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Why does Mars have such a thin atmosphere? Scientists have long hypothesized that chemical reactions among water, carbon dioxide, and rock turned the original thick Martian atmosphere into carbonate mineral solids. The carbonates were never recycled back into carbon dioxide gas because Mars, being so small, cooled quickly and its volcanoes---which might have released dissolved gases back into the atmosphere---stopped erupting. However, so far only a single small area of carbonate rock has been found on Mars, and this outcrop probably formed in warm subsurface waters. Moreover, the carbonate theory offers no explanation for why Mars has so little nitrogen or noble gases. Escape provides a better answer. The atmosphere did not get locked away as rock, it dissipated into space.