There are at least one, and maybe two, serious alternate theories proposing that the gullies on Mars are not water-produced — and one of them states that the “fluid” that created them isn’t even liquid! The first was described in some detail by Dr. Michael Carr of the U.S. Geological Survey at the press conference in which the discovery was first announced. Carr is arguably the world’s leading authority on the subject of water on Mars, but he is seriously skeptical about these gullies being water-produced.