272 Encyclopedia of the Solar System
spectrometers can determine very small noble gas concen-
trations in a meteorite and, in addition, measure the isotopic
composition. Most analyses are carried out on meteorite
samples of<100 mg, but, with effort, samples as small as
6 μg provide essential data. By 2004, about 7400 analyses
of the light noble gases—He, Ne, and Ar—were reported
for all meteorite types.
Noble (rare) gases in meteorites have different origins
and each component has a specific isotopic or elemental
composition. Some components like the radiogenic gases
were produced in situ in meteorites. Radiogenic^40 Ar is
produced by spontaneous, radioactive decay of long-lived,
naturally occurring^40 K (half-life,t 1 / 2 =1.28 Ga), while
(^4) He is produced similarly from (^232) Th, (^235) U, and (^238) U decay
(t 1 / 2 =14.1, 0.704, and 4.47 Ga, respectively). Fission Kr
and Xe components derive from spontaneous or induced
fission of heavy nuclei (e.g., long-lived U isotopes), each
with a characteristic fission-fragment distribution. In addi-
tion, decay products of extinct radionuclides (e.g.,^129 I and
(^244) Pu:t 1 / 2 =5.7 and 81 Ma, respectively) exist in meteorites
(Section 6.5).
Other in situ produced gases are cosmogenic nuclides
formed by nuclear reactions of high-energy galactic or so-
lar particles with meteoroids. The specific nuclear reaction
depends on the particle energy and the chemical compo-
sition of the target material. Nuclear reactions of primary
(GeV energies) particles involve initiation of a cascade of
secondary particles with smaller energies so that the iso-
topic or elemental ratios of cosmogenic noble gas isotopes
depend on the meteorite’s position within the meteoroid
and on its size. Cosmogenic nuclides are limited to the
surface (<1 m depth) of larger bodies and to meter-sized
objects in space. Inert gases found in iron meteorites are
mainly cosmogenic, but stony meteorites contain a mix of
many components.
Trapped gases include a whole family of noble gas com-
ponents that were not produced in situ but incorporated in
the meteoroid when it formed. Trapped gases are of three
main varieties, solar, planetary, and “exotic.” Elemental so-
lar gas ratios are similar to those observed in the Sun. Solar
gases are introduced into meteoritic mineral grains by di-
rect implantation of solar wind ions or more energetic solar
flare particles in the regolith of atmosphere-less surfaces
of parent bodies (e.g., the Moon). The planetary noble gas
pattern shows a systematic fractionation in which the light-
est noble gases—He and Ne—are depleted relative to Ar,
Kr, and Xe. Different meteorite types or individual min-
eral separates have characteristic isotopic and elemental
signatures that differ, for example, from those of terrestrial
atmospheric noble gases.
Each event depicted in Fig. 2 can, in principle, alter plan-
etary matter mineralogically and/or chemically. To illustrate
this qualitatively, consider an element like Ne, whose con-
centration in meteorites reflects any or all of the events
in Fig. 2. As a light noble gas, it forms physical bonds in
meteorites rather than chemical bonds. The three stable
Ne isotopes (20, 21, and 22) were created by several stel-
lar nucleosynthetic processes, and a mixture of them was
introduced into the presolar nebula with other nucleosyn-
thesized nuclides. Some proportion of this Ne (with its char-
acteristic^20 Ne/^22 Ne and^21 Ne/^22 Ne ratios) was trapped by
condensing and accreting nebular material. Presolar grains
incorporated into the material, and not subsequently de-
stroyed, contain another component (Ne-E)—pure^22 Ne—
produced by decay of the now-extinct, radionuclide^22 Na
(t 1 / 2 = 2 .60 years).
Partial or total Ne degassing accompanied heating of
the primitive parent body interior and transformation into
a more evolved form, with the Ne components escaping
into space or being redeposited into cooler parent body
material nearer the surface. Fine-grained matter on the
parent body’s surface could acquire solar wind and solar
flare Ne, which has distinct isotopic compositions that are
implanted in regolith (Section 2.4.3). Impacts repeatedly
churned (“gardened”) the regolith so that a multisource Ne
mixture (cosmogenic, solar, trapped) could be present in
any sample. Finally, an impact occurred that removed a
meter-sized meteoroid, thus starting the CRE “clock” that
accumulated a new batch of cosmogenic Ne and other nu-
clides, including radionuclides.
Because a meteorite can sample Ne from any or all
of these sources, its isotopic composition represents a
weighted average of the isotopic compositions of its compo-
nent sources. These can be recognized on a three-isotope
plot (Fig. 15). A sample consisting of essentially one com-
ponent is represented by one point in such a diagram, while
a neon mix of two components will lie on a line connecting
the isotopic compositions of these components. Included
in Fig. 15, as an example, is the Ne isotopic composition
of samples of the meteoritic breccia ALH 85151, which
contains both solar and cosmogenic gas. Lunar soils also
contain solar Ne, but this is a mixture of Ne from the low-
energy solar wind and from more energetic solar particles,
each differing in isotopic composition. The solar Ne isotopic
composition extrapolated from the ALH 85151 data lies al-
most midway between the Ne isotopic components from
these two solar sources. Addition of Ne from other sources,
like Ne-E, can complicate this picture. A mixture of three
Ne components will fall within a triangle whose apexes each
have the Ne isotopic composition of a pure component. In
addition, many chondrites contain one or more trapped Ne
components, examples of which are in the Fig. 15 inset.
A similar picture can be drawn for Kr and Xe with
many isotopes and several possibilities for three-isotope
plots, but He and Ar have additional individual compli-
cations. For He, only two stable isotopes exist—^3 He and
(^4) He—so no three-isotope plot is possible. Furthermore,
an additional monoisotopic component, radiogenic^4 He,
can exist in meteorites (see Section 6.3). Argon differs
from He in having three stable isotopes. In most stony