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!

• 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

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

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!

• 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

• 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

• 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

• 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

• 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

• 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

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

• 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

• 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

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.

• 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

• 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

• 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

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.

• 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

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…