A Search For Life On Earth From The Galileo Spacecraft (Carl Sagan et al 1993)

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.

Carl Sagan, W. Reid Thompson, Robert Carlson, Donald Gurnett & Charles Hord
Nature volume 365, pages 715–721 (21 October 1993)
https://www.nature.com/articles/365715a0

“A search for life on Earth from the Galileo spacecraft” (1993) #CarlSagan et al full paper (PDF)

Astrobiology