Transcript Moon and Rings - Mid

Moon and Rings
13
Astronomy Picture of the Day
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. 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
42 km
Icebergs?
Dark deposits along cracks
suggest “volcanic” eruptions of
water with dust/rock mixed in.
What is source of heat?
Europa
Io Jupiter
Jupiter
Europa
Ganymede
What is source of heat? Similar to Io: resonance orbits
with Ganymede and Io.
Lack of cratering => process is ongoing
Europa
Warm ocean => life?
Io Jupiter
Jupiter
Europa
Ganymede
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?