So there is evidence that impacts have may have tossed early life onto the moon: http://www.telegraph.co.uk/science/science-news/3297506/Why-... And, there have been experiments that suggest that some life can survive interplanetary trips: http://www.nytimes.com/1985/08/13/science/science-watch-spor... And impacts as well: http://www.sciencedirect.com/science/article/pii/S0019103500... But the trip is very tough: http://www.ncbi.nlm.nih.gov/pubmed/11770260 So, please scratch that 'every year' part! But that said, I still think that given the time scales allowed, organisms ejected from earth have fallen upon Mars, and probably when Mars had liquid water on it. Mars had liquid water ~3.5-3.8 BYA, and life on Earth started ~3.8-4.0BYA. I'll still keep my bet, but I think I'll lower my wager. :) EDIT: Or, is was it the other way around?!: http://hubski.com/pub?id=1427