270 Encyclopedia of the Solar System
Meteoritic terrestrial ages are generally based upon
decay of cosmogenic radionuclides (Fig. 4). Non-desert-
cluster finds have been on Earth for up to∼20 ka, but
the oldest one actually dated is the Tamarugal IIIA octa-
hedrite that has a terrestrial age of 3.6 Ma. As is discussed
in Section 6.1, terrestrial ages for meteorite cluster finds
from hot deserts usually range up to 50 ka: many Antarctic
meteorites are much older. A few dozen fossil meteorites
found in Ordovician seabed layers in several Swedish quar-
ries have∼480 Ma terrestrial ages (Section 6.4).
The oldest Antarctic meteorite is Lazarev, an Antarc-
tic octahedrite that is not part of any established iron–
meteorite chemical group. Its terrestrial age is 5 Ma. Al-
though an Antarctic chondrite has a terrestrial age of
∼2 Ma, the more typical terrestrial ages for these are in the
0.1- to 1-Ma range (averaging 0.3 Ma for the population
from the Allan Hills, Victoria Land; Section 6.1.1). Conceiv-
ably, the meteorite population landing on Earth during that
time window could have differed from the contemporary
one. The number of iron and stony-iron observed falls is
comparable with those from Victoria Land (Table 1); how-
ever, Antarctic achondrites and chondrites are more numer-
ous. Additional differences exist in the details. For exam-
ple, samples from Victoria Land have, on average, smaller
masses than do those from more contemporary falls; small
samples are readily detected in Antarctica. Meteorites of
rare types—like achondrites—are easily recognized, even in
hand-specimen, and pieces can be readily paired with others
of the same fall. Hence, in the Victoria Land (ANSMET)
population, the numbers of different Antarctic achondrites
are reliable (Table 1). When small populations are com-
pared, the results are always suspect. We note that the
number of aubrites and howardites are comparable but
the number of ureilites and lunar meteorites are larger
in the Victoria Land population. A difference may exist
for C1 chondrites, but they are typically friable and might
not survive pulverization in the Antarctic ice sheet. At face
value, Antarctic ordinary chondrites seem very numerous,
but their pairing uncertainties are particularly serious. Or-
dinary chondrites differ only subtly from each other—even
as falls—so the apparent excess of Antarctic LL chondrites
is clouded. Numerous studies of Antarctic meteorites re-
veal many preterrestrial genetic differences between them
and falls, but detailed interpretations of these differences
remain controversial.
The 16,500 fragments collected from Queen Maud
Land, Antarctica, by Japanese meteorite recovery teams
include quite a few fragments of rare or unique meteorite
types. These include 6 different lunar meteorites (9 frag-
ments), 4 martian meteorites (6 fragments), 6 thermally
metamorphosed (open-system) C1–C3 chondrites, and a
unique C1M or C2I chondrite. In general, Queen Maud
Land samples have terrestrial ages of up to 0.3 Ma, averag-
ing 0.1 Ma (i.e., intermediate between those of contempo-
rary falls and Antarctic samples from Victoria Land) and are
of smaller mass, on average, than even those from Victoria
Land. The Queen Maud Land population is less well char-
acterized than the Victoria Land population, so, except for
lunar and martian meteorites, we do not list any of them in
Table 1.4. Meteorites from Larger Bodies
During the early Apollo program, NASA decided to quaran-
tine lunar samples and the astronauts that brought them to
prevent contamination of Earth by some hypothetical “An-
dromeda Strain.” This quarantine cost much and proved
ineffective. Years beforeApollo 11(in 1969), E. Anders
argued that because lunar escape velocity was so low
(2.38 km/s) and shock-induced ejecta velocities were so
high, lunar samples must already be on Earth to contami-
nate us (if they were going to). To eliminate this unneces-
sary expenditure, he offered to eat the first gram of lunar
sample brought byApollo 11. His offer was not accepted:
quarantine ended withApollo 12, and the first meteorite
recognized as lunar (ALH A81005) was found in Antarctica
in 1982. Yamato 791197 was recovered in 1979 but its lunar
origin was not recognized then.
Today, 31 lunar meteorites are known as are 32 martian
meteorites (Table 1) and NASA plans an expensive quar-
antine to protect humankind from another hypothetical
“Andromeda Strain” if and when they bring martian samples
to Earth. One author of this chapter (MEL) offers to eat the
first gram of that sample to demonstrate that an expendi-
ture of between $10 million and $1 billion ($1,000,000,000)
for quarantine is unwarranted.
These two are the only likely large-body sources for me-
teorites. Other solar system bodies have escape velocities
comparable to that of Mars’ (e.g., Mercury, Pluto, and the
satellites of giant planets like Jupiter or Saturn), but their
distances from Earth and their proximities to much larger
objects with greater gravitational attraction makes Earth-
capture of ejecta from them virtually impossible. Thus, we
need only consider the Moon or Mars as meteorite sources.
(For additional information, see the websites listed in the
footnote to Table 1).4.1 Lunar Meteorites
The minerals, textures, chemical compositions, and iso-
tope ratios of these 31 individuals (each between2gand
1.8 kg) are similar to those of samples brought to Earth by
the Apollo and Luna missions [seeTheMoon] and un-
like those of terrestrial rocks or martian and other mete-
orites (Fig. 14). Only their fusion crust differentiates them
from Apollo and Luna samples. Most are regolith, fragmen-
tal, or melt breccias from the lunar highlands: 7 are Mare
basalts, 3 of which include regolith breccias or cumulate
clasts. Their cosmic ray exposure ages range up to 10 Ma