Mars has fascinated humans for thousands of years. Even when we knew the planets only as wanderers in the sky, Mars stood out. It was prominent and distinctively, brilliantly red. As the years passed, it became even more familiar. We gazed at its surface through telescopes and found dried riverbeds and deep canyons. We discovered that Mars' year is twice as long as ours is. We also learned that Mars has four seasons and a 24-hour day. With the...