Just days after starting its science mission, a new spacecraft orbiting Mars has struck pay dirt, detecting vast fields of ice that scientists say provide evidence of sufficient water to make it possible for the planet to have harbored life. The discovery is a coup for NASA, whose leaders are using a “follow the water” strategy to understand the evolution of Mars and look for signs of past and present life there. The presence of water would also be key to any future attempt to have astronauts explore the Martian surface. “Water is vital to life. Water has changed the surface of Mars in the past. And water is essential to the future exploration of Mars,” R. Stephen Saunders, the Jet Propulsion Laboratory’s project scientist for the Odyssey orbiter, said at a news conference Friday in Pasadena to release the findings.