Scientists have zero evidence for past life on Mars, but they do have another theory. At one point in its distant past, Mars had both water and a protective magnetic field, just as Earth does now. However, once the magnetic field disappeared, radiation ionized the water, splitting it into hydrogen and oxygen. Much of the oxygen was absorbed by the planet's famous red iron-oxide rocks, but manganese-oxide requires a lot more oxygen to form. That has led scientists to believe that in its early history, Mars had more oxygen than they thought.

Curiosity spotted manganese-oxide in the Gale Crater region.

The data could be disappointing for alien-seekers that believe planetary oxygen is a surefire sign of life. "It's important to note that this idea represents a departure in our understanding for how planetary atmospheres might become oxygenated," Lanza says. The theory will be hard to prove, but it's the best one they have for now, unless Curiosity's next discovery is a Martian bug.