QUESTION:
What are the dark streaks on the surface of Europa?

ANSWER from Ken Klaasen of the Galileo Imaging Team write:
The question of the origin of the dark streaks on Europa is still puzzling
planetary scientists.  The surface of Europa appears to be composed primarily
of water ice (>85%) combined with small amounts of other minerals.  The dark
streaks are most likely areas where darker sub-surface material has filled in
fractures, cracks, or gaps in the water-ice crust.  These cracks could be
produced by tidal or interior volcanic stresses or by meteoritic impacts.
Debris-laden materials could then have been transported, perhaps as muddy
slurries, through Europa's icy crust to the surface.  In this model, Europa
would have an outer ice shell surface no thicker than a few tens of kilometers.
One high-priority objective of the Galileo orbiter imaging investigation is to
obtain high-resolution images of some of the dark streaks on Europa to try to
understand better their origin and composition.