Biosignatures as revealed by spectropolarimetry of Earthshine

Abstract

Low-resolution intensity spectra of Earth’s atmosphere obtained from space reveal strong signatures of life (‘biosignatures’), such as molecular oxygen and methane with abundances far from chemical equilibrium, as well as the presence of a ‘red edge’ (a sharp increase of albedo for wavelengths longer than 700 nm) caused by surface vegetation1. Light passing through the atmosphere is strongly linearly polarized by scattering (from air molecules, aerosols and cloud particles) and by reflection (from oceans and land2). Spectropolarimetric observations of local patches of Earth’s sky light from the ground contain signatures of oxygen, ozone and water, and are used to characterize the properties of clouds and aerosols3,4. When applied to exoplanets, ground-based spectropolarimetry can better constrain properties of atmospheres and surfaces than can standard intensity spectroscopy5,6,7,8,9. Here we report disk-integrated linear polarization spectra of Earthshine, which is sunlight that has been first reflected by Earth and then reflected back to Earth by the Moon10,11,12,13. The observations allow us to determine the fractional contribution of clouds and ocean surface, and are sensitive to visible areas of vegetation as small as 10 per cent. They represent a benchmark for the diagnostics of the atmospheric composition, mean cloud height and surfaces of exoplanets.

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Figure 1: Polarization spectra observed on 25 April 2011.
Figure 2: Polarization spectra observed on 10 June 2011.

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Acknowledgements

This Letter is based on data collected with the Very Large Telescope under ESO programme 87.C-0040(A) and (B). E.P. acknowledges support from the Spanish MICIIN, grant CGL2009-10641.

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Contributions

M.F.S. was the principal investigator of the programme, contributing to observation preparation and execution, data analysis and interpretation, comparison with models, and writing of the manuscript. S.B. was a co-investigator, contributing to observation preparation and execution, data reduction and interpretation, error analysis, comparison with models, and also contributing to the manuscript. E.P. was a co-investigator, contributing to observation preparation, data interpretation, and derivations of actual Earth surface fractions during the observations.

Corresponding author

Correspondence to Michael F. Sterzik.

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The authors declare no competing financial interests.

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This file contains Supplementary Text, Supplementary Figure 1 and legend and Supplementary References. (PDF 153 kb)

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Sterzik, M., Bagnulo, S. & Palle, E. Biosignatures as revealed by spectropolarimetry of Earthshine. Nature 483, 64–66 (2012). https://doi.org/10.1038/nature10778

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