Mawrth Vallis contains an open book of Mars' history. The exposed valley walls date back some 3.7 billion years. "There's this stack hundreds of meters thick, with exposed sedimentary rocket and lots of clay minerals," said Golombek.
The clays, known as phyllosilicates, form in the presence of water, believed to be a necessary ingredient for life.
But Mawrth also presents a significant puzzle. Scientists don't know how it formed, so any water, for example, that once flowed in the area could have been too acidic for life to blossom. "Certain places with water are more conducive to having all the things you need for life than others," said Golombek.