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Europa is one of the four moons of Jupiter discovered by Galileo Galilei in 1610.  Galileo called it Jupiter II.  His naming system was to give the moons numbers in Roman Numerals starting with the one closest to Jupiter.  His naming system was used for about two centuries, but was becoming unworkable as more Jovian moons were discovered.
Simon Marius also discovered these moons in 1610.  He used a different naming system, calling the 4 moons after mortals who had been abducted by Jupiter (Zeus).  Simon Marius is sometimes suspected of having plagiarised Galileo’s discoveries of these moons.  However, this suspicion does not fit with Herr Marius’s statement that the naming system he used had  been  suggested  by  Johannes Kepler
If Simon Marius was the type of man who would steal discoveries, I would have expected him to keep quiet about Herr Kepler’s suggestion and take all the credit for the naming system.
Europa was a Phoenician princess.  She was abducted by Zeus in the form of a bull and taken to Mount Olympus.  She and Zeus had three sons.  Eventually a continent was named after Europa.
Structure of Europa
Measurements by the six probes that have visited the Jovian system suggest very strongly that Europa has a moderately dense iron nickel core, with a rocky mantle round it.  Above this is water.  It has been established nearly conclusively that the surface of Europa is water ice with many other substances.  There is also a very high probability that under this ice there is liquid water.  It is quite uncertain how thick the ice is, and how deep the water underneath is.
Possibly the ice is between 10 Kilometres and 30 kilometres thick, with perhaps 100 Kilometres of liquid water under it.  The thicknesses of the different layers are quite uncertain, but evidence for this basic structure is mounting.
The maximum depth of the Earth’s oceans is roughly 11 kilometres, but most of our oceans and seas are much shallower than this.  Although Europa is a little smaller than Earth’s own Moon, it probably has more water on it than the Earth.  Some estimates suggest that the moon Europa has twice as much water as the planet Earth.
If this structure is correct, there is no solid connection between the solid core and the solid crust of Europa, and they are only connected by liquid.  This raises the possibility that the core and the crust could be rotating at different speeds.  Measurements by two of the probes that have visited the Jovian system have been interpreted by a few people as indicating that this is happening, and that the core might be rotating so that it turns completely in relation to the surface every 10,000 years or so.
Tidally Locked
Many moons become tidally locked to their planet.  This means that only one side of the moonever faces the planet.  The Earth’s own Moon is tidally locked to the Earth so we only ever see one side of it.  The other side used to be referred to as the dark side of the Moon because we had no idea what it was like until a spacecraft flew round it and took pictures.  In fact in many cases the tidal locking is not complete.  Our Moon has a wobble that allows us to see rather more than half of its surface.
The four large moons of Jupiter are all tidally locked to Jupiter to varying extents.  Europa is not completely locked either.
The main sources of heat for Europa and the Earth are similar, but are quite different in relative importance.
The biggest source of heat for the Earth is the Sun’s rays hitting our planet.  This is also a source of heat for Europa, but the concentration of the Sun’s energy is only about a thirtieth of that hitting the Earth, so it is much less important for Europa.
Radioactive Decay
The second biggest source of heat for the Earth is the energy released by the decay of radioactive elements, particularly Potassium, Thorium and Uranium.  This also occurs in Europa, but the amount is probably less than that in the Earth, because there is probably a higher concentration of heavy elements like Uranium in the Earth.
Tidal Heating
Tidal Heating occurs in the Earth.  We have tides raised by our Moon, the Sun, and to tiny extents by other astronomical bodies.  These tides occur in both our seas and in the land.  The ones in the land are much smaller, but they are measurable.  Heating must occur in both the land and the sea from this tidal energy, but it is not as important on the Earth as solar energy. 
In Europa it is the biggest source of heat.  These tides are partly because the orbit of Europa is not circular, but is elliptical.  Europa is elongated by the gravitational pull of Jupiter because the face nearer Jupiter is subjected to a greater force simply because it is closer.  There are other types of tides on Europa as well from its interactions with Jupiter, the Sun and with the other moons.
Europa has an induced magnetic field because of the very powerful magnetic field of Jupiter.  This provides one of the types of evidence that Europa has a liquid salt water ocean. It would also produce  heat.
Greenhouse Effect
This is not a source of heat, but is an atmospheric effect that can slow down the loss of heat from the surface of a planet or moon.  On the Earth, it is of extreme importance.  Without this effect the Earth would be too cold for many of the types of life we have here.
Europa has an Oxygen atmosphere, but the air pressure is only about a billionth of that of the Earth.  With so little atmosphere there is no significant greenhouse effect.
Europa has the brightest surface of any moon.  That is, it reflects light very well.  This is a consequence of water ice being the main thing on the surface.  It also has a very smooth surface.  Although the surface is basically white, there are many streaks of colour. 
These may be from reactions between sulphur dioxide and water.  Experiments suggest that reactions will happen even under the cold conditions on Europa.
Although these reactions are a plausible explanation for the streaks of colour, they remind me (on a gigantic scale) of the discolorations produced by some types of micro-organisms on the Earth, in aquatic environments.
The surface of Europa is bombarded by about 540 rems of radiation.  This is around the fatal level for Humans, but there are other terrestrial organisms that can thrive in the presence of this high level of radiation.  Most notable of these is Deinococcus radiodurans.
Ice will stop radiation fairly effectively and an organism would not need to be very deep in the ice to be protected.
The temperature of the surface of Europa varies between about minus 220 degrees C and minus 150 degrees C (between minus 370 degrees F and minus 300 degrees F).  This compares with the coldest temperature ever recorded on Earth (At Vostok in the Australian Antarctic Territory) of minus 89 degrees C.  So the surface of Europa is quite cold.  Life as we know it requires liquid water, so there are probably no living things on the Europan surface.
The liquid ocean of Europa must be at least the melting point of salty water under the pressure conditions inside Europa.
The Europan surface gets about one thirtieth of the concentration of the Sun’s light that the Earth gets.  This is well within the amount needed for photosynthesis, but there is not likely to be enough light penetrating to a depth where the ice is warm enough for life. 
However, we do not really know about the temperature distribution of the ice.  The enormous tidal flexing of the ice may mean that it does not simply get warmer steadily until it is warm enough to be liquid water.  It is possible that there are major convection currents within the ice bringing warm ice to the top.  This could be aided by the tidal flexing.
The Europan atmosphere appears to be of Oxygen with a pressure of about a billionth of the Earth’s sea level pressure.  The oxygen probably comes from the breakdown of water molecules by radiation.  The hydrogen part of the water would escape much faster than the Oxygen.
Life on Europa
The Earth is the only place that we know has life on it, but we can speculate about life in other places.  Probably the most likely place other than the Earth to have life is Mars.  Personally I think that there is a very high probability that Mars has living things.  After Mars, perhaps the most likely abode of life is Europa.
Life near the surface of Europa is possible, but a much better habitat for life is the liquid ocean.  Although there would probably not be enough of the Sun’s light penetrating for photosynthesis, life is this ocean is completely possible.  The water may contain a higher concentration of dissolved Oxygen that the Earth’s deep oceans.
In our oceans there are organisms using inorganic chemical energy.  It is reasonable to expect that this type of energy is also available on Europa.













Enhanced colour image by NASA, combining infrared, green and violet, images to enhance the colour differences in the  icy crust of Europa
Natural colour Europan picture by NASA
Inside of Europa

The core of Europa is probably high in Iron and Nickel.  Surrounding this is probably a rocky mantle, with the ocean round this.  There may not be any solid connection between the mantle and the icy crust.


Reddish spots and shallow pits pepper the ridged surface of Europa in this view combining information from images taken by NASA's Galileo spacecraft during two different orbits around Jupiter.

The spots and pits visible in this region of Europa's northern hemisphere are each about 10 kilometers (6 miles) across.
NASA/JPL/University of Arizona/University of Colorado.
Europa's  surface features suggest an actively churning ice shell above a salty liquid water ocean.
Heat is created within the floating ice shell by flexing and squeezing due to rising and falling tides as Europa orbits the giant planet Jupiter. It is this tidal heat that keeps the ocean liquid at a temperature near 0°C (32°F). The cold near-surface ice can crack or crumble, while the tidal heat keeps the base of the ice shell close to the ice melting temperature, so the ice there can slowly flow, like a glacier.
This composite was made by the Jet Propulsion Laboratory.

Frozen sulfuric acid on Jupiter's moon Europa is depicted in this image produced from data gathered by NASA's Galileo spacecraft. The brightest areas, where the yellow is most intense, represent regions of high frozen sulfuric acid concentration.

This image is based on data gathered by Galileo's near infrared mapping spectrometer.

Although in this picture the impurities are interpreted as being sulphuric acid, they could also be salts similar to the ones in the oceans of the Earth.  In both  cases they suggest sub surface conditions suitable for life.

Although speculation about life in the massive ocean of Europa is often about Microscopic life, based on what has happened on the Earth there is no reason to expect life to be confined to microbes.  IF microscopic life exists in Europa, then more complex life is likely.  Arthur C. Clarke and other authors have explored this idea.