Lecture 34

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The Outer Planets. The Moon. The Origin of the Moon The Outer Planet Family. Lecture 34. Chapter 16.9  16.16. Summary of Terrestrial Planets. All the planets are relatively small (diameter < 13,000 km) Composition: rocks and metals (mean densities 3-5 g/cm 3 ) Density of water is 1 g/cm 3

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The Outer Planets. The Moon.

The Origin of the Moon

The Outer Planet Family

Lecture 34

Chapter 16.9  16.16

Summary of Terrestrial Planets

All the planets are relatively small (diameter < 13,000 km)

Composition: rocks and metals (mean densities 3-5 g/cm3) Density of water is 1 g/cm3

The planets are closely spaced (mean distance between them is 0.4 A.U.)

1 Astronomical Unit (A.U.) is the mean distance between the Sun and the Earth – 150 million kilometers

The outermost planet, Mars, is located 1.5 A.U. from the Sun

The Moon

The Moon is the largest satellite of a planet in the inner solar system.

The mean distance from the Earth is 384,000 km.

Diameter 3476 km (27% of the Earth’s diameter)

Mass is 1/81 of the Earth’s mass

Average density is 3.3 g/cm3 (60% of the Earth’s)

• Hypotheses of the Moon’s origin:
• Forming together with Earth and splitting
• Captured by the Earth fully formed
• Double-planet
• Collision
What Do We Need to Learn?

Why is there a large gap between the two families of planets?

What did cause the differences in the structures, compositions, and surroundings of the two families?

Jovian Planets: Basics
• Distance: 5-30 AU
• Much farther from Sun than terrestrial planets
• Much colder (100-50 K)
• Mass: 10-100 Earth masses
• Much more massive than terrestrial planets
• Jupiter & Saturn are similar
• Size (about 10 Earth diameters)
• Composition: mostly hydrogen and helium
• Uranus & Neptune are similar
• Smaller than Jupiter & Saturn
• Less hydrogen and helium
Appearance
• Jovian planets show “banded” appearance
• due to atmosphere
• we see only cloud tops
• Rotation quite fast (hours)
• Jupiter: 10 hrs
• Saturn: 11 hrs
• Uranus: 17 hrs
• Neptune: 16 hrs
Clouds
• Clouds on Jupiter & Saturn composed of ammonia ice (NH3)
• different colors due to differing cloud composition Saturn’s clouds deeper; less visible
• Clouds on Uranus & Neptune
• composed of methane (CH4)
• produces blue-green color
Jovian Planet Atmospheres

No solid surfaces

Jupiter atmosphere

Content: Almost entirely H and He + trace amounts of methane (CH4), ammonia (NH3), and H20.

Jupiter’s weather occurs in troposphere where clouds can be formed of ammonia crystals and other compounds

Jovian Planet Moons

There are more than 100 known moons orbiting Jovian planets (J-52, S-30, U-21, N-11)

Three main groups of jovian moons:

Small moons - less than 300 km in diameter

Medium-size - 300-1500 km

Large - more than 1500 km

Medium and large moons have circular orbits that lie close to the equatorial planes of their parent planets

Jupiter Moons

Pre-visit expectations: cold and geologically dead

Voyager missions: the moons are active!

Four Galilean moons: Io, Europa, Ganymede, Callisto

Io has many volcanoes and no impact craters

Europa – no craters, fractured surface, icebergs

Ganymede – grooves on surface, magnetic field

Callisto – a heavily cratered iceball

Io

Eruptions erased all Io’s impact craters.

Reasons for Geological Activity

Io has an additional heating source – tidal heating

Tidal heating is due to the Io’s orbit ellipticity.

Io is continuously flexed by Jupiter.

Source of the orbit ellipticity – orbital resonances

Periodical lining up of the three closest satellites of Jupiter (Io – 4 orbits, Europa – 2 orbits, Ganymede – 1 orbit)

Rings and Gaps

Two of Saturn’s rings can be seen from Earth

In fact, there are as high as 100,000 individual rings and gaps

Rings and gaps are caused by grouping of particles at some orbital distances which are being forced out at others.

Gaps can be created by gap moons located within rings. They clear up gaps around their orbits.

Summary of Jovian Planets
• Jovian planets larger, more massive than terrestrial
• Composition:
• mostly hydrogen (H) and helium (He)
• dominated by hydrogen
• also large amounts of ices (water, ammonia, methane)
• Why So Large?
• basic reason is distance from sun
• cooler temps allowed ices (volatiles) to freeze
Role of Volatiles
• Inner solar system is hot:
• volatiles are gaseous; not available for planet core formation
• planet cores
• only rock (no ice)
• smaller, less massive (1 earth mass)
• Outer solar system is cold:
• volatiles are solid; available for planet core formation
• planet cores
• both rock and ice
• bigger, more massive (10 earth masses)
• Massive cores have larger gravity; can capture gas
• Jovian planets have massive atmospheres (lots of H and He)
• Terrestrial planets have minimal atmosphere (little H and He)
Summary

The Earth’s Moon seems to be a result of a collision with a large proto-planet (Mars size)

Jovian planets are dynamic worlds with rapid winds, huge storms, strong magnetic fields, and interiors where common materials strangely behave.

Jovian moons are geologically active because of their ice compositions.

Ring systems were formed from small moons.