Galilean moons

Marius discovered the moons independently at nearly the same time as Galileo, 8 January 1610, and gave them their present individual names, after mythological characters that Zeus seduced or abducted, which were suggested by Johannes Kepler in his Mundus Jovialis, published in 1614.

[4] Their discovery showed the importance of the telescope as a tool for astronomers by proving that there were objects in space that cannot be seen by the naked eye.

As a result of improvements that Galileo Galilei made to the telescope, with a magnifying capability of 20×,[5] he was able to see celestial bodies more distinctly than was previously possible.

[6] Galileo's discovery proved the importance of the telescope as a tool for astronomers by showing that there were objects in space to be discovered that until then had remained unseen by the naked eye.

[8] Galileo's 13 March 1610, Sidereus Nuncius (Starry Messenger), which announced celestial observations through his telescope, does not explicitly mention Copernican heliocentrism, a theory that placed the Sun at the center of the universe.

Galileo, seeking patronage from his now-wealthy former student and his powerful family, used the discovery of Jupiter's moons to gain it.

[6] In his dedicatory introduction, Galileo wrote: Scarcely have the immortal graces of your soul begun to shine forth on earth than bright stars offer themselves in the heavens which, like tongues, will speak of and celebrate your most excellent virtues for all time.

Indeed, it appears the Maker of the Stars himself, by clear arguments, admonished me to call these new planets by the illustrious name of Your Highness before all others.

Then there was Ganymede, the handsome son of King Tros, whom Jupiter, having taken the form of an eagle, transported to heaven on his back, as poets fabulously tell...

The numbers run from Jupiter outward, thus I, II, III and IV for Io, Europa, Ganymede, and Callisto respectively.

The longitude problem was so important that large prizes were offered for its solution at various times by Spain, Holland, and Britain.

By the time the present generation formed, the gas in the proto-satellite disk had thinned out to the point that it no longer greatly interfered with the moons' orbits.

[29] If a surface data or collection vessel were to land on Io in the future, it would have to be extremely tough (similar to the tank-like bodies of the Soviet Venera landers) to survive the radiation and magnetic fields that originate from Jupiter.

[14] It has a smooth and bright surface,[31] with a layer of water surrounding the mantle of the planet, thought to be 100 kilometers thick.

[33] The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, which could conceivably serve as an abode for extraterrestrial life.

[34] Heat energy from tidal flexing ensures that the ocean remains liquid and drives geological activity.

So far, there is no evidence that life exists on Europa, but the likely presence of liquid water has spurred calls to send a probe there.

The reddish-brown color of the markings is theorized to be caused by sulfur, but because no data collection devices have been sent to Europa, scientists cannot yet confirm this.

It is named after the Greek mythological nymph Callisto, a lover of Zeus who was a daughter of the Arkadian King Lykaon and a hunting companion of the goddess Artemis.

The moon does not form part of the orbital resonance that affects three inner Galilean satellites and thus does not experience appreciable tidal heating.

[47] Callisto is composed of approximately equal amounts of rock and ices, which makes it the least dense of the Galilean moons.

It is one of the most heavily cratered satellites in the Solar System, and one major feature is a basin around 3000 km wide called Valhalla.

Ganymede reveals past tectonic movement of the ice surface which required partial melting of subsurface layers.

The current model is that the moons experience tidal heating as a result of the gravitational field of Jupiter in inverse proportion to the square of their distance from the giant planet.

[55] By the time the present (possibly fifth) generation formed, the disk had thinned out to the point that it no longer greatly interfered with the moons' orbits.

[20] The current Galilean moons were still affected, falling into and being partially protected by an orbital resonance which still exists for Io, Europa, and Ganymede.

[55] Tidal dissipation in the Jovian system is still ongoing and Callisto will likely be captured into the resonance in about 1.5 billion years, creating a 1:2:4:8 chain.

[59] The maximum angular separations of the moons are between 2 and 10 arcminutes from Jupiter,[60] which is close to the limit of human visual acuity.