Astr 1050     Mon., Apr. 25, 2005

	Today:  Solar System Overview

Chapter 16: Origin of the Solar System

	Solar Nebula Hypothesis
		Context for Understanding Solar System
	Extrasolar Planets
		Dust Disks, Doppler Shifts, Transits and Eclipses
	Survey of the Solar System
		Terrestrial Planets
		Jovian Planets
		Other “Stuff” including apparent patterns with application to the nebular hypothesis

Patterns in Motion

	All planets orbit in almost the same plane (ecliptic, AKA Zodiac)
	Almost all motion is counterclockwise as seen from the north:
		All planets orbit in this direction
		*Almost* all planets spin in same direction
			with axes more-or-less perpendicular to ecliptic
		Regular moons (like our own moon) orbit in this direction, too
	Planets are regularly spaced
		steps increasing as we go outward

Solar Nebula Model

	Planets form from disk of gas surrounding the young sun
		Disk formation expected given angular momentum in collapsing cloud
		Naturally explains the regular (counterclockwise) motion
	Makes additional explicit predictions
		Should expect planets as a regular part of the star formation process
		Should see trends in composition with distance from sun
		Should see “fossil” evidence of early steps of planet formation

Extra-Solar Planets

	Hard to see faint planet right next to very bright star
	Two indirect techniques available (Like a binary star system but where 2nd “star” has extremely low mass)
		Watch for Doppler “wobble” in position/spectrum of star
		Watch for “transit” of planet which slightly dims light from star
	About 100 planets discovered since 1996   See http://exoplanets.org/
	Tend to be big (³Jupiter) and very close to star    (easier to see)

Characteristics of “Planets”

	Two types of planets
		Terrestrial Planets: small, rocky material: inner solar system
		Jovian Planets: large, H, He gas outer solar system
	Small left-over material provides “fossil” record of early conditions
		Asteroids   –  mostly between orbits of Mars and Jupiter
		Comets   –  mostly in outermost part of solar system
		Meteorites –  material which falls to earth

Slide 7

Patterns in Composition

	Terrestrial Planets
		Relatively small
		Made primarily of rocky material:
			Si, O, Fe, Mg  perhaps with Fe cores (Note – for earth H2O is only a very small fraction of the total)
	Jovian Planets
		Relatively large
		Atmospheres made of H2, He, with traces of CH4, NH3, H2O, ...
		Surrounded by satellites covered with frozen H2O
	Within terrestrial planets inner ones tend to have higher densities (when corrected for compression due to gravity)        Planet Density Uncompressed Density (gm/cm3)        (gm/cm3)
		Mercury     5.44          5.30
		Venus     5.24          3.96
		Earth     5.50          4.07
		Mars     3.94          3.73
		(Moon)     3.36          3.40

Equilibrium Condensation Model

	Start with material of solar composition material
		(H, He, C, N, O, Ne, Mg, Si, S, Fe ...)
	Material starts out hot enough that everything is a gas
		May not be exactly true but is simplest starting point
	As gas cools, different chemicals condense
		First high temperature chemicals, then intermediate ones, then ices
	Solids begin to stick together or accrete
		snowflakes Þ snowballs (“Velcro Effect”)
	Once large enough gravity pulls solids together into planetesimals
		planetesimals grow with size
	At some point wind from sun expels all the gas from the system
		Only the solid planetesimals remain to build planets
		Composition depends on temperature at that point (in time and space)
		Gas can only remain if trapped in the gravity of a large enough planet

Growth of the Planetisimals

	Once a planetisimal reaches critical size gravity takes over

Evidence of Assembly Process?    Craters

Craters evident on almost all small “planets”

Clearing of the Nebula

	Radiation pressure  (pressure of light)
		Will see present day effects in comets
	Solar Wind
		Strong solar winds from young T Tauri stars
		Will see present day effects in comets
	Sweeping up of debris into planets
		Late Heavy Bombardment
	Ejection of material by near misses with planets
		Like “gravity assist maneuvers” with spacecraft
		Origin of the comets

Chapter 17: Terrestrial Planets

	Earth
		History, Interior, Crust, Atmosphere
	The Moon
		In particular origin
	Mercury
	Venus
	Mars
		Including water (and life ?)

“Comparative Planetology”

	Basis for comparisons is Earth
	Properties of Earth
	Similarities and differences with Mars and Venus help us understand Earth better (e.g., life, greenhouse effect, etc.)
	Won’t spend much class time on basic properties (size, gravity, orbital period, length of day, number of moons, etc.) but you should have some relative ideas about these (see “Data Files” in text).

Four Stages of Planetary Development

Timeline

The Active Earth

	Plate tectonics, volcanoes, etc., a lot of action!

Earth’s Atmosphere: Greenhouse Effect

The Moon and Mercury

	No atmosphere
	Cratering is evidence of final planet assembly – lots to be learned from craters

Patterns in Geologic Activity

	Judge age of surface by amount of craters: more craters Þ more ancient surface (for some objects, have radioactive age dates)
		Moon “dead” after about 1 billion years
		Mercury “dead” early in its lifetime
		Mars active through ~1/2 of its lifetime
		Venus active till “recent” times
		Earth still active
	Big objects cool off slower
		Amount of heat (stored or generated) proportional to Volume ( so R3)
		Rate of heat loss proportional (roughly) to Surface Area          (so R2)
		Heat/(Unit Area) µ R3/R2 = R     so activity roughly proportional to R
	Same reason that big things taken out of oven cool slower than small things     (cake cools slower than cookies)

Formation of an impact crater

	Crater caused by the explosion
		Impactor is melted, perhaps vaporized  by the kinetic energy released
	Temporary “transient” crater is round
	Gravity causes walls to slump inward forming “terraces”
	Movement of material inward from all sides (trying to fill in the hole) may push up central peak in the middle.
	Final crater is typically ~10 times  the size of the impactor

Examples of craters on the moon

	Images on line at The Lunar and Planetary Institute: http://www.lpi.usra.edu/expmoon/lunar_missions.html
	Detailed record of Apollo work at: http://www.hq.nasa.gov/office/pao/History/alsj/frame.html

Effects of late impacts

Moon: Giant Impact Hypothesis

	Explains lack of large iron core
	Explains lack of “volatile” elements
	Explains why moon looks a lot like earth’s mantle, minus the volatiles
	Explains large angular momentum in the earth-moon system

Venus

Expect Venus to be similar to Earth?
(It isn’t!)

	Venus only slightly closer to sun, so expect about same initial composition
	Venus only slightly smaller than Earth, so expect about same heat flow
	Venus atmosphere is dramatically different
		Very thick CO2 atmosphere
		Virtually no water in atmosphere or on surface
	Venus shows relatively recent volcanic activity, but no plate tectonics
	Both probably related to its slightly closer position to the sun which caused loss of its critical water
	Thick atmosphere and clouds block direct view so information from:
		Orbiting radar missions  (Magellan in early 90’s)
		Russian landers (as in previous photo)

Why does Venus have much more CO₂ in atmosphere than Earth?

	Amount of CO2 in atmosphere on Venus roughly equal to amount of CO2 in limestone on Earth
	With no oceans, don’t have a way to get CO2 out of atmosphere and back into rocks
	Runaway effect, because high T causes faster loss of water to space.
		If H2O gets into upper atmosphere it is broken down into O, H by UV sunlight
		H is so light it escapes to space
		On Earth cooler T traps H2O in lower atmosphere (it condenses if it gets to high)
	Location closer to the sun pushed Venus “over the edge”  compared to Earth

Surface Relief of Venus from Radar

	Venus does show evidence of “recent” volcanism
	It does not show linear ridges, trenches, or rigid plates
		In a few spots there are weak hints of this – but clearly different

Volcanoes

	Sapas Mons
		Lava flows from central vents
		Flank eruptions
		Summit caldera
	Size:
		250 miles diameter
		1 mile high

Lava Channels

	Large!
		100’s of miles long
		1.2 miles wide
	High Venus temperatures may allow very long flows
	Composition could also be different

Pancake Domes

	Pancake domes formed from very viscous lava

Slide 33

Lots of Martian Science Fiction

	Best, most recent and scientifically accurate is probably Kim Stanley Robinson’s series:
	Red Mars, Blue Mars, Green Mars
	Terraforming/colonization of Mars

Mars and the Pattern of Geologic Activity
and Atmospheric Loss

	Expect intermediate geologic activity based on size
		RMars = 0.53 REarth         RMoon = 0.27 REarth
		Earth still active but lunar mare volcanism ended ~3 billion years ago
	Expect intermediate atmospheric loss
		Smaller size will make atmospheric escape easier
		Cooler temperature (farther from sun) will make astmospheric escape harder
	In some ways Mars is most “Earth-like” planet
		Has polar caps
		Has weather patterns
		Had (in past) running water
			May have had conditions necessary for development of life

Which planets can retain which gasses?

Mars atmosphere today

	Pressure is only ~1% of Earth’s
	Composition:  95% CO2    3% N2    2% Ar
	Water:
		Pressure too low for liquid water to exist
			Water goes directly from solid phase to gas phase
			CO2 (dry ice) acts like this even at terrestrial atmospheric pressure
		Water seen in atmosphere
		Water seen in polar caps
		Evidence of running water in past
	Carbon dioxide (CO2)
		Gets cold enough for even this to freeze at polar caps
		Unusual meteorology, as atmosphere moves from one pole to other each “year”

Mars dust storm

Sand Dunes on Mars

	Spacecraft in Mars orbit
		Mars Global Explorer
		Mars Odyssey
	Even though atmosphere is thin, high winds can create dust storms

Water ice clouds

Ancient River Channels?
(note channels older than some craters – by superposition)

Recent liquid water?
(water seeping out of underground “aquifer” ?)

Layered Deposits

Where is the water today?

	Much may have escaped to space
	Some is locked up in N Polar Cap
	Much could be stored in subsurface ice (permafrost)
	Mars Missions making progress this semester:
			http://www.nasa.gov/vision/universe/solarsystem/mer_main.html
	Location of water critical to knowing where to search for possible past life

“Comparative Planetology”

	Think about how Venus, Earth, and Mars started out so similarly
	Think about what properties led to the very different environments today
	Think about how these issues may apply to the future of Earth, and even our prospects for terraforming (and there is a debate about whether we should terraform at all!).

Chapter 18: Worlds of the Outer Solar System

	Jupiter
			Condensation model
			Atmospheric winds
			Atmospheric chemistry
			Magnetic fields
		Other Jovian Planets (Saturn, Uranus, Neptune)
			will only cover major differences from Jupiter
		Satellites (i.e. Moons)

Jovian Planets

Ice+Rock Core    H+He “Atmosphere”

Details of the atmosphere

	Mostly made of H, He
	Trace amounts of C, N, O, S
	CH4 present as gas
	NH3, NH4SH, H2O can condense in colder upper regions Þ clouds
	Colors from unknown trace chemicals
	Density of gas smoothly increases with depth till point where it is indistinguishable from liquid   Þ no real “surface”
	At very high temperatures and pressures hydrogen becomes a “metal” and conducts electricity  Þ generates magnetic field

Jupiter as seen by Cassini

Magnetic fields and trapped particles

Aurora on Jupiter

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

Saturn as seen by the Hubble Space Telescope

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

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

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

Possible 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 dropped a probe in early 2005.
	“Code of the Lifemaker” by James P. Hogan, good sf

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

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

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”

Types of 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

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

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

Pluto and Charon