1. Ancient Mars could have had the right chemistry to support living microbes. Curiosity has found carbon, hydrogen, oxygen, phosphorus, sulfur and hints that there was once fresh, flowing water.
2. Evidence of an ancient streambed. Smooth, rounded rocks were found that suggest a steady stream of flowing water about knee deep.
3. Radiation levels on Mars exceed NASA’s career limit for astronauts. Curiosity detected the Red Planet could be dangerous for humans explorers if they’re not properly protected.
4. The lack of methane in the air (so far). Living organisms release methane so Curiosity was hoping to detect it, but so far has sniffed none.