The Galilean Satellites of Jupiter
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Overview
Easily seen with Earth-based telescopes – look like pinpoints of light.
Orbit Jupiter in nearly the same plane as the planet’s equator.
From Earth we see this plane nearly edge on, so these 4 moons appear to move
back and forth relative to Jupiter.
Orbital data. Distances from the center of Jupiter and orbital periods.
Each of them is bright enough to be seen by naked eye, but the angular distance
between them and the planet is small, so they are hidden in the glare of Jupiter.
Estimations of the angular distance between Io and Jupiter and Callisto and
Jupiter using the small angle formula.
The brightness of the satellites varies slightly as they orbit the planet - tells us
that they also spin on their axes.
Each of them goes through one complete cycle of brightness during one orbital
period. Ratio (orbital period) / (rotational period) = 1/1 – synchronous rotation.
The synchronous rotation is due to the gravitational force Jupiter exerts on the
satellites. Tidal force.
Explanation of the tidal force (similar to Earth-Moon system).
Orbital periods of Io, Europa and Ganymede – special relation 1:2:4 – due to the
gravitational force that they exert on one another.
Overview of the physical parameters – diameters, masses and densities;
comparisons to Moon, Mercury and Mars.
Diameter (km)
Io
3642 (Diam. of Moon = 3476)
Europa
3120
Ganymede 5268 (b/w Merc. & Mars)
Callisto
4800 (Merc., or 1.5 Moon)
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Mass (Moon=1)
1.2
0.65
2.02
1.5
Density (g/c cm)
3.5
3.02
1.9
1.85
Ganymede and Callisto - the most massive satellites in the whole Solar System.
Internal structure – the greater the distance from Jupiter, the greater the
proportion of ice within the satellites and the lower the average density. The
internal structure and geological behavior are different for different moons.
o Ganymede and Callisto – low density, made of roughly equal parts of
rock and water ice
o Io and Europa – made mainly of rocky material
Io
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Because the size and density of Io are very similar to those of Moon, we expect Io
to have the same internal structure like Moon – not at all!
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No impact craters
Many volcanoes – the surface of Io is a result of intense volcanic activity
Intense gravitational forces exerted on Io by Jupiter, Europa and Ganymede; the
force is variable and causes a lot of stress on Io, changing its shape. The tidal
force is so intense that causes tidal heating of the interior of Io.
Io’s interior is kept hot by tidal forces and this is the cause of the intense volcanic
activity.
Io is by far the most volcanic place in the solar system – about 300 active
volcanoes – due to that there are no long-lived surface features on Io.
Color of the surface - white spots (sulfur dioxide), red rings (sulfur), black spots
(10-50 km in diameter; cover 5% of the surface; active volcanic vents).
Io is located very deep in the Jupiter’s magnetosphere, which rotates very quickly
– generates a voltage of 40 000 volts across Io and causes a huge electric current
to flow through Io.
Io has a week magnetic field .
Internal structure of Io.
Europa
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The smoothest solid body in the Solar System, icy surface.
Europa’s crust is pure frozen water; the brown areas contain deposit of rocky
material; how this water got to the surface, we don’t know.
Very few highlands.
No impact craters.
The surface we see now is very young.
The dominant surface feature we see are cracks – dark lines (12 to 40 km wide)
crossing the surface.
Like Io, this is an exception of the general rule that a small planet/satellite should
be cratered and geologically dead.
Interior – rocky material below the surface – makes 85% of the mass of Europa.
It has internal heat source due to the tidal force of Jupiter (but much less intense
in comparison to Io).
The surface fractures may be a result of the crust’s being stretched and
compressed by the tidal force.
The water beneath surface is probably liquid .
Interior – 100-200 km surface of ice, warm subsurface ocean, Rocky mantle,
Metallic core.
Very thin atmosphere of oxygen.
The only body in the solar system with liquid water (except Earth).
Ganymede
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Cratered surface – like Moon, but the craters are made of ice
Dark parts (older, higher density of craters) and bright parts (younger, lower
density of craters)
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Internal structure – ice shell (800 km thick); rocky mantle; metallic core (500 km
diameter)
Located far from Jupiter - should not have much internal heat due to tidal force
Surprisingly, it has very strong magnetic field – the reason is not known –
probably warm and molten inside
Callisto
1. Orbital Data
 Distance from Jupiter – 1883000 km
 Period of revolution – 17 days
 It rotates in the same period as it revolves, so it always keeps the same face
toward Jupiter ( day = month = 17 days)
2. Physical Data
 Diameter = Diameter of Mercury = 4820 km
 Mass = 1/3 Mass of Mercury
 Density (Mass / Volume) = 1/3 Density of Mercury
 Internal structure - not much rock and iron
 The material inside Callisto has not fully differentiated – it does not have a dense
core
 Surface temperature = 130 K = -143 C – icy surface
3. Surface
 The reflectivity of the surface is less in comparison to that of Europa and
Ganymede
 The reflectivity is 2 times larger than that of Moon (rocky surface), so we can
conclude that the surface of Callisto is made of ice
 This is the darkest of any of the 4 Galilean satellites
 Many impact craters – very old surface
 Callisto is a geologically dead body – the tidal heating has never been significant
there
 Some of the craters are destroyed – due to the process of sublimation of ice
(transformation from solid into gaseous phase)
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Very thin atmosphere of carbon dioxide
4. Curious (unexplained) facts about Callisto
 very few craters with diameter < 1 km (could be explained with sublimation)
 The surface appears to be covered with dark deposit (could be explained with
sublimation)
 Has magnetic field ( we don’t know why - no liquid layer beneath the crust should
exist to create magnetic field)