Astronomy Picture of the Day Video on Saturn and Its Moons

Moons of Jovian Planets Jupiter alone has over 60 We will focus on large: Diameter > 2,500 km Why study the Jovian moons?

Moons of Jovian Planets Jupiter alone has over 60 We will focus on large: Diameter > 2,500 km Why: Similarity to terrestrials, possibility of life!

The Galilean Moons of Jupiter (sizes to scale) Closest to Jupiter Furthest from Jupiter Europa - slightly smaller than our Moon Ganymede - largest moon in Solar System, slightly larger than Mercury Would you expect Jupiter to be hotter or cooler very early in its history? What might this imply for the Jupiter-Galilean moon system?

The Galilean Moons of Jupiter (sizes to scale) Closest to Jupiter Furthest from Jupiter Europa - slightly smaller than our Moon Ganymede - largest moon in Solar System, slightly larger than Mercury The closer to Jupiter, the higher the moon density (similar to behavior of terrestrial planets) => like mini “planetary” system! Intense heat of young Jupiter played role of sun.

Io's Vulcanism More than 80 active volcanoes have been observed. Can last months or years. Largest is bigger than Maryland - emits more energy than all Earth volcanoes combined! Io is about the size of our moon. Why is it's volcanic activity surprising?

Volcanic activity requires internal heat. Io is a small body Volcanic activity requires internal heat. Io is a small body. Should be cold and geologically dead by now. What is source of heat? Io and Europa are in a resonance orbit => Io stretched/squeezed => friction Start the clock now Jupiter Europa Io One Orbit of Io Jupiter Europa Io Two Orbits of Io Jupiter Europa Io

Europa may have Warm Ocean beneath Icy Surface Fissures suggest large moving ice sheets. 860 km Dark deposits along cracks suggest “volcanic” eruptions of water with dust/rock mixed in. 42 km Icebergs?

What is source of heat? Europa Io Jupiter Ganymede Jupiter Europa

Lack of cratering => process is ongoing Warm ocean => life? What is source of heat? Similar to Io: resonance orbits with Ganymede and Io. Lack of cratering => process is ongoing Warm ocean => life? Europa Io Jupiter Ganymede Jupiter Europa

Rings What are the rings of Saturn made of? Are they solid?

Saturn's Rings (all Jovians have ring systems) - Inner radius 60,000 km, outer radius 300,000 km. Thickness ~100 m! - Composition: icy/rocky particles, <1 mm to >10m in diameter. Most a few cm. - A few rings and divisions distinguishable from Earth.

Question How did Saturn's rings form?

Origin of Saturn's Rings: If a large moon gets too close to Saturn, the tidal force breaks it apart into small pieces. The radius where this happens is called the Roche Limit. Total mass of ring particles equivalent to moon 250 km in diameter.

Neptune’s Triton Retrograde orbit => slowly spiralling towards Neptune, will be torn apart by gravity in 100 million years Why does a moon with a retrograde orbit slowly spiral closer to the planet?

Voyager probes found that rings divide into 10,000's of ringlets. What could maintain this ringlet structure?

Shepherd Moons Some ringlets maintained by gravitational influence of “shepherd moons” Keep particles from straying outside the path of the ringlet How can we tell the ages of the ringlets?