Overview of the Solar System

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Outline

• General properties of planets and Solar System• Spectroscopy, composition, density of planets• Why do some have atmospheres?• Terrestrial/Jovian planet distinction• Clues to interiors and history

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You live in a special age

• Landings on Moon, Venus, Mars, Titan, an asteroid and a comet

• Returned rocks from the Moon• Atmospheres probed on Venus, Mars, Jupiter, Titan• Fly-bys past all planets and Pluto• Venus and Titan surfaces revealed by radar mapping

All of this in past ~45 years - and more to come!

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• Sun in center, contains most of the mass• Planets• Moons• Rings• Asteroids – mostly between Mars and Jupiter.

Mostly rocky material• Comets – High eccentricity orbits. Icy material• “Trans-Neptunian Objects”• Meteoroids• Gas and dust

Contents of the Solar System

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Key questions• How did the Solar System and planets form?• How are planets similar to and different from

Earth?• What are the planets made of and how do we

know?• What range of properties do their moons show?• What is the origin and fate of rings?• What can we learn from asteroids and comets?

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Solar System objects to scale

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The Solar System is BIG!

• It is difficult to make a correctly scaled model

• Most of the Solar System is (nearly) empty space

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If the Solar System were 10 km across…

Object Distance Diameter Size example

The Sun 0 km 1.55 m A large beach ball

Mercury 65 m 5.4 mm A pebble

Venus 121 m 1.35 cm A small marble

Earth 167 m 1.42 cm A small marble

Earth's Moon 43 cm 3.9 mm A pebble

Mars 254 m 7.6 mm A pea

Jupiter 868 m 15.9 cm A softball

Saturn 1.6 km 13.4 cm A softball

Uranus 3.2 km 5.7 cm A hen's egg

Neptune 5 km 5.5 cm A hen's egg

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The Sun

• Average-sized star (mostly H, He)• 99.8% of the mass of the Solar System• ~4.6 Gyr old (middle-age)• Surface (photosphere) about 5800 K (emits

mostly in visible, UV, IR)• Hot because of nuclear fusion in core

– Builds He nuclei from H nuclei, a process that releases energy

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Planetary orbits• All planets orbit the Sun in

same direction and almost same plane

• Orbits are close to circular

Main exception is Mercury:

orbital tilt 7o eccentricity 0.21

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• All spin in same sense as orbit, except Venus and Uranus

Key concept: Density

= mass/volume. Depends on composition, compression by gravity.

Can compare to density of water:water = 1000 kg/m3 = 1 g/cm3

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Planet Diameter Mass (Earth=1) (Earth=1) (g/cm3)

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Mercury 0.383 0.055 5.4

Venus 0.949 0.815 5.2

Earth 1.000 1.000 5.5

Mars 0.533 0.107 3.9

Jupiter 11.21 317.8 1.3

Saturn 9.45 95.2 0.7

Uranus 4.01 14.5 1.3

Neptune 3.88 17.2 1.6

Inner four are rocky, outer four are mostly gas and liquid. Must giveclue to formation. 12

(“Terrestrial”Planets)

(“Jovian”Planets)

How do we know what planets, moons, etc., are made of?

• Chemical composition - determined by spectroscopic observations or sometimes direct chemical analysis.

• The spectrum of a planet with a thick atmosphere reveals the atmosphere’s composition

• If there is no atmosphere, the spectrum indicates the composition of the surface.

• To a large extent, we must infer what the interiors are made of.

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Visible light from planets, moons, comets, etc. is dominated by reflected sunlight. In IR, might see emitted blackbody radiation. 14

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For context, consider overall abundances of elements in our part of the Milky Way

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Where in the Solar System is this kind of

abundance found?• Sun, Jupiter, Saturn, Uranus, Neptune

(except planet cores)

Where is it NOT found?• Mercury, Venus, Earth, Mars, Moons,

comets, asteroids

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H and He abundant on the Jovian planets. The terrestrial planets are composed mostly of heavier elements (e.g. Fe, Si, Mg, O, Ca, Al).

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Planetary Atmospheres

• Why do some planets have atmospheres, and others don't?

• Why do they have different atmospheric compositions?

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m

kTV

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Recall

V is the typical speed of particles of mass m in a gas of temperature T

Now recall escape speed from a planet

Vesc 2GM

rIf a particle is moving too fast, the planet can't retain it

Rule of thumb: a gas will be retained in the atmosphere if Vesc > 6V 20

Example: For room temperature (293 K)

• VH2 = 1.9 km/s, and 6 times 1.9 km/s = 11.4 km/s

• VO2 = 0.5 km/s, and 6 times 0.5 km/s = 3 km/s

The escape speed from surface of Earth is 11.2 km/s.

Molecular oxygen is easily retained, but hydrogen is not.

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Guess which planets have atmospheres:

Planet Vesc (km/s)__________________________________

Mercury 4.3

Venus 10.4

Earth 11.2

(Moon) 2.4

Mars 5.0

Jupiter 59.5

Saturn 35.5

Uranus 21.3

Neptune 23.522

Two Kinds of Planets"Terrestrial"

Mercury, Venus,Earth, Mars

"Jovian"

Jupiter, Saturn, Uranus, Neptune

Close to the SunSmall (D=5000-13000 km)

Far from the SunLarge (D=50,000-143,000 km)

Few MoonsNo RingsMain Elements Fe, Si, Mg, O

Mostly RockyHigh Density (3.9 -5.5 g/cm3)Slow Rotation (1 - 243 days)

Mostly Gas and LiquidLow Density (0.7 -1.6 g/cm3)

Many MoonsRingsMain Elements H, He

Fast Rotation (0.41 - 0.72 days)

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Also in the Solar System• Seven giant moons: size ~ size of Mercury

• Many smaller moons• Comets• Asteroids (most orbit between Mars and Jupiter)• Meteoroids• “Trans-Neptunian” objects

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Chunks of rock and ice• Asteroids: small, rocky objects, most orbiting between Mars and

Jupiter

• Comets: small, dirty ice balls whose orbits bring them into inner Solar System

• Trans-Neptunian Objects – icy bodies beyond Neptune’s orbit, including Pluto and Eris

• Kuiper belt – zone 30-50 AU from Sun containing most of the TNOs

• All debris left over from planet making process

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Trans-Neptunian Objects

Orbit of Eris26

More definitions!

• A planet is a spherical object orbiting a star that is not astar itself, and has swept out its path

• A dwarf planet is a spherical object orbiting a star that has not swept out its path (Pluto, Eris, Ceres, a few other TNOs), and isnot a satellite. Note Pluto and Eris are also TNOs, and Ceresis an asteroid.

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Cratering on terrestrial planets• Result of impacts from interplanetary debris (butsome are volcanic)

• Geologic activity =>– Many craters means old surface and low geological

activity– Smaller objects lose heat faster: more cratered 28

Olympus Mons on Mars – largest volcano in Solar System. Volcanicflows can fill impact craters. Its crater is a caldera. 29

Magnetic Fields – a direct indication of interior

• The presence of a global, regular, magnetic field indicates a liquid, conducting interior

• Need circulating currents to generate magnetic field, like in an electromagnet

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• The global magnetic field of the Earth is produced by metals, mainly iron, in the liquid state

• The stronger fields of the Jovian planets are generated by liquid metallic hydrogen or by water with ionized molecules dissolved in it

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Solar system formation

• All objects formed from the same cloud of gas and dust

• Composition determined by cosmic history

• Different objects formed in different environments depending on their distance to the Sun

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Problem 6.36

• The four largest moons of Jupiter are roughly the same size as our Moon and are about 628 million km from Earth at opposition.

• What is the size in km of the smallest surface feature that the HST can detect (resolution 0.1")?

• How does this compare with the smallest feature that can be seen on the Moon with the unaided human eye (resolution 1')?

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