1	Astr 1050 Mon., May 3, 2004 Today: Chapter 19, Pluto and “Debris” Wednesday: Evaluations and Review (HW 10 due) Friday: Exam #4, Ch. 16-19
2	Chapter 19: 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
3	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
4	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.
5	The larger asteroids
6	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
7	Another Galileo Asteroid: Gaspra
8	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”
9	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?
10	Origin of different asteroid types Carbonaceous = undifferentiated? Stones and Metals from differentiated planetesimals? S = mantles M = cores Try to sort out using meteorite samples
11	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
12	Large Meteor over the Tetons (1972)
13	The Leonids 2001 APOD site: Picture by Chen Huang-Ming
14	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
15	Comets: Hale-Bopp in April 1997
16	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
17	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
18	Hale-Bopp clearly shows components
19	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 M_Earth Passing stars deflect comets in from the cloud
20	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
21	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
22	Pluto and Charon
23	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