The figure shows how surface air pressure changed throughout Martian history. A bar at 4 billion years ago denotes a lower limit shown by this study. Constraints suggested by other studies are also shown by arrows. Credit: Image courtesy of
Exploration missions have suggested that Mars once had a warm climate, which sustained oceans on its surface. To keep Mars warm requires a dense atmosphere with a sufficient greenhouse effect, while the present-day Mars has a thin atmosphere whose surface pressure is only 0.006 bar, resulting in the cold climate it has today. It has been a big mystery as to when and how Mars lost its dense atmosphere.
An old meteorite has been known to contain the ancient Martian atmosphere...
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