an asteroid belt? Thought Question Which explanation for the belt seems the most plausible? A. The belt is where all the asteroids happened to form. B. The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C. The belt is where all the asteroids happened to survive. Thought Question Which explanation for the belt seems the most plausible? A The belt is where all the asteroids happened to A. form. B. The belt is the remnant of a large terrestrial planet that used to be between Mars and Jupiter. C. The belt is where all the asteroids happened pp to survive. But WHY didn’t they f form a little littl planet? l t? Origin of Asteroid Belt • Rockyy planetesimals p between Mars and Jupiter did not accrete into a planet. planet p gravity, g y, • Jupiter’s through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet. Orbits of known Asteroids Kirkwood’s Gaps • The asteroid orbits are not evenly distributed throughout the asteroid belt between Mars and Jupiter. • There Th are severall gaps where h no asteroids t id are ffound: d Kirkwood’s gaps (purple bars below) These correspond to resonances of the orbits with the orbit of Jupiter. p Example: 2:3 resonance Orbital Resonances • Asteroids in orbital resonance with Jupiter experience periodic nudges n dges • Eventually those nudges move asteroids out of resonant orbits, leaving gaps in belt Non-Belt Asteroids Not all asteroids orbit within the asteroid belt. Apollo Amor Apollo-Amor Objects: Asteroids with elliptical orbits, reaching into the inner solar system. Some potentially colliding g with Mars or Earth. Trojans: Sharing stable t bl orbits bit along the orbit of Jupiter: Trapped in the Lagrangian points of Jupiter. The Search for N B lt Asteroids Non-Belt A t id Dedicated telescopes like th LINEAR ttelescope the l monitor the sky for non-belt asteroids that could potentially collide with Earth. Survey covers mostly regions away from the Galactic plane: faint asteroids are difficult to detect against the background of the Milky Way. Where do meteorites come from? Meteor Terminology • Meteorite: A rock from space that falls through g Earth’s atmosphere p • Meteor: The bright trail left by a meteorite Meteors burn in atmosphere SPORADIC METEOR It’s a METEORITE—IF IT HITS THE GROUND CAN I GET HIT? A dog in Cairo A lady’s hip in Alabama M t it Impact Meteorite I t Chicago, March 26, 2003 Barringer Meteorite Crater One of Earth’s youngest IMPACT craters Earth hit by a 62 Meter Nickel Iron Meteorite 50 000 years ago 1 mile 50,000 il across How do we know that this is not a volcano? Analysis of Meteorites 3b broad d categories: t i • Iron meteorites • Stony meteorites • Stony Stony--Iron meteorites Nickel-Iron Meteorite 6% of Falls 66% of finds! Why? Polished and etched with acid we see the Widmanstatten patterns Cooled very slowly so Nickel-iron crystals grew. Few deg/ 106 yrs FROM A PLANETESMAL CORE Stoney Meteorites 92% of falls, 26% finds This is a CHONDRITE Veryy old CHONDRULES NOTE: CHONDRULES ARE SMALL GLASSY ROCK BITS PRESENCE IMPLIES CHONDRITES NEVER MELTED! From a region of molten rock and iron Planetesimal core / mantle boundary! STONY-IRON METEORITE—pallasite great FOR Jewelry Typical Meteorite Jewelry and knives for outer of this world gifts! 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 exist. Asteroids Stones St Stony-Irons I Irons Finding Meteorites M t meteorites Most t it are smallll and dd do nott produce d significant i ifi t craters. t 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. Meteorites from Moon and Mars • A few meteorites arrive from the Moon and Mars • Composition differs from the asteroid fragments. fragments • A cheap (but slow) way to acquire moon rocks and Mars rocks. rocks 12.2 Comets Our goals for learning: • What are comets like? • Where do comets come from? What are comets like? Comet move slowly over several weeks across the sky! Can they Kill me? Throughout history, comets have been considered as portents of doom, even very recently: Appearances of comet Kohoutek (1973), doom (1973) Halley (1986), (1986) and Hale-Bopp (1997) caused great concern among superstitious. Comet Facts • Formed beyond the frost line, comets are icy counterparts to asteroids. • Nucleus of comet a “dirty snowball” • Most comets do not have tails. • Most comets remain perpetually frozen in y the outer solar system. • Only comets that enter the inner solar y ggrow tails. system S Sun-grazing i Comet C t The first signs of an approaching comet COMA -vast cloud of gas &dust up to 106 Km (80 earth diameters!) Comet Ikeya-Seki 12km diameter 1965 dawn sky WWII Battleship Binoculars great for comet hunting Discover a Comet You Name it! Hyakutaki You discover it you Name it! Hyakutaki core from Astrophysical Observatory at CSI METEOR SHOWER LEONIDS 1833 LEONIDS 1966 Meteor Showers Most meteors appear in showers, peaking periodically at specific dates of the year. Why are there Meteor Showers? RADIENT—SHOWER METEORS ALL FROM SAME SPOT Meteors in a shower appear to emanate from the same area of sky because of Earth’s Earth s motion through space and they are all traveling parallel together RADIENT= ALL SHOWER METEORS ARE IN SAME ORBIT MOST SHOWERS HAVE BEEN LINKED TO COMETS METEORS ARE COMET PIECES EARTH CROSSES COMETS ORBIT! WHAT HAPPENS IF THE PARENT COMET IS AT THE INTERSECTION POINT Comets eject small particles that follow the comet around in its orbit and cause meteor showers when Earth crosses the comet’s orbit. orbit Growth of Tail Anatom of a Comet Anatomy • Coma is atmosphere p that comes from heated nucleus • Plasma tail is gas escaping p g from coma,, pushed by solar wind • Dust tail is pushed by photons Gas Tail Spectrum Emission Lines of ionized Gases excited By UV of SUN CO+,, CO2+ CO CH+,CN+,N2+ C+,Ca+ COMA spectrum UV Breakup parts Of H2O,CO O CO2,CH CH4 NH3 &Na, Fe, K, Ca, etc Comet Dust Fills Solar system COMET PARTS TWIN TAILS TYPE I Gas pushed by Solar Wind Gas Tails have Wisps & twists Because! Ions effected by M Magnetic ti field fi ld In the Solar wind TYPE II Dust pushed Gently by Light pressure Dust tails obey gravity And gently curve COMA Or HEAD ICY NUCLEUS few KM diameter Gas and Dust Tails of Comet Mrkos in 1957 Comet Hale Bopp (1997) N cle s of Comet: Tempel 1 Nucleus • A “dirty snowball” • Source of material for comet’s tail Deep Impact Mission • Mission to studyy nucleus of Comet Tempel 1 • Projectile hit surface on Julyy 4,, 2005 • Many telescopes studied aftermath of impact 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 p g 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. Where do comets come from? Only a tiny number of comets enter the inner solar system - most stayy far from the Sun Oort cloud: On random orbits extending to about 50 000 AU 50,000 Kuiper belt: On orderly orbits from 30-100 AU in disk of solar system 100,000AU Oort Cloud Origin of Long Period Comets 10,000AU Note: 1 ly=63,000AU Passing star sends a Cometary Shower into the Solar system? How did they get there? • Kuiper belt comets formed in the Kuiper belt: flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the planets. • Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational i t interactions ti with ith jovian j i planets: l t spherical h i l distribution, orbits in any direction. 12.3 Pluto: Lone Dog No More Our goals for learning: • How big g can a comet be? • What are the large objects of the Kuiper belt like? • Are Pluto and Eris planets? How big can a comet be? Pluto’ss Orbit Pluto • Pl Pluto t will ill never hit Neptune, N t even though th h their th i orbits bit cross, because of 3:2 orbital resonance • Neptune orbits three times during the time Pluto orbits twice Is Pluto a Planet? • • • • • By far the smallest planet. Not a gas giant like other outer planets. planets Has an icy composition like a comet. H a very elliptical, Has lli i l inclined i li d orbit. bi Pluto has more in common with comets than with the eight major planets What is Pluto like? • Its moon Charon is nearly as large as Pluto itself (p (probablyy made byy a major j impact) p ) • Pluto is very cold (40 K) • Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun. Sun Other Icy Ic Bodies • There are manyy icyy objects like Pluto on elliptical, inclined orbits beyond Neptune. Neptune • The largest of these, Eris, was discovered in summer 2005, and is even larger g than Pluto. • Note it has a Moon • Eris is no longer called Xena or 2003 UB313 Pluto and Eris • Pluto’s size was overestimated after its discovery in 1930, and nothing of similar size was discovered for several decades • Now other large objects have been discovered in Kuiper Belt, including Eris • The International Astronomical Union (IAU) now classifies Pluto and Eris as dwarf planets • The gravity of a dwarf planet is not strong enough to make it round K iper Belt Objects Kuiper • These large, g , icyy objects have orbits similar to the smaller objects in the Kuiper Belt that become short period p comets • So are they very large comets or very small planets? Other Kuiper Belt Objects • Most have been discovered very recently so little is known about them. • NASA’s New Horizons mission will study Pluto and a few other Kuiper Belt object in a planned flyby. 12 4 Cosmic Collisions: Small Bodies 12.4 Versus the Planets Our goals for learning: • Have we ever witnessed a major j impact? p • Did an impact kill the dinosaurs? • Is the impact threat a real danger or media hype? • How do the jovian planets affect impact rates and life on Earth? Have we ever witnessed a major impact? Comet SL9 caused a string of violent impacts on Jupiter in Summer of 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter Summer of 1994 SL9 on direct collision course with Jupiter! Impact plume from a fragment of comet SL9 rises high above Jupiter’ss surface Jupiter Dusty debris at an impact site Jupiter’s Moon Callisto Why do we Think we are Seeing FRESH CRATERS? What’s that Bode for us? Did an impact kill the dinosaurs? ICE Hungarian Stamp Mass Extinctions • Fossil record shows occasional large dips in the diversity y of species: p mass extinctions. • Most recent was 65 million years ago, ending the reign of the dinosaurs dinosaurs. Iridium: Evidence of an Impact • Iridium is very rare in Earth surface rocks but often found in meteorites. • Luis and Walter Alvarez found a worldwide layer containing iridium iridium, laid down 65 million years ago, probably by a meteorite impact. impact • Dinosaur fossils all lie below this layer Iridium Layer No dinosaur fossils in upper rock layers Thin layer containing the rare element iridium Dinosaur fossils in l lower rockk layers l Consequences of an Impact • Meteorite 10 km in size would send large amounts of debris into atmosphere. p • Debris would reduce sunlight reaching Earth’ss surface Earth surface. • Resulting climate change may have caused mass extinction. extinction Red hot rock fell all over the earth. All forests were on fire! Dust plunged Earth into a winter that lasted years Likel Impact Site Likely • Geologists g have found a large subsurface crater abo t 65 million about years old in Mexico Comet or asteroid about 10 km in diameter approaches Earth Is the impact threat a real danger or media hype? Facts about Impacts • Asteroids and comets have hit the Earth. j impact p is onlyy a matter of time: not IF but • A major WHEN. • Major impacts are very rare. • Extinction level events ~ millions of years. j damage g ~ tens to hundreds of years. y • Major Impact features from space. Bosumtwi impact feature Clearwater impact craters Deep Bay impact crater Manicougan Impact feature (Canada) from shuttle Wolf impact crater Tunguska, Siberia: June 30, 1908 j disintegrated g and exploded p in A ~40 meter object the atmosphere The Tunguska Event • The Tunguska event in Siberia in 1908 d t destroyed d an area th 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 g ground. Area of destruction from the Tunguska event superimposed p p on a map p of Washington, D.C. and surrounding beltway. •Energy release comparable to a 12megaton nuclear weapon! Meteor Crater, Arizona: 50,000 years ago (50 meter object) The asteroid with our name on it • We haven’t seen it yet. • Deflection is more p probable with years y of advance warning. • Control is critical: breaking g a bigg asteroid into a bunch of little asteroids is unlikely to help. • We get less advance warning of a killer comet… NEO’s NEO’S Long period comet The worst problem How do the jovian planets affect impact rates and life on Earth? Influence of Jovian Planets Gravity G i off a jjovian i planet l (especially ( i ll Jupiter) J i ) can redirect di a comet Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud. Was Jupiter necessary for life on Earth? Impacts can extinguish life life. But were they y necessary y for “life as we know it”? If the Dinosaurs were here would we be? Freq enc of Impacts Frequency • Small impact p happen almost daily. • Impacts large enough to cause mass extinctions are many millions of years apart Impact videos • Video 1 KT event simulation Video 2 Apophis p p p possibilities FOR MORE INFORMATION NASA Impact Hazard Website: What are we doing about it? http:// impact.arc.nasa.gov NASA NEO Program Website http:// neo.jpl.nasa.gov So you want to hunt Asteroids IAU:MINOR PLANET CENTER http://CFA-WWW.HARVARD.EDU