**Icy Quartet**

Icy Quartet


Galileo discovered Europa in January 1610, along with three other large moons that have been named Galilean moons in his honor: Io, Ganymede and Callisto. On several clear starlit winter nights, Galileo climbed up to the roof of his house in Padua, and discovered the distant Jovian quartet. He accomplished this using only a primitive early telescope that he called a “spyglass”–one of the first telescopes to be used for astronomical purposes. This discovery is also important historically because it was the first time that a moon had been found in orbit around a planet other than Earth. Up until that time, Earth’s Moon was the Moon, the only moon known to exist. Ganymede is the largest moon in our Solar System, and Europa, Ganymede, and Callisto are icy and rocky bodies. However, Io–which is far from being icy–stands out in the crowd. This is because it is a tiny moon with certain hell-like features. Volcanic little Io–the innermost Galilean moon–displays a very colorful surface that some astronomers have compared to a “pepperoni pizza”. The surface of Io is splotched and scarred by many eruptions of lava emitted from numerous powerful and fiery volcanoes. The moon is also generously endowed with an abundance of sulfur.

Galileo made the first reported discovery of both Io and Europa on January 7, 1610. However, during the first observation of the duo, he was unable to separate Io and Europa into two entirely separate moons. This was because of the poor magnification abilities of his primitive “spyglass”–and so the pair appeared to be a single point of light. The next night, Io and Europa revealed themselves to be two separate objects, and Galileo went on with his historic observation of the Jovian system. However, the four Galilean moons may have been discovered independently by the German astronomer Simon Marius (1573-1625).

The Galilean moons are thought to have been born from material leftover after Jupiter itself condensed out of our Solar System’s original natal cloud, composed of gas and dust, that swirled around our newborn Sun. The four moons are probably approximately the same age as the rest of our Solar System–4.5 billion years. Indeed, the formation of Jupiter’s “mini solar system” is similar to the way our entire Solar System came to be. First, each of the four major planets dwelling in the warm and well-lit inner region of our Solar System (Mercury, Venus, Earth and Mars) is less dense than its inner planet neighbor–hence, Mars is less dense than Earth, which is less dense than Venus, which is less dense than Mercury. The Galilean quartet follow the same pattern, becoming less dense the farther they are from Jupiter. This reduction of density with increasing distance is probably the result of temperature. That is because denser, rocky and metallic material condenses out first, close to Jupiter or the Sun, while lighter-weight icy material can only condense out at greater distances where the temperature is colder.

The distance from Jupiter also determines the amount of tidal heating a particular Galilean moon must endure. This means that fiery little Io, which is the closest of the four moons to Jupiter, is heated so much that it triggered its volcanic nature. In fact, this little tormented moon-world, is the most volcanically active body in our entire Solar System. For this reason, it is considered likely that, long ago, this intense tidal heating drove off any water Io had when it was born.

Europa has a layer of ice and water resting on top of a rocky and metallic interior. In contrast, both Ganymede and Callisto sport greater proportions of water ice and, thus, have lower densities.

Like Earth, Europa likely has an iron core, a rocky mantle and an ocean of salty water. However, in contrast to our own planet, Europa’s ocean sloshes beneath a shell of ice that is thought to be about 10 to 15 miles thick and has a probable depth of 40 to 100 miles.

Europa’s icy shell is criss-crossed by long, linear fractures.