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Exoplanet science is one of the most thriving fields of modern astrophysics. A major goal is the atmospheric characterization of dozens of small, terrestrial exoplanets in order to search for signatures in their atmospheres that indicate biological activity, assess their ability to provide conditions for life as we know it, and investigate their expected atmospheric diversity. None of the currently adopted projects or missions, from ground or in space, can address these goals. In this White Paper, submitted to ESA in response to the Voyage 2050 Call, we argue that a large space-based mission designed to detect and investigate thermal emission spectra of terrestrial exoplanets in the mid-infrared wavelength range provides unique scientific potential to address these goals and surpasses the capabilities of other approaches. While NASA might be focusing on large missions that aim to detect terrestrial planets in reflected light, ESA has the opportunity to take leadership and spearhead the development of a large mid-infrared exoplanet mission within the scope of the "Voyage 2050" long-term plan establishing Europe at the forefront of exoplanet science for decades to come. Given the ambitious science goals of such a mission, additional international partners might be interested in participating and contributing to a roadmap that, in the long run, leads to a successful implementation. A new, dedicated development program funded by ESA to help reduce development and implementation cost and further push some of the required key technologies would be a first important step in this direction. Ultimately, a large mid-infrared exoplanet imaging mission will be needed to help answer one of humankind's most fundamental questions: "How unique is our Earth?", Competing Interests: Conflict of interestThe authors declare that there is no conflict of interest., (© The Author(s) 2021.)

We describe how environmental context can help determine whether oxygen (O 2 ) detected in extrasolar planetary observations is more likely to have a biological source. Here we provide an in-depth, interdisciplinary example of O 2 biosignature identification and observation, which serves as the prototype for the development of a general framework for biosignature assessment. Photosynthetically generated O 2 is a potentially strong biosignature, and at high abundance, it was originally thought to be an unambiguous indicator for life. However, as a biosignature, O 2 faces two major challenges: (1) it was only present at high abundance for a relatively short period of Earth's history and (2) we now know of several potential planetary mechanisms that can generate abundant O 2 without life being present. Consequently, our ability to interpret both the presence and absence of O 2 in an exoplanetary spectrum relies on understanding the environmental context. Here we examine the coevolution of life with the early Earth's environment to identify how the interplay of sources and sinks may have suppressed O 2 release into the atmosphere for several billion years, producing a false negative for biologically generated O 2 . These studies suggest that planetary characteristics that may enhance false negatives should be considered when selecting targets for biosignature searches. We review the most recent knowledge of false positives for O 2 , planetary processes that may generate abundant atmospheric O 2 without a biosphere. We provide examples of how future photometric, spectroscopic, and time-dependent observations of O 2 and other aspects of the planetary environment can be used to rule out false positives and thereby increase our confidence that any observed O 2 is indeed a biosignature. These insights will guide and inform the development of future exoplanet characterization missions. Key Words: Biosignatures-Oxygenic photosynthesis-Exoplanets-Planetary atmospheres. Astrobiology 18, 630-662.

Near-infrared images of the Venus night side show bright contrast features that move from east to west, in the direction of the cloud-top atmospheric superrotation. Recently acquired images of the Venus night side along with earlier spectroscopic observations allow identification of the mechanisms that produce these features, their level of formation, and the wind velocities at those levels. The features are detectable only at wavelengths near 1.74 and 2.3 micrometers, in narrow atmospheric windows between the CO(2) and H(2)O bands. The brightest features have brightness temperatures near 480 Kelvin, whereas the darkest features are more than 50 Kelvin cooler. Several factors suggest that this radiation is emitted by hot gases at altitudes below 35 kilometers in the Venus atmosphere. The feature contrasts are produced as this thermal radiation passes through a higher, cooler, atmospheric layer that has horizontal variations in transparency. The 6.5-day east-west rotation period of the features indicates that equatorial wind speeds are near 70 meters per second in this upper layer. Similar wind speeds have been measured by entry probes and balloons at altitudes between 50 and 55 kilometers in the middle cloud layer. The bright features indicate that there are partial clearings in this cloud deck. The presence of these clearings could decrease the efficiency of the atmospheric greenhouse that maintains the high surface temperatures on Venus.