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Our Moon and other moons of the Solar System

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Starter 1/25/13

• What do you already know about our Moon?

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What is a moon?

• A moon is a natural satellite of a planet or dwarf planet

• There are over 170 moons around the planets and dwarf planets in our solar system

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Planets and numbers of moonsPlanet Number of known moonsMercury 0Venus 0Earth 1Mars 2Jupiter 63Saturn 63Uranus 27Neptune 13

Dwarf planetPluto 3 (maybe 4)Eris 1

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How do we explain the existence of our Moon?

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Giant Impact Theory• Between 4.4 and 5 billion years ago an object

about the size of Mars struck a very young Earth and blasted material into orbit around Earth– The “youngest” Moon rocks are 4.4 billion years old

• The material from the collision came together by gravity (called accretion) and became our Moon.

• The Moon was much closer to the Earth when it first formed– It was between12,000-18,000 miles away– Now it is 240,000 miles away and moving away from

us by 1.5 inches/year

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Giant Impact

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Why does the Moon look the way it does?

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Impact Cratering

• Most cratering happened soon after the solar system formed – “the Late Heavy Bombardment”

• Craters are about 10 times wider than object that made them.

• Small craters greatly outnumber large ones.

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Impact Craters

Meteor Crater (Arizona) Tycho Crater (Moon)

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Impact Craters on Mars

“Standard” crater Impact into icy ground

Eroded crater

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Thought question

Shouldn’t the Earth be covered in craters too? Why isn’t it?

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Volcanism

• Volcanism happens when molten rock (magma) finds a path through the Earth’s crust (lithosphere) to the surface.

• Molten rock is called lava after it reaches the surface.

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Lava and Volcanoes

Runny lava makes flat lava plains.

Slightly thicker lava makes broad shield volcanoes.

Thickest lava makes steep stratovolcanoes.

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The Lunar Maria were formed by volcanism

• Smooth, dark lunar maria are less heavily cratered than lunar highlands.

• Maria were made by floods of runny lava.

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Formation of Lunar Maria

Large impact crater weakens crust.

Heat build-up allows lava to well up to surface.

Early surface is covered with craters.

Cooled lava is smoother and darker than surroundings.

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The Moon is geologically dead• Moon is considered

geologically “dead” because geological processes have virtually stopped.

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Do the Moon and Mercury have any atmosphere?

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Exospheres of the Moon and Mercury

• Sensitive measurements show that the Moon and Mercury have virtually no atmosphere.

• The little gas there comes from impacts and the solar wind that eject surface atoms.

Earth’s Moon Mercury

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The Moon’s surface conditions

• No atmosphere• No liquid water – frozen

water at the poles• Because of the lack of an

atmosphere the moon experiences extreme temperatures– Daytime = 130C (265°F)– Nighttime = -190C (-310 F)

• The moon has 1/6th of Earth’s gravity

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Starter 2/7

• Identify one similarity and one difference between our Moon and the other moons of the solar system?

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What kinds of moons orbit the other planets of the solar system?

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Sizes of Moons

• Small moons (< 300 km) “irregular”– No geological activity

• Medium-sized moons (300–1500 km) “regular”– Geological activity in the past (mostly)

• Large moons (> 1500 km) “regular”– Ongoing geological activity (mostly)

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Small (or irregular) Moons

• These are far more numerous than the medium and large moons.

• They do not have enough gravity to be spherical: Most are “potato-shaped.”

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• They are captured asteroids or comets, so their orbits do not follow usual patterns – *retrograde and tilted orbits

Small (or irregular) Moons

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Captured Moons

• Mars has two moons (Phobos and Deimos) that are thought to be captured asteroids from the asteroid belt

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Medium and Large (regular)

Moons• Enough self-gravity to be spherical

• Often have substantial amounts of ice

• Except for our Moon they formed in orbit around jovian planets (gas giants) *by accretion

• Circular orbits in same direction as planet rotation *around equator

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Jupiter’s Galilean moons

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Io

• Io is the most volcanically active body in the solar system

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Io’s Volcanoes

• Volcanic eruptions continue to change Io’s surface.

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Europa

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Europa’s interior also warmed by tidal heating producing a possible ocean under the ice.

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Ganymede

• Largest moon in the solar system

• Clear evidence of geological activity

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Callisto

• “Classic” cratered iceball

• No evidence of geologic activity

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Other interesting moons in the solar system

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Titan

• Saturn’s largest moon Titan is the only moon in the solar system to have a thick atmosphere.

• It consists mostly of nitrogen with some argon, methane, and ethane.

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Titan’s Surface

• Huygens probe provided first look at Titan’s surface in early 2005.

• It found liquid methane and “rocks” made of ice.

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Enceladus• Enceladus is a

medium sized moon of Saturn

• Ice fountains of Enceladus suggest it may have a subsurface ocean.

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Charon

• The largest moon of Pluto is also the largest moon relative to the size of its planet

• Charon is ½ the size of Pluto, and is thought to be ice-covered like Pluto as well

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Top 12 largest moons in the solar system

1. Ganymede2. Titan3. Calisto4. Io5. The Moon6. Europa

7. Titania8. Rhea9. Oberon10. Iapetus11. Charon12. Umbriel

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Top 10ish moons in the solar system (with other terrestrial bodies for comparison)