Transcript Document

This set of slides.
• This material covers an overview of our solar
system, some comparative planetology, the
Jovian planets (Jupiter, Saturn, Uranus and
Neptune), planetary magnetic fields.
• Units covered: 32, 42, 43, 44
Components of the Solar System
•
The vast majority of the Solar
System’s mass resides in the Sun.
– All the planets, asteroids and
comets make up less than 1/700
of the mass of the Solar System!
•
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The rocky inner planets (Mercury,
Venus, Earth and Mars) are called
the terrestrial planets.
The gaseous outer planets
(Jupiter, Saturn, Uranus and
Neptune) are the Jovian planets.
An asteroid belt lies between the
inner and outer planets.
The outermost icy planet, Pluto, is
in a class called Trans-Neptunian
Objects (TNO). It’s a dwarf
planet.
The Kuiper Belt
• Outside the orbit
of Neptune lies
the Kuiper Belt.
– Located about
40 AU from the
Sun.
– Home of TNO’s
– Many objects
smaller and
larger than Pluto
have been found
here.
• So is Pluto a planet or not?
How to Be a Planet
• Once upon a time, be a wanderer in the night sky.
• Since 2006,
– Be massive enough that your own gravity pulls you into a
spheroid shape.
– Be the dominant mass in your orbital neighborhood.
• Pluto makes the cut in the first category but not the
second.
• Meet the first criterion, you can be a dwarf planet.
The Oort Cloud
• The Solar System is
surrounded by a cloud
of cometary bodies.
– Located about 50,000
AU from the Sun.
– Gravitational
influences from
passing stars
occasionally send
comets into the Solar
System.
Rotation and Revolution in the Solar System
• Because of the conservation of
angular momentum, all planets
revolve around the Sun in the
same direction and in more or
less the same plane.
– Mercury’s orbit is tipped by 7
degrees.
– Pluto’s is tipped by 17 degrees.
• Most of the planets rotate in the
same direction.
– Counterclockwise as viewed
from above.
– Venus rotates clockwise as
viewed from above.
– Uranus and Pluto’s rotational
axes are tipped significantly.
• Any model of solar system
formation must explain all of
these oddities.
Composition of the Solar System Objects
• Spectrum analysis shows us the Sun is 71% hydrogen,
27% helium, 2% everything else.
• Jovian planets have similar composition. Much in ice,
frozen methane, ammonia, and water.
• Inner planets are rocky, silicon oxide, aluminum, etc.
• Spectroscopy tells us surface composition. We need
other info to determine below the surface structure.
Calculating a Planet’s Density
• Calculate the planet’s mass (M) by
observing its satellite’s orbital distance
(d) and period (P).
• Use Newton’s modified form of
• If we know the distance to the planet,
we can measure its angular diameter
and calculate its linear diameter (or
radius, R), and then its volume:
4
V  R 3
3
Kepler’s 3rd Law:
4d
M
GP 2
3
• The planet’s average density, , is
then:
M

V
Average Density tells us a lot
• Inner planets have
high average
densities (~5 kg/liter)
– Small bodies
– Mostly rock and iron
• Outer planets have
lower densities (~1
kg/liter)
Again, any model of solar
system formation must explain all
of this!
– Larger bodies
– Gasses, ices and
other volatiles
The Role of Mass and Radius
• Mass and size of a planet
help determine its
environment.
– Small planets cool
quickly, leading to dead
worlds with little activity.
– Small planets also have
trouble holding an
atmosphere. (low gravity)
– Larger planets hold on to
their heat, and have active
interiors and surfaces.
– Mars is right in the
middle, not too large, and
not too small.
• Once had water and an
active surface.
• Now is cold and dead.
The Role of Water and Biological Processes
• The presence or absence of
water helps determine the
nature of the atmosphere.
– Water acts as a sink for carbon
dioxide, removing it from the
atmosphere.
– Water helps lock CO2 into rocks
as well.
– Too much CO2 can lead to a
runaway greenhouse effect (as
with Venus).
– Too little CO2 can lead to
cooling (as on Mars).
• Biological activity impacts the
environment, too.
– Animals remove oxygen from the
atmosphere (and get carbon from
plants), and release CO2 (and
methane.)
– Plants remove CO2 from the
atmosphere, and with sunlight and
water, converts it into our food,
and release oxygen.
– Burning (wood, fossil fuels)
releases CO2 into the air.
The Role of Sunlight
• A planet’s distance from
the Sun determines how
much sunlight it receives.
– Venus receives ¼ of the
energy per square meter
that Mercury does.
– Planets in eccentric orbits
receive varying amounts of
sunlight.
– The axial tilt of a planet
determines its seasons.
• Sunlight warms a planet, but
the atmosphere has an impact,
too
– Venus’s atmosphere warms the
surface to 750 K, but it would be
very warm even without the CO2
– Mercury is closer to the Sun, but
still cooler than Venus.
– The Moon is cooler than the
Earth, even though they are at the
same distance from the Sun.
• Sunlight also determines the
makeup of the planets.
– Inner planets are rocky. (iron)
– Outer planets are gaseous.
The Outer Planets
• Far from the Sun, temperatures
are cold enough that water
vapor can condense into ices.
• Beyond this frost line, planets
are primarily composed of
hydrogen and ices.
• The low temperatures allowed
the outer planets to capture
hydrogen and helium gas, and to
grow to immense sizes.
• The outer planets have no
surfaces.
– Pressures steadily climb (moving
inward), turning gases into
liquids and eventually metals.
Equatorial Bulges
• The outer planets
rotate much
faster than their
terrestrial
cousins.
– These faster
rotational speeds
make the outer
planets much
wider at the
equator.
Other Differences
• Each gas giant has a set
of rings.
– Some are easy to see, like
Saturn’s.
– Others are harder, like
Neptune’s.
• The gas giants have many
more moons, as well.
– The number of moons
discovered goes up all the
time.
Jupiter and Saturn
• Jupiter
– 5 AU from the Sun
– 11x Earth’s diameter
– 300x Earth’s mass
• Saturn
– 9.5 AU from the Sun
– 9.5x Earth’s diameter
– 100x Earth’s Mass
The Appearance of Jupiter
• Parallel bands of clouds
– Dark belts
– Light zones
• 90% H2, 10% He, traces of
methane, ammonia and water.
• Outer atmosphere has a
temperature of 160K.
• Rotates once every 9.9
hours.
• Visibly flattened.
The Appearance of Saturn
• Parallel bands of clouds.
– Similar to Jupiter’s, but not
as distinct.
• 96% H2, 4% He, traces of
hydrogen-rich compounds.
• Outer atmosphere has a
temperature of 130K.
• Rotates once every 10.7
hours.
• Even flatter than Jupiter.
The Interiors of the Gas Giants
Coriolis Effect
• Coriolis Effect is due to the different rotational speeds
at different latitudes. A spinning sphere rotates at
higher speed at the equator than north or south.
• Coriolis Forces DO cause weather patterns (for
example) to move in the directions they do –
hurricanes and tornadoes turn counterclockwise in the
Northern hemisphere, clockwise in Southern.
• Coriolis Forces are too small and insignificant to the
water in your toilet bowl.
Winds
• Rapid rotation
gives rise to strong
Coriolis forces,
and very high
winds.
– Measured max
wind speeds of
500 km/hr at
Jupiter, and faster
at Saturn.
• Bands of clouds
move in opposite
directions, creating
very large wind
shears.
The Great Red Spot
• On Jupiter, these wind
shears give rise to
enormous vortices, or
storms, seen as white,
brown or red ovals in its
clouds.
• The Great Red Spot on
Jupiter is one such vortex.
– Rises 50 km above
surrounding clouds
– Wind speeds of 500 km/hr. • The Great Red Spot is a storm that has
lasted for at least 300 years.
– Galileo saw it, and it hasn’t changed much.
Storms on Saturn
• Saturn, though it
appears calmer,
has storms as well
– Higher wind
speeds than
Jupiter
– Storms are
deeper in its
atmosphere
Magnetic Fields
• The liquid metallic
hydrogen in Jupiter and
Saturn can carry electrical
currents, similar to the
liquid core of the Earth.
• These currents generate
very large magnetic fields.
– Jupiter’s is 20,000 times as
strong as Earth’s, and if it
were visible, would appear
larger than the full Moon in
our sky.
– Saturn’s field is 500 times as
strong as Earth’.
• Both Jupiter and Saturn
experience auroras.
The Discovery of Uranus
• In 1781 a new planet
was discovered by
W. Herschel
– Originally thought to
be a comet.
– Herschel named it
Georgium Sidus
(George’s Star) after
King George III.
– Name changed to
Uranus to stay
consistent with the
mythological names
of the other planets.
A New Method of Discovery
• Uranus was not
following its
calculated orbit.
– Another planet must
be effecting its orbit.
– Scientists calculated
where the unseen
planet should be.
– Astronomers looked
at this location, and
found Neptune.
– Galileo saw Neptune
but didn’t realize
what it was.
The Atmospheres of Uranus and Neptune
• The atmospheres of both
Uranus and Neptune are rich
in hydrogen and helium.
– Both have larger amounts of
methane, giving them their
blue color.
– Methane crystals scatter blue
light, and methane gas
absorbs red light.
• Both planets are very cold
– Uranus: 80K
– Neptune: 75K
• Densities:
– Uranus: 1.3 kg/liter
– Neptune: 1.6 kg/liter
• Their interiors are probably
ordinary water mixed with methane
and ammonia, surrounding a core
of rock and iron-rich material.
Interior of Uranus
Storms
• High winds lead to storms
on Neptune.
• Neptune has a Great Dark
Spot, which disappeared
recently.
Uranus’s Axial Tilt
• Uranus is tipped almost 90
degrees to the ecliptic plane.
• Possible that a collision early
in its history tipped the axis,
and broke out material that
formed its moons.
• This inclination means that for
half of Uranus’ orbit, one
hemisphere is in uninterrupted
daylight, while the other
hemisphere is in darkness.
Odd Magnetic Fields
• Both Uranus and
Neptune have strong
magnetic fields.
– Uranus: 47xEarth
– Neptune: 25xEarth
– Possibly generated
by currents in the
liquid water in their
interiors.
– Not centered on the
center of the planet
and tipped in odd
directions.
Earth’s Magnetic Field
• Earth’s magnetic north pole and the “north pole”
(i.e., north end of axis) are not in the same location.
• Earth’s magnetic north (and south) pole aren’t
fixed but change over time.
• The poles have “flipped” throughout history.
• We may be “due” for a flip again.
• Results not likely to be catastrophic but could be
interesting if so…