# Earth as an Exoplanet. II. Earth's Time-variable Thermal Emission and Its Atmospheric Seasonality of Bioindicators

**Authors:** Jean-Noel Mettler, Sascha P. Quanz, Ravit Helled, Stephanie L. Olson, Edward W. Schwieterman  
**arXiv:** [2210.05414](https://arxiv.org/abs/2210.05414)  
**Date:** 2022-10-11  
**Summary page:** https://www.summarizepaper.com/en/paper/2210.05414v3/

## Key points

- The study investigates Earth's disk-integrated mid-infrared thermal emission spectrum
- A dataset of 2690 disk-integrated thermal emission spectra for four different full-disk observing geometries over four consecutive years was compiled using the Atmospheric Infrared Sounder aboard the Aqua satellite
- Significant seasonal variability in Earth's thermal emission spectrum was found, and the strength of spectral features of bio-indicators depends strongly on both season and viewing geometry
- There is a strong spectral degeneracy with respect to viewing geometry, and multi-epoch measurements may be required to fully characterize planetary environments
- Mid-infrared observations remain advantageous over reflected light observations since they provide consistent results irrespective of when the planet is measured
- There is no representative, disk-integrated thermal emission spectrum of Earth due to significant seasonal variability in its appearance caused by different surface types contributing differently depending on their abundance fraction and distribution as well as their thermal properties.
- The study highlights the importance of considering seasonality when characterizing exoplanets using mid-infrared observations.

## Summary

In this study, the authors investigate the dependence of Earth's disk-integrated mid-infrared thermal emission spectrum on observation geometries and seasonality. They compiled an exclusive dataset containing 2690 disk-integrated thermal emission spectra for four different full-disk observing geometries over four consecutive years using the Atmospheric Infrared Sounder aboard the Aqua satellite. The spectra were derived from 2378 spectral channels in the wavelength range from 3.75 to 15.4 micron and were recorded at a high temporal resolution. The authors found that there is significant seasonal variability in Earth's thermal emission spectrum, and the strength of spectral features of bio-indicators, such as N2O, CH4, O3, and CO2 depends strongly on both season and viewing geometry. They also found a strong spectral degeneracy with respect to viewing geometry and multi-epoch measurements may be required to fully characterize planetary environments. Despite variations in flux and strength of absorption features in the disk-integrated data being small (typically ≤10%), disentangling these variations from noise in future exoplanet observations will be challenging. However, mid-infrared observations remain advantageous over reflected light observations since they provide consistent results irrespective of when the planet is measured. The authors also investigated how Earth's MIR spectral appearance changes as a function of viewing geometry, seasons, and phase angles. They found that there is no representative, disk-integrated thermal emission spectrum of Earth due to significant seasonal variability in its appearance caused by different surface types (oceans, deserts, ice as well as vegetation and clouds) contributing differently depending on their abundance fraction and distribution as well as their thermal properties. Overall, this study provides valuable insights into how observation geometries impact Earth's thermal emission spectrum and highlights the importance of considering seasonality when characterizing exoplanets using mid-infrared observations. By compiling an exclusive dataset containing 2690 disk-integrated thermal emission spectra for four different full-disk observing geometries over four consecutive years using the Atmospheric Infrared Sounder aboard the Aqua satellite at a high temporal resolution, this study has enabled us to better understand how Earth’s MIR spectral appearance changes due to seasonal variability caused by different surface types contributing differently depending on their abundance fraction and distribution as well as their thermal properties; thus providing valuable insights into how observation geometries can affect exoplanet characterization via mid-infrared observations.