Lecture 34 The Outer Planets. The Moon. The Origin of the Moon The Outer Planet Family 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 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/cm 3 (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 ( K) Mass: 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

Basic Parameters PlanetDist. A.U. Mass M Earth Radius R Earth Density g/cm 3 Composition Jupiter H, He Saturn H, He Uranus H compounds rock, H, He Neptune H compounds rock, H, He

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 (NH 3 ) different colors due to differing cloud composition Saturn’s clouds deeper; less visible Clouds on Uranus & Neptune – composed of methane (CH 4 ) produces blue-green color

Planet Interiors Jupiter

Planet Density

Jovian Planet Atmospheres No solid surfaces Jupiter atmosphere Content: Almost entirely H and He + trace amounts of methane (CH 4 ), ammonia (NH 3 ), and H 2 0. Jupiter’s weather occurs in troposphere where clouds can be formed of ammonia crystals and other compounds

The Great Red Spot

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 km Large - more than 1500 km Medium and large moons have circular orbits that lie close to the equatorial planes of their parent planets 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)

Tidal Heating

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.

Rings of Saturn Telescopic view

Rings of Saturn (Voyager image)

Rings of Saturn (close up)

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.