Kim Hyatt wrote:
Joe, I'm just remembering some basic high school physics, but you're right - standing water can't exist on the surface of Mars under today's conditions due to low atmoshperic pressure. However, I don't recall that anyone has yet put a timeline to these features...
The picture, when it was first released, caused a lot of discussion on the web in part because the event that caused the gullies had to have happened quite recently (months as opposed to centuries). The "newness" is indicated by the colors shown. Any ancient feature on Mars tends to have that familiar oxide coating that constantly rains down on the surface. Remember the rover that was buzzing about on the surface a few years back? After being there only a few days a coating of dust was already seen. However, while few argued the features were new, there was a lot of debate about what caused them. One idea that seemed to have a lot of support was liquid CO2. Personally I'm hoping for water but that is countered by the argument that says that Mars should be permanently frozen from the surface down to several kilometers so there's just no way that a large underground lake could be lucking just below the surface and occasionally gush out in a large enough torrent to etch the features we see before evaporating. Patrick