Analysis and Interpretation of Mars Dayglow Altitude Profiles

The Mariner 4 (1965), Mariner 6 and 7 (1969), Mariner 9 (1971-72), and Viking 1 and 2 (1976-80) missions provided the first quantitative information about the structure, energetics, and dynamics of the Mars atmosphere. Not until more than 20 years later did new generations of landers and orbiters revisit the planet.The initial Mariner dayglow observations [1] 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 all four dayglow emitting states:(1) hν + CO₂ → O(¹S), CO(a³Π), CO₂⁺(A²Π_u & 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 [1, 2] provided weak statistics [3]. Addition of 4 months of Mars Express dayglow data [4], and including radio occultation and mass spectrometry data from other missions, have made the analyses and conclusions more robust.The CO(a³Π) and CO₂⁺(B²Σ_u⁺) altitude profiles are consistent with Reaction (1). In contrast, the O(¹S) scale heights are much larger and are consistent with source Reaction (2).(2) O₂⁺ + e^– → O(¹S)Both sets of scale heights change with respect to solar activity roughly as suggested by modeling studies [5, 6].Acknowledgements: Supported by NASA Mars Data Analysis grant NNX06AE04G. SRI document MP 09-050.