Transcript Lecture 08a: Galilean moons - Sierra College Astronomy Home Page

The Moons of the Jovian Planets
Goals
Discover the 150+ moons in the outer solar system;
• Jupiter’s Galilean satellites as a place for life;
• Europa, Europa, Europa!
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The Moons of the Outer Solar System
Jupiter
1. Outer moons: ~55, eccentric orbits, many are retrograde, captured asteroids.
2. Inner moons: 4, very close to Jupiter, probably fragmented moonlets.
3. Galilean moons: 4, nearly circular orbits, smallest is 5000 times more massive
than the largest non-Galilean moons.
Saturn
1. Major moons: 7 larger than 200 km radius.
2. Minor moons: about 54.
Uranus
1. Larger moons: 5 larger than 230 km radius.
2. Minor moons: about 22.
Neptune
1. Larger moons: 3 larger than 170 km radius.
2. Minor moons: about 10.
Jovian planets formed as mini-solar systems!
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The moons of the outer solar system
Shapes
– Large satellites are spherical;
– Small satellites (250 km radius) are irregular.
Composition of large satellites
– Jupiter’s big four: ordered higher density to lower density;
– Rock, metal, and ice;
– The ice in Jupiter’s satellites is H2O; satellites of outer planets
include other ices such as NH3 and CO2.
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Their orbits
Orbits
– Large satellites (usually) have nearly circular, prograde orbits;
– Small satellites (especially outer ones) may have elliptical,
retrograde orbits.
Tidal locking
– Nearly all moons are synchronously locked with their central
planet (spin-orbit locking);
– Many satellites are synchronously locked with each other
(orbit-orbit locking);
– Tidal locking can maintain ellipticities in orbits that would
otherwise be circular.
– Tidal heating results from non-circular orbits.
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Jupiter’s Galilean satellites
Io
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Size: 1.05× Earth’s moon, mass: 1.22× Earth’s moon;
The closest Galilean satellite to Jupiter;
Extreme tidal forces;
200× the heat per gram, compared to radioactivity on Earth);
The most volcanically active place in the Solar System;
Typical temperatures are 90-130K;
Constant volcanic eruptions, local temperatures reaching 2000ºC
(3600ºF);
Certainly, not a candidate for a habitable world!
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Jupiter’s Galilean satellites
Europa
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Size: 0.90× Earth’s moon, mass: 0.65× Earth’s moon;
Shiny, spidery cracks, few craters;
Typical surface temperature is 150K;
Probably iron core, rock sheath, 80-170 km H2O cover;
A magnetic field, one that changes with Jupiter’s as Europa passes
through it;
– Salty deposits on the ice surface.
What can all this mean?
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Jupiter’s Galilean satellites
Europa
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Tidal heating may be keeping Europa warm;
This salty, conductive ocean produces an induced magnetic field;
Brittle H2O skin is underlain by convective ice?
Core activity produces hydrothermal vents;
All this results in an active surface without craters.
Model #1
A vast ocean lurks under the brittle ice (all 80-170 km thick);
Model #2
The brittle ice surface is reformed merely because of a mantle of
convecting ice? (Largely discounted because of the magnetic fields.)
Model #3
A crust of brittle ice lays atop a mantle of convective ice (together
about 20-25 km thick), and a lower layer of water (55-150 km thick),
all over the solid core?
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Jupiter’s Galilean satellites
More evidence
Domes such as at the Conamaran Chaos can be up to 200 m tall, and
are highly fragmented.
Antarctica Lake Vostok (250×50 km) may be an excellent analogy, as
long as the highly criticized research effort does not destroy what it seeks
to study! The hole is about 3600 m (11800 ft) deep!
Sample analysis is in early stages.
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Jupiter’s Galilean satellites
Europa
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Carbon-based, organic compounds?
Liquid medium
Energy source.
Tidal heating produces only small amounts of energy, is there anything else
that could be an energy source?
– Sunlight is far weaker than on Earth (1/25 Earth levels);
– Also, the surface ice is reflective, and ultimately opaque;
– Exotic energy sources—UV radiation, high energy particles trapped in
Jupiter’s magnetic fields, may provide energy.
Such exotic sources might produce complicated organic molecules, that
might be churned via surface processes into the Europan ocean?
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Jupiter’s Galilean satellites
Ganymede
– Size: 1.52× Earth’s moon, mass: 2.02× Earth’s moon;
– Weaker tidal forces still;
– Old, dark, cratered terrain;
– Young, grooved terrain (water flows and tectonics?);
– Weak magnetic field (molten core?)
– Weak, induced component of the magnetic field;
– Salty deposits on surface
Perhaps there is an ocean under 150+ km of ice, kept warm by weak
tidal heating, radioactivity, and Ganymede’s large thermal inertia?
Deep ocean deposits of exotic, high pressure forms of H2O ice may
encase the rocky core, diminishing the chance of rock-water interfaces
with hydrothermal vents.
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Jupiter’s Galilean satellites
Callisto
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Size: 1.38× Earth’s moon, mass: 1.46× Earth’s moon;
Ice and rock, with outer several hundred km of H2O ice?
It never probably warmed enough to differentiate;
Yet Galileo spacecraft detected an induced magnetic field;
Not locked in orbital resonance (no tidal heating);
Smallest craters are missing (erosion);
Soot surface, residue after gases leave;
Some craters show slush rise post impact?
Oceanic layer seems possible, kept warm by internal radioactivity?
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Jupiter’s Galilean satellites
Summary
– Tidal heating plays varying roles in keeping them warm;
– Europa, Ganymede, and Callisto have indications of oceans;
– It is difficult to support life so far from the traditional habitable
zone, close to the central star in a solar system.
– But life is persistent….
– But could we ever get to it…
…and not pollute it with our own attempts?
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