Extraterrestrial Chemistry in the Chemistry Department
The 1960s and early 1970s were the time of the race to the Moon, and space-related research was very popular and well funded. At this time, the BNL Chemistry Department already had a great deal of expertise in low-level counting developed during the search for neutrinos from the sun, and in analytical chemistry, both of which made analysis of extraterrestrial objects, meteors and samples brought back by astronauts from the Moon a natural extension of programs already underway.
The Chemistry Department's experts had great experience in low-level counting of small aliquots of radioactive gases. Under a contract with NASA, members of the Chemistry Department built, tested and operated a system at the Johnson Space Center to measure radioactivity in separated rare-gas fractions of samples of lunar atmosphere and extracted from lunar rocks collected by Apollo 11 and 12 astronauts. Measurements were made of 3H, 37Ar, and 39Ar activities. The 37Ar/39Ar ratio from Apollo 12 samples was more than double that observed in Apollo 11 samples, a result attributable to a solar flare eighteen days before the Apollo 12 collection. Measurements also led to conclusions that the collected rocks had lain on the lunar surface for 110 million (!) years.1
1. "Cosmic Ray Production of Rare Gas Radioactivities and Tritium in Lunar Material" R. W. Stoenner, W. J. Lyman and R. Davis, Jr., Science 167 553 (1970).
37Ar and 39Ar activities were measured in samples from the iron meteorite that fell in Alandroal, Portugal on November 14, 1968. The amount of 37Ar activity is proportional to the cosmic ray flux to which the meteorite was exposed for the sixty days just prior to falling to earth. The amount of 39Ar activity is proportional to the average cosmic ray activity to which the meteorite was exposed for the last five hundred years. Since meteors have eccentric orbits, the the ratio of the activities is a measure of the variation of the cosmic ray flux in different parts of the solar system. Results showed that the cosmic-ray intensity near the earth at the time of the meteorite's fall was about one-half that of the average cosmic-ray intensity exposure during the meteorite's orbit, attributable to modulation of the cosmic-ray flux by the solar wind.2
2. Cosmic Ray Gradient Measured by the Argon-37/Argon-39 Ratio in the Lost City Meteorite" M. A. Forman, R. W. Stoenner and R. Davis, Jr., J. Geophys. Res. 76 4109 (1971).
The Pueblo de Allende meteorite, a type-3 cabonaceous chondrite fell in Mexico on February, 9, 1969. The meteorite contained chondrules, seed-sized droplets thought to have condensed from the disk of matter that later coagulated into our solar system. Dating this meteorite therefore provides insight into the age of the solar system. The chondrules were shown to be gas-tight, and therefore allowed measurement of their age by two separate, independent methods. Uranium is an alpha emitter; alpha particles are Helium nuclei, so that measurement of Helium and Uranium concentrations permit dating the meteorite. Radioactive 40K yields stable 40Ar, so that measurement of 40K and 40Ar concentrations form another, independent clock. Dating by both methods yielded ages of 4.2 ± 0.2 x 109 and 4.44 ± 0.14 x 109 years, in remarkable agreement.
Last Modified: June 28, 2012