The BBC has a DVD out called Voyage To The Planets.  This is an imaginary grand tour based on science facts.  They go to Venus, Mars, Jupiter, Saturn and Pluto.  I plan on showing it to my students a little later in the school year.
JPL just recently announced that any oceans on Mars would have been incredibly salty - MUCH saltier than our present oceans.  According to JPL, there are only a handful of terrestrial lifeforms that could have survived in that sea.
As I recall, Mars was struck by an asteroid about 20km across about 2 to 2.5 billion years.  The energy of this impact could have vaporized any remaining oceans.  As perspective, a 10km asteroid impact on Earth is generally regarded as a sterilization even - one that would kill off all life on our planet.
Chaunte