Mars

Science Daily: Mars once had a moderately dense atmosphere - Scientists suggest the fingerprints of early photochemistry provide a solution to the long-standing mystery

summary - Scientists suggest that 3.8 billion years ago, Mars might have had only a moderately dense atmosphere. The scientists have identified a photochemical process that could have helped such an early atmosphere evolve into the current thin one without creating the problem of 'missing' carbon and in a way that is consistent with existing carbon isotopic measurements.