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Astr 1050 Wed., March. 22, 2017
Today: Chapter 12, Pluto and “Debris”
March 24: Exam #2, Ch. 5-12 (9:00-9:50) March 27: Mastering Astronomy HW Chapter 11 & 12
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Chapter 12: Meteorites, Asteroids, Comets
• Small bodies are not geologically active • They provide “fossil” record of early solar system
– Asteroids • Mostly from region between Mars and Jupiter • Left over small debris from accretion, never assembled into a large
planet • Meteorites come mostly from asteroids
– Comets • “Stored” on large elliptical orbits beyond planets • Thought to be “planetesimals” from Jovian planet region, almost
ejected from solar system in its early history
• Meteorites provide only samples besides Apollo – With sample in hand, can perform very detailed analysis:
detailed chemistry; radioisotope age; other isotope info
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Asteroids • Most located between
Mars and Jupiter • Largest is Ceres
– 1/3 diameter of moon – Most much smaller
• >8,000 known • Total mass << Earth • A few make it to earth
– source of the meteorites
The New Solar System, Beatty et al.
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Meteorites from Asteroids • If meteorite speed and
direction is observed as it enters Earth’s atmosphere, you can work backwards to find its orbit.
• Almost all of the meteorites with well determined orbits have most distant part of orbit ellipse within the asteroid belt.
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The larger asteroids
The New Solar System, Beatty et al.
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Are Asteroids Primitive?
• Ida (56 km diam.) and its moon Dactyl (1.5 km diam.) – Colors have been “stretched” to show subtle differences
• Imaged by Galileo on its way out to Jupiter • Presence of craters indicates great age
– Absolute age requires knowledge of cratering rate – uncertain – Not spherical – gravity too weak to pull it into a sphere
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Another Galileo Asteroid: Gaspra
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Phobos & Deimos: Two “misplaced” asteroids?
• Phobos and Diemos are small (~25 km and ~15 km diam.) moons of Mars • Look like captured asteroids rather than moons formed in place • Are “C” class – i.e. dark “Carbonaceous” type “asteroids”
Grooves seem related to the large crater called “Stickney”
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Clues from Meteorites
• Three main kinds of meteorites – Carbonaceous chondrites: Most primitive material –
dark because of C – Stones Similar to igneous rocks – Irons Metallic iron – with peculiarities
• Why do we have different kinds? – How are the main types of meteorites related to the
asteroids?
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Origin of different asteroid types
• Carbonaceous = undifferentiated?
• Stones and Metals from differentiated planetesimals? – S = mantles – M = cores
• Try to sort out using meteorite samples
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Meteors vs. Meteorites • Meteor is seen as streak in sky • Meteorite is a rock on the ground • Meteoroid is a rock in space
• Meteor showers (related to comet orbits) rarely
produce meteorites – Apparently most comet debris is small and doesn’t
survive reentry
• Meteorites can be “finds” or “falls” – For a fall – descent actually observed and sometimes
orbit computed – Most have orbits with aphelion in asteroid belt
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Large Meteor over the Tetons (1972)
Aphelion distance 2.3 AU Diameter 3 to 10 m Seen at height of ~50 km – skipped out of atmosphere
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The Leonids
2001 • APOD site:
Picture by Chen Huang-Ming
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Meteor Showers and Comets • Meteor showers caused
by large amount of small debris spread out along comet orbits
• Almost none makes it to the ground – no meteorites
• Occur each year as earth passes through orbit of comet
• Appears to come from “radiant point” in sky
• Leonids: Mid November
From our text Horizons, by Seeds
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Comets: Hale-Bopp in April 1997
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Comet characteristics • Most on long elliptical orbits
– Short period comets – go to outer solar system • “Jupiter family” still ~ in plane of ecliptic • “Halley family” are highly inclined to ecliptic
– Longer period ones go out thousands of AU • Most of these are highly inclined to ecliptic
• Become active only in inner solar system – Made of volatile ices and dust – Sun heats and vaporizes ice, releasing dust – “Dirty snowball” model
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Comet structure • Gas sublimates from nucleus • Dense coma surrounds nucleus • Ion tail is ionized gas points
directly away from sun – shows emission spectrum – ions swept up in solar wind
• Dust tail curves slightly outward from orbit – shows reflected sunlight – solar radiation pressure gently
pushes dust out of orbit
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Hale-Bopp clearly shows components
Ion Tail
Dust Tail
Coma (Nucleus too small to see)
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Where do comets come from? Long period comets: The Oort Cloud
• Most (original) orbits have aphelions of >1000 AU
• Need ~6 trillion comets out there to produce number seen in here
• Total mass of 38 MEarth
• Passing stars deflect comets in from the cloud
The New Solar System, Beatty et al.
¼ to 1 parsec
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Formation of Oort cloud comets
• Composition indicates formation in region between Jupiter and Neptune
• Ejected to the Oort cloud by near collisions as Jovian planets formed
• Most probably lost from solar system – a few have just barely closed orbits
• Occasional passing stars perturb more comets into orbits passing in close to sun
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Where do the Jupiter family comets come from?: The recently discovered Kuiper Belt
• Material beyond Neptune never ejected into the Oort cloud
• Pluto and Charon the biggest members – now also Quarar, Sedna
• Very hard to detect because very faint – far from the sun so little
illumination – comets not active at that
distance – Hubble and new large
telescopes have recently detected ~100
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Pluto and Charon
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Importance of comets
• Evidence of solar nebula • Source of H2O and CO2 for earth • Impacts continue
– Impacts on Earth • Extinction of the dinosaurs
– SL-9 impact on Jupiter