Airglow optical emissions from planetary atmospheres provide remotely observable signatures of atmospheric composition, energy deposition processes, and the resulting chemical reactions. We may one day be able to detect airglow emissions from extrasolar planets. Reliable interpretation requires quantitative understanding of the energy sources and chemical mechanisms that produce them. The ultraviolet dayglow observations by the Mariner 6 and 7 (1969) and Mariner 9 (1971–72) motivated numerous modeling studies and laboratory experiments. The most obvious source reaction is photodissociation and photoionization of ambient CO2, which is known in the laboratory to produce the four strong dayglow emitting states: hν + CO2 → O(1S), CO(a3Π), CO+2(A2Πu & B2Σ+u) If this simplest of models were sufficient, then the high altitude dayglow emissions would all share the same scale height, which would be that of CO2. The few Mariner dayglow observations provide weak statistics. Addition of 4 months of Mars Express dayglow data, and including radio occultation and mass spectrometry data from other missions, have made the analyses and conclusions more robust. The CO(a3Π) and CO+2(B2Σ+u) dayglow altitude profiles are consistent with the first source reaction. In contrast, the O(1S) dayglow scale heights are much larger and are consistent with a second source reaction: O+2 + e− → O(1S) Both sets of scale heights change with respect to solar activity roughly as suggested by modeling studies.