Transcript The Galilean Moons of Jupiter

Astronomy Picture of the Day
Question
Jovian planets are composed of fluids, which
means:
A) They are composed of a liquid interior layer and a gaseous
outer layer.
B) They have no rocky surface of any kind.
C) They can exhibit convection.
D) All of the above
Question
What are the three processes which contribute to the
banded structure observed on Jovian planets'
surfaces ?
A) convection, rapid rotation, differential rotation
B) retrograde motion, convection, plate tectonics
C) differential rotation, plate tectonics, ring motion
Moons of Jovian Planets
• Jupiter alone has over 60, number discovered is growing due to better
& better telescopes
• We will focus on the 6 largest: Diameter > 2,500 km & spherical
• There are also 12 “medium-sized” moons, massive enough to be
spherical rock and water ice, which exhibit large amounts of cratering
and approximately synchronous orbits. Most – no evidence of
geological activity.
Many “small” moons,
most probably captured
interplanetary debris.
Why study Jovian
moons?
Moons of Jovian Planets
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?
The Galilean Moons of Jupiter
(sizes to scale)
Closest to Jupiter
Furthest from Jupiter
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 (Galileo Flyby of Io)
plume
volcano
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! Most geologically active object in the solar system.
Causes a thin atmosphere and smooth surface.
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
(Jupiter's Moon Europa Video)
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? Similar to Io:
resonance orbits with Ganymede and Io.
Lack of cratering => process is ongoing
Europa
Evidence of a warm subsurface ocean from
measurements of a weak changing magnetic
field.
Io Jupiter
Warm ocean => life?
Jupiter
Europa
Ganymede
Ganymede and Callisto:
• Both heavily cratered
• Ganymede is differentiated but Callisto is not
•Ganymede shows evidence of ancient plate tectonics
• Neither show any obvious sign of recent geologic activity
•Unknown why two such similar bodies evolved differently!
Saturn's Large Moon – Titan – A moon with a thick
atmosphere (lower temp than Jupiter's moons)
One of the main reasons Scientists find Titan so interesting is it may present an opportunity to
study the kind of chemistry which occurred billions of years ago on Earth.
Neptune’s Triton
• Retrograde orbit → slowly
spiralling towards Neptune
due to location of tidal bulge
and will be torn apart by
gravity in 100 million years.
• Inclined orbit + retrograde
orbit → may be a captured
Kuiper belt object.
• Currently geologically active
→ smooth icy, reflective
surface.
Rings
All the Jovian planets have a system of rings – but
what are they 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/rock/dust particles,
<1 mm to >10m in diameter. Most a
few cm “dirty snowballs”
- A few rings and divisions
distinguishable from Earth.
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 (approximatly 2.5 x planet radius). Satellite must be held togther
solely by its own gravity + must have similar density to planet for this
to be an appropriate limit.
Total mass of ring particles equivalent to moon 250 km in diameter.
All ring systems in Solar System within or close to Roche Limit.
Voyager probes found that rings divide into 10,000's of ringlets.
What could maintain this ringlet structure?
Gaps in Rings:
Narrow gaps: Swept clean by small moonlets embedded
within the rings. Moonlets are much larger than largest
ring particles -> simply attract ring material as they
orbit, leaving a less dense area.
Cassini Division due to gravitational force of Saturn's
innermost medium sized moon – particles are deflected
into new orbits.
(Ring Animation)
Shepherd Moons
• Some ringlets maintained
by gravitational influence
of “shepherd moons”
• Keep particles from
straying outside the path of
the ringlet
• Can cause complicated
structure within the rings
(i.e. braided ringlets)
Seasonal variations in our view of Saturn's Rings
Over time Saturn’s rings change their appearance to terrestrial observers as the tilted ring
plane orbits the Sun. The roughly true-color images (inset) span a period of several years
from 1996 to 2000, showing how the rings change from our perspective, from almost edge-on
to more nearly face-on.
Formation of Rings:
Rings are young -> must either be newly
formed or periodically replenished.
Possible explanations: replacement by moon
fragments chipped off by meteoric impacts
and/or moon torn apart by tidal forces.
Recall this will be the fate of Neptunes moon
Triton.