Abstract
Abstract Current investigations of exoplanet biosignatures have focused on static evidence of life, such as the presence of biogenic gases like O2 or CH4. However, the expected diversity of terrestrial planet atmospheres and the likelihood of both “false positives” and “false negatives” for conventional biosignatures motivate exploration of additional life detection strategies, including time-varying signals. Seasonal variation in atmospheric composition is a biologically modulated phenomenon on Earth that may occur elsewhere because it arises naturally from the interplay between the biosphere and time-variable insolation. The search for seasonality as a biosignature would avoid many assumptions about specific metabolisms and provide an opportunity to directly quantify biological fluxes—allowing us to characterize, rather than simply recognize, biospheres on exoplanets. Despite this potential, there have been no comprehensive studies of seasonality as an exoplanet biosignature. Here, we provide a foundation for further studies by reviewing both biological and abiological controls on the magnitude and detectability of seasonality of atmospheric CO2, CH4, O2, and O3 on Earth. We also consider an example of an inhabited world for which atmospheric seasonality may be the most notable expression of its biosphere. We show that life on a low O2 planet like the weakly oxygenated mid-Proterozoic Earth could be fingerprinted by seasonal variation in O3 as revealed in its UV Hartley–Huggins bands. This example highlights the need for UV capabilities in future direct-imaging telescope missions (e.g., LUVOIR/HabEx) and illustrates the diagnostic importance of studying temporal biosignatures for exoplanet life detection/characterization.
Highlights
The search for life beyond our solar system will focus on the identification of biosignature gases in exoplanet atmospheres or perhaps surface signatures of life (Des Marais et al 2002; Schwieterman et al 2018)
Atmospheric seasonality is biologically modulated on Earth and is likely to occur on other inhabited worlds regardless of specific metabolic substrates or products
Atmospheric seasonality is a biologically modulated phenomenon on Earth, and biogenic seasonality may be common among inhabited worlds
Summary
The search for life beyond our solar system will focus on the identification of biosignature gases in exoplanet atmospheres or perhaps surface signatures of life (Des Marais et al 2002; Schwieterman et al 2018). Earth’s oxygenation trajectory demonstrates that extensive biological production of O2 in surface habitats need not result in detectable levels of O2 in the atmosphere, and productive biospheres may be “invisible” to conventional biosignature analysis (Gebauer et al 2017; Reinhard et al 2017). This potential for both false positives and negatives highlights the need to develop alternative biosignatures, including consideration of time-dependent signals. Atmospheric seasonality is biologically modulated on Earth and is likely to occur on other inhabited worlds regardless of specific metabolic substrates or products. We end with a discussion of both the advantages and observational challenges facing seasonal biosignatures
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