302 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 64 



these diverse planetary environments — if, indeed, there are inliab- 

 itants — to have familiar forms. They have made other adaptations 

 to other environments. But the anticipated diversity and unf amiliar- 

 ity of extraterrestrial organisms provide a profound challenge and 

 a supreme opportunity for biologists. 



What, then, are our neighbors in the solar system like ? What are 

 these planetary environments ? 



Mercury and the moon are similar in many ways: little or no 

 atmosphere, no surface water or other likely solvents, and extremes 

 of temperature. With no atmosphere, the moon receives intense ultra- 

 violet radiation and proton bombardment from the sun, and no 

 terrestrial organism could survive, improtected, on the lunar surface 

 for more than a few hours. But conditions are much milder below 

 the lunar surface. Here, there is no solar radiation, the temperature 

 variations are small, at some depths the average temperature is mild, 

 and there may be liquid water trapped below a layer of permafrost. 

 Nevertheless, the likelihood of subsurface life on the moon seems re- 

 mote, because in the absence of sunlight there is no convenient energy 

 source for living systems. 



The planet Venus emits radio waves characteristic of a body at a 

 temperature of 600 or 700° F. Until recently, however, no one knew 

 for certain whether this high-temperature emission came from the 

 surface of the planet, or from some region high in its atmosphere. 

 The voyage of the NASA space vehicle Mariner II to the vicinity of 

 Venus, in 1962, helped solve this problem. Aboard Mariner was a 

 sensitive radiometer designed by five scientists, including A. E. 

 Lilley of the Harvard College Observatory, which radioed back to 

 Earth the news that the radio emission arises from the surface of 

 Venus. The planet is therefore too hot for any familiar biochemicals, 

 and a terrestrial organism placed there would fry. Indigenous life 

 on Venus is very unlikely. 



Between Mars, of which we will speak presently, and Jupiter are 

 fragments of stone and rock known as the asteroids. Chips off the 

 asteroids occasionally intercept the orbit of the Earth, and fall to its 

 surface as meteorites. Meteorites are the only samples of extra- 

 terrestrial material now available for laboratory analysis. A few 

 meteorites, known as the carbonaceous chondrites, contain a few per- 

 cent of very complex organic matter. It is not known whether this 

 organic matter was produced in the absence of life, by chemical 

 processes similar to those invoked for the origin of life on Earth, or 

 whether — more interestingly, but less likely — it was produced by liv- 

 ing organisms on the parent bodies of the chondrites. Inclusions 

 which superficially resemble microorganisms have also been found in 

 these meteorites. But some have been shown to be inorganic, and 

 others, to result from Earthly contamination — for example, by rag- 



