with observations obtained on a coma produced by gas-release experiments and 

 in large-scale laboratory experiments, will contribute substantially to our knowl- 

 edge of the structure and composition of cometary nuclei. Among the questions 

 that will be addressed are: How are gases occluded in the nucleus and how are 

 they released? What gases or grains and what sequence of their release could 

 produce the species distribution observed in real cometary comae? What is the 

 contribution of gas-laden ice grains in the coma to its chemistry? What proper- 

 ties of grains are conducive to the formation of dusty mantles? 



5.4 Microbial Survival in Space 



Panspermia is the concept that life on Earth arose not de novo but from an 

 inoculum that reached the Earth from space. If panspermia were true, the prob- 

 lem of the origin of life would be transferred from the relatively known environ- 

 ment of Earth to a relatively unknown environment elsewhere. If panspermia has 

 occurred it is required that 



1. Life arose somewhere else in the universe and flourished. 



2. Some of the living organisms in this location left their origin and entered 

 outer space. 



3. Some of these latter organisms survived space travel and reached Earth. 



4. Some of these surviving organisms penetrated the Earth's atmosphere in 

 viable form. 



5. Some (or at least one) of these viable organisms found conditions favora- 

 ble on Earth and flourished. 



6. The descendants of this xenobiont led to the vast array of organisms on 

 Earth today. 



It would, of course, be difficult to prove unambiguously that panspermia 

 occurred. However, it might be possible to determine by measurement, calcula- 

 tion, and experiment how likely it is that panspermia could occur, that it is 

 impossible to disprove panspermia experimentally, or that the probability of 

 panspermia is vanishingly small. (In this discussion, directed panspermia is not 

 being considered as there is yet no evidence that intelligent life exists in the 

 universe outside Earth.) 



In light of the above scenario, there are several approaches for investigating 

 certain aspects of the concept of panspermia: 



1. Measurement or calculation of the rate at which organisms could escape 

 from a planet 



2. Measurement or calculation of the rate at which organisms would be 

 killed during space travel 



3. Calculation of the time it would take for xenobionts to reach Earth from 

 various sources in outer space 



4. Calculation of the concentrations of xenobionts that might be expected in 

 the Earth's orbit 



96 



