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.Testing 2D temperature models in Bayesian retrievals of atmospheric properties from hot Jupiter phase curves
Abstract:
Spectroscopic phase curves of transiting hot Jupiters are spectral measurements at multiple orbital phases, giving a set of disc-averaged spectra that probe multiple hemispheres. By fitting model phase curves to observations, we can constrain the atmospheric properties of hot Jupiters, such as molecular abundance, aerosol distribution, and thermal structure, which offer insights into their atmospheric dynamics, chemistry, and formation. We propose a novel 2D temperature parametrization consisting of a dayside and a nightside to retrieve information from near-infrared phase curves and apply the method to phase curves of WASP-43b observed by HST/Wide Field Camera 3 and Spitzer/Infra-Red Array Camera. In our scheme, the temperature is constant on isobars on the nightside and varies with cosn(longitude/ϵ) on isobars on the dayside, where n and ϵ are free parameters. We fit all orbital phases simultaneously using the radiative transfer package NEMESISPY coupled to a Bayesian inference code. We first validate the performance of our retrieval scheme with synthetic phase curves generated from a Global Circulation Model and find that our 2D scheme can accurately retrieve the latitudinally averaged thermal structure and constrain the abundance of H2O and CH4. We then apply our 2D scheme to the observed phase curves of WASP-43b and find: (1) The dayside temperature–pressure profiles do not vary strongly with longitude and are non-inverted. (2) The retrieved nightside temperatures are extremely low, suggesting significant nightside cloud coverage. (3) The H2O volume mixing ratio is constrained to 5.6 × 10−5–4.0 × 10−4, and we retrieve an upper bound for CH4 mixing ratio at ∼10−6.