Exploratour - Carbon Cycle of Mars
This drawing represents several features of the atmosphere on Mars including: dust storms, an atmospheric pressure much lower than found on Earth, and a composition primarily of carbon dioxide.
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Click on image for full size
Lower Atmosphere
The atmosphere of Mars is composed mainly of carbon dioxide (95.3%). So, the Martian atmosphere is what you would call a major source of carbon. Scientists say there may be more carbon inside the Martian clouds. However, Mars has very little cloud cover compared to most planets, so there can't be a lot of carbon trapped here.