Geology Reference
In-Depth Information
of the Moon, that is, in the lunar regolith. These estimates
of
T
2π
, and particularly the values greater than 100 Ma,
should be regarded with some reserve. Ample evidence
from studies of samples returned by the
Apollo
missions
suggests that meteoroid bombardment “gardens” the
lunar regolith. The bombardment can move material
either horizontally or vertically. Vertical motion increases
or decreases shielding or, equivalently, the nuclear par-
ticle fluxes and production rates of cosmogenic nuclides;
horizontal motion can bring the protometeorite into or
out of the cosmic-ray shadows cast by the surrounding
topography, which also affects shielding. In a complex
irradiation scenario of this kind,
T
2π
and
D
2π
represent
averages over all the conditions of lunar irradiation.
Why should lunar meteorites, unlike both asteroidal
and martian meteorites, tend to record preirradiation on
the parent body? Launches from the Moon take less
energy. Lower energy means that impactors may be
smaller (and more numerous) and implies a lower proba-
bility of excavating deeper-lying material [
Warren
, 1994;
Mileikowsky et al.
[2000];
Wieler
, 2002;
Nyquist et al.
,
2001;
Herzog and Caffee
, 2014].
dispersion into sporadic meteoroids. The maximum par-
ticle radius of 10 cm considered in these calculations is
not so different from preatmospheric radii estimated for
many Antarctic meteorites, which suggests that the
influence of the original stream may persist for up to
50 ka.
Pauls and Gladman
[2005] carried out detailed sim-
ulations following the orbital parameters of stream frag-
ments in Earth-intersecting orbits. They concluded that
dissipation occurs in time periods of ~10
4
to 10
5
years.
The association of meteorites with meteor streams
based on trace element evidence has met with limited
acceptance. Although of high quality, the trace element
data are discussed in a way that is difficult for non-
statisticians to comprehend. Other debated criticisms
have focused on the possibilities that (1) differences in
the trace-element profiles may reflect weathering rather
than orbital effects; and (2) unrecognized pairs were
erroneously counted separately, thereby muddling the
find statistics [see
Koeberl and Cassidy
, 1991].
In our view, however, the most significant challenges to
the stream hypothesis have come from the implications of
cosmogenic nuclide measurements.
Schultz et al
. [1991]
and
Schultz and Weber
[1996] measured cosmogenic
noble gases in H chondrites from Victoria Land. The
range of CRE ages, from 4 to 70 Ma, requires a major
stretch of estimates of stream life times cited above. In
addition the concentration patterns of cosmogenic
3
He,
21
Ne, and
38
Ar and radiogenic
4
He and
40
Ar show that
some of the meteoroids in this group of H chondrites lost
3
He and
4
He in the last few Ma, probably while at or near
perihelion, while others did not. It is difficult to see how
a single astronomical stream could accommodate objects
with such different orbital elements.
9.3.3. Meteor Streams
During the 1990s, M. E. Lipschutz, S. P. Wolf, and their
coworkers analyzed and interpreted the trace element con-
centrations of numerous H and other chondrites from the
Antarctic. They inferred that non-Antarctic meteorites
had thermal histories different from those of meteorites
from Victoria Land, but not from Queen Maud Land
[
Dennison and Lipschutz
, 1987;
Wolf and Lipschutz
, 1995a,
b;
Michlovich et al
., 1995, and references therein]. In dis-
cussing the difference,
Denison and Lipschutz
[1987] argued
that “As the mean terrestrial ages of the two groups
(Victoria Land and non-Antarctic meteorites) differ by
about 10
5
yr, a change in meteoroid flux with time could be
invoked.” More broadly, the authors suggested that the
Earth samples “streams” of meteoroids (objects with
similar orbital elements) whose fluxes may ebb or flow.
While astronomical evidence establishes firmly the
existence of cometary and asteroidal meteoroid streams
[
Lipschutz et al
., 1997;
Wiegert et al
., 2009;
Williams and
Jones
, 2007], the stream lifetimes have been controversial.
Studies of L chondrites especially show that the terres-
trial fluxes of meteorites from a single collisional event
may continue for up to 500 Ma [
Schmitz and Häggström
,
2003;
Heck et al
., 2004], but to our knowledge no one has
suggested that streams can preserve their orbital elements
for so long.
Wetherill
[1986] asserted streams may not
persist for more than 10 ka. Modeling by
Wiegert et al
.
[2009] for particles with radii up to 10 cm follows the evo-
lution of the terrestrial meteoroid flux from a single event
for over 50 ka, that is, from the creation of a stream to its
9.4. CONCLUSIONS
From the 1970s onwards, the return of Antarctic
meteorites gave an enormous boost to the study of
meteoritics in general and of cosmogenic nuclides and
cosmic-ray exposure histories in particular. With the
investigation of Antarctic lunar meteorites has come
improved understanding of meteoroid ejection process
and the transit to Earth. Through their greater numbers
and clustering, the exposure ages of Antarctic martian
meteorites have helped to identify major launch events
from Mars. Newly documented and surprisingly short
exposure ages for many CI and CM chondrites have
pointed strongly to a source in a nearby orbit [
Nishiizumi
and Caffee
, 2012]. The distribution of terrestrial ages of
Antarctic finds tells us that to first order, the Earth has
been sampling a similar population of meteorites for
up to 2 Ma and serves as a warning or perhaps an invi-
tation to consider the influence of terrestrial weathering
on meteoroid arrival rates or fluxes as a function of
