166 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
oceans must have been formed from the decomposition of heavier 
compounds after the earth achieved its present size. 
To summarize the best geological evidence: the earth is at least 3 
billion years old and its surface conditions of temperature and atmos- 
phere have not changed materially in 1 billion years and not radically 
in 3 billion years. Its stratified layers from density about 3 at the 
surface to density about 13 at the center could result from plastic 
flow and chemical compaction whether or not the earth were originally 
molten. Finally, the earth lost most of its gases early in life, probably 
because it was at one time in pieces of too small mass to hold on to 
light gas molecules. 
SHOOTING STARS 
An important bridge between geology and astronomy is provided 
by the meteors. Millions of these small chunks of rock and iron 
collide with the earth each day, most of them burning up high in the 
atmosphere. Some of the larger, slower-moving ones reach the 
ground; there the few collected are the only material from outside the 
earth available for detailed study. Are they a few remaining planetes- 
imals—or are they visitors from outside the solar system? 
Measures of meteor speeds by Whipple at Harvard have established 
that they are at least members of the solar system. If they came from 
outside they would be moving much faster than observed. Radio- 
activity measurements (as in dating rocks, but corrected for the effects 
of cosmic rays which form extra helium) show that the meteors are 
between 2 and 3 billion years old, in startling agreement with the 
earth’s age. Their high iron and nickel content has supported the 
assumption that the earth’s core is nickel-iron (so that earth and 
meteors would have the same over-all composition). 
Furthermore, Harrison Brown’s recent studies of the chemical 
compounds present in meterorites show that they were probably at 
one time under the high pressures and temperatures of a planet’s 
interior. It would seem that, far from being planetesimals, the 
meteors are the remains of a fair-sized planet which was formed at 
the same time as the earth, and which broke up in some large-scale 
interplanetary collision at a later date. 
THE GAMUT OF SPECULATION 
The astronomer, in his approach to the problem of the earth’s 
origin, started by recognizing a certain order and regularity among 
the planets, their satellites, and the smaller asteroids, all moving about 
the sun. The emphasis is shifted from the origin of the earth, as one 
of the planets, to the origin of the solar system asa whole. The latest 
trend goes even further in linking the origin of the solar system with 
