O₂(a¹Δ_g) dayglow limb observations on Mars by SPICAM IR on Mars-Express and connection to water vapor distribution

International audienceThe 1.27-μm O2(a1Δg) dayglow on Mars is a product of the ozone photolysis by solar UV radiation. The intensity of the O2(a1Δg) emission rate depends on ozone concentration, atmospheric density and kinetic parameters of involved photochemical reactions. In turn, the distribution of ozone is sensitive to the vertical and spatial distribution of water vapor, which is an effective destructor of O3. SPICAM IR on the Mars-Express mission measures the O2(a1Δg) dayglow with spectral resolving power of 2200. The results of 147 limb observations from 2004 to 2013 are reported. Limb resolution of the instrument is variable and exceeds the scale height of the atmosphere. The slant emission rate reaches a maximum at the high northern latitudes at northern and southern springs Ls=0-50° and 160-190°, respectively and a minimum in middle and low latitudes at southern summer Ls=200-300°. We have compared the SPIVAM O2(a1Δg) limb profiles with the General Circulation Model simulation by the Laboratoire de Meteorologie Dynamique reduced to the vertical resolution of the instrument. The GCM includes the radiative effect of the water clouds and an interactive dust scheme, and well reproduces Martian Climate Sounder (MCS) temperature profiles. The model underestimates the emission for Ls=0-50°, Ls=160-180° and overestimates it from Ls=60° to Ls=150° at high northern latitudes. In the Southern hemisphere the model underestimates the emission for Ls = 170-200° and overestimates it for Ls = 200-230° at high southern latitudes. The disagreement could be related to the water vapor distribution as the model reproduces it. The most recent version of the LMD GCM including microphysical representation of cloud formation taking into account the effect of dust scavenging by water ice clouds gives much better agreement with SPICAM O2(a1Δg) dayglow limb observations. Characterization of the Mars water cycle by GCMs continues to improve, and the observations of the O2(a1Δg) dayglow offer a powerful tool for its validation