538 research outputs found
Prebiotic Vitamin B Synthesis in Carbonaceous Planetesimals
Aqueous chemistry within carbonaceous planetesimals is promising for
synthesizing prebiotic organic matter essential to all life. Meteorites derived
from these planetesimals delivered these life building blocks to the early
Earth, potentially facilitating the origins of life. Here, we studied the
formation of vitamin B as it is an important precursor of the coenzyme
NAD(P)(H), which is essential for the metabolism of all life as we know it. We
propose a new reaction mechanism based on known experiments in the literature
that explains the synthesis of vitamin B. It combines the sugar precursors
glyceraldehyde or dihydroxyacetone with the amino acids aspartic acid or
asparagine in aqueous solution without oxygen or other oxidizing agents. We
performed thermochemical equilibrium calculations to test the thermodynamic
favorability. The predicted vitamin B abundances resulting from this new
pathway were compared with measured values in asteroids and meteorites. We
conclude that competition for reactants and decomposition by hydrolysis are
necessary to explain the prebiotic content of meteorites. In sum, our model
fits well into the complex network of chemical pathways active in this
environment.Comment: Accepted for publication in ChemPlusChem. The authors Klaus Paschek
and Mijin Lee contributed equally. 18 pages, 7 figures (all colored).
Supporting Information is available at
https://doi.org/10.6084/m9.figshare.2432657
Day Plots of Bacterial Magnetite from Sediments of Shira Lake (Khakassia, Russia)
The domain state of magnetite detected in sediments of Shira lake (Khakassia, Russia) was examined by means of magnetic hysteresis. Analysis of experimental data obtained on samples from diο¬erent parts of bottom sediment cores in terms of Day plots allowed us to conclude that magnetite particles are in the pseudo-single-domain state. This indicates respectively small size of magnetite particles (< 100 nm) and reveals their bacterial origin. Biogenic magnetite buried in the bottom sediments can indicate the climatic changes in the Shira lake level in the Late Holocen
Using HCO isotopologues as tracers of gas depletion in protoplanetary disk gaps
The widespread rings and gaps seen in the dust continuum in protoplanetary
disks are sometimes accompanied by similar substructures seen in molecular line
emission. One example is the outer gap at 100 au in AS 209, which shows that
the HCO and CO emission intensities decrease along with the
continuum in the gap, while the DCO emission increases inside the gap.
We aim to study the behavior of DCO/HCO and DCO/HCO
ratios in protoplanetary disk gaps assuming the two scenarios: the gas
depletion follows the dust depletion and only the dust is depleted.
We first modeled the physical disk structure using the thermo-chemical model
ANDES. This 1+1D steady-state disk model calculates the thermal balance of gas
and dust and includes the FUV, X-rays, cosmic rays, and other ionization
sources together with the reduced chemical network for molecular coolants.
Afterward, this physical structure was adopted for calculations of molecular
abundances with the extended gas-grain chemical network with deuterium
fractionation. Ideal synthetic spectra and 0th-moment maps were produced with
LIME.
We are able to qualitatively reproduce the increase in the DCO intensity
and the decrease in the HCO and CO intensities inside the
disk gap, which is qualitatively similar to what is observed in the outer AS
209 gap. The corresponding disk model assumes that both the gas and dust are
depleted in the gap. The model with the gas-rich gap, where only the dust is
depleted, produces emission that is too bright in all HCO isotopologues and
CO.
The DCO/HCO line ratio can be used to probe gas depletion in
dust continuum gaps outside of the CO snow line. The DCO/CO line
ratio shows a similar, albeit weaker, effect; however, these species can be
observed simultaneously with a single ALMA or NOEMA setup.Comment: 12 pages, 7 figures, Accepted for publication in Astronomy and
Astrophysic
Effect of the Compaction Pressure and Ni Content on the Modified Aluminum-Based Perovskite Synthesis Designed to Immobilize the Radioactive Waste in Combustion Mode
The article deals with the synthesis of perovskite-like ceramics matrix material for immobilization of radioactive waste by SHS method. The dependence of the compaction pressure on the synthesis of the samples was established. Synthesis conditions for the matrix with the desired properties of the composition were determined that is acceptable for reliable isolation of radionuclides throughout the long-term storage of waste. The maximum amount of aluminum perovskite is observed when the initial mixture compaction pressure equal to 30 MPa and 25% wt. Nickel
Chemistry of a Protoplanetary Disk with Grain Settling and Ly_ Radiation
We present results from a model of the chemical evolution of protoplanetary disks. In our models, we directly calculate the changing propagation and penetration of a high energy radiation field with Ly_ radiation included. We also explore the effect on our models of including dust grain settling. We find that, in agreement with earlier studies, the evolution of dust grains plays a large role in determining how deep the UV radiation penetrates into the disk. Significant grain settling at the midplane leads to much smaller freeze-out regions and a correspondingly larger molecular layer, which leads to an increase in column density for molecular species such as CO, CN, and SO. The inclusion of Ly_ radiation impacts the disk chemistry through specific species that have large photodissociation cross sections at 1216Β Γ
. These include HCN, NH 3 , and CH 4 , for which the column densities are decreased by an order of magnitude or more due to the presence of Ly_ radiation in the UV spectrum. A few species, such as CO 2 and SO, are enhanced by the presence of Ly_ radiation, but rarely by more than a factor of a few.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90767/1/0004-637X_726_1_29.pd
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