especially adapted for dissemination over the biosphere. Endolithic life forms, 

 such as cyanobacteria, algae, fungi, and lichens of unusual organization, have 

 been detected inside rocks from the Antarctic dry valleys as well as inside desert 

 sandstone. They represent an example of a simple ecosystem with a favorable 

 microclimate surrounded by an extreme environment of low humidity, tempera- 

 ture extremes, and a high influx of solar radiation. Resistant organisms could be 

 sought by the wholesale exposure to extreme conditions of samples taken from a 

 variety of such inhospitable localities on Earth. 



The study of any given organism (or small group of organisms) has the disad- 

 vantage that the results obtained may not be representative of the total picture. 

 Misleading "dogma" can be established through the rigorous and in-depth study 

 of a few easy-to-handle organisms. Therefore, the variability in levels of resis- 

 tance for a variety of microorganisms should be systematically examined. 



Suggestions for Further Reading 



Wood, J. A.; and Chang, S., eds.: The Cosmic History of the Biogenic Ele- 

 ments and Compounds. NASA SP-476, 1985. 



Davies, R. E.: Panspermia: Unlikely, Unsupported, But Just Possible. Acta 

 Astronautica, in press. 



Bar-Nun, A.; Herman, G.; Laufer, D.; and Rappaport, M. L.: Trapping and 

 Release of Gases by Water Ice and Implications for Icy Bodies. Icarus, vol. 63, 

 1985, p. 317. 



Whipple, F. L.; and Huebner, W. F.: Physical Processes in Comets. Annual 

 Review of Astronomy and Astrophysics, vol. 14, 1976, p. 143. 



99 



