Presentation on theme: "Chapter 12: Remnants of Rock & Ice"— Presentation transcript:
1Chapter 12: Remnants of Rock & Ice Asteroids, Comets & Pluto
2Small Bodies in the Solar System Small bodies, the leftover “scraps” from the formation of the Solar System, fall into three distinct groups:asteroidsrocky or metallic in compositionmost are located between the orbits of Mars and Jupiter
3Small Bodies in the Solar System Kuiper belt cometsmade mostly of iceorbit the Sun beyond Neptuneorbit in same direction and plane as the planetsOort cloud cometsorbit at the outer fringe of the Solar Systemspherically distributed about the Sun
4A Note on Namesasteroid – a rocky leftover planetesimal orbiting the Suncomet – an icy leftover planetesimal orbiting the Sun, regardless of its size or whether it has a tailmeteor – a flash of light in the sky caused by a particle entering the atmosphere, regardless of its originmeteorite – any piece of rock that fell to the ground from space, regardless of its origin
5Properties of Asteroids They are small in size.the largest one, Ceres, is only 1,000 km acrossThey are not spherical in shape.shaped more like “potatoes”gravity not strong enough to compress rocky materialodd shapes imply that some are fragments from asteroid collisions
6Properties of Asteroids Asteroid orbits are more elliptical & inclined than planetary orbits.Most asteroids are located in the asteroid belt.between the orbits of Mars & JupiterSome share Jupiter’s orbit.two swarms at 60º in front of and behind Jupiterknown as Trojan asteroidsA few cross Earth’s orbit.they are called Near-Earth asteroids
7The Asteroid BeltJupiter’s gravity disrupted the orbits of those asteroids whose periods were an integer fraction of Jupiter’s.orbital resonances created gaps… like in Saturn’s ringsknown as the Kirkwood gapsThis explains why no planet formed in the asteroid belt.tugs from Jupiter’s gravity prevented the planetesimals from accreting into a planet
8Measuring Asteroid Properties Sizethe larger the asteroid, the more sunlight it will reflectmeasuring the brightness and knowing reflectivity & distance gives us the size.reflectivity is calculated from the visual & IR brightnessMassmeasure the effect gravity has on a passing spacecraft or a moonuse Kepler’s & Newton’s lawsDensitycalculate from mass & sizemass volume
9Measuring Asteroid Properties Shapesmeasure the asteroid’s changes in brightness as it rotatesthe asteroid’s shape can be reconstructed from thiswe can bounce radar signals off of Near-Earth asteroids
10Measuring Asteroid Properties Compositionexamine the spectrum of sunlight reflected off the asteroidlook for non-Solar absorption lines in the spectrumThree categories of asteroid composition:very dark asteroids which contain Carbon-rich materialsfound in outer regions of the asteroid beltbrighter asteroids which contain rocky materialsfound in inner regions of the asteroid beltasteroids which contain metals such as Iron
11Rocks Falling from the Sky meteor – a flash of light caused by a particle which enters Earth’s atmosphere.most of these particles are the size of a peathey completely burn up in Earth’s atmospheremeteorite – a rock which is large enough to have survived its fall to Earththey caused a brighter meteor…sometimes called a fireballHow can you tell that you have a meteorite?they have a higher metal content than terrestrial rocksthey contain Iridium and other isotopes not found in terrestrial rocks
13Types of MeteoritesBased on composition, meteorites fall into two basic categories:primitiveabout 4.6 billion years oldaccreted in the Solar nebulaprocessedyounger than 4.6 billion yearsmatter has differentiatedfragments of a larger object which processed the original Solar nebula materialEach type of meteorite can be divided into two subcategories:primitive meteorites can be either stony, containing rocky minerals & metals, or Carbon-rich, containing Carbon compounds or even waterprocessed meteorites can be either metallic, high-density Iron/Nickel like Earth’s core, or rocky, containing low-density material similar to earth’s crust
14Origin of MeteoritesPrimitive meteorites condensed and accreted directly from the Solar nebula.the stony ones formed closer than 3 AU from the Sunthe Carbon-rich ones formed beyond 3 AU from the Sun, where it was cold enough for Carbon compounds to condenseProcessed meteorites come from large objects in the inner Solar System.the metallic ones are fragments of the cores of asteroids which were shattered in collisionsthe rocky ones were chipped off the surfaces of asteroids, Mars, and the Moon by impacts
15Comets Recent Comets: One of the most beautiful sights in the sky. Throughout human history, these “hairy” stars would appear.like planets, they moved with respect to the fixed starsunlike planets, they were not confined to the ecliptic and disappeared after several weeksThey were taken as omens of good or bad fortune.Recent Comets:1986 Halley’s Comet1996 Comet Hyakutake1997 Comet Hale-BoppDozens per year too dim to be seen by eyeHale-BoppHyakutake
16Comets The Orbits of Comets: Edmund Halley (1656 – 1742) first to realize that comets orbit the Sunpredicted the return of a comet which had been seen every 76 yearsthe comet returned in 1758 and now bears his nameThe Orbits of Comets:
17Composition and Structure of Comets Comets are “dirty snowballs”…ice mixed with rock and dust.ices are H2O, CO2, CO, NH3, CH4nucleusthe “dirty snowball”how the comet appears far from the Suncomasurrounds nucleus when near the Sunsublimated gas & dustplasma tailionized gas swept back by Solar winddust taildust particles swept back more slowly by radiation
20A Comet’s JourneyThe SOHO telescope observed Comet NEAT (C/2002 V1) round the Sun on Feb 16, 2002.courtesy of SOHO/LASCO consortium. SOHO is a project of ESA and NASA.A comet can only visit the Sun a few hundred times before losing all its ice to sublimation.the comet may then disintegrateor the rocky remains may stick together as an asteroid
22The Origin of CometsWe can tell where comets originate by measuring their orbits as they visit the Sun.Most approach from random directions and do not orbit in the same sense as the planets.they come from the Oort cloud 50,000 AU distantOthers orbit along the ecliptic plane in the same sense as the planets.they come from the Kuiper belt AU distant
24Pluto and Neptune Pluto has an eccentric and inclined orbit. For 20 of its 248-year orbital period, it is actually closer to the Sun than Neptune.such was the case between 1979 and 1999An orbital resonance between the two planets keeps them from ever colliding!ResonanceNeptune completes three orbits for every two orbits that Pluto makes.
25Pluto and Charon Pluto’s moon, named Charon, was discovered in 1978. it orbits Pluto every 6.4 daysThis allows us to measure the mass of Pluto using Kepler’s Law #3.Eclipses allow us to measure the diameters of both Pluto & Charon.Hubble ST image of Pluto & CharonPluto’s density (2 gm/cm3) is larger than expected for an icy world.Charon is less dense (1.6 gm/cm3)Explanation similar to Earth/MoonCharon formed from large impactPluto lost lower density outer layersReconstructed image of Pluto
27Planet or Kuiper Belt Comet? The classification of Pluto has recently come into question.Pluto has many properties in common with Kuiper belt comets.it orbits in the vicinity of the Kuiper beltseveral Kuiper belt comets have orbital resonances with Neptuneits composition of ice and rock is similar to cometsit has an atmosphere of Nitrogen which sublimes when Pluto is closest to the Sunsome Kuiper belt comets have moonsPluto has some properties which differ from Kuiper belt comets.its surface is much brighter; presumable because the Nitrogen atmosphere refreezes on the surface rather than escapingit is much larger than most Kuiper belt cometsBut…it is smaller than Triton, which presumably once roamed the Kuiper belt!This is why Pluto is part of a new class of Dwarf Planets
29Comet Shoemaker-Levy 9This comet was discovered in orbit about Jupiter in 1992.a previous encounter with Jupiter broke the nucleus into a string of fragmentsthe comet was on a collision course with JupiterSomething similar had happened to Callisto.This crater chain is evidence that a string of nuclei once impacted it.
30Comet Shoemaker-Levy 9One by one, each fragment collided with Jupiter in July 1994.infrared cameras observed hot plumes ejected from the planetmaterial from deep inside Jupiter was ejected, and fell… left dark spotsSuch impacts probably occur on Jupiter once every 1,000 years.This was a reminder to us that impacts still occur in the present!!
31Meteor ShowersEarth is impacted by an estimated 25 million small particles each day which cause meteors.When the Earth passes through the trail of a comet, the number of particles impacting the Earth’s atmosphere increases.We call this a meteor shower.You can see upward of 1 meteor per minute from one location.Showers occur on the same dates each year, corresponding to when the Earth crosses a given comet’s orbit.The meteors appear to emanate from one point in the sky.
32Meteor ShowersMeteors appear to shoot from the point directly ahead in the direction that the Earth is moving.
34Meteor ShowersMeteor showers are named after the constellation from which the meteors appear to emanatei.e., the constellation which lies in the direction of the Earth’s motion.
35Impacts and Mass Extinctions on Earth We know that larger objects have impacted EarthMeteor Crater in northern Arizonacaused by a 50-meter asteroidimpact occurred 50,000 years ago65 million years ago, many species, including dinosaurs, disappeared from earthSedimentary rock layer from that time shows:Iridium, Osmium, Platinumgrains of “shocked quartz”spherical rock dropletssoot from forest fires
36Impacts and Mass Extinctions on Earth Elements like Iridium, rare on Earth, are found in meteorites.Shocked quartz, found at Meteor Crater, forms in impacts.Rock droplets would form from molten rock “rain.”Forest fires would ensue from this hot rain.All this evidence would imply that Earth was struck by an asteroid 65 million years ago.In 1991, a 65 million year old impact crater was found on the coast of Mexico.200 km in diameterimplies an asteroid size of about 10 km acrosscalled the Chicxulub crater
38Impacts and Mass Extinctions on Earth We have a plausible scenario of how the impact led to mass extinction.debris in atmosphere blocks sunlight; plant die…animals starvepoisonous gases form in atmosphere
39Could it happen again?This chart shows how frequently objects of various sizes will impact Earth.The odds of a large impact are small … but not zero!
40Influence of the Jovian Planets Gravity of a jovian planet (especially Jupiter) can redirect a comet.Jupiter has directed some comets toward Earth but has ejected many more into the Oort cloud.
44What have we learned?What are the three major groups of small bodies in the Solar System?Asteroids, comets of the Kuiper belt, comets of the Oort cloud. The groups are distinguished by their orbits.Describe asteroid sizes, shapes, and orbits.Most asteroids are small, potato-shaped, and orbit in the asteroid belt. Trojan asteroids share Jupiter’s orbit. Near-Earth asteroids have orbits that pass near Earth’s orbit.Why didn’t a planet form in the region of the asteroid belt?Orbital resonances with Jupiter disrupted orbits of planetesimals, preventing them from accreting into a planet. Resonances also cause the gaps in the asteroid belt today.
45What have we learned? How do we measure asteroid properties? Orbits from observations of asteroid motion and application of the law of gravity. Sizes by comparing infrared and visible brightness, combined with distance. Masses from gravitational effects on moons or passing spacecraft. Densities from size and mass. Shapes from brightness changes with rotation or radar. Composition from spectroscopy.How is a meteor different from a meteorite?meteor: a flash of light caused by a small particle entering our atmosphere. meteorite: a rock that survives the plunge from space to reach the ground.
46What have we learned? How are meteorites categorized? Two major categories: primitive meteorites are remnants from the Solar nebula and processed meteorites are fragments of larger objects that underwent differentiation. Primitive meteorites may be either stony or carbon-rich. Processed meteorites may be similar in composition either to Earth’s crust or to its mantle or core.Why do meteorites differ from one another?Whether a primitive meteorite is stony or carbon-rich depends on where it formed. Carbon-rich meteorites formed farther from the Sun (beyond about 3 AU) where it was cool enough for carbon compounds to condense. Processed meteorites made of crust-like material are chips from the surface of larger objects. Processed meteorites made of mantle- or core-like material are fragments from the interior of a shattered asteroid.
47What have we learned? What are comets made of? Ice mixed with rocky dust, giving them a “dirty snowball” composition.What happens to a comet as it approaches the Sun?The nucleus – all there is when the comet is far away and frozen – heats up and gases begin to sublime from its surface. Escaping gases carry some dust along. The gas and dust form a coma and tails: a plasma tail of ionized gas and a dust tail.
48What have we learned?How do we know that vast numbers of comets reside in the Oort cloud and Kuiper belt?Analysis of orbits shows where comets in the inner Solar System have come from. Based on the number of comets seen in the inner Solar System and the relatively short times during which comets can survive in the inner Solar System, we conclude that the Oort cloud and Kuiper belt must contain enormous numbers of comets.Why don’t Pluto and Neptune collide?A stable orbital resonance ensures that Pluto always remains a safe distance from Neptune, even though it is sometimes closer to the Sun than Neptune.
49What have we learned?What is surprising about Pluto’s density, and how might it have come to be?It is slightly higher than expected for material that condensed in the outer Solar System. It may be the result of a giant impact the blasted away Pluto’s low-density outer layers and led to the formation of its moon, Charon.Why do we think Pluto is a Kuiper belt comet?Both its composition and orbit are more similar to Kuiper belt comets than to other planets. Even its size is not that much bigger than other known Kuiper belt comets, and it is smaller than one object that almost certainly once roamed the Kuiper belt –Neptune’s moon Triton.
50What have we learned? What happened to Jupiter in 1994? It was struck by a string of nuclei that were all fragments of comet Shoemaker-Levy 9. Such impacts probably occur on Jupiter only once every 1,000 years, on average.How often do small particles impact Earth?Constantly – an estimated 25 million small particles create meteors each day. Even more hit the Earth during meteor showers.
51What have we learned?Why do we think the dinosaurs were driven extinct by an impact?Careful analysis of sediments from the time of the dinosaurs’ demise shows evidence of a major impact. An impact crater has been found, and we have a plausible scenario to describe how the impact led to the mass extinction.Do future impacts pose a real threat to our civilization?The probability of a major impact in our lifetimes is very low, but not zero. The threat is still being assessed.