The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610.
The four moons were discovered sometime between 1609 and 1610 when Galileo made improvements to his telescope, which enabled him to observe celestial bodies more distinctly than had ever been possible before.
On January 7, 1610, Galileo wrote a letter containing the first mention of Jupiter’s moons.
At the time, he saw only three of them, and he believed them to be fixed stars near Jupiter. He continued to observe these celestial orbs from January 8 to March 2, 1610. In these observations, he discovered a fourth body, and also observed that the four were not fixed stars, but rather were orbiting Jupiter.
Galileo initially called his discovery the Cosmica Sidera ("Cosimo's stars"), in honour of Cosimo II de' Medici (1590–1621). At Cosimo's suggestion, Galileo changed the name to Medicea Sidera ("the Medician stars"), honouring all four Medici brothers (Cosimo, Francesco, Carlo, and Lorenzo).
The names that eventually prevailed were chosen by Simon Marius, who discovered the moons independently at the same time as Galileo: he named them at the suggestion of Johannes Kepler after lovers of the god Zeus (the Greek equivalent of Jupiter): Io, Europa, Ganymede and Callisto, in his Mundus Jovialis, published in 1614.
The satellites discovered by Galileo with his small telescope form a small, "mini" solar system around Jupiter. They each have special characteristics related to their formation process, but have the following traits in common:
- all orbit Jupiter
- they all are tidally locked to Jupiter
- they all have radii larger than our Moon
- the inner moons have densities higher than outer moons (implies that Jupiter was much warmer in the past, such that the moons formed near Jupiter have less of the volatile elements such as CO2 and H2O)
Io is the innermost world, closest to Jupiter and can be classified as one of the most unusual moons in our solar system. Its unique properties include:
- Surface of Io is a mottled yellow, reds and blackish-browns = sulfur compounds, where different colors correspond
to regions with different temperatures. The mean temperature on Io's surface is about -143 C (-230 F).
A hot spots associated with a volcanic features measure about 17 C (60 F). Scientists believe the hot spots may be lava lakes, although the temperature indicates the surface is not molten.
- Thin patchy atmosphere of SO2
- Active plumes (not volcanos since there is no mantle), geysers of molten sulfur and SO2.
The mechanism for heating Io's core is tidal friction from Jupiter. Jupiter's gravitational field is much stronger than,
say, the Moon's on the Earth. The tides pull and squeeze the core of the moons around Jupiter producing mechanical friction.
This friction is turned into heat, which makes the inner moons much more active than the outer moons. When liquid sulfur
on surface flows into vents and makes contact with the hot interior, there is an explosive conversion into gas producing
- Io is composed primarily of rocky material with very little iron. Io has the "youngest surface" in the Solar System,
constantly changing, as the above two images demonstrate. The one on the left is from Voyager, the one of the right
is from the Galileo probe, 20 years later.
- No impact craters, which means that the surface of Io is very young, less than a million years old = lots of
Europa is the next world out from Io and is considered a strong candidate to find primitive life. Its characteristics are:
- Icy world with high albedo, smooth surface with only a few hills/mountains and craters, meaning the surface is
young and renewing itself
- Large long cracks on the surface, due to ice plate tectonics floating on water mantle
- Visible surface is actually H2O ice mixed with rock (dirty icefield) about
100 km thick
Ganymede is much less dense than Europa or Io. Its interesting characteristics are:
- Very diversified surface with dark regions, valleys, mountains, evidence of past tectonic activity and lots
of vertical relief
- Low mean density implies a very deep ice mantle, about 1000 km thick
- White craters and rims = impact exposes mantle ice
- Ghost craters = smoothed by ice flow
- Thin oxygen atmosphere, possible aurora at its poles (magnetic field?)
Callisto is the outermost of the 4 primary satellites:
- Similar surface to Ganymede but with 10 times more cratering, which means less tectonic activity, cooler
- Little vertical relief = craters destroyed landscape = weak surface material (low density)
- Large impact basins
Summary of Galilean Moons:
- Mini-Solar System model where the closer moons are rocky, high density and the outer moons are icy,
- Geological activity decreases with distance from Jupiter, due to the decreasing tidal friction from
Jupiter on interior of moons - this has a marked influence on the appearance of their surfaces
- Little or no atmospheres
- High icy component to crust of all moons