Billions of years ago, Mars probably looked more like Earth does now, with clouds and oceans and a much thicker atmosphere. It may even have had some type of microbes. But now it's a barren, frozen desert.
So what happened? Where did the air and water go?
NASA launched a new spacecraft to try to find out. It's called MAVEN, which stands for Mars Atmosphere and Volatile Evolution. It's the first mission dedicated to studying the red planet's upper atmosphere.
"We expect to learn how the modern Mars works, really in detail. To see its climate state, to understand how the atmosphere is lost to space -- how Mars may have lost a magnetic field -- to take that information and map it back in time, " said NASA's James Garvin.
MAVEN lifted off Monday, November 18, at 1:28 p.m. ET from Cape Canaveral Air Force Station in Florida. A United Launch Alliance Atlas V rocket will launch the probe into space on a 20-month trip to Mars. It's scheduled to arrive September 22, 2014.
The solar-powered probe is about the length of school bus -- 37.5 feet (11.43 meters) long -- and will weigh about 5,410 pounds (2,454 kg) at launch.