THERE was great excitement last week when news leaked out that a spacecraft has seen signs that water might have recently flowed on Mars’s surface. But experts are puzzled by how water could be liquid in the frigid Martian climate and warn that concrete proof will be hard to come by. “If these results prove true, that there is water on Mars near the surface, it has profound implications for the possibility of life on Mars,” NASA’s associate administrator for space science Ed Weiler told a press conference in Washington DC last week. “Just about any place biologists find liquid water, organic molecules and energy, they find life, whether it’s on the surface of the Earth or 10 000 feet below.”