Nature 365, 715 - 721 (21 October 1993); doi:10.1038/365715a0

A search for life on Earth from the Galileo spacecraft

Carl Sagan^*, W. Reid Thompson^*, Robert Carlson^†, Donald Gurnett^‡ & Charles Hord^§

^* Laboratory for Planetary Studies, Cornell University, Ithaca, New York 14853, USA
^† Atmospheric and Cometary Sciences Section, Jet Propulsion Laboratory, Pasadena, California 91109, USA
^‡ Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242-1479, USA
^§ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80309, USA

In its December 1990 fly-by of Earth, the Galileo spacecraft found evidence of abundant gaseous oxygen, a widely distributed surface pigment with a sharp absorption edge in the red part of the visible spectrum, and atmospheric methane in extreme thermodynamic disequilibrium; together, these are strongly suggestive of life on Earth. Moreover, the presence of narrow-band, pulsed, amplitude-modulated radio transmission seems uniquely attributable to intelligence. These observations constitute a control experiment for the search for extraterrestrial life by modern interplanetary spacecraft.

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