this set of slides. this material covers an overview of our solar system, some comparative...
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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
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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!
• 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.
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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?
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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.
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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.
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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.
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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.
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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
Kepler’s 3rd Law:
• 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:
• The planet’s average density, , is then:
2
34
GP
dM
3
3
4RV
V
M
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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) – Larger bodies
– Gasses, ices and other volatiles
Again, any model of solar system formation must explain all
of this!
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• 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 Mass and Radius
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• 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 Water and Biological Processes
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• 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 Role of Sunlight
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• 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.
The Outer Planets
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• The outer planets rotate much faster than their terrestrial cousins.– These faster
rotational speeds make the outer planets much wider at the equator.
Equatorial Bulges
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• 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.
Other Differences
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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
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• 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 Jupiter
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• 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 Appearance of Saturn
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The Interiors of the Gas Giants
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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.
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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.
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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.
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Storms on Saturn
• Saturn, though it appears calmer, has storms as well– Higher wind
speeds than Jupiter
– Storms are deeper in its atmosphere
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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.
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• 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.
The Discovery of Uranus
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• 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.
A New Method of Discovery
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• 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.
The Atmospheres of Uranus and Neptune
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Interior of Uranus
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Storms
• High winds lead to storms on Neptune.
• Neptune has a Great Dark Spot, which disappeared recently.
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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.
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• 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.
Odd Magnetic Fields
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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…