Chapter 12 12.1 Asteroids and Meteorites Remnants of Rock and Ice Asteroids, Comets, and the Kuiper Belt • Our goals for learning • What are asteroids like? • Why is there an asteroid belt? • Where do meteorites come from? Asteroid What are asteroids like? Facts • Asteroids are rocky leftovers of planet formation. • Largest is Ceres, diameter ~1,000 km • 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 terrestrial planet. Asteroids with Moons • Some large asteroids have their own moon • Asteroid Ida has a tiny moon named Dactyl Asteroids are cratered and not round 1
Density of Asteroids Asteroid Orbits • Most asteroids orbit • Measuring orbit of in a belt between asteroid’s moon tells Mars and Jupiter us asteroid’s mass • Trojan asteroids • Mass and size tell us follow Jupiter’s asteroid’s density orbit • Some asteroids are • Orbits of near-Earth solid rock; others asteroids cross Earth’s orbit just piles of rubble Orbital Resonances Why is there an asteroid belt? • Asteroids in orbital resonance with Jupiter experience periodic nudges • Eventually those nudges move asteroids out of resonant orbits, leaving gaps in belt Origin of Asteroid Belt Where do meteorites come from? • Rocky planetesimals between Mars and Jupiter did not accrete into a planet. • Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet. 2
Meteorite Impact Meteor Terminology • Meteorite: A rock from space that falls through Earth’s atmosphere • Meteor: The bright trail left by a meteorite Chicago, March 26, 2003 Primitive Meteorites Meteorite Types 1) Primitive: Unchanged in composition since they first formed 4.6 billion years ago. 2) Processed: Younger, have experienced processes like volcanism or differentiation. Processed Meteorites Meteorites from Moon and Mars • A few meteorites arrive from the Moon and Mars • Composition differs from the asteroid fragments. • A cheap (but slow) way to acquire moon rocks and Mars rocks. 3
What have we learned? What have we learned? • What are asteroids like? • Where do meteorites come from? – They are rocky leftovers from the era of – Primitive meteorites are remnants from planet formation solar nebula • Why is there an asteroid belt? – Processed meteorites are fragments of – Orbital resonances with Jupiter prevented larger bodies than underwent planetesimals between Jupiter and Mars from differentiation forming a planet 12.2 Comets What are comets like? • Our goals for learning • What are comets like? • Where do comets come from? Sun-grazing Comet 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 the outer solar system. • Only comets that enter the inner solar system grow tails. 4
Nucleus of Comet Deep Impact • Mission to study • A “dirty snowball” nucleus of Comet Tempel 1 • Source of material for comet’s tail • Projectile hit surface on July 4. 2005 • Many telescopes studied aftermath of impact Growth of Tail Anatomy of a Comet • Coma is atmosphere that comes from heated nucleus • Plasma tail is gas escaping from coma, pushed by solar wind • Dust tail is pushed by photons Meteors in a shower appear to emanate from the same area of sky Comets eject small particles that follow the comet around in its because of Earth’s motion through space orbit and cause meteor showers when Earth crosses the comet’s orbit. 5
Only a tiny number of comets enter the inner Where do comets come from? solar system - most stay far from the Sun Oort cloud: On random orbits extending to about 50,000 AU Kuiper belt: On orderly orbits from 30-100 AU in disk of solar system What have we learned? How did they get there? • What are comets like? – Comets are like dirty snowballs • Kuiper belt comets formed in the Kuiper belt: flat – Most are far from Sun and do not have tails plane, aligned with the plane of planetary orbits, – Tails grow when comet nears Sun and nucleus orbiting in the same direction as the planets. heats up • Where do comets come from? • Oort cloud comets were once closer to the Sun, – Comets in plane of solar system come from but they were kicked out there by gravitational Kuiper Belt interactions with jovian planets: spherical – Comets on random orbits come from Oort distribution, orbits in any direction. cloud How big can a comet be? 12.3 Pluto: Lone Dog No More • Our goals for learning • How big can a comet be? • What are the large objects of the Kuiper belt like? • Is “Planet X” a planet? 6
Pluto’s Orbit Is Pluto a Planet? • By far the smallest planet. • Not a gas giant like other outer planets. • Has an icy composition like a comet. • Has a very elliptical, inclined orbit. • Pluto has more in common with comets than • Pluto will never hit Neptune, even though their orbits with the eight major planets cross, because of 3:2 orbital resonance • Neptune orbits three times during the time Pluto orbits twice Other Icy Bodies Kuiper Belt Objects • There are many icy • These large, icy objects like Pluto on objects have orbits elliptical, inclined similar to the orbits beyond smaller objects in Neptune. the Kuiper Belt that become short period • The largest of these, comets “Planet X” was discovered in • So are they very summer 2005, is large comets or very even larger than small planets? Pluto What are the large objects of the What is Pluto like? Kuiper belt like? • Its moon Charon is nearly as large as Pluto itself (probably made by a major impact) • 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. 7
HST’s view of Pluto & Charon 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. Is “Planet X” a planet? Pluto and “Planet X” • Pluto’s size was overestimated after its discovery in 1930 • It was considered a planet, and nothing of similar size was discovered for several decades • Now other large objects have been discovered in Kuiper Belt, including “Planet X” • Some scientists consider all of those objects planets; others consider none of them planets. What have we learned? 12.4 Cosmic Collisions: Small Bodies Versus the Planets • How big can a comet be? – The Kuiper belt from which comets come • Our goals for learning contains objects as large as Pluto. • Have we ever witnessed a major impact? • What are the large objects of the Kuiper • Did an impact kill the dinosaurs? belt like? • Is the impact threat a real danger or media – Large objects in the Kuiper belt have orbits and icy compositions like those of comets. hype? • Is “Planet X” a planet? • How do the jovian planets affect impact – It remains a matter of opinion because rates and life on Earth? scientists have not yet settled on a definition of the minimum size of a planet. 8
Have we ever witnessed a major impact? Comet SL9 caused a string of violent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen. Tidal forces tore it apart during a previous encounter with Jupiter Impact plume from a fragment of comet SL9 rises high above Jupiter’s surface This crater chain on Callisto probably came from another comet that tidal forces tore to pieces Dusty debris at an impact site Artist’s conception of SL9 impact 9
Several impact sites Impact sites in infrared light Did an impact kill the dinosaurs? Mass Extinctions • Fossil record shows occasional large dips in the diversity of species: mass extinctions . • Most recent was 65 million years ago, ending the reign of the dinosaurs. Iridium: Evidence of an Impact Iridium Layer • Iridium is very rare in Earth surface rocks No dinosaur fossils but often found in meteorites. in upper rock layers • Luis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 Thin layer million years ago, probably by a meteorite containing the rare element iridium impact. • Dinosaur fossils all lie below this layer Dinosaur fossils in lower rock layers 10
Likely Impact Site Consequences of an Impact • Geologists have found a large • Meteorite 10 km in size would send large subsurface crater amounts of debris into atmosphere. about 65 million • Debris would reduce sunlight reaching years old in Mexico Earth’s surface. • Resulting climate change may have caused mass exinction. Comet or asteroid about 10 km in diameter approaches Earth 11
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