No. 42: Nov-Dec 1985
There is good evidence that life appeared on earth just 200-400 million years after the crust had cooled (assuming conventional methods of measuring age). Two hundred million years seems a bit on the short side for the spontaneous generation of life, although no one really knows just how long this process should take (forever?). The apparent rapidity of the onset of terrestrial life has led to a reexamination of the old panspermia hypothesis, in which spores, bacteria, or even nonliving "templates" of life descended on the lifeless but fertile earth from interstellar space.
P. Weber and J.M. Greenberg have now tested spores (actually Bacillus subtilis) under temperature and ultraviolet radiation levels expected in interstellar space. They found that 90% of the spores under test would be killed in times on the order of hundreds of years -- far too short for panspermia to work at interstellar distances. However, if the spores are transported in dark, molecular clouds, which are not uncommon between the stars, survival times of tens or hundreds of million years are indicated by the experiments. Under such conditions, the interstellar transportation of life is possible.
But perhaps the injection and capture phases of panspermia might be lethal to spores. Weber and Greenberg think not -- under certain conditions. The collision of a large comet or meteorite could inject spores from a life-endowed planet into space safely, particularly if the impacting object glanced off into space pulling ejecta after it. The terminal phase, the capture of spores from a passing molecular cloud by the solar system and then the earth, would be nonlethal if the spores were somehow coated with a thin veneer of ultraviolet absorbing material. In sum, the experiments place limits on panspermia, but do not rule it out by any means.
(Weber, Peter, and Greenberg, J. Mayo; "Can Spores Survive in Interstellar Space?" Nature, 316:403, 1985.)
Comment. Weber and Greenberg do not discuss the possible existence of dense, low-temperature regions in molecular clouds where conditions might be conducive to the development of large molecules. Does life have to have the proverbial warm, sunlit pond to develop?