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Orbit: 670,900 km from Jupiter
Orbital period: 85 hours
Gravity: 1.315 m/s²
Diameter: 3138 km
Mass: 4.80e22 kg
Europa, is the sixth closest moon of the planet Jupiter, and the smallest of its four Galilean satellites, but still the sixth-largest moon in the Solar System.

Europa       was  a Phoenician princess abducted to Crete by Zeus,       who had assumed the form  of a white bull, and       by him the mother of Minos.

Discovered by Galileo      and  Marius in 1610.

Europa and Io are somewhat       similar in bulk  composition to the        terrestrial planets:  primarily composed of        silicate rock. Unlike Io, however, Europa has a thin outer layer of ice. Recent data from Galileo indicate that Europa has  a layered  internal structure perhaps with a small metallic core.

But Europa’s surface is not at all like anything in the inner solar system.       It is exceedingly smooth:       few features more than a few  hundred meters high       have been seen. The prominent markings seem to be only         albedo features with very low  relief.

Jupiters moon Europa

There are very few craters on Europa; only three        craters larger than 5 km in diameter have been found.       This would seem to  indicate a young and       active surface.        However, the Voyagers mapped only a        fraction of the surface at high resolution.       The precise age of Europa’s surface is an open question.

The images of Europa’s surface strongly resemble images of sea ice on  Earth.      It is possible that beneath Europa’s surface ice there is a layer of       liquid water, perhaps as much as 50 km deep, kept liquid by        tidally generated heat.       If so, it would be the only place in the solar  system besides       Earth where liquid water exists in significant quantities.

Europa's dark streaks

Europa’s most striking aspect is a series of dark streaks       crisscrossing the entire globe. The larger ones are roughly 20 km across with  diffuse outer edges and a       central band of lighter material.     The latest  theory of their origin is that they are produced by a series of     volcanic  eruptions or geysers.

Recent observations with HST reveal that       Europa has a very tenuous atmosphere (1e-11 bar) composed of oxygen.  Of  the many       moons in the solar system only five others (Io,        Ganymede,    Callisto,      Titan and Triton)      are known to have       atmospheres.  Unlike  the oxygen in Earth’s atmosphere,       Europa’s is  almost certainly  not of biologic origin.       It is most likely generated by  sunlight and       charged particles hitting Europa’s icy surface producing  water vapor which       is subsequently split into hydrogen and oxygen.  The  hydrogen escapes leaving       the oxygen.

Europa rafts taken from Voyager

The Voyagers didn’t get a very good look at Europa.      But it is a  principal focus       of the Galileo mission.       Images   from  Galileo’s first two close encounters with Europa      seem to confirm earlier  theories that Europa’s surface is very young:      very few craters are seen,  some sort of  activity is obviously occurring.      There are       regions   that look very much like      pack-ice on polar seas during  spring thaws on Earth.      The exact nature of Europa’s surface and interior is  not yet clear but the evidence      is now strong for a subsurface ‘ocean’.

Galileo has found that Europa has a  weak magnetic field (perhaps 1/4 of the strength of Ganymede’s).  And most  interestingly, it varies  periodically as it passes thru Jupiter’s massive magnetic field.  This is  very strong evidence that there is a conducting material beneath Europa’s  surface, most likely a salty ocean.




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