Observations of the Icy Universe Boogert, A.C. Adwin; Gerakines, Perry A; Whittet, Douglas C.B
Annual review of astronomy and astrophysics,
08/2015, Letnik:
53, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Freeze-out of the gas-phase elements onto cold grains in dense interstellar and circumstellar media builds up ice mantles consisting of molecules that are mostly formed in situ (H
2
O, NH
3
, CO
2
, ...CO, CH
3
OH, and more). This review summarizes the detected infrared spectroscopic ice features and compares the abundances across Galactic, extragalactic, and Solar System environments. A tremendous amount of information is contained in the ice band profiles. Laboratory experiments play a critical role in the analysis of the observations. Strong evidence is found for distinct ice formation stages, separated by CO freeze-out at high densities. The ice bands have proven to be excellent probes of the thermal history of their environment. The evidence for the long-held idea that processing of ices by energetic photons and cosmic rays produces complex molecules is weak. Recent state-of-the-art observations show promise for much progress in this area with planned infrared facilities.
In the denser and colder (≤20 K) regions of the interstellar medium (ISM), near-infrared observations have revealed the presence of submicron-sized dust grains covered by several layers of ...H2O-dominated ices and "dirtied" by the presence of other volatile species. Whether a molecule is in the gas or solid-phase depends on its binding energy (BE) on ice surfaces. Thus, BEs are crucial parameters for the astrochemical models that aim to reproduce the observed evolution of the ISM chemistry. In general, BEs can be inferred either from experimental techniques or by theoretical computations. In this work, we present a reliable computational methodology to evaluate the BEs of a large set (21) of astrochemical relevant species. We considered different periodic surface models of both crystalline and amorphous nature to mimic the interstellar water ice mantles. Both models ensure that hydrogen bond cooperativity is fully taken into account at variance with the small ice cluster models. Density functional theory adopting both B3LYP-D3 and M06-2X functionals was used to predict the species/ice structure and their BEs. As expected from the complexity of the ice surfaces, we found that each molecule can experience multiple BE values, which depend on its structure and position at the ice surface. A comparison of our computed data with literature data shows agreement in some cases and (large) differences in others. We discuss some astrophysical implications that show the importance of calculating BEs using more realistic interstellar ice surfaces to have reliable values for inclusion in the astrochemical models.
Abstract
Laboratory results of the optical properties of vapor-deposited water ice, specifically the refractive index and extinction coefficient, are available mainly for a selective set of ...wavelengths and a limited number of deposition temperatures. Experimental limitations are the main reason for the lack of broadband data, which is unfortunate as these quantities are needed to interpret and predict astronomical and planetary observations. The goal of this work is to address these lacking data, using an experimental broadband method that is capable of rapidly providing reliable water ice data across the entire UV–visible range. This approach combines the simultaneous use of a monochromatic HeNe laser and a broadband Xe-arc lamp to record interference fringes of water ice during deposition at astronomically relevant ice temperatures. The ice thickness is typically more than 20
μ
m. Analyzing the period and intensity patterns combining both the monochromatic and broadband interference patterns allows the determination of the wavelength-dependent refractive index and extinction coefficient. We present accurate refractive index and extinction coefficient graphs for wavelengths between 250 and 750 nm and ices deposited between 30 and 160 K. From our data, we find a possible structural change in the ice in the 110–130 K region that has not been reported before. We also discuss that the data presented in this paper can be used to interpret astronomical observations of icy surfaces.
Abstract
It is reasonable to assume that the structure of a planet and the interior distribution of its components are determined by its formation history. We thus follow the growth of a planet from ...a small embryo through its subsequent evolution. We estimate the accretion rate range based on a protoplanetary disk model at a large-enough distance from the central star for water ice to be a major component. We assume the accreted material to be a mixture of silicate rock and ice, with no H–He envelope, as the accretion timescale is much longer than the time required for the nebular gas to dissipate. We adopt a thermal evolution model that includes accretional heating, radioactive energy release, and separation of ice and rock. Taking the Safronov parameter and the ice-to-rock ratio as free parameters, we compute growth and evolutionary sequences for different parameter combinations, for 4.6 Gyr. We find the final structure to depend significantly on both parameters. Low initial ice-to-rock ratios and high accretion rates, each resulting in an increased heating rate, lead to the formation of extended rocky cores, while the opposite conditions leave the composition almost unchanged and result in relatively low internal temperatures. When rocky cores form, the ice-rich outer mantles still contain rock mixed with the ice. We find that a considerable fraction of the ice evaporates upon accretion, depending on parameters, and assume it is lost, thus the final surface composition and bulk density of the planet do not necessarily reflect the protoplanetary disk composition.
Abstract
The absolute absorption cross section of dangling OH bonds in water ice, a free OH stretch mode by three-coordinated surface water molecules, is derived experimentally as 1.0 ± 0.2 × 10
−18
...cm
2
at 3696 cm
−1
for amorphous water at 90 K using infrared multiple-angle incidence resolution spectrometry (IR–MAIRS). The integrated absorption cross section (band strength) of the dangling OH bond at 90 K (1.4 ± 0.3 × 10
−17
cm molecule
−1
at 3710–3680 cm
−1
) is found to be more than 1 order of magnitude smaller than those in bulk ice or liquid water. This indicates that a lack of hydrogen-bonding significantly decreases the band strength of dangling OH bonds. The present study also provides average molecular orientations of dangling OH bonds at 10 K and 90 K, because both the surface-parallel (in-plane) and surface-perpendicular (out-of-plane) vibration spectra of dangling OH bonds are quantitatively measured by IR–MAIRS. The intensity ratio of the dangling-OH peaks between in-plane to out-of-plane spectra shows the isotropic nature (random orientation) of the two- and three-coordinated dangling OH bonds in microporous amorphous water prepared at 10 K; however, the three-coordinated dangling OH bonds in nonporous amorphous water prepared at 90 K are dominantly located at the top ice surface and oriented perpendicular to it. These findings provide fundamental insights into the relationship between the structure and optical properties of ice surfaces, and aid quantitative understanding of the surface structure of interstellar ices and their laboratory analogs.
Numerous investigations have measured and identified properties of water and exotic ices. These properties are measured at various pressures and temperatures; however, these disparate studies are ...sometimes difficult to find, confusing, or even contradictory. Here, we amass and present a catalogue of some temperature dependent equations on ice density, latent heat of fusion, constant values for latent heat of sublimation, specific heat capacity at constant pressure, thermal conductivity, and Arrhenius flow law parameters for water ice, carbon dioxide ice, and solid nitrogen. The equations are either directly cited from the sources or are fitted from data in past publications.
A new experimental setup, INterStellar Ice-Dust Experiment (INSIDE), was designed for studying cosmic grain analogs represented by ice-coated carbon- and silicate-based dust grains. With the new ...instrument, we can simulate the physical and chemical conditions prevailing in interstellar and circumstellar environments. The setup combines ultrahigh vacuum and low-temperature conditions with infrared spectroscopy and mass spectrometry. Using INSIDE, we plan to investigate physical and chemical processes, such as adsorption, desorption, molecule formation, on the surface of dust/ice samples. First experiments on the photodesorption of water ice molecules from the surface of silicate and carbon grains by UV photons revealed a strong influence of the surface properties on the desorption yield, in particular in the monolayer regime. In the second experiment, the thermal desorption of cometary ice analogs composed of six molecular components was studied for the first time. Codesorption of CO2 and CH3OH with O2 indicates that at high O2 concentrations in cometary or interstellar ices, "heavy" ice molecules can be partly trapped in O2 and enter the gas phase much earlier than expected. This effect could explain astronomical detections of complex organic molecules in cold dense interstellar clouds.
Pore Accessibility in Amorphous Solid Water Carmack, Rebecca A.; Tribbett, Patrick D.; Loeffler, Mark J.
The Astrophysical journal,
01/2023, Letnik:
942, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
The porous nature of amorphous solid water (ASW) can significantly effect the chemical evolution of any planetary or astrophysical surface it forms on due to its ability to trap and retain ...volatiles. The amount of volatiles that can enter an ASW grain or mantle is limited by how interconnected the pores are to each other and to the exterior surface. Previous laboratory studies examined the interconnectivity of ASW pores in thin ASW films relevant to ice mantles on interstellar grains. Here, we investigate to what extent the interconnectivity of pores and subsequent gas absorption properties of ASW change as one moves toward thicker samples (up to ∼10
19
H
2
O cm
−2
or ∼4
μ
m) more representative of icy material found in the outer solar system. We find that for all film thicknesses studied, the internal pores are accessible from the sample’s surface, and the amount of gas needed to fill the pores increases linearly with the ASW column density. This linear relation supports that the interconnectivity to the surface will persist in ices that are much thicker than those we were able to study, suggesting that the amount of contaminant gas trapped within ASW can significantly alter the chemical evolution of a variety of ASW-rich surfaces in the outer solar system.
Over the last 16 years, the Mars Climate Sounder (MCS) onboard the Mars Reconnaissance Orbiter (MRO) has acquired a large body of surface observations at visible and thermal infrared wavelengths. ...Primary differentiators between the MCS record and other surface datasets include mission duration, regular global coverage, and high emission angles associated with most observations. These data have been analyzed to generate a global median apparent thermal inertia map that smooths out artificial spatial variability (i.e., streaking along orbital ground tracks) common in other maps. At mid and low latitudes, high emission angle observations yield similar nighttime thermal inertia values compared to nadir observations, even if grazing angles should favor vertically rough materials oriented towards MCS (i.e., material poking out of the ground, presumably associated with high thermal inertia rocks and scarps). This result independently confirms that fines control the Martian thermal inertia, not rocks, and also suggests that most rock and bedrock exposures should be characterized by low aspect ratios (i.e., appear platy or flat, regardless of subsurface shape). Selected global temperature maps are also presented. They show the influence of polar processes, the latitudinal distribution of exposed ices, and physical properties of the Martian regolith on surface temperatures. Temperature controls at regional and global scales include the variation of insolation with season and latitude, atmospheric composition, global circulation, the presence of snow precipitations, and various regolith properties (i.e., albedo and thermal inertia).
•Leveraging 16 years of Mars Climate Sounder observations, we have generated new thermal inertia and global temperatures maps.•Their analysis confirms that the thermal inertia of the martian surface layer is controlled by the fines, not by rocks.•Generally, Martian rocks must be platy/oblate to match both nadir and high emission angle thermal observations.
PDF
