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The millibars on Mars

What happened to Mars' atmosphere and water has been a well guarded cosmic mystery.

One idea was that the martian atmosphere was eroded away over billion years by powerful solar winds. A more controversial idea is that Mars' atmosphere was blown into space by a catastrophic asteroid or comet impact in the remote past. A space rock at least 6 miles (10 kilometers) wide could have been enough to obliterate Mars's once abundant atmosphere. However, in this writer's opinion, scientists may often overplay the impact hand when explaining planetary changes.

What's in the Sky: On Sunday, Dec. 14, the Moon showcases the stellar twins Castor and Pollux in the constellation Gemini. Best view will be mid evening. Pollux is at the upper left of the Moon and Castor is off to Pollux's side.

Lou Varricchio, M.Sc., is a former NASA senior science writer. He is involved with the NASA/JPL Solar System Ambassador program in Vermont and is a senior member of the U.S. Civil Air Patrol.

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