Four billion years ago, Mars may have looked completely different. Water could have flowed across the planet's surface. There might have been life. To support these conditions, the planet's atmosphere must have been very different.

A NASA mission to investigate that atmosphere – and why it changed – is about to enter orbit around the Red Planet. Led by scientists at the University of Colorado Boulder, the mission, called MAVEN (short for Mars Atmosphere and Volatile EvolutioN), consists of a satellite that will orbit the planet.

"The MAVEN mission is all about trying to learn what the history of the atmosphere has been," said Bruce Jakosky, the CU-Boulder scientist heading up the mission.

The mission launched in November 2013, and will enter orbit Sunday evening, Sept. 21. The mission is scheduled to spend a year orbiting the planet and learning about the upper atmosphere. If that goes well, the satellite has enough fuel to remain in orbit for a decade more, said Jakosky.

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KUNC sat down with Jakosky to ask him some questions about the MAVEN project.

Why do scientists think that Mars had life and water?

"We see features on the surface that look like they were carved by flowing water, river channels. We see minerals that can only form in the presence of liquid water and temperatures that are warmer than what we see today. And they are widespread enough that we think there must have been liquid water."

Why isn't there water now?

"You have to ask what was different back then. And the best explanation is that there was a greenhouse atmosphere, that there was more carbon dioxide in the Mars atmosphere and that the warm temperatures allowed liquid water. At that point, the question becomes, where did the CO2, where did the water go? Did it go down into the crust or did it go up into the top of the atmosphere where it was removed into space?"

How does MAVEN help answer these questions?

"MAVEN is the first mission really devoted to studying this part of the planet, the upper atmosphere, the interactions with the solar wind. And we have 8 different instruments that are devoted to these measurements to see how it all fits together."

What exactly are those instruments doing?

"What we want to do is to learn how the sun drives escape to space. How it strips off gases from the top of the atmosphere. So we need to know what's coming in from the sun. We'll make measurements of the solar extreme ultraviolet and the solar wind. All of these can drive processes that lead to [gas] escape. And then we'll look at the response of the atmosphere…How do interactions between solar wind and the upper atmosphere drive gasses off? We'll measure that today. Really we'd like to know how they varied over the last 4 billion years, but we measure it today and we learn to extrapolate backwards."

When will scientists start getting answers to these questions?

"In terms of getting the measurements that will answer our questions: 'Where did the water go? Where did the CO2 go?' we think that is going to take several months. We're looking at winter when we get to talk about our preliminary results."