Our moon and other moons of the solar system
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Our Moon and other moons of the Solar System. Starter 1/25/13. What do you already know about our Moon?. 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. Planets and numbers of moons.

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Our Moon and other moons of the Solar System

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Our Moon and other moons of the Solar System

Starter 1/25/13

  • What do you already know about our Moon?

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

Planets and numbers of moons

PlanetNumber of known moons







Uranus 27


Dwarf planet

Pluto3 (maybe 4)


How do we explain the existence of our Moon?

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

Giant Impact

Why does the Moon look the way it does?

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.

Impact Craters

Meteor Crater (Arizona)

Tycho Crater (Moon)

Impact Craters on Mars

“Standard” crater

Impact into icy ground

Eroded crater

Thought question

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


  • 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.

Lava and Volcanoes

Runny lava makes flat lava plains.

Slightly thicker lava makes broad shield volcanoes.

Thickest lava makes steep stratovolcanoes.

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.

Formation of Lunar Maria

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

Cooled lava is smoother and darker than surroundings.

Early surface is covered with craters.

Large impact crater weakens crust.

The Moon is geologically dead

  • Moon is considered geologically “dead” because geological processes have virtually stopped.

Do the Moon and Mercury have any atmosphere?

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


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 = 130C (265°F)

    • Nighttime = -190C (-310 F)

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

Starter 2/7

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

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

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)

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.”

Small (or irregular) Moons

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

Captured Moons

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

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 jovianplanets (gas giants) *by accretion

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

Jupiter’s Galilean moons


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

Io’s Volcanoes

  • Volcanic eruptions continue to change Io’s surface.


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


  • Largest moon in the solar system

  • Clear evidence of geological activity


  • “Classic” cratered iceball

  • No evidence of geologic activity

Other interesting moons in the solar system


  • 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.

Titan’s Surface

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

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


  • Enceladus is a medium sized moon of Saturn

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


  • 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

Top 12 largest moons in the solar system

  • Ganymede

  • Titan

  • Calisto

  • Io

  • The Moon

  • Europa

7. Titania

8. Rhea

  • Oberon

  • Iapetus

  • Charon

  • Umbriel

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

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