“I Swallowed a Martian” and Water on Mars

NASA’s Perseverance rover, which has been exploring the Jezero crater on Mars since 2021 has just logged 26 miles of total travel, the equivalent of a marathon. This crater was chosen as the destination for the rover, with its advanced sensing instruments, because at one end there is a clear river delta and the crater itself was at one time believed to be under water. And indeed, as the rover has explored the terrain, it continues to gather more and more evidence that the red planet had a much more watery past. Today, Mars has a very thin atmosphere (your first deep breath would be your last deep breath) and water can only exist there as water vapor or frozen ice. When the air pressure is really low and nothing presses on them, water molecules tend to fly apart as gas. But astronomers know that billions of years ago, Mars had a much thicker atmosphere and ample pressure to keep water liquid. Indeed, we see evidence of dry river beds and lake beds from that era. Because Mars is a…