the former is supplied by certain carbon-bearing compounds found in carbona- 

 ceous meteorites, characterized by a 12 C/ 13 C ratio of 42, compared with the 

 canonical solar system value of 89. An origin by formation in the atmosphere of 

 a red giant star is inferred for such grains. An example of evidence for presolar 

 chemical processing is the deuterium content of meteoritic organic matter, with 

 D/H values over 20 times the galactic value. This is commonly attributed to ion- 

 molecule reactions at the low temperatures of interstellar clouds. The presence 

 of such material in meteorites is evidence that some presolar organic matter was 

 able to survive entry into the solar nebula. 



Suggestions for Further Reading 



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

 ments and Compounds. NASA SP-476, 1985. 



3.2 Remote Observations 



Most of our information concerning extraterrestrial objects and environments 

 comes from the analysis of electromagnetic radiation. The intensity and polariza- 

 tion of the radiation, at a particular frequency, in a certain direction, and at a 

 specific time, can be measured to provide information of particular interest to 

 exobiology, for example: 



1. The existence of extra-solar planets that might serve as suitable hosts for 

 the chemical evolution of life 



2. The abundances and distribution of biogenic substances throughout the 

 cosmos 



3. The conditions under which complex organic molecules form or are 

 destroyed 



4. The conditions under which solids composed of the biogenic elements 

 form from gases 



5. The processes in circumstellar, interstellar, and nebular stages of physical- 

 chemical evolution which govern the composition and distribution of biogenic 

 matter during its transit from stage to stage 



Exobiologists need access to observing platforms in space. Observing from 

 spacecraft has clear advantages over ground-based viewing because of the opacity 

 of the atmosphere in many regions of the electromagnetic spectrum (fig. 3-2): 

 the gamma-ray, X-ray, ultraviolet, parts of the infrared, most of the submilli- 

 meter, and parts of the millimeter. Even in those spectral regions where the 

 atmosphere is generally transparent, there are specific frequencies where absorp- 

 tion or emission by species in the air precludes ground-based observations. 



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