<What is known of Europa:
* it is a moon of Jupiter
* it is a semi-spherical structure
* it has a solid surface
* it has a tenuous atmosphere (1e-11 bar) that contains oxygen
* it appears to be slightly blue, perhaps aquamarine, in color
* it has a very weak magnetic field
* it is a very cold, very distant place

What is not known of Europa:
* it may have a water-ice cover
* if it has an ice cover (water or not), the sheet may extend downward in layers for several kilometers
* if it has an ice cover (water or not), there may be a liquid (water or not) interspersed below the surface

It is believed that the variation in Europa’s period as it passes through Jupiter's magnetic field is due to the existence of some conductive material beneath the moon’s surface. This material is thought to be (more correctly “hoped” to be) a liquid, salty ocean.

Dan