Lecture on Planetary Moons

Introduction

• Moons are not a measure of a planet's worth.
• Example: Jupiter has many moons, Pluto has five, Earth has one.
• Science fiction often features multiple moons or suns, unlike Earth's single moon.

Moon Formation

• Earth's lone moon is a result of solar system formation.
• Planets categorized into terrestrial (rocky) and jovian (gas giants).

Terrestrial Planets

• Mars: 2 moons (Phobos, Deimos)
• Earth: 1 moon (The Moon or Luna)
• Mercury and Venus: 0 moons

Jovian Planets

• Jupiter: >60 moons
• Saturn: >60 moons
• Uranus: 27 moons
• Neptune: 14 moons
• Pluto (Dwarf Planet): 5 moons

Moon Acquisition Theories

1. Impact Theory: A collision causes a body to orbit a planet.
   • Example: Earth's Moon
2. Capture Theory: A planet's gravity captures a passing body.
   • Example: Mars' moons (proximity to asteroid belt)
3. Accretion Disk Theory: Moons form from a disk of material around the planet.
   • Example: Jupiter's moons

Specific Cases

• Pluto's moons vary in size; smallest, like Nix and Hydra, captured by gravity.
• Venus may have had a moon that crashed into it after a spin reversal.
• Mercury is too small and too close to the Sun; solar winds prevent moon formation.

Discoveries

• Galileo discovered Jupiter’s four largest moons in 1610 (Galilean moons: Io, Europa, Ganymede, Calisto).
• Mercury confirmed to have no moons in the 1970s (Mariner 10 mission).

Modern Studies

• Increased understanding of moons has led to proposing missions to study moons specifically, not just their parent planets.