Astr 1050    Fri., Dec. 13, 2002

	Today: Astronomy Articles
	Homework #11
	Finish Ch. 18, Chapter 19, Pluto and “Debris”

Homework #11

	Q1:  If you were an astronomer in the Alpha Centauri system (assume 4.2 light years from Earth) looking toward the solar system, what would be the maximum angular separation between Jupiter and the sun you could ever see? (Hint: 1 ly equals 63,000 AU).
		Small Angle formula, Jupter 5.2 AU from sun, yields 4 arcseconds
	Q2:  If the Atlantic Seafloor is spreading at 30 mm/year and is now 6400 km wide, how long ago were the continents in contact?
		6400 km/30 mm/year = 210 million years.
	Q3:  Which effect is the most important for clearing the remaining gas and dust in the solar nebula following planet formation?
		Radiation pressure.
	Q4:  Europa has few craters because
		It has erased craters nearly as fast as they form.
	Q5:  A meteor shower is produced when
		Earth passes through the orbital path of a comet.
	Q6:  An asteroid has an orbital period around the sun of 5.2 years. How far from the sun is this asteroid?
		Kepler’s Law.  P2 = a3.  P = 5.2 years, so a in AU is the cube root of 5.2 squared, which is 27, making a = 3 AU.

Comparison of Jovian Planets

	Variation in distance presumably ultimate causes other effects
		P:      Kepler’s third law
		T:      Falloff mostly just result of falling solar energy
				But Neptune hotter because more internal heat
		M:     Clue to details of solar nebula mode
				Less material in outer solar system – or perhaps less efficient capture
		r:      Should drop with mass because less compression
				Works for Saturn vs. Jupiter
				Increase for Uranus, Neptune indicates less H, He and more heavy material

Effects of T (and E) on Atmospheres

	Saturn’s bands much less distinct than Jupiter’s
		Temp. lower on Saturn Þ cloud condense lower
		Deeper clouds Þ markings less visible
	Differences at Uranus and Neptune
		Even colder Þ clouds even deeper
		So cold CH4 can condense
		Little solar energy to drive weather
			Uranus has strange seasons – tipped on its side
			Neptune has strong internal heat source, so it still can have weather
		Large amounts of heavy elements compared to amount of H, He on Jupiter, Saturn
			Large amounts of CH4 gas absorb red,  make planets appear blue

Implications of M, r for nebula

	Relative amount of H, He (compared to heavy elements) drop for Saturn then drop dramatically at Uranus and Neptune
	Why were these outer planets so less efficient at capturing H, He?
		Their mass is still great enough to do this, especially given low temperatures  in the outer solar system
	May be a problem of timing
		Accretion takes longer in the outer solar system because
			The velocities of all objects there are much less
			The distances between objects are greater
			This is the same reason the periods of the orbits are so long
		Uranus and Neptune may have only started to grow to critical size by the time the H, He gas was being driven out of the solar system

The Jovian planets are miniature solar systems

	Because planets captured gas as they were forming they had small “solar” nebulae Þ tests of nebula theory
		Regular satellite systems
			Large moons in direct (i.e. counterclockwise), equatorial orbits
			Preserve solid material from time of formation
			Jupiter shows solar-system like density gradients
		Rings
	Moons unusual and interesting objects in their own right
		Io – most volcanically active body in solar system
		Europa – also active, may have liquid water ocean (and life!)
		Titan – only satellite with a thick atmosphere
	Many properties are due to tidal forces and resonances

The Roche Limit
When can tides tear a moon apart?

	As a planetary body get close to another object, tidal forces distort the body more and more.
		Remember, Earth raises tides on the Moon just like it raises tides on the Earth
	If the distortion gets large enough, the moon will be pulled apart
		Happens at “Roche Limit” when moon is ~2.44 ´ radius of planet away
		At that point, tidal force pulling up on surface of moon is greater than moon’s gravity pulling down
	Only matters for objects held together by gravity
		Astronaut in orbit will not be pulled apart
			Is held together by much stronger chemical forces
		Astronaut standing on the outside of the shuttle, hoping the shuttle’s gravity would hold her there, will be pulled away from the shuttle

Saturn as seen by the Hubble Space Telescope

Rings are individual particles all orbiting separately

	Each particle – dust to golf ball to boulder size – is really a separate moon on its own orbit
	Orbit with Keplerian velocities:  high in close, slow farther out
	Nearby relative velocities are low – so particles just gently bump into each other – slowly grinding themselves up
	Structure in rings largely caused by gravity of moons

Resonances:  Properly timed gravitational “pushes”

	Like someone pushing kid on a swing
		Timing of pushes just as important as force used
			Pushing at random times has little effect
			Pushing at just right point in each cycle can produce big effect

Cassini division at 1:2 resonance with Mimas

Comparison of Rings

	All within Roche limit
	Details controlled by Resonances and Shepard Satellites

Jupiter as a miniature solar system

	Four large moons (Io, Europa, Ganymede, Callisto)
	Regular (equatorial, circular) orbits
	Pattern of changing density and composition with distance
		Inner two (Io, Europa) mostly rocky
		Outer two (Ganymede, Callisto) more icy

Io, Europa break rules about activity

	Io most volcanically active body in solar system
	Europa shows new icy surface with few craters

Tidal heating explains activity

	Large tides from Jupiter flex satellites
	Friction from flexing heats interiors
	Important for Io, Europa, some other outer solar system satellites

Probable H₂O ocean on Europa

	Tidal heating may keep H2O liquid under ice cover
	Perhaps a location where life could evolve
	“Europa Orbiter” Mission being planned to determine if ocean exists

Callisto not active

Comparison of Satellites

Titan

	Largest moon of Saturn
	Has thick atmosphere
		Pressure ~ 1 earth atmosphere
		Mostly N2, some CH4
		Gas held because of low T
	UV acting on CH4 Þ smog
		Ethane produced – Lakes?
		Can “see” surface only in IR
	Cassini will drop probe in Fall 2004
	“Code of the Lifemaker” by James P. Hogan, good sf

Triton

	Largest moon of Neptune
	In unusual retrograde orbit
		Probably captured after it formed
		Tides during capture may have caused heating
	Does have thin atmosphere
	Shows recent “activity”
		Not volcanic – rather volatile related
		Ices migrate with seasons
		“Geysers” caused by heated ices

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

Asteroid Belt Structure

	As Jupiter formed it stirred up velocities in what would become the asteroid belt
	Higher velocities meant planetesimals destroying each other rather than accreting
	Gaps and concentrations occur at resonances with Jupiter

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?

Types of asteroids observed

	Simple classification by albedo and color
	Three main types
		C  (carbonaceous?)
		S   (stones?)
		M (metals?)
	Finer classification by spectra

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 MEarth
	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
	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

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

Chapter 16-19 Review

	Solar Nebula
	Terrestrial Planets
		Properties of Earth
		Greenhouse Effect (cf. Venus, Mars)
		Cratering, origin of moon
	Jovian Planets
		Properties of Jupiter, composition, atmosphere
		Rings
	“Debris”
		Asteroids and Comets

Chapter 16-19 Review

	We’ve covered this material fast – exam will not cover subtle concepts or obscure facts.  Very basic information and only the most fundamental ideas.
	Things you should know include:
		Order of planets in solar system, general sizes of orbits, sizes and compositions of the planets (also asteroids and comets in general, notable moons).
		How these items fit into the solar nebula picture.

Chapter 16-19 Review

	Example questions:
	True/False:
		Jupiter was probably influential in preventing the formation of a planet at the location of the asteroid belt.
		The dirty snowball theory suggests that the head of a comet is composed of ices.
		Jupiter radiates more heat than it absorbs from the sun.
		Venus is very hot because its atmosphere is rich in CO2.
		The Greenhouse effect occurs because gases like carbon monoxide are opaque to IR radiation.
		The Jovian planets have lower densities than the terrestrial planets.

Chapter 16-19 Review

	Example questions:
	True/False:
		Meteorites appear to be composed of material similar to that found in comets.
		Jupiter’s interior is mostly liquid helium.
		Saturn’s rings are composed of metallic dust grains.
		Flow channels on Venus suggest it was once rich in water.
		The oxygen in Earth’s atmosphere was outgassed by volcanic explosions.
		Mars is the third rock from the sun.

Chapter 16-19 Review

	Example questions:
	Multiple choice:
		On a photograph of the moon, the moon measures 30 cm in diameter and a small crater measures 0.2 cm.  The moon’s physical diameter is 1738 km.  What is the physical diameter of the small crater?
			About 1738 km
			About 12 km
			About 520 km
			About 350 km
			About 3.5 km

Chapter 16-19 Review

	Example questions:
	Multiple choice:
		Though Titan is small, it is able to retain an atmosphere because?
			It is very cold.
			It is very dense.
			It rotates very slowly.
			It attracts gas from the solar wind.
			It has a very strong magnetic field.

Final Exam

	30 Multiple Choice questions, 15 true/false, 3 essay/written questions, plus 1 follow-up extra credit problem (computational and meant to be challenging).
	About 1/3 of the questions the same as or slightly modified from previous exams
	About 1/3 of the questions covering the solar system
	About 1/3 of the questions completely new but covering old material
	Questions mostly cover the basics and are not intended to be subtle or tricky.

Final Exam

	List of possible topics for essay questions:
		Dark Matter
		Cosmic Microwave Background Radiation
		The Evolution of the Sun on the H-R diagram
		The Solar Nebula
		Extrasolar Planets
		Comparative Planetology of Venus, Earth, and Mars
		The Seasons
		Phases of the Moon
		The Distances to Astronomical Objects (Distance Ladder)