"We reimaged it several times at different sun angles to be sure it wasn't just a trick of different illumination conditions," says Ken Edgett, a member of the research team that made the discovery. More than just a geologic curiosity, finding water on Mars has major implications for the search for life, because the presence of H2O greatly increases the odds that living organisms once thrived on the planet, and perhaps still inhabit it today. "We didn't expect this to happen in our lifetime, let alone our 10-year mission," Edgett says.
But is the liquid flowing down these gullies really water? Probably, say experts at NASA's Jet Propulsion Laboratory, but more evidence is needed to seal the case. "You can never be certain from orbit," Edgett says. "But this is the best evidence yet of liquid water being present on Mars right now."