Screening Constitutional Isomers of C3H2O2 for Thermodynamic Properties Obtained from Systematic Electron Structure Calculations

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The Enigma of Methane on Mars

International audienceBetween 2004 and 2012, four independent groups reported detections of low levels (10–60 ppbv) of methane on Mars. If true, these constitute the first observations of a potential biosignature on that planet and would be an important finding and addition to the inventory of minor species in its atmosphere. However, these claims for the presence of methane have been highly controversial. In 2014, the most robust search for methane on Mars was performed by the rover Curiosity. The latest measurements by Curiosity indicate a background CH4 level of 0.2–0.7 ppbv, except during a two-month period between November 2013 and January 2014, when high mixing ratios of around 7 ppbv were observed. These observations immediately raise the question of the origin of methane on Mars, but also pose fundamental challenges to our current understanding of Martian atmospheric physics and chemistry

Atmospheric Biosignatures

Life has likely coevolved with the Earth system in time in various ways. Our oxygen-rich atmosphere and the protective ozone layer are mainly the result of photosynthetic activity. Additionally, bacteria emit greenhouse gases such as methane and nitrous oxide into the atmosphere, and vegetation can emit a variety of organic molecules. In an exoplanetary context, it is important to consider whether such gas-phase species – so-called atmospheric biosignatures – could be detected spectroscopically and attributed to extraterrestrial life. Another signature of life on Earth is the so-called redox disequilibrium of its atmosphere. This refers to the presence of simultaneously oxidizing and reducing species (e.g., molecular oxygen and methane). Without life, such species would react and be removed on relatively fast timescales. Since Earth’s atmosphere has changed considerably during its history, we will also consider atmospheric biosignatures in the context of the early Earth. This chapter will present a brief literature review of atmospheric biosignatures. We will discuss the main photochemical responses of such species in the modern and early Earth’s atmosphere and their potential to act as atmospheric biosignatures in an exoplanetary context