Atmospheric stability and its implication on prebiotic chemistry on early Venus, Earth, and Mars

The early evolution of the atmospheres of Venus, Earth, and Mars, and hence their potential habitability, is strongly linked to the early evolution of the Sun, which was significantly more active during the first ~1 billion years than it is at present. The high solar X-ray and extreme ultraviolet (XUV) surface flux received by the three planets during this time heated their upper atmospheres, inducing thermal and non-thermal atmospheric escape rates that are assumed to be significantly higher than at present. However, whether an atmosphere is stable against escape into space depends not only on the incident solar/stellar XUV surface flux but also on planetary size and atmospheric composition. CO₂ and other infrared coolants, for instance, cool the thermosphere, making atmospheres with larger CO₂ mixing ratios less susceptible to atmospheric erosion. This implies that atmospheres with certain compositions (e.g., CO₂-dominated atmospheres) were more likely to survive the harsh conditions of the early Solar System than others (e.g., N₂-dominated atmospheres). This, in turn, has implications for prebiotic chemistry and the origin of life since different atmospheric compositions, but also the therewith connected tropospheric temperature and the hydrology of a planet, further affect the photochemical production and rainout of prebiotic molecules into ancient oceans or shallow ponds. This highlights the importance of considering both the upper atmosphere/thermosphere to evaluate atmospheric stability and the lower atmosphere/homosphere to evaluate prebiotic chemistry and climatic conditions if we want to better understand the habitability and evolution of the rocky planets in the Solar System and beyond. We first discuss atmospheric stability on early Venus, Earth, and Mars and investigate the atmospheric compositions needed for the three planets to host stable atmospheres. By investigating the photochemical production and rainout of prebiotic molecules (with a focus on formaldehyde) within thermally stable atmospheres, we finally assess the prebiotic potential and early habitability of the three planets.