95 research outputs found
A Systematic Study of the Correlations Between Meteorite Impacts and Soot Formation
A massive extinction of more than 50 percent of existing life forms on Earth occurred 65 million years (Ma) ago. This event is marked in the geological record by the Cretaceous-Tertiary (KT) boundary and corresponds to the Chicxulub meteorite impact in the Yucatan Peninsula. Since 1985, large quantities ofreduced elemental carbon in the form of characteristic spheroidal clusters of soot have been found in twelve KT boundary sites from across the globe. Because ofthe wide geographic distribution ofthese sites, the data was interpreted to indicate that deposition ofsoot was a global phenomenon. The source of this global soot layer is suspected to be eolian (airfall) deposition of fine-grained particles resulting from widespread wildfires. A global soot concentration of 2.2 ± 0.7 . mg/cm2 has been estimated from these studies.
As a systematic study of the correlation between meteorite impact and soot formation, five sedimentary sequences ofmeteorite impact-related samples were analyzed for the presence ofelemental carbon in the form of soot. Two KT boundary sedimentary sequences from the Berwind, Colorado, and Madrid, Colorado sites were analyzed as continuations of previous KT boundary studies. Soot concentrations of 3000 ± 0 parts per million (ppm) and 780 ± 90 ppm, respectively, were found in the KT boundary sediments ofthese sequences. These concentrations are similar to those found in previous KT boundary studies and further refine the KT boundary global soot concentration value.
Likewise, samples from Deep Sea Drilling Project (DSDP) Core 465-A were analyzed as a determination ofthe global nature ofthe soot layer. A soot concentration of 550 ± 750 ppm was found, again similar to those found in previous KT boundary studies. The mid-Pacific location of Core 465-A rules out the possibility ofsoot deposition from\u27 groundwater runoff and indicates that eolian transport is the most probable mecharllsm for soot deposition. Therefore, the presence of soot in boundary sediments in DSDP Core 465-A supports the theory that the KT boundary soot layer is global.
In addition, samples related to two other impact events of differing size and age were analyzed for evidence ofwildfires. In the first study, samples were analyzed from the Sudbury, Ontario impact structure. This structure is the result of an impact similar in size to the Chicxulub event occurring 1850 Ma ago. These sediments were found to contain soot similar in concentrations to those found in KT boundary studies, ranging from 2300 ± 200 ppm soot to 3000 ± 300 ppm soot across the post-impact sedimentary sequence. The presence of soot in these sediments further strengthens the correlation between meteorite impact and soot formation.
In a second study, samples were analyzed from the Gardnos impact structure, Norway. This structure is the result ofan impact at least one order ofmagnitude smaller than the Chicxulub event occurring between 900 and 400 Ma ago~ No appreciable amounts of soot were found in sediments directly related to the impact event. The absence ofsoot in Gardnos samples suggests that an impact event ofthis size is below the impact threshold required for ignition of soot-producing wildfires
MILLIMETER AND SUBMILLIMETER STUDIES OF O(1D) INSERTION REACTIONS TO FORM MOLECULES OF ASTROPHYSICAL INTEREST
While both the number of detected interstellar molecules and their chemical complexity continue to increase, understanding of the processes leading to their formation is lacking. Our research group combines laboratory spectroscopy, observational astronomy, and astrochemical modeling for an interdisciplinary examination of the chemistry of star and planet formation. This talk will focus on our laboratory studies of O(D) insertion reactions with organic molecules to produce molecules of astrophysical interest. By employing these reactions in a supersonic expansion, we are able to produce interstellar organic reaction intermediates that are unstable under terrestrial conditions; we then probe the products using millimeter and submillimeter spectroscopy. We benchmarked this setup using the well-studied O(D) + methane reaction to form methanol. After optimizing methanol production, we moved on to study the O(D) + ethylene reaction to form vinyl alcohol (CHCHOH), and the O(D) + methyl amine reaction to form aminomethanol (NHCHOH). Vinyl alcohol measurements have now been extended up to 450 GHz, and the associated spectral analysis is complete. A possible detection of aminomethanol has also been made, and continued spectral studies and analysis are underway. We will present the results from these experiments and discuss future applications of these molecular and spectroscopic techniques
A theoretical study of the conversion of gas phase methanediol to formaldehyde
Methanediol, or methylene glycol, is a product of the liquid phase reaction of water and formaldehyde and is a predicted interstellar grain surface species. Detection of this molecule in a hot core environment would advance the understanding of complex organic chemistry in the interstellar medium, but its laboratory spectroscopic characterization is a prerequisite for such observational searches. This theoretical study investigates the unimolecular decomposition of methanediol, specifically the thermodynamic and kinetic stability of the molecule under typical laboratory and interstellar conditions. Methanediol was found to be thermodynamically stable at temperatures of <100 K, which is the characteristic temperature range for interstellar grain mantles. The infinite-pressure RRKM unimolecular decomposition rate was found to be <10^(−18) s^(−1) at 300 K, indicating gas phase kinetic stability for typical laboratory and hot core temperatures. Therefore, both laboratory studies of and observational searches for this molecule should be feasible
Aminomethanol water elimination: Theoretical examination
The mechanism for the formation of hexamethylenetetraamine predicts the formation of aminomethanol from the addition of ammonia to formaldehyde. This molecule subsequently undergoes unimolecular decomposition to form methanimine and water. Aminomethanol is the predicted precursor to interstellar glycine, and is therefore of great interest for laboratory spectroscopic study, which would serve as the basis for observational searches. The height of the water loss barrier is therefore useful in the determination of an appropriate experimental approach for spectroscopic characterization of aminomethanol. We have determined the height of this barrier to be 55 kcal/mol at ambient temperatures. In addition, we have determined the infinite-pressure Rice-Ramsperger-Kassel-Marcus unimolecular decomposition rate to be < 10^(-25) s^(-1) at 300 K, indicating gas-phase kinetic stability for typical laboratory and hot core temperatures. Therefore, spectroscopic characterization of and observational searches for this molecule should be straightforward provided an efficient formation mechanism can be found
A CSO Search for -CH: Detection in the Orion Bar PDR
The results of a Caltech Submillimeter Observatory (CSO) search for
-CH, first detected by Pety et al. (2012) in observations toward the
Horsehead photodissociation region (PDR), are presented. A total of 39 sources
were observed in the 1 mm window. Evidence of emission from -CH is
found in only a single source - the Orion Bar PDR region, which shows a
rotational temperature of 178(13) K and a column density of 7(2) x
cm. In the remaining sources, upper limits of ~10
cm are found. These results are discussed in the context of guiding
future observational searches for this species.Comment: 9 pages, 8 figures, 4 table
A search for ortho-benzyne (o-C6H4) in CRL 618
Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential
carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar
bands (DIBs). PAHs are not likely to form by gas-phase or solid-state
interstellar chemistry, but rather might be produced in the outflows of
carbon-rich evolved stars. PAHs could form from acetylene addition to the
phenyl radical (C6H5), which is closely chemically related to benzene (C6H6)
and ortho-benzyne (o-C6H4). To date, circumstellar chemical models have been
limited to only a partial treatment of benzene-related chemistry, and so the
expected abundances of these species are unclear. A detection of benzene has
been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but
no other benzene-related species has been detected in this or any other source.
The spectrum of o-C6H4 is significantly simpler and stronger than that of C6H5,
and so we conducted deep Ku-, K- and Q-band searches for o-C6H4 with the Green
Bank Telescope. No transitions were detected, but an upper limit on the column
density of 8.4x10^13 cm^-2 has been determined. This limit can be used to
constrain chemical models of PPNe, and this study illustrates the need for
complete revision of these models to include the full set of benzene-related
chemistry.Comment: 13 pages, 4 figures, to be published in The Astrophysical Journal
Letter
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