Download “Planet-sized” Moons sized” Moons

11.2 Jovian Satellites
“Planet--sized” Moons
“Planet
• Miniature solar systems
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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?
Outer Solar System Moons
Small
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
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.
• Very numerous
g mass
• Not enough
to self-round
• Many are likely
captured asteroids
or comets.
• Unusual orbits
common.
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 Jupiter Mission
Europa interior
LaPlace or JIMO (2020 launch)
Ganymede
Callisto
• Largest moon in the
solar system
• Clear evidence of
geological activity
• Evidence of internal
heating from tidal
flexing plus heat from
radioactive decay?
50+ small “moons”
• Many likely
captured asteroids
of comet nuclei
• Also impact and
collision debris
• Transients in
unstable orbits
• Cratered iceball
• No tidal heating, no
orbital resonances.
• But it has magnetic
field !
Out to Saturn
Earth
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Family Portrait
Medium-sized Moons of Saturn
Enceladus
~500 km @ 4 Rs
Density = 1.6
(rock w/ice crust)
Tiger Stripes
Almost all show evidence of past or current volcanism and/or tectonics
Enceladus to scale
• Like Europa, a
good candidate
g
for having
liquid water
beneath a
frozen ocean
surface
Enceladus
Eruptions from
Tiger Stripes
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Titan
Enceladus
cold volcanism
5,150 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
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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Iapetus
Huygens
Lander
~1,500 km @ 59 Rs
Density = 1.08 (ice)
Moons of Uranus
Iapetus surface
• 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
Neptune’s
Triton
Miranda
• Mantle & Core
visible at surface
– The racetracks
• Large-scale
g
tectonic
features
• Very few craters
• Shattered and
reassembled in not
too distant past?
•
•
•
•
Thin N2
atmosphere
Similar to Pluto, but larger (2,700 km)
Almost certainly a captured KBO
Active nitrogen geysers
Density ~ 2.0
2 0 (rock + ice crust)
+ 13 small
moons.
Maybe
chunks of
each other
Geyser plumes
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Redux: Comparative Planetology
Chapter 12
Remnants of Rock and Ice
Asteroids, Comets, and the Kuiper Belt
• Rock melts at higher
• Ices forms lower temp magmas
temperatures
• Tidal heating melts ice &
compounds driving activity
• Only large rocky planets
have enough internal heat • Much higher impacts rates but
lower numbers of craters
for tectonic activity
12.1 Asteroids and Meteorites
What are asteroids?
• Our goals for learning
– What are asteroids?
– What is the asteroid belt?
– The Asteroid = Meteorite connection
Vesta – Ceres - Moon
• Small Solar
System Bodies
• Rocky or metallic
p
composition
• Regular orbits
• Small (<500 km)
• Most are between
Mars and Jupiter,
but…
• ECAs number in
the tens of
thousands
Asteroid
Facts
• Asteroids are rockyy leftovers of planet
p
formation.
• 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:
9/2/2008
• Rosetta - European
Space Agency mission
• Spacecraft en route to
rendezvous with Comet
67P/ChuryumovGerasimenko
• Steins is mainbelt
asteroid (2.4 AU from Earth)
Density of Asteroids
Asteroids with Moons
• Some large asteroids have their own moon
• Asteroid Ida has a tiny moon named Dactyl
Orbits
• Measuring orbit of
asteroid’s moon tells
us asteroid’s mass
• Most asteroids orbit
in a belt between
Mars and Jupiter
• Mass and size tell us
asteroid’s density
• Trojan asteroids
follow Jupiter’s
orbit
• Some asteroids are
solid rock; others
just piles of rubble
• Orbits of near-Earth
asteroids cross
Earth’s orbit
Orbital Resonances
Origin of Asteroid Belt
• Asteroids in orbital
resonance with
Jupiter experience
periodic nudges
• Rocky planetesimals
between Mars and
Jupiter did not
accrete into a planet.
• Eventually those
nudges move
asteroids out of
resonant orbits,
leaving gaps in belt
• Jupiter’s gravity,
through influence of
orbital resonances,
stirred up asteroid
orbits and prevented
their accretion into a
planet.
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Meteorites
Meteor Terminology
• Meteorite: A rock from space that
falls through Earth’s atmosphere
• Meteor:
M t
Th bbright
The
i ht ttrail
il lleft
ft by
b a
meteorite
• Both of pieces of solar system debris
– Chunks of asteroids or comets
Meteorite Types
Meteorite Impacts
1) Primitive: Unchanged in composition
since they first formed 4.6 billion years
ago.
g
Chicago, March 26, 2003
2) Processed: Younger, have experienced
processes like volcanism or
differentiation. Ergo……..
Primitive Meteorites
Processed Meteorites
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Meteorites from Moon and Mars
The Space Environment
• A few meteorites arrive from the Moon and
Mars (SNCs)
• Composition differs from the asteroid
fragments.
• A cheap (but slow) way to acquire samples
from the Moon and Mars.
1) LDEF left in orbit for 6 years, > 32K orbits
2) Millions of micrometeorite collisions
12.2 Comets
• Our goals for learning
Comet Facts
• Formed beyond the frost line,
comets are icy counterparts to asteroids.
• Nucleus of comet a “dirty
dirty snowball”
snowball
• Most comets do not have tails
tails..
• Most comets remain perpetually frozen in
the outer solar system.
• Only comets that enter the inner solar
system grow tails.
– What are comets?
– Where do they come from?
Nucleus of Comet
• A “dirty snowball”
• Source of material
for comet
comet’ss tail
Deep 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 Comet
Growth of Tail
• Coma is atmosphere
that comes from
heated nucleus
• Plasma tail is gas
escaping from coma,
pushed by solar
wind
• Dust tail is pushed
by photons
Meteor Showers from comet dust
From the hinterlands…
Only a tiny number of
comets enter the inner
solar system - most
stay far from the Sun
Oort cloud:
On random orbits
extending to about
50,000 AU
Kuiper belt:
On orderly orbits
from 30-100 AU in
disk of solar system
• Comets eject small particles that follow the comet’s orbit
• Cause meteor showers when Earth crosses the orbit.
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
planets.
l
• 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 More
• Our 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 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
Other Icy Bodies
Is Pluto a Planet?
• What is a planet?
• Not a gas giant like other
outer planets.
• Has an icyy composition
p
like a
comet.
• Has a very elliptical, inclined
orbit.
• Pluto has more in common
with comets than with the
eight major planets
Kuiper Belt Objects
• These large, icy
objects have orbits
similar to the
smaller objects in
the Kuiper Belt that
become short period
comets
• 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
• So are they very
large comets or very
small planets?
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Pluto
Best Image of Pluto
• 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.
HST view of Pluto & Charon
+ two smaller moons
Other Kuiper Belt Objects
Postcards from the Edge
“Planet X” a planet?
• 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
object
bj in
i a 2015 flyby.
fl b
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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 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
Bodies Versus the Planets
Have we ever witnessed a major
impact?
• 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?
Comet SL9 caused a stringg of
violent 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
Did an impact kill the
dinosaurs?
Impact sites in infrared light
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Mass Extinctions
• Fossil record shows
occasional large dips
in the diversity of
species:
i mass
extinctions.
• Most recent was 65
million years ago,
ending the reign of the
dinosaurs.
Iridium: Evidence of an Impact
• Iridium is very rare in Earth surface rocks
but often found in meteorites.
• Luis 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 Layer
Consequences of an Impact
• Meteorite 10 km in size would send large
amounts of debris into atmosphere.
• Debris would reduce sunlight reaching
Earth’s surface.
• Resulting climate change may have caused
mass exinction.
No dinosaur fossils
in upper rock layers
Thin layer
containing the rare
element iridium
Dinosaur fossils in
lower rock layers
Likely K/T Impact Site
• Geologists have
found a large
subsurface crater
about 65 million
years old in Mexico
Comet or
asteroid
about 10
km in
diameter
approaches
Earth
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Impact Facts
• Asteroids and comets have hit the Earth.
• A major impact is only a matter of time: not IF but
WHEN.
• Major impacts are very rare.
• Extinction level events
– Millions of years.
• Major damage
– Every 10
10--100 years.
30 Megaton airburst
Meteor Crater, Arizona: 35-50,000 years ago (50 meter object)
Is the best-preserved impact on Earth
Tunguska, Siberia: June 30, 1908
A ~40 meter object disintegrated and exploded in the atmosphere
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Frequency of Impacts
ECA Families
• Small impact
happen almost daily.
Atens, Earth-crossers
• Impacts large
enough to cause
mass extinctions are
many millions of
years apart
Apollos average orbital
Apollos,
radii > 1 AU but perihelia
inside Earth's orbit.
Amors, Mars-crossers
Near Misses
And Hits…
• 1989FC (Asclepius ) missed Earth by
700,000 km in 1989
– Passed through point where Earth was 6 hours
previously.
– Mean orbital speed = 28 km/sec
• Unnamed 10 m object
exploded over the
Mediterranean Sea
on June 6, 2002
– 26 kiloton yield
Peru, September 2007
Torino
Scale
•
•
•
•
Apophis (2004MN4)
Combines collision probability and Impact Energy
Green/Yellow = Normal background impacts level
Orange = Very Close Encounter
Red = Certain Impact, Localized to Global Destruction
April 2029 trajectory
• Atens class
• 350m diameter
• April 13
13, 2036
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.
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An impact with our name on it?
•
•
•
•
We haven’t seen it yet.
Deflection requires detection.
detection.
More probable with years of advance warning.
Control is critical: breakingg a bigg asteroid into a
bunch of little asteroids is unlikely to help.
• We get less advance
warning of a killer
comet…can come from any
direction and VERY fast
What are we doing about it?
• LINEAR Program
– USAF, NASA, MIT robotic search program
– Has detected 211,849 new objects
• 1,622
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 Planets
Gravity of a jovian planet (especially Jupiter)
can redirect a comet
Was Jupiter necessary
for life on Earth?
water
Impacts can and do
extinguish life.
But were they
necessary for “life as
we know it”?
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!
vacuum cleaner
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Next Week
• Extrasolar Planetary
Systems
• Distribute final averages
• Open class period for
exam prep and discussion
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