Transcript Lecture 7 - University of Minnesota

Lecture 7
Astro 1001
6/13/07
Jovian Planets Overview
• Much larger than terrestrial
planets
– Jupiter is over 300x more
massive than Earth
– About 1/5 of the density of
Earth
• We know a lot about the
planets now
– Pioneer and Voyager visits in
the 70s
– Recently, Galileo and
Cassini visits
Composition
• Jupiter and Saturn are almost
entirely hydrogen and helium
– Jupiter is often called a “failed
star”
• Uranus and Neptune are
smaller, contain much less
hydrogen and helium
– Made up of hydrogen
compounds
• Why are they different sizes?
Planets and Pillows
• Difference between densities in Neptune
and Uranus vs Saturn and Jupiter should
now be obvious
• Size is not necessarily an indication of mass
– More mass compresses things more, increasing
density but not planetary radius
Rotation Rates
• Jovian planets rotate very
quickly
– Difficult to measure
• Fast rotation makes the
planets bulge
– Saturn’s equator is about
10% wider than its poles
– Extra equatorial material
keeps moons and rings
aligned with bulge
The Interior
• We can say things
about the interior of
jovian planets
because of
experiments and
computer
simulations
• Interior is very
dense and hot
– Galileo dropped
probe that only
survived for 200 km
The Other Jovian Planets
• Saturn is very similar to Jupiter
• Uranus and Neptune don’t have metallic or
liquid layers, although their cores might be
liquid
Internal Heat
• Jupiter emits about twice as much energy as
it gets from the Sun
• Cannot be accounted for by accretion,
differentiation
• Heat is probably coming from gravitational
contraction
• Neptune is a bit mysterious
Weather and Clouds
• Jovian planets are typically
colorful due to their clouds
– We see lots of methane,
ammonia, water bands
• Jupiter has similar layers to
what the Earth has
• Neptune and Uranus have
different behaviors and can
form methane snow
Color and Winds
• Trace amounts of chemicals
produce the colors and Saturn
and Jupiter
– Saturn has more subdued colors
because its clouds are lower
• Methane is responsible for the
bluishness of Uranus and
Neptune
• Stripes on Jupiter due to
Coriolis effect
• Great Red Spot is a giant
hurricane
Magnetic Fields
• The jovian planets all have
magnetic fields
– Jupiter’s magnetic field is
20,000 stronger than Earth’s
– Jupiter has very spectacular
aurora
– Causes atmospheres on the
moons
• Neptune and Uranus have odd
magnetic fields in that they
aren’t aligned with the poles
Moons
• All of the jovian planets have moons (and
rings)
–
–
–
–
–
Jupiter has over 60 moons
Ganymede and Titan are larger than Mercury
Lots of ice
Larger moons probably formed near the planets
Smaller moons are probably captured asteroids
Io
• Most volcanically
active world in the
solar system
• Shoots some of its
volcanic materials
into space
• Tidal heating is why
Io is so active
Europa
• Covered by water ice
• Interior might be
water or convecting
ice
• Magnetic field data
indicates its probably
liquid water
• Might be heat sources
beneath the surface
Group Work
• Europa is a fascination moon that has a
fairly strong chance for supporting life.
However, NASA no longer has any plans to
visit it with a probe. Why do you think
there are no plans to visit Europa?
Ganymede and Callisto
• Ganymede
– Largest moon in the solar system
– Has both young and old features
– Features probably erased due to
liquid water welling up
• Callisto
– Heavily cratered
– No significant internal heat
Titan
• Second largest moon in the
solar system
• Atmosphere is very thick
– Mostly nitrogen
– No oxygen, but lots of
hydrogen compounds
– Created by the surface
sublimation or evaporating
• Recently explored with the
Huygens probe
Triton
• Coldest world in the
solar system
• Almost certainly
captured
• Probably had past
geological activity
Rings
• Made up of countless
small particles
• Particles are icy in nature
• Might be thinnest known
astronomical structure
• Rings have gaps
– Might be because of
shepherd moons
– Might be because of orbital
resonances
Other Ring Systems
• Other ring systems are
much darker, smaller
• Always go about the
planet’s equator
• Uranus’s rings are
slightly tilted,
elliptical
How Did the Rings Form?
• Large planets have enough gravity to rip
things apart that get too close
– Difficult to explain why this would frequently
happen
• Rings were formed from leftover material,
but are resupplied from the moons of the
planets
Asteroids
• Small rocky bodies
– Discovered only about 200 years ago
– Took 50 years to discover the first 10
– 150,000 known asteroids now
• Ceres is the largest, under 1000km in
diameter
• Total mass is probably less than that of the
moon
Asteroids Continued
• Shape depends on mass
• Thousands of asteroids
have been analyzed
through spectroscopy
– Made up of metal and
rock, with perhaps a big
of ice
• Asteroids vary greatly
in density
The Asteroid Belt
• Vast majority of
asteroid are in the
asteroid belt
– Individual asteroids
are separated by
millions of kilometers
• Jupiter’s gravity
causes two groups of
asteroids called
Trojan Asteroids
Meteorites
• Meteors are just particles
coming through the
atmosphere
– Usually around pea sized
objects
• Meteorites actually hit
the ground
– Can tell use a great deal
about how the solar
system formed
Types of Meteorites
• Primitive
– Very old (4.6 billion years old)
– Are made up of stone, or a combination of
stone and carbon compounds
• Processed
– Were once a part of another object
– Can be made of metals, or rock
Comets
• Comets formed outside of the
frost line
– Thus, are made up of lots of ice
• Comets do not race across the
sky
• Vast majority of comets do
not have tails and never get
anywhere close to Earth
• Some comets are knocked
into the inner solar system by
various sources of gravity
Composition
• Made up of chunks of
ice mixed with rocky
dust and complex
chemicals
– “Dirty snowballs”
• Starting to get more and
more details about
comets
– Deep Impact
– Stardust
Structure of a Comet
• Nucleus is the actual icy core
– Typically about 20km across
• Might have a dusty
atmosphere called a Coma
• Two tails
– Plasma tail consists of the
gasses escaping the comet
– Dust tail is made up of dust
sized particles
Where Do Comets Come From?
• Left over material was
flung far out into the
solar system by the
gravity from the jovian
planets
• This lead to the Oort
Cloud
• Slightly beyond the
solar system, remnants
stayed put and formed
the Kuiper Belt
Big Icy Things
• Lots of Pluto type objects
in the Kuiper belt
– Perhaps 1000s of km in
diameter
– “Xena” (Eris) is a good
example
– Not really comets
• These objects are very
cold, but might have
atmospheres
• Will be visited by a probe
in a decade or so
Collisions
• In 1994, a comet whacked
into Jupiter
– Each fragment had the
energy of a million H-bombs
• A massive collision is
probably (at least partly)
responsible for the
extinction of the dinosaurs
– A layer of irridium was the
initial evidence
– Later found the Chicxulub
crater
Do We Have to Worry?
• We witnessed a
probably collision in
1908
– Had the force of
several atomic bombs
– No more than 40
meters across
• The asteroid Apophis
will pass very close
to Earth soon