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Pb-204 ratio and the Pb-207/Pb-204 ratio in some specimen today and one knows
or can reasonably assume what those ratios were when the specimen originated,
onecancalculate itsagefromthatinformationalone.Neithertheamountoforigin-
al lead nor the Pb-208 from thorium comes into the calculation. Thus if one knew
the lead-isotope ratios of modern lead minerals and of the primordial Earth, one
could calculate the age of the Earth.
Therewasjustoneproblem:howcouldscientistsdiscoverthelead-isotopecom-
position of the primordial Earth? They couldn't, but they could do the next best
thing: search for lead minerals that have the lowest ratios of Pb-206/Pb-204 and
Pb-207/Pb-204: in other words, that have the most primordial, least radiogenic
lead. Those oldest and most primitive types of lead, when compared with the
youngest, would allow scientists to calculate a minimum age for the Earth.
The pioneer in lead mass spectrometry, the technique used to measure isotope
ratios, was Alfred Nier of the University of Minnesota. In 1941, Nier and his col-
leagues published analyses of galena, a lead sulfide mineral, of different ages.
Galena contains no detectable uranium or thorium and thus should preserve its ori-
ginal lead-isotope ratios.
Nier found that a galena from Ivigtut, Greenland, had the least radiogenic lead.
In 1942, the Russian academician E. K. Gerling used Nier's data to calculate the
time required for lead with the ratios of the Ivigtut galena to evolve to the lead
in one of Nier's youngest galenas. The calculation gave 3,950 million years. This
led Gerling to write that the age of the Earth “is not under 3,000-4,000 million
years.” 1
After the war, two other scientists made similar calculations using slightly dif-
ferent approaches. One was Arthur Holmes, who calculated that “the most prob-
able age of the earth is about 3,350 million years.” 2 The other was a German phys-
icist named Friedrich G. Houtermans (1903-1966). 3 Because of wartime secrecy,
neither was aware of Gerling's work.
One of the most remarkable characters in the history of science, Houtermans
could have stepped from the pages of an Eric Ambler spy novel. He emigrated
from Germany to England in 1933, then a year later, out of allegiance to the ex-
pressed ideals of the Soviet Union, moved to Kharkov. As Stalin's reign of terror
reached its peak, in 1937 the NKVD arrested and tortured Houtermans. To allow
his wife and children to escape, unaware they had already slipped out of Moscow
to Riga, Houtermans confessed to spying for Nazi Germany. After Hitler and Stal-
in signed their infamous pact in August 1939, the NKVD extradited Houtermans
back to Germany, handing him over to the Gestapo at the border. This put Houter-
mans in the unenviable position of being able to compare the torture techniques of
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