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11.2 Jovian Satellites

• Miniature solar systems• Overview of outer solar system moons

• Geologic activity on Galilean moons

• The special case of Titan

• Open question: Why are small icy moons more geologically active than small rocky planets?

“Planet“Planet--sized” Moonssized” Moons

Outer Solar System Moons

• Small moons (< 300 km)– No geological activity

• Medium-sized moons (300-1,500 km)– Past Geological activity

• Large moons (> 1,500 km)– Ongoing geological activity

Small Moons

• Very numerous

• Not enough mass gto self-round

• Many are likely captured asteroids or comets.

• Unusual orbits common.

Medium & Large Moons

• Enough self-gravity to be spherical

• Substantial amounts of ice mixed with rock.

• Mostly formed in orbit around jovian planets.

• Circular orbits in same direction as planet rotation.

The Galilean Moons of Jupiter

• “Planet-sized” objects in regular orbits

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Galilean Moons

• Follow density vs. distance trend• Easily visible from Earth in small telescopes• Very precise orbits (used as “clocks” in 17th century)

Interior Structures

Studied by Pioneer (2X), Voyager (2X) and Galileo missions

Io Volcanism

• Io is the most volcanically active body in the solar system

Europa: The Frozen Ocean Planet

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Europa interior Europa Jupiter MissionLaPlace or JIMO (2020 launch)

Ganymede

• Largest moon in the solar system

• Clear evidence of geological activitygeological activity

• Evidence of internal heating from tidal flexing plus heat from radioactive decay?

Callisto • Cratered iceball

• No tidal heating, no orbital resonances.

• But it has magnetic field !

50+ small “moons”

• Many likely captured asteroids of comet nuclei

• Also impact and collision debris

• Transients in unstable orbits

Out to Saturn

Earth

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Family Portrait

Medium-sized Moons of Saturn

Almost all show evidence of past or current volcanism and/or tectonics

Enceladus~500 km @ 4 Rs

Density = 1.6 (rock w/ice crust)

Tiger StripesTiger Stripes

Enceladus to scale

• Like Europa, a good candidate for having gliquid water beneath a frozen ocean surface

EnceladusEruptions from Tiger Stripes

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Enceladuscold volcanism

Titan5,150 km @ 20 Rs Density = 1.8 (rock core w/ice crust)

Titan’s Atmosphere

• Titan is the only moon with a thick (1.5 bar) atmosphere

• Consists mostly of nitrogen with some argon, methane, and ethane

Cassini Orbital Gallery

Optical cameras (VIMS)

Surface Lander

Optical cameras (VIMS)

Radar Imager

Titan Structure Titan’s Surface

• Huygens probe soft-landed on Titan’s surface in early 2005

• Liquid methane “rocks” made of flammable ice

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HuygensLander

Iapetus~1,500 km @ 59 Rs

Density = 1.08 (ice)

Iapetus surface Moons of Uranus

• Only visited by Voyager 2• Varying amounts of geological activity• Moon Miranda has large tectonic features and very few craters

– Miranda likely shattered and reassembled

• Total of 27 smaller moons – mostly in irregular orbits

Miranda• Mantle & Core

visible at surface– The racetracks

• Large-scale tectonic gfeatures

• Very few craters

• Shattered and reassembled in not too distant past?

Neptune’s Triton

• Similar to Pluto, but larger (2,700 km)

• Almost certainly a captured KBO

• Active nitrogen geysers

• Density 2 0 (rock + ice crust)

Thin N2

atmosphere

• Density ~ 2.0 (rock + ice crust)

Geyser plumes

+ 13 smallmoons.

Maybe chunks ofeach other

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ReduxRedux: Comparative Planetology: Comparative Planetology

• Rock melts at higher temperatures

• Only large rocky planets have enough internal heat for tectonic activity

•• Ices forms lower temp Ices forms lower temp magmasmagmas

•• Tidal heating melts ice & Tidal heating melts ice & compounds driving activitycompounds driving activity

•• Much higher impacts rates but Much higher impacts rates but lower numbers of craterslower numbers of craters

Chapter 12Remnants of Rock and Ice

Asteroids, Comets, and the Kuiper Belt

12.1 Asteroids and Meteorites12.1 Asteroids and Meteorites

• Our goals for learning– What are asteroids?– What is the asteroid belt?– The Asteroid = Meteorite connection

Vesta – Ceres - Moon

What are asteroids?What are asteroids?

• Small Solar System Bodies

• Rocky or metallic compositionp

• Regular orbits• Small (<500 km)• Most are between

Mars and Jupiter, but…

• ECAs number in the tens of thousands

Asteroid Asteroid FactsFacts

• Asteroids are rocky leftovers of planet formation.y p• Largest is Ceres, diameter ~1,000 km • 150,000 in catalogs, and probably over a million with

diameter >1 km.• Small asteroids more common than large asteroids.• All the asteroids in the solar system wouldn’t add up to

even a small terrestrial planet.Asteroids are cratered and not round

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Steins flyby: Steins flyby: 9/2/20089/2/2008

• Rosetta - European Space Agency mission

• Spacecraft en route to• Spacecraft en route to rendezvous with Comet 67P/Churyumov-Gerasimenko

• Steins is mainbelt asteroid (2.4 AU from Earth)

Asteroids with MoonsAsteroids with Moons• Some large asteroids have their own moon

• Asteroid Ida has a tiny moon named Dactyl

Density of AsteroidsDensity of Asteroids

• Measuring orbit of asteroid’s moon tells us asteroid’s mass

• Mass and size tell us asteroid’s density

• Some asteroids are solid rock; others just piles of rubble

OrbitsOrbits• Most asteroids orbit

in a belt between Mars and Jupiter

• Trojan asteroidsfollow Jupiter’s orbit

• Orbits of near-Earth asteroids cross Earth’s orbit

Orbital ResonancesOrbital Resonances• Asteroids in orbital

resonance with Jupiter experience periodic nudges

• Eventually those nudges move asteroids out of resonant orbits, leaving gaps in belt

Origin of Asteroid Origin of Asteroid BeltBelt• Rocky planetesimals

between Mars and Jupiter did not accrete into a planet.

• Jupiter’s gravity, through influence of orbital resonances, stirred up asteroid orbits and prevented their accretion into a planet.

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MeteoritesMeteorites Meteor TerminologyMeteor Terminology

• Meteorite: A rock from space that falls through Earth’s atmosphere

M t Th b i ht t il l ft b• Meteor: The bright trail left by a meteorite

• Both of pieces of solar system debris– Chunks of asteroids or comets

Meteorite ImpactsMeteorite ImpactsChicago, March 26, 2003

Meteorite TypesMeteorite Types1) Primitive: Unchanged in composition

since they first formed 4.6 billion years ago.g

2) Processed: Younger, have experienced processes like volcanism or differentiation. Ergo……..

Primitive MeteoritesPrimitive Meteorites Processed MeteoritesProcessed Meteorites

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Meteorites from Moon and MarsMeteorites from Moon and Mars

• A few meteorites arrive from the Moon and Mars (SNCs)

• Composition differs from the asteroid fragments.fragments.

• A cheap (but slow) way to acquire samples from the Moon and Mars.

The Space EnvironmentThe Space Environment

1) LDEF left in orbit for 6 years, > 32K orbits

2) Millions of micrometeorite collisions

12.2 Comets12.2 Comets• Our goals for learning

– What are comets?– Where do they come from?

Comet FactsComet Facts•• Formed beyond the frost line, Formed beyond the frost line,

comets are icy counterparts to asteroids.comets are icy counterparts to asteroids.•• Nucleus of comet a “dirty snowball”Nucleus of comet a “dirty snowball”•• Nucleus of comet a dirty snowballNucleus of comet a dirty snowball•• Most comets do not have Most comets do not have tailstails..•• Most comets remain perpetually frozen in Most comets remain perpetually frozen in

the outer solar system. the outer solar system. •• Only comets that enter the inner solar Only comets that enter the inner solar

system grow tails.system grow tails.

Nucleus of CometNucleus of Comet

• A “dirty snowball”

• Source of material for comet’s tailfor comet s tail

Deep Impact missionDeep Impact missionDeep Impact missionDeep Impact mission• Mission to study

nucleus of Comet Tempel 1

• Projectile hit surface on July 4. 2005

• Many telescopes studied aftermath of impact

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Anatomy of a CometAnatomy of a Comet• Coma is atmosphere

that comes from heated nucleus

• Plasma tail is gasPlasma tail is gas escaping from coma, pushed by solar wind

• Dust tail is pushed by photons

Growth of TailGrowth of Tail

Meteor ShowersMeteor Showers from comet dustfrom comet dust

• Comets eject small particles that follow the comet’s orbit• Cause meteor showers when Earth crosses the orbit.

Oort cloud:On random orbits

Only a tiny number of comets enter the inner solar system - most stay far from the Sun

From the hinterlands…From the hinterlands…

Kuiper belt:On orderly orbits from 30-100 AU in disk of solar system

extending to about 50,000 AU

How did they get there?How did they get there?

• Kuiper belt comets formed in the Kuiper belt: in a flat plane, aligned with the plane of planetary orbits, orbiting in the same direction as the lplanets.

• Oort cloud comets were once closer to the Sun, but they were kicked out there by gravitational interactions with jovian planets: spherical distribution, orbits in any direction.

12.3 Pluto: Lone Dog No More12.3 Pluto: Lone Dog No More

• Our goals for learningOur goals for learning– A long awakening– What are Kuiper Belt Objects?– Planets X, Y, Z…oops, out of letters!

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Pluto’s OrbitPluto’s Orbit

• Pluto will never hit Neptune, even though their orbits cross, because of 3:2 orbital resonance

• Neptune orbits three times during the time Pluto orbits twice

Is Pluto a Planet?Is Pluto a Planet?• What is a planet?• Not a gas giant like other

outer planets.• Has an icy composition like a y p

comet.• Has a very elliptical, inclined

orbit.• Pluto has more in common

with comets than with the eight major planets

Other Icy BodiesOther Icy Bodies

• There are many icy objects like Pluto on elliptical, inclined orbits beyond Neptune.

• The largest of these, “Planet X” was discovered in summer 2005, is even larger than Pluto

Kuiper Belt ObjectsKuiper Belt Objects• These large, icy

objects have orbits similar to the smaller objects in the Kuiper Belt thatthe Kuiper Belt that become short period comets

• So are they very large comets or very small planets?

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PlutoPluto• Its moon, Charon, is nearly as

large as Pluto itself (probably made by a major impact)

• Pluto is very cold (40 K)

• Pluto has a thin nitrogen atmosphere that will refreeze onto the surface as Pluto’s orbit takes it farther from the Sun.

Best Image of PlutoBest Image of Pluto

HST view of Pluto & CharonHST view of Pluto & Charon+ two smaller moons+ two smaller moons

Postcards from the EdgePostcards from the Edge

Other Kuiper Belt ObjectsOther Kuiper Belt Objects

• T.N.O.s = Trans-Neptunian Objects

• Most have been discovered very recently so little is known about them.

• New Horizons mission will study Pluto and a few other Kuiper Belt

bj i 2015 fl bobject in a 2015 flyby.

“Planet X” a planet?“Planet X” a planet?

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Pluto and “Planet X”Pluto and “Planet X”

• Pluto’s size was overestimated after its discovery in 1930

• It was considered a planet, and nothing of similar size was discovered for several decadessize was discovered for several decades

• Now other large objects have been discovered in Kuiper Belt, including “Planet X”

• Some scientists consider all of those objects planets; others consider none of them planets.

12.4 Cosmic Collisions: Small 12.4 Cosmic Collisions: Small Bodies Versus the PlanetsBodies Versus the Planets

• Our goals for learning– Major impacts in modern times– Did an impact kill the dinosaurs?– Is the impact threat a real danger or media

hype?– How do the jovian planets affect impact rates

and life on Earth?

Have we ever witnessed a major impact?

Comet SL9 caused a string of gviolent impacts on Jupiter in 1994, reminding us that catastrophic collisions still happen.

Tidal forces tore it apart during a previous encounter with Jupiter

This crater chain on Callisto probably came from another comet that tidal forces tore to pieces

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Impact plume from a fragment of comet SL9 rises high above Jupiter’s surface

Dusty debris at an impact site

Artist’s conception of SL9 impact Several impact sites

Impact sites in infrared light

Did an impact kill the Did an impact kill the dinosaurs?dinosaurs?

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Mass ExtinctionsMass Extinctions• Fossil record shows

occasional large dips in the diversity of

ispecies: mass extinctions.

• Most recent was 65 million years ago, ending the reign of the dinosaurs.

Iridium: Evidence of an ImpactIridium: Evidence of an Impact

• Iridium is very rare in Earth surface rocks but often found in meteorites.

• Luis and Walter Alvarez found a worldwideLuis and Walter Alvarez found a worldwide layer containing iridium, laid down 65 million years ago, probably by a meteorite impact.

• Dinosaur fossils all lie below this layer

Iridium LayerIridium Layer

No dinosaur fossils in upper rock layers

Dinosaur fossils in lower rock layers

Thin layer containing the rare element iridium

Consequences of an ImpactConsequences of an Impact

• Meteorite 10 km in size would send large amounts of debris into atmosphere.

• Debris would reduce sunlight reachingDebris would reduce sunlight reaching Earth’s surface.

• Resulting climate change may have caused mass exinction.

Likely K/T Impact SiteLikely K/T Impact Site• Geologists have

found a large subsurface crater about 65 million years old in Mexicoyears old in Mexico

Comet or asteroid about 10 km in diameter approaches EarthEarth

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Impact Facts•• Asteroids and comets have hit the Earth.Asteroids and comets have hit the Earth.

•• A major impact is only a matter of time: not IF but A major impact is only a matter of time: not IF but WHEN.WHEN.

•• Major impacts are very rare.Major impacts are very rare.

•• Extinction level events Extinction level events –– Millions of years.Millions of years.

•• Major damage Major damage –– Every 10Every 10--100 years.100 years.

Meteor Crater, Arizona: 35-50,000 years ago (50 meter object)Is the best-preserved impact on Earth

Tunguska, Siberia: June 30, 1908A ~40 meter object disintegrated and exploded in the atmosphere

30 Megaton airburst

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Frequency of ImpactsFrequency of Impacts• Small impact

happen almost daily.

• Impacts large enough to causeenough to cause mass extinctions are many millions of years apart

Atens, Earth-crossers

Apollos average orbital

ECA FamiliesECA Families

Apollos, average orbital radii > 1 AU but perihelia inside Earth's orbit.

Amors, Mars-crossers

Near Misses• 1989FC (Asclepius ) missed Earth by

700,000 km in 1989– Passed through point where Earth was 6 hours

previously.– Mean orbital speed = 28 km/secMean orbital speed 28 km/sec

• Unnamed 10 m object exploded over the Mediterranean Sea on June 6, 2002

– 26 kiloton yield

And Hits…

Peru, September 2007Peru, September 2007

Torino Torino ScaleScale

• Combines collision probability and Impact Energy

• Green/Yellow = Normal background impacts level

• Orange = Very Close Encounter

• Red = Certain Impact, Localized to Global Destruction

Apophis (2004Apophis (2004MN4MN4))

• Atens class• 350m diameter• April 13 2036 1 in• April 13, 2036, 1 in

45,000 probability of impact

• Was briefly a Torino 4, now a Torino 0

• Not a danger, but each pass changes orbit slightly.

April 2029 trajectory

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An impact with our name on it?An impact with our name on it?•• We haven’t seen it yet.We haven’t seen it yet.

•• DeflectionDeflection requires requires detectiondetection..

•• More probable with years of advance warning. More probable with years of advance warning.

•• Control is critical: breaking a big asteroid into a Control is critical: breaking a big asteroid into a g gg gbunch of little asteroids is unlikely to help.bunch of little asteroids is unlikely to help.

•• We get less advance We get less advance warning of a killerwarning of a killercomet…can come from anycomet…can come from anydirection and VERY fastdirection and VERY fast

What are we doing about it?What are we doing about it?

• LINEAR Program– USAF, NASA, MIT robotic search program– Has detected 211,849 new objects

• 1 622 near earth asteroids• 1,622 near earth asteroids • 142 were comets.

• Stay tuned to http://impact.arc.nasa.gov

• B612 Foundation

How do the jovian planets affect impact rates and life on Earth?

Influence of Jovian PlanetsInfluence of Jovian Planets

Gravity of a jovian planet (especially Jupiter)can redirect a comet

Was Jupiter necessary Was Jupiter necessary for life on Earth?for life on Earth?

water

Impacts can and do extinguish life.

But were they necessary for “life as we know it”?

water

vacuum cleaner

What have we learned?What have we learned?• Orderly view of 9 planets in empty

space is just plain wrong.• Small objects outnumber large objects

by millions to one.• The outer solar system is crowded

with a new class of objects - KBOs• Most impacts happened early in solar

system history; but many are still happening today, esp. in outer solar system.

• Solar Systems everywhere!

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Next WeekNext Week•• Extrasolar Planetary Extrasolar Planetary

SystemsSystems

•• Distribute final averagesDistribute final averages

•• Open class period for Open class period for exam prep and discussionexam prep and discussion