Geology Reference
In-Depth Information
Band sawing frequently leaves dark streaks on the cut
face of a meteorite, and the identity of this dark material
was unknown for many years. To learn more about the
effects of band sawing and to identify the components
of the dark streaks, a band-sawed pristine
Apollo
sample
known to have low levels of organics [
Allton
, 1999;
Clemett et al.
, 2005] was studied using a variety of micros-
copy techniques. Little organic contamination was found,
but significant Ni and C contamination from the saw
blade was found on the cut surface (C was also found by
Wright et al
. [1993]). These are some known examples of
contaminants introduced during the handling of meteor-
ites. There may be others depending upon materials and
procedures used by various groups. The curators of the
collection have always been open to modifying proce-
dures and approaches when new information comes to
light about potential contaminants.
Focusing on a specific organic compound (amino
acids) illustrates that the clean room environment for
Antarctic meteorites has proven acceptable for detailed
studies of carbonaceous chondrites. Because Antarctic
meteorites are recovered from blue ice in Antarctica and
have a finite residence time (up to 1,200,000 years) on
Earth before recovery, they may obtain organic contami-
nation from their terrestrial environment. Therefore, it
might be prudent to compare the total organic carbon
(TOC) contents of
Apollo
lunar sample cabinets to those
in the Antarctic meteorite lab. In 2000 the TOC content
of an
Apollo
16 pristine sample cabinet in B31N (PSL-38)
was 6.28 ng/cm
2
, and in 2007, TOC of the returned sample
processing cabinet in B31N was measured at 5.3 ng/ cm
2
(
J. Allton
, pers. communication). In both of these mea-
surements, a silicon wafer was exposed in the cabinet for
48 hours. These measurements are in line with the low
levels of organic contamination estimated from the early
Apollo
days (1-10 ng/cm
2
) as well as the very low levels of
organics detected in
Apollo
samples especially after
Apollo
12 [
Allton
, 1999]. Finally,
Brinton and Bada
[1996]
measured 15 ppb of amino acids in an
Apollo
17 sample,
which was stored for ~23 years at JSC before analysis.
Some of the measured amino acids were clearly indige-
nous to the sample, which means the amount due to ter-
restrial contamination was much lower, perhaps 5 ppb. In
comparison to the lunar cabinets, the carbonaceous
chondrite cabinet TOC was measured at 0.4 ng/cm
2
, even
smaller than that measured in
Apollo
lunar sample
processing cabinets. It was from this environment in the
carbonaceous chondrite processing cabinet that samples
of the CM2 chondrite LEW 90500 and ungrouped carbo-
naceous chondrite LON 94102 were processed and sent
out to various investigators, including
Glavin and Dworkin
[2009], who measured from 2 to 65 ppm total amino acids
in these and many other carbonaceous chondrite samples
and types, the majority of which were extraterrestrial in
origin (Figure 3.5). In addition, amino acids in lunar
meteorite MAC 88105 (Plate 68), processed on an open
flow bench in the Antarctic meteorite lab, were below
blank levels, <10-20 ppb [
Brinton and Bada
, 1996].
Thus, it is clear that the environments maintained in
JSC GN2 cabinets have acceptably low levels of TOC
such that contamination of amino acids does not inter-
fere with measurements of the low concentrations of
martian meteorites, or the much higher concentrations in
carbonaceous chondrites.
3.2.8. Long-Term Storage
By the mid-1980s, the collection had grown to >5,000
samples and JSC had reached capacity in terms of storage
space. The MWG initial vision provided guidelines for
managing the long-term storage of meteorites at the
Smithsonian. Scientific interest in EOCs is lower than
other groups, and because there are so many EOCs
(~90%) in the U.S. Antarctic meteorite collection, proce-
dures were defined for formal transfer of such samples on
a regular basis to the Smithsonian to make more space at
JSC for the active research collection. Currently, if an
EOC specimen at JSC has not been requested or allo-
cated for ~4 to 5 years, it is a candidate for transfer. A
representative sampling of meteorites from each icefield,
however, is kept at JSC for long-term storage, so that
there is a set of samples available for study that have been
stored in the same environment over a long period of
time. Long-term storage of all meteorites at the
Smithsonian (except irons and pallasites) is located at
the Museum Support Center in Suitland, MD, USA
(established in 1983). Nearly 20,000 meteorites are now
stored in a Class 10,000 clean room (completed in 2011)
with 14 stainless steel GN2 glove boxes, outfitted with
hypalon gloves (Figure 3.2). Meteorites are segregated
into cabinets and pans by class. Samples, which are only
removed for allocations, are enclosed in the same style
and materials as those used at JSC. Sampling takes place
on a clean room laminar flow bench (Figure 3.2). Tools,
sample containers, and bags are made of stainless steel,
aluminum, Teflon, nylon, or polyethylene zip lock bags.
Polyvials are also used as sample containers (polystyrene
vials with polyethylene lids).
3.3. SAMPLES: CASE STUDIES
The value of detailed curation of samples in a con-
trolled minimal contamination environment becomes
clear when considering several case histories. The
program had just been established when the collection
received one of its most famous samples: the first recog-
nized lunar meteorite ALH A81005 (Plate 64). The chal-
lenges this sample posed to the MWG have been best
described elsewhere [
Cassidy
, 2003], but the detailed sub-
division of this lunar meteorite is described later in this
chapter. Although collected before ALH A81005, EET
A79001 (Plate 70) was not proposed to be a piece of
