This article begins a series of articles that will consider the possibilities for life in the Jovian system, especially on Jupiters moon Europa. We begin with an introduction to Jupiter and its four largest moons.

Despite being called a gas giant, most of Jupiter is not made up of material in a gas state, but of a strange substance called liquid metallic hydrogen. This is a type of hydrogen that can only form under extremely high pressures, and as far as we know it is only found in the interiors of Jupiter and Saturn. Above the liquid hydrogen is a layer of liquid helium, and above that layer is an atmosphere of more normal gaseous hydrogen and helium with traces of other elements mixed in. The stunning cloud layers visible from space are due to small amounts of sulfur mixed into the clouds, which produces bands ranging in color from reds to yellows to whites.

Jupiter orbits the Sun at a distance about 5.2 times the distance from the Earth to the Sun. The energy that drives Jupiters strong storm systems and other weather comes mostly from internal sources, since Jupiter gets much less solar energy at its distance from the Sun than we do here on the Earth. Jupiter also has a very strong magnetic field, resulting in lots of energetic particles being trapped near it. These high-energy particles result in a huge amount of radiation, causing problems for spacecraft like Galileo and almost certainly threatening any astronauts who might try to explore Jupiter or its moons. This radiation environment also results in some interesting chemistry being performed at Europas surface as charged particles impact and interact with the icy surface. This topic will be discussed in a future article.

Jupiters four large satellites were first seen by Galileo in 1610. The paradigm-shifting discovery of objects orbiting a celestial body other than Earth eventually disrupted the geocentric view of the universe in favor of the heliocentric model, with great religious and scientific consequences. The four large satellites are called the Galilean moons in honor of Galileo. In order from Jupiter they are Io, a brightly-colored volcanic body; Europa, an ice-covered world; Ganymede, a rock and ice body with some old cratered areas and some younger terrains; and Callisto, an old, cratered body resembling Earths Moon.

The four Galilean satellites resemble a miniature solar system, and are thought to have condensed out of leftover material surrounding Jupiter in much the same way as the planets condensed out of leftover material in the disk surrounding the young Sun. There is also a gradation in composition and density in the Galilean satellites, from rocky, high-density, almost water-free Io, to Europa, with an ice/water layer over rock, to Ganymede, with a thick water ice mantle over a large rocky/metal core, to lower-density Callisto, which may not be completely differentiated and could have a mixture of rock and ice throughout.

This gradation is similar to the planets change in volatile content and density with distance from the Sun, from rocky, high-density Mercury to the icy outer solar system. The solar systems difference in volatile content and density is thought to come from a temperature gradient in the primordial disk from which the planets condensed; the "snow line," where water and other volatiles could condense out of the cloud of gas and dust, is thought to have been out near the orbit of Jupiter, which is why the moons of the outer solar system have a much higher volatile content than the planets of the inner solar system. This could be the case for formation within the Jupiter system as well, as the Galilean satellites formed from a disk of leftover material surrounding the proto-Jupiter. Another possibility is that the volatiles were simply driven off of Io, and perhaps even Europa to some extent, by the intense geologic activity driven by tidal heating (to be discussed in a future article).