Figure 12. Carbon oxide species released during a TPD of a CO ice exposed to keV electrons. Species sublimating into the gas-phase are detected by a residual gas analyzer (RGA). Shown are the profiles of parent/daughter ions/fragment ions for C 3 O 2
The
organic inventories of carbonaceous chondrites (CCs) provide
insights into the physicochemical environments involved in the Solar
System’s formation and the postaccretionary evolution of meteorite
parent bodies. Studying changes in these inventories across samples
that have experienced varying degrees of aqueous/hydrothermal alteration
untangles one aspect of such complex records. Here, the polycyclic
aromatic hydrocarbon (PAH) and heterocyclic aromatic compound (HAC)
contents of 15 CCs representing CI1, CM1, CM2, C2u, C3u, and CO3 classes
were probed with two-step laser mass spectrometry (L2MS)
and subjected to a comparative principal component analysis (PCA),
a multivariate analysis method. PAHs with mass-to-charge ratios (m/z) of 128 to >300 were detected, with
large PAHs (4–7 rings) dominating the spectra of the most aqueously
altered samples like Ivuna (CI1) and NWA 12328 (CM1), while a larger
relative fraction of smaller PAHs (3–4 rings) was observed
in less altered samples like Chwichiya 002 (C3.00u). The same trend
was observed across CM2 samples with varying degrees of aqueous alteration
with the exception of Jbilet Winselwan (CM2.3-2.5), which has experienced
impact shock. Alkylated homologues of C14H10 and C16H10 were detected in all samples, with
the latter showing a stronger correlation with aqueous alteration.
Additionally, oxygen-containing PAHs and thiophenes were detected.
These experiments demonstrate that aqueous alteration induces aromatic
condensation in CCs of multiple chemical groups, and many of the 4–7-ring
PAHs (>215 m/z) in CCs may be
products
of aqueous alteration. Thus, PAH size distributions are an additional
parameter to consider when evaluating a meteorite’s alteration
history.
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