r 
Type of Meteorite 
Approximate Percentage of 
Known Meteorites 
chondrite. 
^ ordinary chondrite 72% 
^.carbonaceous chondrite 13% 
Stony 
achondrite 
8% 
Iron 
6% 
Stony-iron 
1% 
The stony meteorites, in particular, 
are subdivided into two major groups, 
chondrites and achondrites. These des- 
ignations come from the small, round 
objects called chondrules that were 
discovered when meteorites were first 
studied under a microscope in 1869. 
Chondrules are tiny blobs of mineral 
material — glass and crystals — about 
one millimeter in diameter. The chon- 
drites contain chondrules; the achon- 
drites do not. 
Although both the chondrites and 
the achondrites are stony meteorites, 
they are very different in appearance. 
No terrestrial rocks contain chon- 
drules, and therefore no earth rocks 
look like the chondrites. The achon- 
drites, on the other hand, often look 
like those formations of igneous earth 
rocks in which small angular frag- 
ments are piled together in a chaotic 
heap whose gaps are filled with still 
smaller fragments. This kind of rock 
formation is called a breccia, and brec- 
cias similar to those of achondrites 
were brought back from the moon by 
the Apollo and Luna missions. Studies 
of the lunar specimens have shown 
that a single breccia can include frag- 
ments of many different types of rock 
instead of only one. Similarly, a va- 
riety of extraterrestrial rock types will 
often be found in a single brecciated 
achondrite, so that the meteorite is 
a particularly rich source of informa- 
tion. The chondrites are the most 
abundant of the known meteorites, 
making up about 85 percent of the 
identified finds. Achondrites consti- 
tute about 8 percent of the total. 
The chondrites are themselves fur- 
ther divided into ordinary chondrites 
and carbonaceous chondrites. Both 
kinds are made of primordial and un- 
differentiated (that is, unseparated) 
material but there are important dif- 
ferences between them. Carbonaceous 
chondrites are the most primitive of 
all the meteorites, containing complex 
organic molecules that may, under ap- 
propriate environmental conditions, be 
the precursors of life. They also con- 
tain water bound structurally to their 
minerals. In addition, carbonaceous 
chondrites have inclusions that were 
created at high temperatures: these 
are the very earliest minerals, pro- 
duced when the solar nebula, a hot 
cloud of gas, began to collapse and 
condense to form the chondritic ma- 
terials from which all the planets and 
the sun evolved. Ordinary chondrites 
are also primitive, being about 4.5 bil- 
lion years old, but unlike the carbo- 
naceous chondrites, they contain nei- 
ther organic components nor water. 
They are presumed to record later 
chapters than the carbonaceous chon- 
drites in the evolution of the solar 
system. 
Iron meteorites are very different 
from stone ones. There are no rocks 
on the surface of the earth that look 
like them. The only place on the planet 
where anything similar might be found 
would be 5,000 kilometers beneath our 
feet in the earth’s semimolten core. 
Because metal is less brittle than rock, 
iron meteorites are less likely than 
stony ones to break up as they travel 
through the atmosphere, so that very 
large pieces can sometimes be found. 
The iron Ahnighito, for example, is 
the largest piece of a meteorite on 
display in any museum in the world. 
A polished ullrathin section of an 
ordinary chondrite shows one of the 
small, rounded masses known as 
chondrules that are typically 
embedded in this type of meteorite. 
Ordinary chondrites are the most 
numerous of all known meteorites. 
This one fell in India in 1976. 
R Beauchamp. Batelle Pacific Northwest Laboratories 
It is not, however, the largest me- 
teorite that has ever been found. An 
iron meteorite called Hoba, which 
may weigh more than 60 tons, lies 
where it fell in Namibia (formerly 
South-West Africa); it is too big to 
be easily moved. More than 500, or 
about 6 percent, of all known me- 
teorites are irons. The stony-irons are 
a smaller group than either the stones 
or the irons. Only about sixty-five to 
seventy of the known meteorites are 
in this group, and like the irons, the 
stony-irons do not resemble any rocks 
of any type found on the earth. 
The studies that are currently being 
conducted on meteorites are largely 
devoted to determining their compo- 
sition, their age, and the processes by 
which their components have been dif- 
ferentiated. Composition and proc- 
esses of differentiation are determined 
by means of chemical analyses. Age 
is determined by measuring the abun- 
dance in a meteorite of certain ra- 
dioactive isotopes that have known 
rates of decay and can serve as clocks. 
The articles that follow in this Spe- 
cial Supplement of Natural History 
describe the improved understanding 
of the nature of meteorites that has 
resulted from these ongoing studies 
and the role that meteorites have come 
to play in informing scientists about 
the development of the planetary sys- 
tem. The oldest meteorites yet dated 
are about 4.6 billion years old, some 
100 million years older than the earth 
or the moon. Since they predate any- 
thing found on the planet, they can 
help fill in the earliest chapters of 
our own history that have been oblit- 
erated on the earth. □ 
54 
