CO DIFFUSION INTO AMORPHOUS H2O ICES

Floating in Space: How to Treat the Weak Interaction between CO Molecules in Interstellar Ices

In the interstellar medium, six molecules have been conclusively detected in the solid state in interstellar ices, and a few dozen have been hypothesized and modeled to be present in the solid state as well. The icy mantles covering micrometer-sized dust grains are, in fact, thought to be at the core of complex molecule formation as a consequence of the local high density of molecules that are simultaneously adsorbed. From a structural perspective, the icy mantle is considered to be layered, with an amorphous water-rich inner layer surrounding the dust grain, covered by an amorphous CO-rich outer layer. Moreover, recent studies have suggested that the CO-rich layer might be crystalline and possibly even be segregated as a single crystal atop the ice mantle. If so, there are far-reaching consequences for the formation of more complex organic molecules, such as methanol and sugars, that use CO as a backbone. Validation of these claims requires further investigation, in particular on acquiring atomistic insight into surface processes, such as adsorption, diffusion, and reactivity on CO ices. Here, we present the first detailed computational study toward treating the weak interaction of (pure) CO ices. We provide a benchmark of the performance of various density functional theory methods in treating the binding of pure CO ices. Furthermore, we perform an atomistic and in-depth study of the binding energy of CO on amorphous and crystalline CO ices using a pair-potential-based force field. We find that CO adsorption is represented by a large distribution of binding energies (200-1600 K) on amorphous CO, including a significant amount of weak binding sites (<350 K). Increasing both the cluster size and the number of neighbors increases the mean of the observed binding energy distribution. Finally, we find that CO binding energies are dominated by dispersion and, as such, exchange-correlation functionals need to include a treatment of dispersion to accurately simulate surface processes on CO ices. In particular, we find the ωB97M-V functional to be a strong candidate for such simulations.

Potential of Good's buffers to inhibit denaturation of myofibrillar protein upon freezing

The current systematic study sought to examine the potential use of three Good's buffers (MES, MOPS and HEPES) in inhibiting myofibrillar protein (MFP) denaturation induced by acidity changes. The highest degree of acidity variation was found in the center and bottom of large bottles due to the freeze-concentration effect. Good's buffer tended to basify during freezing, and it could prevent the crystallization of sodium phosphate (Na-P) buffer. Acidification upon freezing Na-P disrupted the natural conformation of MFP and induced the formation of large proteins aggregates with tight packing. The 15 mM MES, 20 mM MOPS, and 30 mM HEPES were respectively added to neutralize the strong acidity drop induced by freezing 20 mM Na-P, and all of them significantly improved the stability of the MFP conformation (P < 0.05). This work is not only critical to meet the growing demand for protein, but also groundbreaking for broadening the applicability of Good's buffers in the food industry.

VIZSLA-Versatile Ice Zigzag Sublimation Setup for Laboratory Astrochemistry

In this article, a new multi-functional high-vacuum astrophysical ice setup, VIZSLA (Versatile Ice Zigzag Sublimation Setup for Laboratory Astrochemistry), is introduced. The instrument allows for the investigation of astrophysical processes both in a low-temperature para-H₂ matrix and in astrophysical analog ices. In the para-H₂ matrix, the reaction of astrochemical molecules with H atoms and H⁺ ions can be studied effectively. For the investigation of astrophysical analog ices, the setup is equipped with various irradiation and particle sources: an electron gun for modeling cosmic rays, an H atom beam source, a microwave H atom lamp for generating H Lyman-α radiation, and a tunable (213-2800 nm) laser source. For analysis, an FT-IR (and a UV-visible) spectrometer and a quadrupole mass analyzer are available. The setup has two cryostats, offering novel features for analysis. Upon the so-called temperature-programmed desorption (TPD), the molecules, desorbing from the substrate of the first cryogenic head, can be mixed with Ar and can be deposited onto the substrate of the other cryogenic head. The efficiency of the redeposition was measured to be between 8% and 20% depending on the sample and the redeposition conditions. The well-resolved spectrum of the molecules isolated in an Ar matrix serves a unique opportunity to identify the desorbing products of a processed ice. Some examples are provided to show how the para-H₂ matrix experiments and the TPD-matrix-isolation recondensation experiments can help understand astrophysically important chemical processes at low temperatures. It is also discussed how these experiments can complement the studies carried out by using similar astrophysical ice setups.

Making good's buffers good for freezing: The acidity changes and their elimination via mixing with sodium phosphate

Solutions of three Good's buffers (HEPES, MOPS, and MES), both pure and mixed with sodium phosphate buffers (Na-P), are investigated in terms of the freezing-induced acidity changes in their operational pH ranges. The Good's buffers have the tendency to basify upon freezing and, more intensively, at lower pHs. The acidity varies most prominently in MES, where the change may reach the value of two. Importantly, the Good's buffers are shown to mitigate the strong acidification in the Na-P buffer. Diverse concentrations of the Good's buffers are added to cancel out the strong, freezing-induced acidity drop in 50 mM Na-P that markedly contributes to the solution's acidity; the relevant values are 3 mM HEPES, 10 mM MOPS, and 80 mM MES. These buffer blends are therefore proposed to be applied in maintaining approximately the acidity of solutions even after the freezing process and, as such, should limit the stresses for frozen chemicals and biochemicals.

On the Origin and Evolution of the Material in 67P/Churyumov-Gerasimenko

Primitive objects like comets hold important information on the material that formed our solar system. Several comets have been visited by spacecraft and many more have been observed through Earth- and space-based telescopes. Still our understanding remains limited. Molecular abundances in comets have been shown to be similar to interstellar ices and thus indicate that common processes and conditions were involved in their formation. The samples returned by the Stardust mission to comet Wild 2 showed that the bulk refractory material was processed by high temperatures in the vicinity of the early sun. The recent Rosetta mission acquired a wealth of new data on the composition of comet 67P/Churyumov-Gerasimenko (hereafter 67P/C-G) and complemented earlier observations of other comets. The isotopic, elemental, and molecular abundances of the volatile, semi-volatile, and refractory phases brought many new insights into the origin and processing of the incorporated material. The emerging picture after Rosetta is that at least part of the volatile material was formed before the solar system and that cometary nuclei agglomerated over a wide range of heliocentric distances, different from where they are found today. Deviations from bulk solar system abundances indicate that the material was not fully homogenized at the location of comet formation, despite the radial mixing implied by the Stardust results. Post-formation evolution of the material might play an important role, which further complicates the picture. This paper discusses these major findings of the Rosetta mission with respect to the origin of the material and puts them in the context of what we know from other comets and solar system objects.

Krypton isotopes and noble gas abundances in the coma of comet 67P/Churyumov-Gerasimenko

The Rosetta Orbiter Spectrometer for Ion and Neutral Analysis mass spectrometer Double Focusing Mass Spectrometer on board the European Space Agency's Rosetta spacecraft detected the major isotopes of the noble gases argon, krypton, and xenon in the coma of comet 67P/Churyumov-Gerasimenko. Earlier, it was found that xenon exhibits an isotopic composition distinct from anywhere else in the solar system. However, argon isotopes, within error, were shown to be consistent with solar isotope abundances. This discrepancy suggested an additional exotic component of xenon in comet 67P/Churyumov-Gerasimenko. We show that krypton also exhibits an isotopic composition close to solar. Furthermore, we found the argon to krypton and the krypton to xenon ratios in the comet to be lower than solar, which is a necessity to postulate an addition of exotic xenon in the comet.

Impact of oxygen chemistry on model interstellar grain surfaces

Temperature-programmed desorption (TPD) and reflection–absorption infrared spectroscopy (RAIRS) are used to probe the effect of atomic and molecular oxygen (O and O₂) beams on amorphous silica (aSiO₂) and water (H₂O) surfaces (porous-amorphous solid water; p-ASW, compact amorphous solid water; c-ASW, and crystalline solid water; CSW).

Simulations and spectra of water in CO matrices

Models for the inclusion of water molecules in carbon monoxide matrices are developed using density functional theory applied to amorphous solid systems. The models cover a large range of systems for smaller or larger CO matrices with different water content, consisting of either individual H₂O molecules or small clusters linked by H-bonds. The vibrational spectra of the samples are predicted at the minimum of their potential energy surface. The spectra allow instances where the water molecules remain isolated or form aggregates to be discerned, and they also provide an indication of the strength of the H-bonding, when present. The calculations support recent experimental observations that linked IR bands at 3707 cm^-1 and 3617 cm^-1 to the presence of unbound water molecules in water-poor CO/H₂O mixed ices. Assignment of some observed bands to water dimers or trimers is suggested as well. The residual static pressure in fixed-volume simulation cells is also calculated.

Surface heterogeneity and inhomogeneous broadening of vibrational line profiles

The surface heterogeneity of amorphous silica (aSiO₂) has been probed using coverage dependent temperature programmed desorption (TPD) of a simple probe molecule, carbon monoxide (CO), and is used to explain the inhomogeneous broadening of the CO stretching vibration in the infrared.

A new model of the chemistry of ionizing radiation in solids: CIRIS

We simulate irradiated O₂ ice, both reproducing measured ozone abundances and predicting the ice thickness of a previous experiment.