We had a substitute teacher today, and he was rather bland and tended to go off on tangents. He started talking about water (which was supposed to be the topic), only water on Mars. He said that there's evidence that water once flowed on Mars, but it's all gone now. I asked why, and he said "The water molecule has such a low molecular weight, that it just drifted away." Now, to me, that sounds like a really, really generalized answer. I talked to him after class, and I told him that the water wouldn't just drift away just because it's lightweight. Even the lightest molecules are still subject to gravity. I told him they still needed some other force to push or pull them away from the planet. He didn't agree, but then later proved my point by saying something along the lines of "No, they can just drift away if they are small enough...it's caused by solar winds and other space weather." I kinda cut it off there (it was lunchtime), but I still have some problems with that theory: 1. Why would that cause the 'light' molecules of water to drift any more than the other solids on mars? The molecule is light, but unless the water water was vapor the molecules would be in the form of compounds. And, if anything, the density or weight of the compounds would cause a bigger effect than the molecular weight. 2. Even if the water was vapor, why would the water disappear and other gases (such as methane) still remain? 3. If the water was vapor, why would there be evidence that it was once there? (From what I know, he made the point that there was once water because of canals and river beds and such. I think there now we have found ice, but I don't think he knew about that). What does everyone think? Are my concerns valid? Is his theory valid?