On the derivation of thermospheric temperatures from dayglow emissions on Mars

The Cameron bands and the UV doublet are two of the most prominent emission systems in the UV in Mars dayside. Their altitude variation has been exploited in the past to derive thermospheric temperatures from measurements obtained by the Mariner 6, 7, and 9 missions, the SPICAM instrument on board Mars Express, and the IUVS instrument on board MAVEN. Here we identify and quantify possible biases in these temperature determinations. For this purpose, we use a global model able to simulate these two emission systems, and we compare the temperature derived from the simulated emission with that predicted by the model at the same location and time. We find that an exponential fit to the scale height of the UV doublet can be used to derive temperatures with an error less than 10 K at altitudes above about 170 km and for low and moderate values of the Solar Zenith Angle. The temperature derived from the Cameron bands is biased towards higher values due to the non-negligible contribution of CO to the emission. We find that, at 170 km, the difference between the temperature derived from the Cameron bands and the UV doublet can be related to the CO abundance. Our results have implications for previous temperature determinations from the Mariners, SPICAM/MEx and IUVS/MAVEN, some of them being biased by about 25 K.

Automated summary

This study identifies and quantifies possible biases in the temperature determinations of Mars' Cameron bands and UV doublet emission systems. By using a global model to simulate these emission systems, a new method for calculating thermospheric temperatures is provided. An exponential fit to the scale height of the UV doublet can be used to derive temperatures accurately at altitudes above 170 km and for low solar zenith angles. The Cameron bands' temperature values are biased towards higher values due to the significant contribution of CO to the emission.