epoch, most of the carbon is observed to take the form either of carbon monox- 

 ide or of solid material in microscopic dust grains. Some of the grains may be 

 made of graphite, while others appear to contain relatively heavy organic mole- 

 cules, rather like those forming the organic residue in carbonaceous meteorites 

 that have been recovered after they have fallen to the Earth's surface, and 

 analysed in the laboratory. How did carbon become part of the Earth? Clearly 

 not as a gas like methane or carbon monoxide, as it is known that the noble 

 gases of even greater molecular weight are enormously depleted relative to their 

 abundances elsewhere. It is more likely that at the time the Earth formed the 

 local temperature and pressure favored the retention of carbon in solid form, 

 possibly in the form of the heavy organic molecules like those in meteorites. 

 Such a hypothesis raises questions open to further study: Were the organics 

 simply those astronomers infer to be part of interstellar dust? Did they survive 

 the heating believed to have accompanied the formation of the Solar System? If 

 so, how? Can such primitive materials be observed at the current epoch, perhaps 

 in comets? 



All these questions are interesting, and many of them can be addressed 

 using the techniques of space research. Infrared telescopes in Earth orbit, like 

 the projected Space Infrared Telescope Facility (SIRTF) and Infrared Space 

 Observatory (ISO), can make more definitive studies of the carbon in molecules 

 and solids in interstellar molecular clouds. They and the Hubble Space Telescope 

 (HST) can provide more accurate information about asteroids thought to be 

 representative of the bodies where carbonaceous meteorites originate. High- 

 flying aircraft can recover interplanetary particles, some of which could be 

 fragments of comets. And a rendezvous mission to a comet, such as the Comet 

 Rendezvous Asteroid Flyby (CRAF), could yield far more information about 

 comets, of which the Comet Halley flyby missions gave tantalizing hints. 



This volume gives a status report on the scientific investigations which can be 

 undertaken in the field of exobiology using instruments in Earth orbit. The 

 reader will find that there is much to be done and a whole host of questions that 

 can be addressed. Every scientist interested in exobiology should consider how 

 his or her work will be affected by the opportunities described here. They 

 should also see what they can do to assist the Nation to reestablish a first-class 

 space-science program, given the constraints imposed by the aftermath of the 

 Challenger accident. 



George Field 

 Harvard-Smithsonian Center for Astrophysics 



VI 



