2. Water on Mars?

Mars simply will not give up its most coveted secrets. Ultimately, the big quest for NASA and all the Mars scientists is about whether there is life, but before that's answered, there is the question of liquid water, a requirement of life as we know it.

Despite two major discoveries of water ice in 2002, nobody can figure out yet whether any of it might exist in the melted state.

Meanwhile, clues mount. In one compelling study released in December, dark streaks on the surface were attributed to salty, running water. But many experts remain unconvinced. NASA's Odyssey spacecraft is circling Mars as you read this, hunting for more evidence.

Next Page: Our mediocre black hole