Robin Wordsworth, assistant professor in environmental science and engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences, and his colleagues used a 3-D atmospheric circulation model to compare a water cycle on Mars under different scenarios 3 to 4 billion years ago. The left rendering looks at Mars as a warm and wet planet with an average global temperature of 10 degrees Celsius (50 Fahrenheit), and the other as a cold and icy world with an average global temperature of minus 48 degrees Celsius (minus 54 Fahrenheit).