This is an artist’s concept of an early Mars with liquid water (blue areas) on its surface. Ancient regions on Mars bear signs of abundant water – such as features resembling valleys and deltas, and minerals that only form in the presence of liquid water. Scientists think that billions of years ago, the atmosphere of Mars was much denser and warm enough to form rivers, lakes, and perhaps even oceans of water. As the planet cooled and lost its global magnetic field, the solar wind and solar storms eroded away to space a significant amount of the planet’s atmosphere, turning...