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. Access through your institution Buy or subscribe This is a preview of subscription content, access via your

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* Learn about institutional subscriptions * Read our FAQs * Contact customer support SIMILAR CONTENT BEING VIEWED BY OTHERS LABORATORY PREDICTIONS FOR THE NIGHT-SIDE SURFACE ICE GLOW OF

EUROPA Article 09 November 2020 POTENTIAL FOR OBSERVING GEOLOGICAL DIVERSITY FROM MID-INFRARED SPECTRA OF ROCKY EXOPLANETS Article 14 November 2024 IN-ICE LIGHT MEASUREMENTS DURING THE

MOSAIC EXPEDITION Article Open access 27 June 2024 REFERENCES * 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  * 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  * 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  * 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  * Stam, D. M. Spectropolarimetric signatures of Earth-like extrasolar planets. _Astron. Astrophys._ 482, 989–1007 (2008) Article  ADS  Google Scholar  *

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  * Keller,

C. U. et al. EPOL: the exoplanet polarimeter for EPICS at the E-ELT. _Proc. SPIE_ 7735, 77356G (2010) Article  Google Scholar  * Bailey, J. Rainbows, polarization, and the search for

habitable planets. _Astrobiology_ 7, 320–332 (2007) Article  ADS  CAS  Google Scholar  * 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  * 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  * 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  * 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

  * 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  * Dollfus, A. Études des planètes par la polarisation de leur lumière. _Suppléments

aux Annales d’Astrophysique_ 4, 3–114 (1957) ADS  Google Scholar  * 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  * Appenzeller, I. et al. Successful

commissioning of FORS1 — the first optical instrument on the VLT. _ESO Messenger_ 94, 1–6 (1998) ADS  Google Scholar  * Giovanni portal.

http://disc.sci.gsfc.nasa.gov/giovanni/overview/index.html * 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  * 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  * 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  * 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  * 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  * 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  * 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  Download references

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. AUTHOR INFORMATION AUTHORS AND AFFILIATIONS * European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile Michael F. Sterzik * Armagh Observatory, College

Hill, Armagh BT61 9DG, UK, Stefano Bagnulo * Instituto de Astrofisica de Canarias, Via Lactea s/n, E38205 La Laguna, Tenerife, Spain, Enric Palle Authors * Michael F. Sterzik View author

publications You can also search for this author inPubMed Google Scholar * Stefano Bagnulo View author publications You can also search for this author inPubMed Google Scholar * Enric Palle

View author publications You can also search for this author inPubMed Google Scholar 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. ETHICS DECLARATIONS

COMPETING INTERESTS The authors declare no competing financial interests. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION This file contains Supplementary Text, Supplementary Figure 1

and legend and Supplementary References. (PDF 153 kb) POWERPOINT SLIDES POWERPOINT SLIDE FOR FIG. 1 POWERPOINT SLIDE FOR FIG. 2 RIGHTS AND PERMISSIONS Reprints and permissions ABOUT THIS

ARTICLE CITE THIS ARTICLE Sterzik, M., Bagnulo, S. & Palle, E. Biosignatures as revealed by spectropolarimetry of Earthshine. _Nature_ 483, 64–66 (2012).

https://doi.org/10.1038/nature10778 Download citation * Received: 16 September 2011 * Accepted: 09 December 2011 * Published: 29 February 2012 * Issue Date: 01 March 2012 * DOI:

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