No. 94: Jul-Aug 1994
Three astronomical events, all within the short span of written human history, lead J. Hartung to warn us that June is a dangerous month for earthlings.
June 18, 1178. On the moon.
"...just after sunset, it was reported by at least five men that the 'upper horn of a new moon split and from the division point fire, hot coals, and sparks spewed out.'"
These observations have been interpreted as eyewitness accounts of the impact on the moon that gouged out the crater named Giordano Bruno, 20 kilometers in diameter.
June 30, 1908. Siberia.
"On the morning of June 30, 1908, a tremendous explosion deep in the Siberian taiga near the Tunguska river caused trees over an area of 40 km in diameter to be flattened in a radial pattern and produced a pressure wave in the atmosphere which circled the Earth."
June 17-27, 1975. On the moon.
"...an unusual meteoroid 'storm' was detected by the array of seismometers placed on the moon during the Apollo missions. The peak impact rate on the moon of 0.5-to-50-kg objects was about 10 times the normal background during this interval. Such a high rate was not recorded at any other time during the 8-year operation of the Apollo passive seismic network."
Hartung links all three events to the comet Encke and the closely related Taurid Complex of naturally occuring space debris. Some chunks in this wide stream of space debris are measured in kilometers and, if they hit the earth, would far outclass the infamous Siberian projectile of 1908.
(Hartung, Jack B.; "Giordano Bruno, the 1975 Meteoroid Storm, Encke, and Other Taurid Complex Objects," Icarus, 104:280, 1993.
Comment. Since we will not mail this issue of SF until the first week in July, you are safe for another year (?) if you are reading this!
No. 94: Jul-Aug 1994
On December 2, 1942, at Stagg Field, in Chicago, the first human-built nuclear reactor went critical. This feat has long been hailed as a triumph of the human intellect. Nature, though, had already beat E. Fermi and his colleagues by 2 billion years. For at Oklo and Bangombe, in the African Republic of Gabon, one finds the "ashes" where some 17 natural nuclear reactors cooked away for hundreds of thousands of years. Operating at temperatures as high as 360°C, they generated about 17,800 megawatt-years of energy.
The Gabon reactors were discovered in 1972 when the French found that uranium ore from Gabon contained anomalously low concentrations of the fissionable isotope 235U as well as fission products. A little excavation work uncovered small pockets, a few meters in length and less than a meter in width, where natural fission had occurred in the Precambrian period.
A geological reconstruction of what probably happened involves: (1) uranium-bearing solutions migrating through the fractured rocks of the region; and (2) the precipitation of the uranium as pitchblende and uranite when the solutions came in contact with kerogen. A critical mass was formed and a chain reaction started. Such a scenario is unlikely today because the concentration of fissionable 235U in natural uranium has declined by a factor of about five in the last 2 billion years. The half life of 235U is only about 700 million years.
(Nagy, Bartholomew; "Precambrian Nuclear Reactors at Oklo," Geotimes, 38: 18, May 1993. Also: Nagy, Bartholomew, et al; "Role of Organic Matter in the Proterozoic Oklo Natural Fission Reactors, Gabon, Africa," Geology, 21:655, 1993.)
Reference. The Oklo Phenomenon is covered in greater detail in ESP13 in the catalog: Anomalies in Geology. To order visit here.