This fragment of a stony meteorite 
was photographed on the ice 
near Mount Baldr where it was 
found in 1976 by a joint 
Japanese-U.S. expedition. 
William A Cassidy 
million years, there is the possibility 
of recovering meteorites that have fal- 
len over a very long period of time. 
(Meteorites falling in temperate and 
tropical areas are likely to disappear 
within a few thousand years because 
of weathering, erosion, or burial.) The 
Antarctic meteorites fell into the 
equivalent of a “deep freeze” and have 
thus been preserved with a minimum 
of terrestrial alteration and contami- 
nation. Given this unique concentra- 
tion of meteorites, there is a good 
chance of finding rare or previously 
undescribed meteorite types. 
How long have the Antarctic me- 
teorites been on the earth? What are 
their terrestrial ages? Conceivably we 
may expect any age from the present 
back to the time of inception of the 
icecap, perhaps twenty million years 
ago. Meteorites carry within them a 
time clock in the form of radioactive 
isotopes produced by cosmic-ray bom- 
bardment in outer space. When me- 
teorites land on the earth, this cosmic- 
ray bombardment is effectively ter- 
minated, and the induced radioactivity 
decays as a function of time. Approxi- 
mate terrestrial ages have been mea- 
sured for twenty-seven Antarctic me- 
teorites, and the calculations range up 
to 700,000 years. This range includes 
the oldest terrestrial ages ever mea- 
sured on stony meteorites. 
The old terrestrial ages of the Ant- 
arctic meteorites have introduced a 
new and exciting possibility, that of 
using meteorites as time probes. Mea- 
surement of radioactivity induced by 
cosmic rays should make it possible 
to span a range of terrestrial ages ex- 
tending back twenty million years to 
the time of the inception of the Ant- 
arctic icecap. The concentrations of 
meteorites now being found near the 
margins of the icecap may well yield 
specimens old enough to provide criti- 
cal information on the formation, ex- 
pansion, and flow directions of the 
great mass of ice blanketing the Ant- 
arctic continent. 
One of the unanswered questions 
that has troubled meteoriticists is 
whether the meteorites collected and 
described over the past two centuries 
are representative of the total influx 
of meteorites. Are there meteorite 
types that have escaped recognition 
and collection? Has the range of me- 
teorite types changed during geologic 
time? The Antarctic meteorites pro- 
vide some clues to answering these 
questions. 
The specimens so far classified from 
the Antarctic collections are: 87 
percent chondrites; 5 percent achon- 
drites; 7 percent irons; and 1 percent 
stony irons. These percentages agree 
remarkably well with those for all re- 
corded meteorite falls. For the 809 
confirmed falls recorded worldwide up 
to 1977 these percentages pertain: 85 
percent chondrites; 8 percent achon- 
drites; 6 percent irons; and 1 percent 
stony irons. Thus the Antarctic col- 
lections appear to be a representative 
sampling of the terrestrial meteorite 
influx. A point of interest is that iron 
meteorites are present in their statis- 
tical proportion. It might be expected 
that because of the great difference 
in density between nickel-iron and ice, 
irons would sink to the bottom of the 
icecap, never to resurface. 
65 
