CO Emissions from Cometary and Planetary Atmospheres as a Marker for CO2

Photodissociation of CO₂ in the atmosphere of Mars leads to dayglow emissions in the 190-250 nm region from the CO(a-X) Cameron bands, the 290-nm CO₂⁺ (B-X) band, the 300-400 nm CO₂⁺ (A-X) system, and the 297-nm O(¹S-³P) line [1]. Very recently, detectors on Venus Express have shown the same emissions at that planet with an order of magnitude higher intensity [2], approximately 2 MR on the limb. It has been generally assumed that production of the CO(a) state is direct, i.e., CO₂ is photodissociated by photons with wavelengths less than the 108-nm threshold to produce CO(a) + O(³P). Experiments at the Advanced Light Source (ALS) in Berkeley indicate that this scenario is incorrect, and that CO(a) production arises mainly from cascading from higher CO triplet states with a threshold of 100 nm [3]. The cascading process results in initial emission in the visible and infrared (IR), followed by the Cameron band emission. As a result, there are discrepancies between the observations and models and, furthermore, there has never been an attempt to monitor the strong unquenched CO dayglow emission in the visible and IR at Mars/Venus. On the other hand, cometary spectra in these wavelength regions are obtained from the ground, and a serious effort should be made to identify the very complex CO bands. [1] C. A. Barth et al., J. Geophys. Res. 76, 2213-2227 (1971). [2] J.-L. Bertaux et al., Geophys. Res. Abstracts, 14, EGU 2012-8097 (2012). [3] K. S. Kalogerakis et al., Icarus 220, 205-210 (2012). The ALS experiments were performed under grant NNX06AB82G from the NASA Outer Planets Research Program to SRI International. Partial support for K.S. Kalogerakis from NSF grants AST-0709173 and AST-1109372 is also acknowledged. M. Ahmed, K.R. Wilson, and the ALS are supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U.S. Department of Energy under contract No. DE-AC02-05CH11231.