TIME SCALE OF OUR UNIVERSE OPIK 213 



years. The higher values represented a puzzle as, for example, they 

 considerably exceeded the upper limit of age for the earth and the 

 solar system as set by the abundances of radioactive isotopes (cf. 

 preceding section) . 



Now came the latest act of the drama. Bauer (29) and Huntley 

 (30) pointed out that part of the helium in meteorites must have been 

 produced by nuclear transmutations, caused by cosmic rays during 

 millions of centuries. This suggestion has now become an estab- 

 lished fact, as otherwise the presence in meteorites of the isotope 

 He* in considerable amounts (18 to 32 percent of He*) camiot be ex- 

 plained: radioactive disintegration leads to He^ only? not to He*. 

 On the other hand, cosmic rays produce both isotopes in the approxi- 

 mate proportion of 10 He^ to 3 He* atoms (Le Couteur). This ratio 

 being given, an analysis by the mass spectrograph leads to the deter- 

 mination of the amomit of purely radiogenic He*, whicli is very much 

 less than the total amount of helium. As a result, the estimated ages 

 of meteorites are greatly reduced and, from the provisional data avail- 

 able, hardly attain 1,000 million years (31). This is much less than 

 the well-established age of the earth and the solar system ; therefore, 

 the method is of no avail in estimating the age of the miiverse. It has 

 been suggested that the meteorites lost their original helium when 

 passing near the sun and melting in its heat ; their orbits are sometimes 

 likely to become highly eccentric from perturbations at close ap- 

 proaches to the planets, in which case near passages to the sun become 

 possible. However, mipublished calculations by the writer show that 

 such happenings are very rare, and that the explanation is invalid. 



Urey (32) pointed out that iron meteorites are unlikely to contain 

 enough radioactive elements to account for measurable amounts of 

 radiogenic helium. The correlation between the total amount of 

 helium and its isotopic ratio in iron meteorites is highly remarkable 

 ( 31 ) . In the opinion of the author of this review the simplest explana- 

 tion of Paneth's results could be that all the helium is produced by 

 cosmic rays, the absolute amount and isotopic ratio depending upon 

 the original thickness of the protective layer, subsequently lost through 

 ablation in our atmosphere. The time of separation of the stone and 

 iron of meteorites, as determined from the isotopic composition of 

 lead, is consistently found to be 4,500 millions years (20). This may 

 refer to a preplanetary stage. Potassium-argon-40 ages of stony 

 meteorites are found to be 1,900 to 3,800 million years (33) and 4,700 

 to 4,800 million years (34). Evidently there has been little or no 

 escape of argon from stony meteorites. The argon ages would date 

 from the moment of last solidification, thus probably from a planetary 

 or postplanetary stage. Also, these high argon ages of stone seem to 

 indicate again that the helium ages of iron inclusions, often con- 

 nected with stone, are unreliable. 



