Risks threatening viable transfer of microbes between bodies in our solar system

A fraction of the number of ejecta expelled from a planet by comet or aster oid impacts end up landing on another planet. If microorganisms were living in the ground before impact, they would be transported inside ejecta to th e target planet. During that perilous trip, they would be subject to four m ain categories of threat to their survival: dynamical stress, excess temper ature, radiation, chemical attack and vacuum. The effect of these, in the f orm of survival fractions as a function of time, as well as approximate num bers of arriving ejecta with viable flight times, have been investigated in a quantitative study we have made. The result shows that viable transfer f rom Mars to Earth and vice versa was highly probable during the first 0.5 G a, and also probable, but with lower frequency, thereafter. Here we follow up with considerations about the consequences of the result regarding the q uestion of whether the ancestor cell of all life on Earth must have origina ted on Earth, or whether it could have originated on Mars, its descendants thereafter moving to Earth. Some other possible consequences are also discu ssed. (C) 2000 Elsevier Science Ltd. All rights reserved.