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

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

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

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.

The larger asteroids

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

Another Galileo Asteroid: Gaspra

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”

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?

Origin of different asteroid types

Carbonaceous = undifferentiated?

Stones and Metals from differentiated planetesimals?

S = mantles

M = cores

Try to sort out using meteorite samples

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

Large Meteor over the Tetons (1972)

The Leonids 2001

APOD site: Picture by Chen Huang-Ming

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

Comets: Hale-Bopp in April 1997

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

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

Hale-Bopp clearly shows components

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

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

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

Pluto and Charon

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


	Today: Chapter 19, Pluto and “Debris”

	Wednesday: Evaluations and Review (HW 10 due)

	Friday: Exam #4, Ch. 16-19


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


	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


	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.


	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


	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”


	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?


	Carbonaceous = undifferentiated?

	Stones and Metals from differentiated planetesimals?
		S = mantles
		M = cores

	Try to sort out using meteorite samples


	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


	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


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


	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


	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


	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


	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


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