Presentation #105.03 in the session Future 1.
The search for life on habitable exoplanets is one of the key objectives of 21st century astrophysics. Due to the impact of life on the geochemical environment and the composition of the atmosphere throughout billions of years of coevolution on Earth, it is thought that alien biospheres should be detectable via spectroscopy. Therefore, the search for life will concentrate on the identification of three types of exoplanet biosignatures which can be found imprinted in a planet’s transmission, reflectance or thermal emission spectrum: biogenic gases, surface signatures and temporal modulations of observable quantities like gas concentration. Current investigations of such biosignatures have focused on static evidence of life, such as the presence and long-term coexistence of biogenic gases like oxygen or methane. Yet, the variety of terrestrial planet atmospheres and the probability of both “false positives” and “false negatives” for conventional biosignatures, motivate the exploration of additional strategies for detecting life, including time-varying signals like seasonal variations in atmospheric abundances (e.g., CO2). It inevitably results from the interplay between the biosphere and time-variable insolation. Despite its significant scientific potential, there have been no extensive studies of seasonality as a biosignature based on empirical Earth-observation data.
Here we present results from our study of Earth’s time variable thermal emission spectrum and its atmospheric seasonality. Using spatially resolved Earth observation data from the AIRS instrument aboard the Aqua satellite, we construct whole-disk views and derive disk-integrated mid-infrared thermal emission spectra (3.75–15.4 μm, 2378 channels) for four different viewing geometries (North & South Pole, Africa & Pacific centred equatorial view) over a time period of four years. We assess the sensitivity of Earth’s disk-averaged thermal emission spectrum to observing geometries, phase angles and integration times much longer than Earth’s rotation period as well as investigate which spectral features of habitability and life are impacted by observable seasonality. We address the question whether such time-varying signals of a “breathing“ atmosphere would be detectable to a remote observer and discuss potential constraints for future instruments and missions.