Europa - a Moon with a Frozen Ocean? The satellites of the giant planets are generally small bodies without atmospheres, basically made of ice, and with surfaces heavily marked by meteoritic collisions. Europa, is one of the four large moons of Jupiter that was first seen by Galileo Galilei in 1610; it is the second from the planet after Io. It is the smallest of the four, with a diameter of about 3125 km, i.e. it is slightly smaller than our own Moon. When visited by the NASA Galileo spacecraft, many images were obtained, some showing very unusual surface features with bright ice-covered plains criss-crossed by an extensive network of darker fractures, and with evidence of outflow (like dirty geysers) situated along the fractures. Calculations show that Europa may still have a liquid mantle and this has lead to speculations that primitive life-forms may be teeming in the mantle. However, there is no clear evidence for this ocean or life in it from current data. We will probably have to wait until the Europa Orbiter visits this moon in 2006 to learn more about this exciting possibility.

The Jovian moon Europa, as photographed by the Galileo spacecraft. Left: in natural colours; right: in enhanced (false) colours to show better the details (NASA). Close-up view of the icy surface of Europa (NASA Galileo Mission)