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When you look at Mars today, it appears to be a barren, dusty world with only a very thin atmosphere.

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However, some scientists think that Mars may have once had a much thicker atmosphere,

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maybe even one more like ours on Earth.  So how does a planet lose so much of its atmosphere?

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NASA's MAVEN spacecraft will help give us clearer answers when it studies Mars's upper atmosphere,

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but scientists think that several processes may have had an impact over billions of years.

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One way that an atmosphere can be lost to space is through a series of what are called Neutral Processes,

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which are processes that involve neutral particles rather than charged particles.

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On Mars for example, atoms and molecules of hydrogen gas can be found in the upper atmosphere

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and they often collide with each other as they make their way around. For the most part, the molecules are still bound to the planet by gravity.

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However, if a fast-moving molecule collides with another molecule at the right angle,

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the molecule may have just enough speed to leave the atmosphere and be lost to space.

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As this process continues over billions of years, it, along with many other processes, can contribute to the disappearance of a planet's atmosphere.

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And since the hydrogen in the atmosphere ultimately comes from water in the lower atmosphere, it may also contribute to the loss of water over time.

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In the end, this cumulative effect could have transformed Mars from a bluer, cloudier planet