3. Could cometary bombardment be at the origin of abiotic O3 on a telluric planet?
3.1. Noll et al.'s hypothesis
A tenuous O2 atmosphere has been found on Europa (Hall et al., 1995) and Ganymede that is attributed to the sputtering of oxygen from H2O ice by energetic particles. Noll et al. (1997) have found the UV spectral signature of in the icy satellites of Saturn , Rhea and Dione. They have explained their observations as the result of particle bombardment of H2O ice in the high radiation flux environment of Saturn's magnetosphere producing O2 and . Then, these authors have stated that ifpre-cometary grains have experienced a similar situation and contain O3 and O2 in their thin outer layers, comets would contain a significant fraction of these gases. As comets have probably brought some fraction of the water content of telluric planets (Owen et al., 1992), Noll et al. have concluded that this could be an important source of abiotic O2 and O3 which would falsify the preceding criterion for detecting photosynthetic activity.
The presence of abiotic O3 on icy planetary objects which are atmosphereless and located at (renormalised) distances larger than the icy frontier (4 AU) could not be confused with the O3 resulting from photosynthesis in a telluric planet atmosphere. The nature of objects will be clearly established: the distance of the planet to its star will be measured and the shape of the IR spectrum will indicate the temperature of the object, 100 K instead of 300 K. The real issue is the proposed input of O2 into telluric planet atmospheres by cometary impacts and subsequent O3 formation.
3.2. The case of the young Earth
The geological record of Earth provides highly significant evidence . Despite any input of O2 by comets, early Earth did not build an O2 rich atmosphere. This was despite a cometary bombardment that was probably much higher near the origin of a telluric planet than later on. Banded iron formations are found in sediments older than 1.9 - 2.2 Gyrs ago (Ga), indicating the absence of a strongly oxidising catmosphere. Quantitatively, Kasting (1987) argued that they tell us that the partial pressure of O2 was lower than 6 mbar. Other estimates point to still lower values.
Another piece of evidence of the very low O2 content of atmosphere of the Young Earth, if any, is the mere fact that we are here and that life has started earlier than 3.6 Ga (presence of stromatolite fossils) and possibly earlier than 3.8 Ga ( indications). There was not a large amount of free O2 in the Earth's early atmosphere, for this would have prevented the spontaneous, reducing chemistry necessary for the origin of life.
Since 2.5 Ga, the O2 content has rapidly increased. It is likely associated with a decrease in the O2 fixation rate due to a decline of volcanic reducing outputs (). This decline was a consequence of progressive oxidation of the upper mantle due to the subduction of H2O, its dissociation followed by outgassing and escape of H2 (Kasting et al 1993b). By no means was it due to an increase of the cometary bombardment.
© European Southern Observatory (ESO) 1999
Online publication: November 26, 1998