A search for life on Earth from the Galileo spacecraft

@article{Sagan1993ASF,
  title={A search for life on Earth from the Galileo spacecraft},
  author={Carl E. Sagan and William R. Thompson and Robert Carlson and Donald A. Gurnett and Charles W. Hord},
  journal={Nature},
  year={1993},
  volume={365},
  pages={715-721}
}
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… 

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