"An artist's concept of MAVEN satellite, pictured, shows Mars in the background. NASA plans to launch the satellite into Mars' orbit on Monday. NewsHour will live stream the launch. Image by NASA/Goddard • Mars is a cold, dry, dusty planet with a thin atmosphere, completely inhospitable to life on the surface. But scientists believe that Mars looked very different four billion years ago. • "What we see on Mars is a planet that seems very Earth-like 4 billion years ago. Not that it had a similar atmosphere, but in terms of what's important for life -- liquid water, access to the elements to build life -- it's possible we had two planets that had the potential for life and they diverged radically," said Bruce Jakosky, the principal investigator for the Mars Atmosphere and Volatile Evolution mission. • So why did Mars become so inhospitable to life while the Earth flourished? Jakosky and his team think the answer is in Mars' missing atmosphere. On Monday, NASA plans to launch the MAVEN satellite, which will orbit Mars and collect data that will explain the planet's past and why it lost its water and carbon dioxide. • PBS NEwsHour will live stream MAVEN's launch, which is planned for Monday afternoon. Watch the player above."