Here's a good article on why they will almost for sure find organic material on Mars: http://www.calacademy.org/calwild/2001spring/stories/panspermia2.html The material will come from the earth! This article also talks about interstellar seeding (panspermia) and interplanetary seeding, Martian meteorites and the like. (The comments about Urey and Miller are grossly biased however!)