But by claiming that life originated on Mars, they aren't really making things more believable. Mars probably had liquid water, but not as much as Earth, and so the number of random chemical reactions goes down considerably, and making spontaneous life from non life even harder. Plus, the sun is nearly twice as far away, giving Mars only a fraction of the energy it gives to Earth. Again, this limits the energy in the Martian system and limits the number of chemical reactions that could have been randomly done. So even if there is molybdenum there, I doubt it would overcome the extra hurdles imposed by the Martian environment.
Sagan reports that complex molecules are constantly arriving on earth from comet and meteor debris. Also, do we have any idea when the asteroid planet blew up, and could it have blown past Mars and carried a lot of debris this way, or just had fragments from the explosion hit the earth, carrying who knows what?