If Mars didn't have water "from the beginning" where and how would it acquire all the water that is said to have covered a fair amount of the surface?? The was

If Mars didn't have water "from the beginning" where and how would it acquire all the water that is said to have covered a fair amount of the surface?? The was there water question seems to be settled, where did it all go? And why? Is the earth subject to that same why?