Abstract
The study of minor chemical species in terrestrial planets’ atmospheres can teach us about the chemistry, dynamics and evolution of the atmospheres through time. Phosphine or methane on terrestrial planets are potential biosignatures, such that their detection may signify the presence of life on a planet. Therefore, the search for these species in the solar system is an important step for the subsequent application of the same techniques to exoplanetary atmospheres. To study atmospheric depletion and the evolution of water abundance in the atmospheres of terrestrial planets, the estimation of the D/H ratio and its spatial and temporal variability is used. We used the Planetary Spectrum Generator (PSG), a radiative transfer suite, with the goal of simulating spectra from observations of Venus, Mars and Jupiter, searching for minor chemical species. The present study contributes to highlight that the PSG is an efficient tool for studying minor chemical species and compounds of astrobiological interest in planetary atmospheres, allowing to perform the detection and retrieval of the relevant molecular species. Regarding detection, it is effective in disentangling different molecular opacities affecting observations. In order to contribute to the scientific community that is focused on the study of minor chemical species in the solar system’s atmospheres, using this tool, in this work, we present the results from an analysis of observations of Venus, Mars and Jupiter, by comparison of observations with simulations in the infrared (IR). The first step was to clearly identify the position of molecular features using our model simulations, since the molecular absorption/emission features of different molecules tend to overlap. For this step, we used the method of the variation of abundances. The second step was to determine the molecular abundances and compare them with values from the literature using the retrieval method and the line depth ratio method. For Venus, our study of SO2-related observations by the Texas Echelon Cross Echelle Spectrograph (TEXES) at 7.4 μm enabled the identification of absorption lines due to sulphur dioxide and carbon dioxide as well as constrain the abundance of SO2 at the cloud top. Phosphine was not detected in the comparison between the simulation and TEXES IR observations around 10.5 μm. For Mars, both a positive and a non-detection of methane were studied using PSG simulations. The related spectra observations in the IR, at approximately 3.3 μm, correspond, respectively, to the Mars Express (MEx) and ExoMars space probes. Moreover, an estimate of the deuterium-to-hydrogen ratio (D/H ratio) was obtained by comparing the simulations with observations by the Echelon Cross Echelle Spectrograph (EXES) onboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) at approximately 7.19–7.23 μm. For Jupiter, the detection of ammonia, phosphine, deuterated methane and methane was studied, by comparing the simulations with IR observations by the Infrared Space Observatory (ISO) at approximately 7–12 μm. Moreover, the retrieval of the profiles of ammonia and phosphine was performed.
Highlights
The sulphur chemical cycle is fundamental to the atmospheric chemistry of Venus [1]
To check whether the detected lines in the simulation were true detections, we did a comparison between the position of the lines in the simulations, corrected for the Doppler shift due to the relative velocity between Earth and Venus at the time of the observations, with the positions extracted from HITRAN [56,64–68] (Table 1)
This work demonstrates that the Planetary Spectrum Generator (PSG) [55] is a highly effective tool to study the detection and perform the retrieval of chemical species in the solar system’s atmospheres in the context of several case studies such as minor chemical species, species of astrobiological interest and species related to atmospheric evolution
Summary
The sulphur chemical cycle is fundamental to the atmospheric chemistry of Venus [1]. Four sulphur species were identified: SO2, SO, OCS and H2SO4 (vapor and aerosols) [2–9].H2 O and SO2 have abundances of 30 ppm and 100–150 ppm below the clouds, respectively [10]. The sulphur chemical cycle is fundamental to the atmospheric chemistry of Venus [1]. Four sulphur species were identified: SO2, SO, OCS and H2SO4 (vapor and aerosols) [2–9]. H2 O and SO2 have abundances of 30 ppm and 100–150 ppm below the clouds, respectively [10]. They are transported upward in the atmosphere to the main cloud deck (40–70 km) by Hadley convection. The cloud top, located at approximately 65–70 km, both species are subject to photodissociation due to solar UV radiation. To form the sulphuric acid clouds, SO2 first reacts with oxygen atoms, forming SO3 and later with H2 O: CO2 + SO2 + hν− > CO + SO3
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