History will note that the guy who discovered liquid water on Mars was an undergraduate at the University of Arizona, a 20-year-old who played guitar in a death-metal band and worked in a planetary science lab. One day, while comparing different satellite images of a single Martian crater taken at various times of year, he noticed something odd: a set of dark streaks in the soil that grew in the Martian summer and shrank in the winter. Learn more about your ad choices. Visit podcastchoices.com/adchoices