It is a pleasure to thank Jim Kasting and Eric Chassefière for valuable discussions and exchanges about the O2 content of planet atmospheres, Tobias Owen for additional arguments about the absence of O2 in comets from the study of Pluto and Triton. We are grateful to Jean Bénit for information about the chemical synthesis by high energy particles, to Fabien Malbet, Pierre Barge and Frank Shu for protoplanetary disk models, to Johannes Geiss for O2 content of comets and to François Raulin for discussions on the relation between evidence for photosynthesis and evidence for life. We also thank Frank Fortuna for his help providing accurate stopping power and range for protons in water ice and Valérie Demuyt for her efficient typing of the manuscript.
© European Southern Observatory (ESO) 1999
Online publication: November 26, 1998