# Earth as an Exoplanet: I. Time variable thermal emission using spatially resolved MODIS data

**Authors:** Jean-Noël Mettler, Sascha P. Quanz, Ravit Helled  
**arXiv:** [2010.02589](https://arxiv.org/abs/2010.02589)  
**Date:** 2020-10-06  
**Summary page:** https://www.summarizepaper.com/en/paper/2010.02589v1/

## Key points

- Study focuses on exploration and understanding of Earth's spectral signatures and variability
- Researchers use MODIS instrument data from Aqua satellite
- Data collected for 15 years in the 3.66-14.40 microns range
- Five different locations studied: Amazon Rainforest, Antarctica, Arctic, Indian Ocean, Sahara Desert
- Analysis of flux levels and variations as a function of wavelength and surface type
- Investigating detection of periodic signals indicating Earth's tilted rotation axis
- Viewing geometry plays a crucial role in analyzing thermal emission data
- Earth's spectrum varies depending on dominant surface type underneath
- Absorption bands from carbon dioxide (15 microns) and ozone (9.65 microns) less pronounced in polar regions' data
- Estimating correct abundance levels for these molecules might be challenging in polar regions
- Time-resolved thermal emission spectrum encodes information about seasons/planetary obliquity
- Significance depends on viewing geometry and spectral band considered
- Importance of exploring Earth's spectral signatures for knowledge of habitable worlds beyond our planet
- Insights into designing future instruments and missions for exoplanet exploration
- Acknowledgment of limitations related to viewing geometry and specific surface types

## Summary

The study focuses on the exploration and understanding of Earth's spectral signatures and variability in order to inform the design of future instruments and missions. The researchers utilize Earth observation data collected by the MODIS instrument aboard the Aqua satellite, specifically 15 years of thermal emission observations in the 3.66-14.40 microns range for five different locations on Earth: Amazon Rainforest, Antarctica, Arctic, Indian Ocean, and Sahara Desert. By analyzing flux levels and variations as a function of wavelength and surface type (climate zone and surface thermal properties), the researchers investigate whether periodic signals indicating Earth's tilted rotation axis can be detected. They find that viewing geometry plays a crucial role in analyzing thermal emission data since Earth's spectrum varies significantly depending on the dominant surface type underneath. Additionally, they observe that typically strong absorption bands from carbon dioxide (15 microns) and ozone (9.65 microns) are less pronounced and partially absent in polar regions' data. This suggests that estimating correct abundance levels for these molecules might be challenging in such cases. Furthermore, the researchers discover that the time-resolved thermal emission spectrum encodes information about seasons/planetary obliquity but note that its significance depends on both viewing geometry and spectral band considered. Overall, this study highlights the importance of exploring and understanding Earth's spectral signatures to enhance our knowledge of habitable worlds beyond our own planet while providing valuable insights into designing future instruments and missions for exoplanet exploration while also acknowledging potential limitations related to viewing geometry and specific surface types.