Long-Term variations in Mars’ ozone column abundances : A study from MY34 to MY38

Measurement of the Mars ozone column abundance provide key insights about our understanding of the planet’s photochemistry, climate dynamics, and potential habitability. Ozone acts as a tracer for odd-hydrogen offering insights into its seasonal and spatial variability —an essential factor in understanding Mars' photochemical stability. Accurate ozone measurements also support atmospheric modelling by providing a “ground truth” for Mars GCMs which currently under- or over-predict ozone abundances.

The Ultraviolet and VIsible Spectrometer (UVIS) is one of the three channels of The Nadir and Occultation for Mars Discovery (NOMAD) instrument, aboard the ExoMars Trace Gas Orbiter (TGO) [1,2]. Since April 2018, UVIS has taken near continuous nadir radiometric measurements of the Mars atmosphere from 200 to 655 nm.  Through inversion of these spectra, UVIS has produced a comprehensive dataset on the aerosol and ozone climatology from Mars Year (MY) 34 (from L_S = 150°) through to MY 36 [3].

The ozone climatology for MY 34, MY 35 and MY 36 follows the well-established seasonal trends evidenced by previous investigations [4] with the largest observed abundances found at the edges of the weakly illuminated polar regions in the spring, autumn and winter hemisphere and associated with cold atmospheric temperatures and low water vapour abundance. At perihelion, a depletion in ozone at equatorial regions is seen in all Mars years where ozone abundances drop below the detection limit of UVIS (<0.7 µm-atm).  An increase in low latitude ozone during the aphelion season was seen in both MY 35 and MY 36 and coincides with the formation of the Aphelion Cloud Belt (ACB), where the freezing of water vapour to form water ice clouds confines the water vapour below the hygropause, allowing ozone to build-up above the clouds. Large variability was observed in the aphelion ozone band, with MY 35 seeing ozone abundances that were ~14% lower compared to MY 36. This reduction in ozone was attributed to the early dust storm between L_S = 30° – 50° in the northern hemisphere of MY35, which led to the near global reduction in ozone around the same time.

We present an extension to the Mars ozone climatology to include MY 37 and MY 38 (present-day). We will show the ozone distribution for all Mars years measured by UVIS and discuss the observed seasonal and interannual distribution and its variability. More specifically we compare the aphelion equatorial ozone band across the four Mars years to determine if the reduced ozone in MY 35 is an isolated event or whether strong variability in ozone exists in this season.