http://antwrp.gsfc.nasa.gov/apod/ap041218.html

Explanation: This bright white swath cutting across
the surface of icy Jovian moon Europa is known as
Agenor Linea. In all about 1000 kilometers long and 5
kilometers wide, only a section is pictured here as
part of a combined color and black and white image
based on data from the Galileo spacecraft. Most linear
features on Europa are dark in color but Agenor Linea
is uniquely bright for unknown reasons. Also unknown
is the origin of the reddish material along the sides.
While these and other details of Europa's surface
formations remain mysterious, the general results of
Galileo's exploration of Europa have supported the
idea that an ocean of liquid water lies beneath the
cracked and frozen crust. An extraterrestrial liquid
ocean holds out the tantalizing possibility of life.