You know kmguru. Just because scientists speculate Mars without a civilization thrown in doesn't mean they are afraid to speculate. The accepted "speculation" at this point is that over the past few billion years solar wind has blown the atmosphere off of Mars given that Mars has only a 1/3 of the gravity to begin with and the lack of a large magnetic field. Throw in a few major impacts to help the process of throwing material off the planet and there you go...a planet without a thick atmosphere. The same thing is happening with Venus as we speak. But given that Venus is larger and has a thicker atmosphere (still minus a large magnetic field) the effect is not as "dramatic" over the timescale given.
One guy confided that just like we have underground bunkers, they hope, there will be some there. Worst case, there could be cave paintings or potteries and digging tools left by Martians.
Hmm...I think I heard that same guy shouting to passer-bys while standing on a milk crate. He had wild bushy hair and reaked of liquor.