Megan A J Brown

and 9 more

We show a positive vertical correlation between ozone and water ice using a vertical cross-correlation analysis with observations from the ExoMars Trace Gas Orbiter’s NOMAD instrument. We find this is particularly apparent during the first half of Mars Year 35 (LS=0-180) at high southern latitudes, when the water vapour abundance is low. This contradicts the current understanding that ozone and water are, in general, anti-correlated. However, our simulations with gas-phase-only chemistry using a 1-D model show that ozone concentration is not influenced by water ice. Heterogeneous chemistry has been proposed as a mechanism to explain the underprediction of ozone in global climate models (GCMs) through the removal of HOX. We find improving the heterogeneous chemical scheme causes ozone abundance to increase when water ice is present, better matching observed trends. When water vapour abundance is high, there is no consistent vertical correlation between observed ozone and water ice and, in simulated scenarios, the heterogeneous chemistry does not have a large influence on ozone. HOX, which are by-products of water vapour, dominate ozone abundance and mask the effects of heterogeneous chemistry on ozone. This is consistent with gas-phase-only modelled ozone, showing good agreement with observations when water vapour is abundant. High water vapour abundance masks the effect of heterogeneous reactions on ozone abundance and makes adsorption of HOX have a negligible impact on ozone. Overall, the inclusion of heterogeneous chemistry improves the ozone vertical structure in regions of low water vapour abundance, which may partially explain GCM ozone deficits.

Manish R Patel

and 25 more

We present ~1.5 Mars Years (MY) of ozone vertical profiles, covering Ls = 163deg; in MY34 to Ls = 320deg; in MY35, a period which includes the 2018 global dust storm. Since April 2018, the Ultraviolet and Visible Spectrometer (UVIS) channel of the Nadir and Occultation for Mars Discovery (NOMAD) spectrometer aboard the ExoMars Trace Gas Orbiter has observed the vertical, latitudinal and seasonal distributions of ozone. Around perihelion, the relative abundance of ozone (and water from coincident NOMAD measurements) increases strongly together below ~40 km. Around aphelion, decreases in ozone abundance exist between 25-35 km coincident with the location of modelled peak water abundances. We report high latitude (above 55deg;), high altitude (40-55 km) equinoctial ozone enhancements in both hemispheres. The northern equinoctial high altitude enhancement is previously unobserved and forms prior to vernal equinox lasting for almost 100 sols (Ls ~350‑40deg), whereas the southern enhancement persists for over twice as long (Ls = ~5-140deg;). Both layers reform at autumnal equinox, with the northern layer at a lower abundance. These layers likely form through a combination of anti-correlation with water and the equinoctial meridional transport of O and H atoms to high-latitude regions. The descending branch of the main Hadley cell shapes the ozone distribution at Ls = 40-60deg;, with the possible signature of a northern hemisphere thermally indirect cell identifiable from Ls = 40-80deg;. The ozone retrievals presented here provide the most complete global description of Mars ozone vertical distributions as a function of season and latitude.