Revised upper limits for abundances of NH3, HCN and HC3N in the Martian atmosphere

Abstract

The Atmospheric Chemistry Suite (ACS) onboard the ExoMars Trace Gas Orbiter (TGO) spacecraft has been studying Mars' atmosphere since 2018. The sensitivity of the middle infrared channel (MIR) allows it to address many ardent topics and it is capable of improving and establishing upper limits for many trace species. In this work we present analysis of transmittance spectra in the 3332.5–3338.6 cm−1 range with 30,000 resolution (λ∕Δλ), covering absorptions lines of three nitrogen-bearing species: ammonia (NH3), hydrogen cyanide (HCN) and cyanoacetylene (HC3N). According to existing models, all of those are not expected to be present in a CO2-rich Martian atmosphere, but outgassing or unknown chemistry sources cannot be discounted. The upper limits of 14, 1.5 and 11 ppbv are obtained for NH3, HCN and HC3N from individual occultation measurements during the warm and dusty perihelion season of martian year 36. For the ammonia and hydrogen cyanide the upper limits are improved compared to previously published results. A search for cyanoacetylene on Mars is reported for the first time.