Simulations of the Martian Airglow and the Recovery of Temperature Profiles From Limb Airglow Observations

Although Mars is our neighbour, the distance between our planets is such that most observation of Mars is done remotely. This limits the amount and types of observations possible. It is thus imperative to develop new techniques to retrieve more and new types of information from observational data. In this work a new method for retrieving temperature and hydroxyl volume emission rates from the Martian atmosphere will be presented. A non linear optimal estimation global fitting method is used to retrieve the profiles from simulated nocturnal airglow measurements. The simulated data is for a limb viewing instrument. The profiles are determined by minimizing the difference between the generated spectrum and the simulated measurements using the Marquardt Levenberg iteration method. The method fits all data from a full simulated scan of the atmosphere rather than one level at a time which greatly reduces the effect of atmospheric interference and instrument noise on the results. A two stage fitting method is needed to retrieve both the volume emission rate and temperature profiles as the fitting method cannot resolve the two simultaneously. Since the retrieval of the hydroxyl emission rates has little dependence on the accuracy of the temperature profile, it is possible to fit first the spectrum for the volume emission rate using an estimated temperature profile. These retrieved rates can then be used to refine the estimate of the temperature profile by using the same fitting method to retrieve temperatures. The two stage fitting method allows the algorithm to be very flexible as it does not require an atmospheric model to retrieve the profiles. This flexibility could be exploited to apply this methodology to the Venetian atmosphere where nighttime hydroxyl airglow has already been detected1. 1G. Piccioni et al. First detection of hydroxyl in the atmosphere of Venus. Astronomy and Astrophysics 483 (3) L29-L33 (2008).