Europa appears to have all conditions necessary for the origin of life: liquid water, organic chemicals and energy. Today we don't know much about the emergence of the moon itself. We can only guess how it was. What we can say is, how Europa is geological build up and out of which elements its atmosphere consists of.

Today scientists think that Europa has a metallic core and a internal structure similar to the Earth's. They also believe that it has a rocky interior. There are two common models which try to describe the construction of the surface including the subsurface layer. 1. This model supposes a brittle ice surface and beneath a convicting ice layer. 2. This model supposes a ice covered surface and beneath a layer of liquid water. For scientists it is very important to know how thick the ice layer , which covers the satellite, is. Because in the next few years they want to send a spacecraft to find out if liquid water and maybe some microbes exist under the surface.

Galileo Galilei first observed the moons of Jupiter (Io, Europa, Callisto and as last Ganymede) on January 7, 1610 through a homemade telescope. He originally thought he saw three stars near Jupiter, strung out in a line through the planet. The next evening, these stars seemed to have moved the wrong way, which caught his attention. Galileo continued to observe the stars and Jupiter for the next week. On January 11, a fourth star (which would later turn out to be Ganymede) appeared. After a week, Galileo had observed that the four stars never left the vicinity of Jupiter and appeared to be carried along with the planet, and that they changed their position with respect to each other and Jupiter. Finally, Galileo determined that what he was observing were not stars, but planetary bodies that were in orbit around Jupiter. This discovery provided evidence in support of the Copernican system and showed that everything did not revolve around the Earth. This four moons are today known as the Galilein Moons.

Simon Marius claimed to have observed Jupiter's moon as early as late November 1609 (about five weeks prior to Galileo) and had begun recording his observations in January 1610 at about the same time Galileo was first making his observations. However, since Marius did not publish his observations right away as Galileo had done, his claims were impossible to verify. Since Galileo's work was more reliable and extensive, he is generally given the credit for discovering the moons of Jupiter. In 1614, Marius did provide the names of the Jupiter's moons that we are familiar with today, based on a suggestion from Johannes Kepler.


Voyager 1 was launched on September 5, 1977 and flew past Jupiter on March 5, 1979.
Voyager 2 was launched on August 20, 1977 and flew past Jupiter on August 7, 1979.



Europa's geologic history is written in its crisscrossed surface. The straight and curved bands and mottled bright and dark patches, tell a complicated story.

The surface of Europa is characterized in several, in some cases totally different units since the Voyager Mission. Traditionally, a geologic unit is defined as a three-dimensional body of material (it has a thickness that extends below the surface) distinguished by its physical features and time of formation. Looking at images of Europa, planetary geologists have used surface shapes, textures, forms, layers, color, and relative brightness to define geologic units. The five primary terrain types now recognized in Galileo images of Europa are plains, chaos, band, ridge, and crater materials.

Image	Name of unit	Description and interpretation
	Plains	Plains material is the most widespread surface type and is subdivided into ridged plains, smooth plains or simply, undifferentiated. Smooth plains are generally dark and occur in circular to irregularly shaped areas. At global resolutions, as shown in the top of the image on the left, plains material is described as undifferentiated because ridges, which are probably there, or any other distinguishing feature simply cannot be detected. Many of the dark bands in this image are more than 1,600 km long. Some ridged plains are the oldest recognized features on Europa. Smooth plains could be the result of a kind of "ice-water" volcanism where the fluid, erupted from below, floods a low-lying area. Most smooth plains appear to be among the youngest materials on Europa. Undifferentiated plains material probably consists of ridged and smooth plains materials, and possibly other units, that are not visible at global scales.
	Chaos	Platy, knobby, fractured, and rotated blocks of crust the size of several city blocks characterize chaos material. Chaos terrain is interpreted as broken pieces of preexisting materials that have been moved and disrupted by inferred internal activity. All current models suggest that chaos terrain formed directly above a heat source. Most chaos terrain is younger than ridged plains.
	Band	Bands are linear, curved or wedge-shaped zones distinguished from the surrounding surface because of a contrast in albedo (or brightness.) Sometimes the margins look like bounding ramps or ridges. Repeated filling along a central spreading axis could account for the bilateral symmetry of features seen along some bands. Perhaps bands are the final stage in the evolution of ridges.
	Ridges	Ridges are topographic highs distinct from surrounding units. Ridges may be straight or curved, single, double (two ridges separated by a trough), or have more complex designs of curved sets of branching or joining segments. Ridges may be structures formed where underlying material broke through the crust along a fracture. They may also be debris piles from "ice-water" volcanism or places where the icy materials were squeezed and heaved upwards.
	craters	Classification of crater materials is based on the clarity of rims and ejecta deposits. In some cases, remnants of impact structures are only recognized by the presence of concentric lineaments. Other features have well-preserved rims and continuous, finely-textured ejecta deposits. Crater materials are considered to be frozen melt formed by impact. Some ejecta may have been thrown out as a fluid. Rough surfaces may be collapsed sections of crater walls.

Current hypotheses for crustal formation generally assume that a liquid water ocean exists below a relatively young, icy shell. Some dark surface areas might be deposits of salty minerals such as sulfates and carbonates. If there is a hidden ocean, then

Is there current geologic activity on Europa?

What kind of activity might we expect

How might we see this activity?

These are the questions asked by Cynthia Phillips and colleagues in their analyses of Galileo images.

Based on their mapping of available Galileo data, planetary geologists have crafted a story of the origin and evolution of Europa's surface. In this story, extrusion of subsurface material onto the surface, tectonic processes (squeezing, pulling, sliding, breaking), and impact bombardment have sculpted Europa's icy crust. Ridged plains form the oldest recognizable unit on Europa. In the high-resolution images, most plains consist of complex networks of crosscutting ridges and ridge sets. The plains materials may represent the ultimate variety of upward, downward, and sideways movement of materials pushed, shoved, and broken across the surface of Europa. Areas of chaos terrain generally overlay the plains. Knobby and platy chaos materials look like broken pieces of pre-existing units. Chaos materials are generally considered to represent the surface expression of localized heat or subsurface activity, such as convection, intrusion by low-density material, or "ice-water" volcanism.

• http://www.astrobio.net/news/print.php?sid=151
• http://photojournal.jpl.nasa.gov/catalog/PIA01669
• http://www.jpl.nasa.gov/galileo/ganymede/discovery.html
• http://www.nineplanets.org/spacecraft.html
• http://www.psrd.hawaii.edu/Feb01/EuropaGeology.html