Ok, not really. It was probably just organic matter that might have been evidence that there was once life on Mars that they might have killed.
Back in 1976, the Phoenix and Viking landers, searching for organic materials in Martian soil, heated soil samples so that the molecules could be analyzed in gas form. But last year the Phoenix lander found something else in the soil: perchlorates.
As it happens, when perchlorates are heated to high temperatures they release alot of Oxygen, which in turn burns up any organic material, making perchlorates great for rocket propulsion and not so great for finding little green men.
[ via New Scientist ]