522 Annals New York Academy of Sciences 



attractive possibility is offered by Wood's (1958, 1962) hypothesis, according to 

 which planetary matter, expelled from the sun at high initial temperatures, 

 cooled by adiabatic expansion, so that progressive expansion could take place. 

 The least volatile constituents would condense to high-temperature minerals 

 (olivine, pyroxene, nickel-iron, and later, magnetite), which would trap some of 

 the surrounding primordial gas. Other substances, e.g., H2O, NH3 , and carbon 

 compounds, would condense on temperature drop. The further accretion of 

 the (now cold) dust into solid bodies, and the separation of the solids from the 

 noncondensable gas would proceed along the path outlined by Urey (1952, 

 1954, 1956, 1957, 1958) or Fish et al. (1960). Incidentally, if such a high-tem- 

 perature stage ever took place, then cometary matter, too, must have passed 

 through it. This raises some new possibilities in regard to the mineral com- 

 position of comets. In particular, the presence in comet tails of metal (or mag- 

 netite?) spherules, inferred from scattered light and polarization measurements 

 (Liller, 1960), is somewhat easier to understand if part of the cometary material 

 had a high temperature history, even though its final accretion occurred at low 

 temperatures. This view gains further support from the discovery in cosmic 

 dust of metal flakes with amorphous organic attachments. The fall dates of 

 these particles seem to be correlated with several meteor showers of cometary 

 origin (Parkin, Hunter, and Brownlow, 1962). Perhaps Herbig's (1961) sug- 

 gestion that the carbonaceous chondrites were derived from comets should be 

 re-examined in the hght of this possibility. 



Aqueous stage and the prerequisites for life. What about the third question, 

 the setting in which the aqueous stage took place? This is one point in which 

 the large planet hypothesis has an advantage over all others. A planet of 

 terrestrial size can hold water vapor gravitationally, and can maintain bodies of 

 liquid water, from ponds to oceans. Surely, the surface temperature must be 

 high enough to allow liquid water to exist, but the temperature is controlled 

 not only by the distance from the sun, but also by the composition of the 

 atmosphere. If Venus, with its CO^-rich atmosphere, were located in the 

 asteroidal belt, it would have a comfortable surface temperature near 300° K., 

 instead of the 600° K. prevailing at its present location. If it were not for the 

 fact that the planetary hypothesis runs into so many other ditficulties (Anders 

 and Goles, 1961), one could stop here. 



Of all the parent bodies discussed, the asteroids are least likely to retain 

 liquid water at their surfaces, owing to their small size and consequent low 

 escape velocities. But there is a way in which they could retain liquid water in 

 their interiors. If the asteroids were ever heated by an internal heat source 

 {e.g., extinct radioactivity), some temperature distribution resembling the 

 curves in figure 6 would result. The surface temperature of the body would 

 be controlled by the amount of solar radiation reaching it, and might be around 

 100 to 200° K. Farther inward, the temperature would rise until the melting 

 point of ice was reached. Liquid water could exist in this zone, down to a 

 depth at which the boiling point at the prevailing pressure was reached. In 

 FIGURE 7 is shown the location of this zone of liquid water for a body with a 

 central temperature of 1900° K. In this case, some 5 per cent of the volume of 

 the body will contain liquid water. 



The water will not last forever, of course. Above the zone of liquid water, 



