Surface and Temporal Biosignatures

Living reference work entry


Recent discoveries of potentially habitable exoplanets have ignited the prospect of spectroscopic investigations of exoplanet surfaces and atmospheres for signs of life. This chapter provides an overview of potential surface and temporal exoplanet biosignatures, reviewing Earth analogues and proposed applications based on observations and models. The vegetation red-edge (VRE) remains the most well-studied surface biosignature. Extensions of the VRE, spectral “edges” produced in part by photosynthetic or nonphotosynthetic pigments, may likewise present potential evidence of life. Polarization signatures have the capacity to discriminate between biotic and abiotic “edge” features in the face of false positives from bandgap-generating material. Temporal biosignatures – modulations in measurable quantities such as gas abundances (e.g., CO2), surface features, or emission of light (e.g., fluorescence, bioluminescence) that can be directly linked to the actions of a biosphere – are in general less well studied than surface or gaseous biosignatures. However, remote observations of Earth’s biosphere nonetheless provide proofs of concept for these techniques and are reviewed here. Surface and temporal biosignatures provide complementary information to gaseous biosignatures and, while likely more challenging to observe, would contribute information inaccessible from study of the time-averaged atmospheric composition alone.



The author gratefully acknowledges support from the NASA Astrobiology Institute and the NASA Postdoctoral Program administered by the Universities Space Research Association. The author also appreciatively acknowledges support from and access to tools developed by the NAI Virtual Planetary Laboratory under Cooperative Agreement Number NNA13AA93A. This chapter was improved by helpful comments from Kim Bott and Victoria Meadows.


  1. Archetti M, Döring TF, Hagen SB et al (2009) Unravelling the evolution of autumn colours: an interdisciplinary approach. Trends Ecol Evol 24:166–173. CrossRefGoogle Scholar
  2. Arney G, Domagal-Goldman SD, Meadows VS (2017) Organic haze as a biosignature in anoxic Earth-like atmospheres. Astrobiology.
  3. Arnold L (2008) Earthshine observation of vegetation and implication for life detection on other planets. Space Sci Rev 135:323–333. ADSCrossRefGoogle Scholar
  4. Arnold L, Gillet S, Lardiere O et al (2002) A test for the search for life on extrasolar planets: looking for the terrestrial vegetation signature in the Earthshine spectrum. Astron Astrophys 237:7. Google Scholar
  5. Arnold L, Bréon F-M, Brewer S (2009) The Earth as an extrasolar planet: the vegetation spectral signature today and during the last Quaternary climatic extrema. Int J Astrobiol 8:81. CrossRefGoogle Scholar
  6. Bailey J (2007) Rainbows, polarization, and the search for habitable planets. Astrobiology 7:320–332. ADSCrossRefGoogle Scholar
  7. Baldridge AM, Hook SJ, Grove CI, Rivera G (2009) The ASTER spectral library version 2.0. Remote Sens Environ 113:711–715. ADSCrossRefGoogle Scholar
  8. Berdyugina SV, Kuhn JR, Harrington DM et al (2016) Remote sensing of life: polarimetric signatures of photosynthetic pigments as sensitive biomarkers. Int J Astrobiol 15:45–56. CrossRefGoogle Scholar
  9. Blankenship RE (2002) Molecular mechanisms of photosynthesis. Blackwell, OxfordCrossRefGoogle Scholar
  10. Blankenship RE (2010) Early evolution of photosynthesis. Plant Physiol 154:434–438. CrossRefGoogle Scholar
  11. Brandt TD, Spiegel DS (2014) Prospects for detecting oxygen, water, and chlorophyll on an exo-Earth. Proc Natl Acad Sci 111:13278–13283. ADSCrossRefGoogle Scholar
  12. Bristow LA, Mohr W, Ahmerkamp S, Kuypers MMM (2017) Nutrients that limit growth in the ocean. Curr Biol 27:R474–R478. CrossRefGoogle Scholar
  13. Buick R (2008) When did oxygenic photosynthesis evolve? Philos Trans R Soc London Ser B 363:2731–2743. CrossRefGoogle Scholar
  14. Buschmann C (2002) Interpretation of the fluorescence signatures from vegetationGoogle Scholar
  15. Chen M, Blankenship RE (2011) Expanding the solar spectrum used by photosynthesis. Trends Plant Sci 16:427–431. CrossRefGoogle Scholar
  16. Cicerone RJ, Oremland RS (1988) Biogeochemical aspects of atmospheric methane. Glob Biogeochem Cycles 2:299–327. ADSCrossRefGoogle Scholar
  17. Clark RN, Swayze GA, Wise R et al (2007) USGS digital spectral library splib06a: In: U.S. geological survey, digital data series 231., Accessed 5 July 2013
  18. Cockell CS (2014) Habitable worlds with no signs of life. Philos Transact A Math Phys Eng Sci 372:20130082. CrossRefGoogle Scholar
  19. Cockell CS, Raven JA (2004) Zones of photosynthetic potential on Mars and the early Earth. Icarus 169:300–310. ADSCrossRefGoogle Scholar
  20. Cockell CS, Kaltenegger L, Raven JA (2009) Cryptic photosynthesis – extrasolar planetary oxygen without a surface biological signature. Astrobiology 9:623–636. ADSCrossRefGoogle Scholar
  21. Cogdell RJ, van Grondelle R (2003) The light-harvesting system of purple bacteria. In: Green BB, Parson WW (eds) Light-harvesting antennas in photosynthesis, vol 13. Springer, Dordrecht, pp 169–194CrossRefGoogle Scholar
  22. Cowan NB (2009) Exo-cartography: time-resolved photometry of exoplanetsGoogle Scholar
  23. Cowan NB, Strait TE (2013) Determining reflectance spectra of surfaces and clouds on exoplanets. Astrophys J 765:L17. ADSCrossRefGoogle Scholar
  24. Crow CA, McFadden LA, Robinson T et al (2011) Views from EPOXI: colors in our solar system as an analog for extrasolar planets. Astrophys J 729:130. ADSCrossRefGoogle Scholar
  25. Dalton JB, Palmer-Moloney LJ, Rogoff D et al (2009) Remote monitoring of hypersaline environments in San Francisco Bay, CA, USA. Int J Remote Sens 30:2933–2949. CrossRefGoogle Scholar
  26. DasSarma S (2006) Extreme halophiles are models for astrobiology. Microbe 1:120–126Google Scholar
  27. Des Marais D (2000) When did photosynthesis emerge on Earth? Science 289:1703–1705. Google Scholar
  28. Des Marais DJ, Walter MR (1999) Astrobiology: exploring the origins, evolution, and distribution of life in the universe. Annu Rev Ecol Syst 30:397–420CrossRefGoogle Scholar
  29. Des Marais DJ, Harwit MO, Jucks KW et al (2002) Remote sensing of planetary properties and biosignatures on extrasolar terrestrial planets. Astrobiology 2:153–181. ADSCrossRefGoogle Scholar
  30. Des Marais DJ, Nuth J, Allamandola LJ et al (2008) The NASA astrobiology roadmap. Astrobiology 8:715–730. ADSCrossRefGoogle Scholar
  31. Dixon JM, Taniguchi M, Lindsey JS (2005) PhotochemCAD 2: a refined program with accompanying spectral databases for photochemical calculations. Photochem Photobiol 81:212–213. CrossRefGoogle Scholar
  32. Domagal-Goldman SD, Meadows VS, Claire MW, Kasting JF (2011) Using biogenic sulfur gases as remotely detectable biosignatures on anoxic planets. Astrobiology 11:419–441. ADSCrossRefGoogle Scholar
  33. Ford EB, Seager S, Turner EL (2001) Characterization of extrasolar terrestrial planets from diurnal photometric variability. Nature 412:885–887. ADSCrossRefGoogle Scholar
  34. Fujii Y, Kawahara H, Suto Y et al (2010) Colors of a second Earth: estimating the fractional areas of ocean, land, and vegetation of Earth-like exoplanets. Astrophys J 715:866–880. ADSCrossRefGoogle Scholar
  35. Fujii Y, Kawahara H, Suto Y et al (2011) Colors of a second Earth. II. Effects of clouds on photometric characterization of Earth-like exoplanets. Astrophys J 738:184. ADSCrossRefGoogle Scholar
  36. Fujii Y, Lustig-Yaeger J, Cowan NB (2017) Rotational spectral unmixing of exoplanets: degeneracies between surface colors and geography. Astron J 154:189. ADSCrossRefGoogle Scholar
  37. Fujii Y, Angerhausen D, Deitrick R et al (2018) Exoplanet biosignatures: observational prospects. Astrobiology (in review)Google Scholar
  38. Gates DM, Keegan HJ, Schleter JC, Weidner VR (1965) Spectral properties of plants. Appl Opt 4:11. ADSCrossRefGoogle Scholar
  39. Haddock SHD, Moline MA, Case JF (2010) Bioluminescence in the sea. Annu Rev Mar Sci 2:443–493. ADSCrossRefGoogle Scholar
  40. Hall CAS, Ekdahl CA, Wartenberg DE (1975) A fifteen-year record of biotic metabolism in the northern hemisphere. Nature 255:136–138. ADSCrossRefGoogle Scholar
  41. Hamdani S, Arnold L, Foellmi C et al (2006) Biomarkers in disk-averaged near-UV to near-IR Earth spectra using Earthshine observations. Astron Astrophys 460:617–624. ADSCrossRefGoogle Scholar
  42. Harman CE, Schwieterman EW, Schottelkotte JC, Kasting JF (2015) Abiotic O2 levels on planets around F, G, K, and M stars: possible false positives for life? Astrophys J 812:137. ADSCrossRefGoogle Scholar
  43. Hegde S, Paulino-Lima IG, Kent R et al (2015) Surface biosignatures of exo-Earths: remote detection of extraterrestrial life. Proc Natl Acad Sci 112:3886–3891. ADSCrossRefGoogle Scholar
  44. Janik E, Grudziński W, Gruszecki WI, Krupa Z (2008) The xanthophyll cycle pigments in Secale cereale leaves under combined Cd and high light stress conditions. J Photochem Photobiol B Biol 90:47–52. CrossRefGoogle Scholar
  45. Joiner J, Yoshida Y, Vasilkov AP et al (2011) First observations of global and seasonal terrestrial chlorophyll fluorescence from space. Biogeosciences 8:637–651. ADSCrossRefGoogle Scholar
  46. Karkoschka E (1994) Spectrophotometry of the Jovian planets and Titan at 300- to 1000-nm wavelength: the methane spectrum. Icarus 111:174–192. ADSCrossRefGoogle Scholar
  47. Keeling RF, Shertz SR (1992) Seasonal and interannual variations in atmospheric oxygen and implications for the global carbon cycle. Nature 358:723–727. ADSCrossRefGoogle Scholar
  48. Keeling CD, Chin JFS, Whorf TP (1996) Increased activity of northern vegetation inferred from atmospheric CO2 measurements. Nature 382:146–149. ADSCrossRefGoogle Scholar
  49. Keeling RF, Stephens BB, Najjar RG et al (1998) Seasonal variations in the atmospheric O2/N2 ratio in relation to the kinetics of air–sea gas exchange. Glob Biogeochem Cycles 12:141–163. ADSCrossRefGoogle Scholar
  50. Kenrick P, Crane PR (1997) The origin and early evolution of plants on land. Nature 389:33–39. ADSCrossRefGoogle Scholar
  51. Khalil MAK, Rasmussen RA (1983) Sources, sinks, and seasonal cycles of atmospheric methane. J Geophys Res Oceans 88:5131–5144. CrossRefGoogle Scholar
  52. Kiang NY, Segura A, Tinetti G et al (2007a) Spectral signatures of photosynthesis. II. Coevolution with other stars and the atmosphere on extrasolar worlds. Astrobiology 7:252–274. ADSCrossRefGoogle Scholar
  53. Kiang NY, Siefert J, Govindjee, Blankenship RE (2007b) Spectral signatures of photosynthesis. I. Review of Earth organisms. Astrobiology 7:222–251. ADSCrossRefGoogle Scholar
  54. Kiang NY, Goddard N, Studies S (2014) Looking for life elsewhere: photosynthesis and astrobiology. Biochemist 36:24–30Google Scholar
  55. Knipling EB (1970) Physical and physiological basis for the reflectance of visible and near-infrared radiation from vegetation. Remote Sens Environ 1:155–159. ADSCrossRefGoogle Scholar
  56. Kobayashi M, Akutsu S, Fujinuma D et al (2013) Physicochemical properties of chlorophylls in oxygenic photosynthesis – succession of co-factors from anoxygenic to oxygenic photosynthesis. In: Dubinsky Z (ed) Photosynthesis. InTech, Croatia, pp 47–90Google Scholar
  57. Kouveliotou C, Agol E, Batalha N et al (2014) Enduring quests – daring visions (NASA astrophysics in the next three decades). 1401:3741Google Scholar
  58. Krissansen-Totton J, Schwieterman EW, Charnay B et al (2016) Is the pale blue dot unique? Optimized photometric bands for identifying Earth-like exoplanets. Astrophys J 817:31. ADSCrossRefGoogle Scholar
  59. Livengood TA, Deming LD, A’Hearn MF et al (2011) Properties of an Earth-like planet orbiting a Sun-like star: Earth observed by the EPOXI mission. Astrobiology 11:907–930. ADSCrossRefGoogle Scholar
  60. Luger R, Lustig-Yaeger J, Fleming DP et al (2017) The pale green dot: a method to characterize Proxima Centauri b using exo-aurorae. Astrophys J 837:63. ADSCrossRefGoogle Scholar
  61. Madhusudhan N, Burrows A (2012) Analytic models for albedos, phase curves, and polarization of reflected light from exoplanets. Astrophys J 747:25. ADSCrossRefGoogle Scholar
  62. Manning AC, Keeling RF, Katz LE et al (2003) Interpreting the seasonal cycles of atmospheric oxygen and carbon dioxide concentrations at American Samoa Observatory. Geophys Res Lett.
  63. Meadows VS (2006) Modelling the diversity of extrasolar terrestrial planets. Proc Int Astron Union 1:25. CrossRefGoogle Scholar
  64. Meadows VS (2017) Reflections on O2 as a biosignature in exoplanetary atmospheres. Astrobiology.
  65. Meadows V, Seager S (2010) Terrestrial Planet Atmospheres and Biosignatures. In: Seager S (ed) Exoplanets. University of Arizona Press, Tucson, AZ, p 526; pp 441–470. ISBN 978-0-8165-2945-2Google Scholar
  66. Meroni M, Rossini M, Guanter L et al (2009) Remote sensing of solar-induced chlorophyll fluorescence: review of methods and applications. Remote Sens Environ 113:2037–2051. ADSCrossRefGoogle Scholar
  67. Miller SD, Haddock SHD, Elvidge CD, Lee TF (2005) Detection of a bioluminescent milky sea from space. Proc Natl Acad Sci 102:14181–14184. ADSCrossRefGoogle Scholar
  68. Montanes-Rodriguez P, Palle E, Goode PR et al (2005) Globally integrated measurements of the Earth’s visible spectral albedo. Astrophys J 629:1175–1182. ADSCrossRefGoogle Scholar
  69. Montanes-Rodriguez P, Palle E, Goode PR, Martin-Torres FJ (2006) Vegetation signature in the observed globally integrated spectrum of Earth considering simultaneous cloud data: applications for extrasolar planets. Astrophys J 651:544–552. ADSCrossRefGoogle Scholar
  70. Moore CM, Mills MM, Arrigo KR et al (2013) Processes and patterns of oceanic nutrient limitation. Nat Geosci 6:701–710. ADSCrossRefGoogle Scholar
  71. Nicholson JAM, Stolz JF, Pierson BK (1987) Structure of a microbiol mat at Great Sippewissett Marsh, Cape Cod, Massachusetts. FEMS Microbiol Lett 45:343–364CrossRefGoogle Scholar
  72. O’Malley-James JT, Kaltenegger L (2016) Biofluorescent worlds: biological fluorescence as a temporal biosignature for flare star worldsGoogle Scholar
  73. Olson JM (2006) Photosynthesis in the Archean era. Photosynth Res 88:109–117. CrossRefGoogle Scholar
  74. Oren A, Dubinsky Z (1994) On the red coloration of saltern crystallizer ponds. II. Additional evidence for the contribution of halobacterial pigments. Int J Salt Lake Res 3:9–13. CrossRefGoogle Scholar
  75. Papageorgiou GC et al (2007) Chlorophyll a fluorescence: a signature of photosynthesis, vol 19. Springer, DordrechtGoogle Scholar
  76. Patty CHL, Visser LJJ, Ariese F et al (2017) Circular spectropolarimetric sensing of chiral photosystems in decaying leaves. J Quant Spectrosc Radiat Transf 189:303–311. ADSCrossRefGoogle Scholar
  77. Pilcher CB (2003) Biosignatures of early Earths. Astrobiology 3:471–486. ADSCrossRefGoogle Scholar
  78. Rasmussen RA, Khalil MAK (1981) Atmospheric methane (CH4): trends and seasonal cycles. J Geophys Res 86:9826. ADSCrossRefGoogle Scholar
  79. Reinhard CT, Olson SL, Schwieterman EW, Lyons TW (2017) False negatives for remote life detection on ocean-bearing planets: lessons from the early Earth. Astrobiology 17:287–297. ADSCrossRefGoogle Scholar
  80. Robinson TD, Meadows VS, Crisp D (2010) Detecting oceans on extrasolar planets using the glint effect. Astrophys J 721:L67–L71. ADSCrossRefGoogle Scholar
  81. Robinson TD, Meadows VS, Crisp D et al (2011) Earth as an extrasolar planet: Earth model validation using EPOXI Earth observations. Astrobiology 11:393–408. ADSCrossRefGoogle Scholar
  82. Robinson TD, Stapelfeldt KR, Marley MS (2016) Characterizing rocky and gaseous exoplanets with 2 m class space-based coronagraphs. Publ Astron Soc Pac 128:25003. ADSCrossRefGoogle Scholar
  83. Rugheimer S, Kaltenegger L, Segura A et al (2015) Effect of UV radiation on the spectral fingerprints of Earth-like planets orbiting M stars. Astrophys J 809:57. ADSCrossRefGoogle Scholar
  84. Sagan C, Thompson WR, Carlson R et al (1993) A search for life on Earth from the Galileo spacecraft. Nature 365:715–721. ADSCrossRefGoogle Scholar
  85. Sanromá E, Pallé E, García Munõz A (2013) On the effects of the evolution of microbial mats and land plants on the Earth as a planet. Photometric and spectroscopic light curves of paleo-Earths. Astrophys J 766:133. ADSCrossRefGoogle Scholar
  86. Sanromá E, Pallé E, Parenteau MN et al (2014) Characterizing the purple Earth: modeling the globally integrated spectral variability of the Archean Earth. Astrophys J 780:52. ADSCrossRefGoogle Scholar
  87. Scheer H (2006) An overview of chlorophylls and bacteriochlorophylls: biochemistry, biophysics, functions and applications. In: Grimm B et al (eds) Chlorophylls and bacteriochlorophylls. Springer, Dordrecht, pp 1–26Google Scholar
  88. Schwartz JC, Sekowski C, Haggard HM et al (2016) Inferring planetary obliquity using rotational and orbital photometry. Mon Not R Astron Soc 457:926–938. ADSCrossRefGoogle Scholar
  89. Schwieterman EW, Cockell CS, Meadows VS (2015) Nonphotosynthetic pigments as potential biosignatures. Astrobiology 15:341–361. ADSCrossRefGoogle Scholar
  90. Schwieterman EW, Meadows VS, Domagal-Goldman SD et al (2016) Identifying planetary biosignature impostors: spectral features of CO and O4 resulting from abiotic O2/O3 production. Astrophys J 819:L13. ADSCrossRefGoogle Scholar
  91. Schwieterman EW, Kiang NY, Parenteau MN et al (2018) Exoplanet biosignatures: a review of remotely detectable signs of life. AstrobiologyGoogle Scholar
  92. Seager S, Whitney BA, Sasselov DD (2000) Photometric light curves and polarization of close-in extrasolar giant planets. Astrophys J 540:504–520. ADSCrossRefGoogle Scholar
  93. Seager S, Turner EL, Schafer J, Ford EB (2005) Vegetation’s red edge: a possible spectroscopic biosignature of extraterrestrial plants. Astrobiology 5:372–390. ADSCrossRefGoogle Scholar
  94. Seager S, Schrenk M, Bains W (2012) An astrophysical view of Earth-based metabolic biosignature gases. Astrobiology 12:61–82. ADSCrossRefGoogle Scholar
  95. Segura A, Kasting JF, Meadows V et al (2005) Biosignatures from Earth-like planets around M dwarfs. Astrobiology 5:706–725. ADSCrossRefGoogle Scholar
  96. Shahmohammadi HR, Asgarani E, Terato H et al (1998) Protective roles of bacterioruberin and intracellular KCl in the resistance of Halobacterium salinarum against DNA-damaging agents. J Radiat Res 39:251–262. CrossRefGoogle Scholar
  97. Shields AL, Ballard S, Johnson JA (2016) The habitability of planets orbiting M-dwarf stars. Phys Rep 663:1–38. ADSMathSciNetCrossRefGoogle Scholar
  98. Sparks WB, Hough J, Germer TA et al (2009a) Detection of circular polarization in light scattered from photosynthetic microbes. Proc Natl Acad Sci 106:7816–7821. ADSCrossRefGoogle Scholar
  99. Sparks WB, Hough JH, Kolokolova L et al (2009b) Circular polarization in scattered light as a possible biomarker. J Quant Spectrosc Radiat Transf 110:1771–1779. ADSCrossRefGoogle Scholar
  100. Stam DM (2008) Spectropolarimetric signatures of Earth-like extrasolar planets. Astron Astrophys 482:989–1007. ADSCrossRefGoogle Scholar
  101. Sterzik MF, Bagnulo S, Palle E (2012) Biosignatures as revealed by spectropolarimetry of Earthshine. Nature 483:64–66. ADSCrossRefGoogle Scholar
  102. Sun Y, Frankenberg C, Wood JD et al (2017) OCO-2 advances photosynthesis observation from space via solar-induced chlorophyll fluorescence. Science.
  103. Takahashi J, Itoh Y, Akitaya H et al (2013) Phase variation of Earthshine polarization spectra. Publ Astron Soc Jpn 65:38. ADSCrossRefGoogle Scholar
  104. Tinetti G, Meadows VS, Crisp D et al (2006a) Detectability of planetary characteristics in disk-averaged spectra. II: synthetic spectra and light-curves of earth. Astrobiology 6:881–900. ADSCrossRefGoogle Scholar
  105. Tinetti G, Rashby S, Yung YL (2006b) Detectability of red-edge-shifted vegetation on terrestrial planets orbiting M stars. Astrophys J 644:L129–L132. ADSCrossRefGoogle Scholar
  106. Tucker CJ (1979) Red and photographic infrared linear combinations for monitoring vegetation. Remote Sens Environ 8:127–150. ADSCrossRefGoogle Scholar
  107. Tucker C, Pinzon J, Brown M et al (2005) An extended AVHRR 8-km NDVI dataset compatible with MODIS and SPOT vegetation NDVI data. Int J Remote Sens 26:4485–4498CrossRefGoogle Scholar
  108. Turnbull MC, Traub WA, Jucks KW et al (2006) Spectrum of a habitable world: Earthshine in the near-Infrared. Astrophys J 644:551–559. ADSCrossRefGoogle Scholar
  109. Walker SI, Bains W, Cronin L et al (2018) Exoplanet biosignatures: future directions. AstrobiologyGoogle Scholar
  110. Wolstencroft RD, Raven JA (2002) Photosynthesis: likelihood of occurrence and possibility of detection on Earth-like planets. Icarus 157:535–548. ADSCrossRefGoogle Scholar
  111. Wood SE, Paige DA (1992) Modeling the Martian seasonal CO2 cycle 1. Fitting the Viking Lander pressure curves. Icarus 99:1–14. ADSCrossRefGoogle Scholar
  112. Woolf NJ, Smith PS, Traub WA, Jucks KW (2002) The spectrum of Earthshine: a pale blue dot observed from the ground. Astrophys J 574:430–433. ADSCrossRefGoogle Scholar
  113. Zhao F, Zeng N (2014) Continued increase in atmospheric CO2 seasonal amplitude in the 21st century projected by the CMIP5 Earth system models. Earth Syst Dynam 5:423–439. ADSCrossRefGoogle Scholar
  114. Zugger ME, Kasting JF, Williams DM et al (2010) Light scattering from exoplanet oceans and atmospheres. Astrophys J 723:1168–1179. ADSCrossRefGoogle Scholar

Authors and Affiliations

  1. 1.University of California, RiversideRiversideUSA

Section editors and affiliations

  • Victoria Meadows
    • 1
  • Rory Barnes
    • 2
  1. 1.Astronomy DepartmentUniversity of WashingtonSeattleUSA
  2. 2.Astronomy DepartmentUniversity of WashingtonSeattleUSA

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