text: These "aliens" very well could have simply just put a peice of shiney metal on the surface and the MGS would have seen it and we would probaly think "hey, this unknown element is here on the surface of a planet where there are no metals". Not only would it make us think that there is indeed metals on Mars but then we would think "hey metals most usually form deep undergroud so there is no reason for this to be on the surface". THAT would be a valid way of letting us know "hey, we exist".