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Biosignatures as revealed by spectropolarimetry of Earthshine

Nature volume 483, pages 64–66 (2012)Cite this article

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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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References

  1. Sagan, C., Thompson, W. R., Carlson, R. & Gurnett, D. &. Hord, C. A search for life on Earth from the Galileo spacecraft. Nature 365, 715–721 (1993)

    Article  ADS  CAS  Google Scholar 

  2. Coffeen, D. L. Polarization and scattering characteristics in the atmospheres of Earth, Venus, and Jupiter. J. Opt. Soc. Am. 69, 1051–1064 (1979)

    Article  ADS  Google Scholar 

  3. Aben, I., Helderman, F., Stam, D. M. & Stammes, P. Spectral fine-structure in the polarization of skylight. Geophys. Res. Lett. 26, 591–594 (1999)

    Article  ADS  CAS  Google Scholar 

  4. Boesche, E. et al. Polarization of skylight in the O2A band: effects of aerosol properties. Appl. Opt. 47, 3467–3480 (2008)

    Article  ADS  Google Scholar 

  5. Stam, D. M. Spectropolarimetric signatures of Earth-like extrasolar planets. Astron. Astrophys. 482, 989–1007 (2008)

    Article  ADS  Google Scholar 

  6. Buenzli, E. & Schmid, H. M. A grid of polarization models for Rayleigh scattering planetary atmospheres. Astron. Astrophys. 504, 259–276 (2009)

    Article  ADS  Google Scholar 

  7. Keller, C. U. et al. EPOL: the exoplanet polarimeter for EPICS at the E-ELT. Proc. SPIE 7735, 77356G (2010)

    Article  Google Scholar 

  8. Bailey, J. Rainbows, polarization, and the search for habitable planets. Astrobiology 7, 320–332 (2007)

    Article  ADS  CAS  Google Scholar 

  9. Williams, D. M. & Gaidos, E. Detecting the glint of starlight on the oceans of distant planets. Icarus 195, 927–937 (2008)

    Article  ADS  Google Scholar 

  10. Arnold, L., Gillet, S., Lardiere, O., Riaud, P. & Schneider, J. A test for the search for life on extrasolar planets. Looking for the terrestrial vegetation signature in the Earthshine spectrum. Astron. Astrophys. 392, 231–237 (2002)

    Article  ADS  CAS  Google Scholar 

  11. Woolf, N. J., Smith, P. S., Traub, W. A. & Jucks, K. W. The spectrum of Earthshine: a pale blue dot observed from the ground. Astrophys. J. 574, 430–433 (2002)

    Article  ADS  CAS  Google Scholar 

  12. Seager, S., Turner, E. L., Schafer, J. & Ford, E. B. Vegetation's red edge: a possible spectroscopic biosignature of extraterrestrial plants. Astrobiology 5, 372–390 (2005)

    Article  ADS  CAS  Google Scholar 

  13. Montanes-Rodriguez, P., Palle, E., Goode, P. R. & Martin-Torres, F. J. Vegetation signature in the observed globally integrated spectrum of Earth considering simultaneous cloud data: applications for extrasolar planets. Astrophys. J. 651, 544–552 (2006)

    Article  ADS  Google Scholar 

  14. Dollfus, A. Études des planètes par la polarisation de leur lumière. Suppléments aux Annales d’Astrophysique 4, 3–114 (1957)

    ADS  Google Scholar 

  15. Wolstencroft, R. D. & Breon, F.-M. in Astronomical Polarimetry: Current Status and Future Directions (eds Adamson, A., Aspin, C., Davis, C. J., & Fujiyoshi, T. ) 211–212 (ASP Conf. Ser. Vol. 343, Astronomical Society of the Pacific, 2005)

    Google Scholar 

  16. Appenzeller, I. et al. Successful commissioning of FORS1 — the first optical instrument on the VLT. ESO Messenger 94, 1–6 (1998)

    ADS  Google Scholar 

  17. Giovanni portal. http://disc.sci.gsfc.nasa.gov/giovanni/overview/index.html

  18. Pallé, E., Zapatero Osorio, M. R., Barrena, R., Montanes-Rodriguez, P. & Martin, E. L. Earth's transmission spectrum from lunar eclipse observations. Nature 459, 814–816 (2009)

    Article  ADS  Google Scholar 

  19. Fox, G. K. et al. Solar system observations by the Wisconsin Ultraviolet Photopolarimeter Experiment — III. The first ultraviolet linear spectropolarimetry of the Moon. Mon. Not. R. Astron. Soc. 298, 303–309 (1998)

    Article  ADS  Google Scholar 

  20. Shkuratov, Y. et al. Optical measurements of the Moon as a tool to study its surface. Planet. Space Sci. 59, 1326–1371 (2011)

    Article  ADS  CAS  Google Scholar 

  21. Goloub, P., Deuze, J. L., Herman, M. & Fouquart, Y. Analysis of the POLDER polarization measurements performed over cloud covers. IEEE Trans. Geosci. Rem. Sens. 32, 78–88 (1994)

    Article  ADS  Google Scholar 

  22. Karalidi, T., Stam, D. M. & Hovenier, J. W. Flux and polarisation spectra of water clouds on exoplanets. Astron. Astrophys. 530, A69 (2011)

    Article  ADS  Google Scholar 

  23. Kokhanovsky, A., Platnick, S. & King, M. D. in The Remote Sensing of Tropospheric Composition from Space (eds Burrows, J. P. et al.)231–257 (Springer, 2011)

    Book  Google Scholar 

  24. Tinetti, G. et al. Detectability of planetary characteristics in disk-averaged spectra II: synthetic spectra and light-curves of Earth. Astrobiology 6, 881–900 (2006)

    Article  ADS  CAS  Google Scholar 

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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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Authors and Affiliations

  1. European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile

    Michael F. Sterzik

  2. Armagh Observatory, College Hill, Armagh BT61 9DG, UK,

    Stefano Bagnulo

  3. Instituto de Astrofisica de Canarias, Via Lactea s/n, E38205 La Laguna, Tenerife, Spain,

    Enric Palle

Authors
  1. Michael F. Sterzik
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  2. Stefano Bagnulo
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  3. Enric Palle
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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.

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Correspondence to Michael F. Sterzik.

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The authors declare no competing financial interests.

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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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