H2O Ice Research Articles

THE Ch-CLASS ASTEROIDS: CONNECTING A VISIBLE TAXONOMIC CLASS TO A 3μm BAND SHAPE

Asteroids belonging to the Ch spectral taxonomic class are defined by the presence of an absorption near 0.7 {\mu}m, which is interpreted as due to Fe-bearing phyllosilicates. Phyllosilicates also cause strong absorptions in the 3-{\mu}m region, as do other hydrated and hydroxylated minerals and H2O ice. Over the past decade, spectral observations have revealed different 3-{\mu}m band shapes the asteroid population. Although a formal taxonomy is yet to be fully established, the "Pallas-type" spectral group is most consistent with the presence of phyllosilicates. If Ch class and Pallas type are both indicative of phyllosilicates, then all Ch-class asteroids should also be Pallas-type. In order to test this hypothesis, we obtained 42 observations of 36 Ch-class asteroids in the 2- to 4-{\mu}m spectral region. We found that 88% of the spectra have 3-{\mu}m band shapes most consistent with the Pallas-type group. This is the first asteroid class for which such a strong correlation has been found. Because the Ch class is defined by the presence of an absorption near 0.7 {\mu}m, this demonstrates that the 0.7-{\mu}m band serves not only as a proxy for the presence of a band in the 3-{\mu}m region, but specifically for the presence of Pallas-type bands. There is some evidence for a correlation between band depth at 2.95 {\mu}m and absolute magnitude and/or albedo. However, we find only weak correlations between 2.95-{\mu}m band depth and semi-major axis. The connection between band depths in the 0.7- and 3-{\mu}m regions is complex and in need of further investigation.

Shallow normal fault slopes on Saturnian icy satellites

AbstractFault dips are a function of the coefficient of internal friction, μi, of the lithospheric material. Laboratory deformation experiments of H2O ice at conditions applicable to icy bodies yield 0 ≤ μi ≤ 0.55 such that normal faults dip between 45° and 59°. We tested the hypothesis that normal faults on icy bodies reflect these values by using digital elevation models to examine geometries of large extensional systems on three Saturnian satellites. Analyzed faults within Ithaca Chasma on Tethys and Avaiki Chasmata on Rhea all exhibit shallower‐than‐predicted topographic slopes across the fault scarp, which we term “fault slopes.” A scarp of Padua Chasmata within Dione's Wispy Terrain also has a shallow fault slope, although three others that make up Palatine Chasmata exhibit steeper slopes as predicted. We infer that viscous relaxation is the most viable explanation for these shallow fault slopes, and we model the potential role of viscous relaxation in creating shallow slopes. Our modeling results support formation of these normal faults with steep dips consistent with deformation experiments, followed by their relaxation due to lithospheric heating events related to radionuclide decay. The steepest fault slopes in this terrain yield 0 ≤ μi ≤ 0.73 for Dione's lithospheric ice, which overlaps the dip range predicted from experiments. Results of this work suggest that viscous relaxation substantially affected fault slopes on Tethys, Rhea, and Dione. By implication, these processes may have also affected fault geometries on other icy satellites.

The mid‐IR Absorption Cross Sections of α‐ and β‐NAT (HNO3 · 3H2O) in the range 170 to 185 K and of metastable NAD (HNO3 · 2H2O) in the range 172 to 182 K

AbstractGrowth and Fourier transform infrared (FTIR) absorption in transmission of the title nitric acid hydrates have been performed in a stirred flow reactor (SFR) under tight control of the H2O and HNO3 deposition conditions affording a closed mass balance of the binary mixture. The gas and condensed phases have been simultaneously monitored using residual gas mass spectrometry and FTIR absorption spectroscopy, respectively. Barrierless nucleation of the metastable phases of both α‐NAT (nitric acid trihydrate) and NAD (nitric acid dihydrate) has been observed when HNO3 was admitted to the SFR in the presence of a macroscopic thin film of pure H2O ice of typically 1 µm thickness. The stable β‐NAT phase was spontaneously formed from the precursor α‐NAT phase through irreversible thermal rearrangement beginning at 185 K. This facile growth scheme of nitric acid hydrates requires the presence of H2O ice at thicknesses in excess of approximately hundred nanometers. Absolute absorption cross sections in the mid‐IR spectral range (700–4000 cm−1) of all three title compounds have been obtained after spectral subtraction of excess pure ice at temperatures characteristic of the upper troposphere/lower stratosphere. Prominent IR absorption frequencies correspond to the antisymmetric nitrate stretch vibration (ν3(NO3−)) in the range 1300 to 1420 cm−1 and the bands of hydrated protons in the range 1670 to 1850 cm−1 in addition to the antisymmetric O–H stretch vibration of bound H2O in the range 3380 to 3430 cm−1 for NAT.

Low-temperature chemistry between water and hydroxyl radicals: H/D isotopic effects

Sets of systematic laboratory experiments are presented – combining Ultra High Vacuum cryogenic and plasma-line deposition techniques – that allow us to compare H/D isotopic effects in the reaction of H2O (D2O) ice with the hydroxyl radical OD (OH). The latter is known to play a key role as intermediate species in the solid-state formation of water on icy grains in space. The main finding of our work is that the reaction H2O + OD → OH + HDO occurs and that this may affect the HDO/H2O abundances in space. The opposite reaction D2O + OH → OD + HDO is much less effective, and also given the lower D2O abundances in space not expected to be of astronomical relevance. The experimental results are extended to the other four possible reactions between hydroxyl and water isotopes and are subsequently used as input for Kinetic Monte Carlo simulations. This way we interpret our findings in an astronomical context, qualitatively testing the influence of the reaction rates.

The interstellar oxygen crisis, or where have all the oxygen atoms gone?

The interstellar medium (ISM) seems to have a significant surplus of oxygen which was dubbed as the "O crisis": independent of the adopted interstellar reference abundance, the total number of O atoms depleted from the gas phase far exceeds that tied up in solids by as much as ~160ppm of O/H. Recently, it has been hypothesized that the missing O could be hidden in micrometer-sized H2O ice grains. We examine this hypothesis by comparing the infrared (IR) extinction and far-IR emission arising from these grains with that observed in the Galactic diffuse ISM. We find that it is possible for the diffuse ISM to accommodate ~160ppm of O/H in micron-sized H2O ice grains without violating the observational constraints including the absence of the 3.1micron O-H absorption feature. More specifically, H2O ice grains of radii ~4micron and O/H = 160 ppm are capable of accounting for the observed flat extinction at ~ 3-8 micron and produce no excessive emission in the far-IR. These grains could be present in the diffuse ISM through rapid exchange of material between dense molecular clouds where they form and diffuse clouds where they are destroyed by photosputtering.

Reflectance spectroscopy of low atomic weight and Na-rich minerals: Borates, hydroxides, nitrates, nitrites, and peroxides

We have measured reflectance spectra (0.35–20μm) of a suite of minerals and synthetic compounds that contain low-Z (⩽Na) elements as the major cation and/or the major anion in oxides/oxyhydroxides, and are relevant to planetary geology and astrobiology. The suite comprises Na-borates, Na-, K-, Ca-hydroxides, nitrates, nitrites, and peroxides. Na-borate spectra exhibit B–O fundamental vibrations between 7 and 14μm, and overtones/combinations of these bands in the 1.55, 1.75, 2.15, and 2.25μm regions. Na-, K-, and Ca-hydroxide reflectance spectra are characterized by OH and metal–OH fundamental vibrations near 3, 8, and 18μm, and a number of overtone and combination absorption bands at shorter wavelengths, and a characteristic metal–OH band near 2.35μm. The nitrate and nitrite spectra exhibit fundamental N–O vibrations in the 7–14μm region and numerous combinations and overtones that are still detectable to as low as ∼1.8μm. Na-peroxide is largely spectrally featureless below 24μm, making its detection problematic, while H-peroxide has many OH-related absorption features below 2.5μm that differ in position from those of H2O ice and liquid. The results of this study indicate that the borates, hydroxides, nitrates, nitrite, and hydrogen peroxide can all be uniquely identified using characteristic absorption features that are present below 2.5μm. However, some of these features are weak, and their detectability will depend on the types and abundances of any accessory phases that may be present.

Impact craters: An ice study on Rhea

The goal of this project is to study the properties of H2O ice in the environment of the Saturn satellites and in particular to measure the relative amounts of crystalline and amorphous H2O ice in and around two craters on Rhea. The craters are remnants of cataclysmic events that, by raising the local temperature, melted the ice, which subsequently crystallized. Based on laboratory experiments it is expected that, when exposed to ion bombardment at the temperatures typical of the Saturn satellites, the crystalline structure of the ice will be broken, resulting in the disordered, amorphous phase. We therefore expect the ice in and around the craters to be partially crystalline and partially amorphous.We have designed a technique that estimates the relative amounts of crystalline and amorphous H2O ice based on measurements of the distortion of the 2-μm spectral absorption band. The technique is best suited for planetary surfaces that are predominantly icy, but works also for surfaces slightly contaminated with other ices and non-ice components. We apply the tool to two areas around the Inktomi and the Obatala craters. The first is a young impact crater on the leading hemisphere of Rhea, the second is an older one on the trailing hemisphere.For each crater we obtain maps of the fraction of crystalline ice, which were overlain onto Imaging Science Subsystem (ISS) images of the satellite searching for correlations between crystallinity and geography. For both craters the largest fractions of crystalline ice are in the center, as would be intuitively expected since the ‘ground zero’ areas should be most affected by the effects of the impact. The overall distribution of the crystalline ice fraction maps the shape of the crater and, in the case of Inktomi, of the rays. The Inktomi crater ranges between a maximum fraction of 67% crystalline ice to a minimum of 39%. The Obatala crater varies between a maximum of 51% and a minimum of 33%.Based on simplifying assumptions and the knowledge that crystalline ice exposed to ion bombardment transforms into amorphous at a known rate, we estimate the age of the Obatala crater to be ∼450Ma.

The complex kinetics of the ice VI to ice XV hydrogen ordering phase transition

The reversible phase transition from hydrochloric-acid-doped ice VI to its hydrogen-ordered counterpart ice XV is followed using differential scanning calorimetry. Upon cooling at ambient pressure fast hydrogen ordering is observed at first followed by a slower process which manifests as a tail to the initial sharp exotherm. The residual hydrogen disorder in H2O and D2O ice XV is determined as a function of the cooling rate. We conclude that it will be difficult to obtain fully hydrogen-ordered ice XV by cooling at ambient pressure. Our new experimental findings are discussed in the context of recent computational work on ice XV.

AKARI NIR spectroscopy of interstellar ices

AbstractThe Infrared Camera (IRC) onboard AKARI has a near-infrared (2--5μm) spectroscopic capability with high sensitivity that allows us to study the major ice components in various objects. In particular, H2O and CO2 ice absorption features have been detected towards nearby galaxies, including several young stellar objects (YSOs) in the Large Magellanic Cloud (LMC), as well as a number of HII region-PDR complexes for the first time by IRC spectroscopy. While observations in the LMC show a high ratio (~0.34) of the CO2 to H2O ice column densities, the ratios in Galactic HII-region-PDR complexes are in the range of 0.1--0.2, being compatible with those found in Galactic massive YSOs in previous studies. The good correlation supports concurrent formation of the two ice species on the grain surface and the higher ratio in the low-metallicity LMC suggests possible environmental effects in the formation process.

Laboratory constraints on ice formation, restructuring and desorption

AbstractIces form on the surfaces of interstellar and circumstellar dust grains though freeze-out of molecules and atoms from the gas-phase followed by chemical reactions. The composition, chemistry, structure and desorption properties of these ices regulate two important aspects of planet formation: the locations of major condensation fronts in protoplanetary disks (i.e. snow lines) and the formation efficiencies of complex organic molecules in astrophysical environments. The latter regulates the availability of prebiotic material on nascent planets. With ALMA it is possible to directly observe both (CO) snowlines and complex organics in protoplanetary disks. The interpretation of these observations requires a detailed understanding of the fundamental ice processes that regulate the build-up, evolution and desorption of icy grain mantles. This proceeding reviews how experiments on thermal CO and N2 ice desorption, UV photodesorption of CO ice, and CO diffusion in H2O ice have been used to guide and interpret astrochemical observations of snowlines and complex molecules.

Effect of Electric Field on Condensed-Phase Molecular Systems. II. Stark Effect on the Hydroxyl Stretch Vibration of Ice

We studied the Stark effect on the hydroxyl stretching vibration of water molecules in ice under the influence of an external electric field. Electric fields with strengths in the range from 6.4 × 107 to 2.3 × 108 V·m–1 were applied to an ice sample using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the field-induced spectral changes of vibrationally decoupled O–H and O–D bands of dilute HOD in D2O and H2O–ice, respectively. The spectral changes of the hydroxyl bands under applied field were analyzed using a model that simulates the absorption of a collection of Stark-shifted oscillators. The analysis shows that the Stark tuning rate of ν(O–D) is 6.4–12 cm–1/(MV·cm–1) at a field strength from 1.8 × 108 to 6.4 × 107 V·m–1, and the Stark tuning rate of ν(O–H) is 10–16 cm–1/(MV·cm–1) at a field strength from 2.3 × 108 to 9.2 × 107 V·m–1. These values are uniquely large compared to the Stark tuning rates of carbonyl or nitrile vibrations in other frozen molecu...

Landscapes of polyphase glaciation: eastern Hellas Planitia, Mars

The mid-latitudes of Mars host numerous ice-related landforms that bear many similarities to terrestrial ice masses. This collection of landforms, termed viscous flow features (VFFs), is composed primarily of H2O ice and shows evidence of viscous deformation. Recent work has hypothesised that VFFs are the diminishing remains of once larger ice masses, formed during one or more previous ice ages, and the landscape therefore records evidence of polyphase glaciation. However, debate persists concerning the former extent and volume of ice, and style of former glaciations. The accompanying map (1:100,000 scale) presents a geomorphic and structural assessment of a glacial landscape in eastern Hellas Planitia, Mars. Here, we present a description of the features identified, comprising four geomorphic units (plains, lobate debris apron, degraded glacial material, and glacier-like form) and 16 structures (craters, moraine-like ridges, flow unit boundaries, arcuate transvers structures, longitudinal surface structures, ring-mold craters, terraces, medial moraine-like ridges, raised textured areas, flow-parallel and flow-transverse lineations, crevasses and crevasse traces, and ridge clusters).

Distribution of CO2 ice on the large moons of Uranus and evidence for compositional stratification of their near-surfaces

The surfaces of the large uranian satellites are characterized by a mixture of H2O ice and a dark, potentially carbon-rich, constituent, along with CO2 ice. At the mean heliocentric distance of the uranian system, native CO2 ice should be removed on timescales shorter than the age of the Solar System. Consequently, the detected CO2 ice might be actively produced. Analogous to irradiation of icy moons in the Jupiter and Saturn systems, we hypothesize that charged particles caught in Uranus’ magnetic field bombard the surfaces of the uranian satellites, driving a radiolytic CO2 production cycle. To test this hypothesis, we investigated the distribution of CO2 ice by analyzing near-infrared (NIR) spectra of these moons, gathered using the SpeX spectrograph at NASA’s Infrared Telescope Facility (IRTF) (2000–2013). Additionally, we made spectrophotometric measurements using images gathered by the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope (2003–2005). We find that the detected CO2 ice is primarily on the trailing hemispheres of the satellites closest to Uranus, consistent with other observations of these moons. Our band parameter analysis indicates that the detected CO2 ice is pure and segregated from other constituents. Our spectrophotometric analysis indicates that IRAC is not sensitive to the CO2 ice detected by SpeX, potentially because CO2 is retained beneath a thin surface layer dominated by H2O ice that is opaque to photons over IRAC wavelengths. Thus, our combined SpeX and IRAC analyses suggest that the near-surfaces (i.e., top few 100μm) of the uranian satellites are compositionally stratified. We briefly compare the spectral characteristics of the CO2 ice detected on the uranian moons to icy satellites elsewhere, and we also consider the most likely drivers of the observed distribution of CO2 ice.

Conditions for water ice lines and Mars-mass exomoons around accreting super-Jovian planets at 1−20 AU from Sun-like stars

Exomoon detections might be feasible with NASA's Kepler or ESA's upcoming PLATO mission or the ground-based E-ELT. To use observational resources most efficiently we need to know where the largest, most easily detected moons can form. We explore the possibility of large exomoons by following the movement of water (H2O) ice lines in the accretion disks around young super-Jovian planets. We want to know how different heating sources in those disks affect the H2O ice lines. We simulate 2D rotationally symmetric accretion disks in hydrostatic equilibrium around super-Jovian exoplanets. The energy terms in our semi-analytical model -- (1) viscous heating, (2) planetary illumination, (3) accretional heating, and (4) stellar illumination -- are fed by precomputed planet evolution tracks. We consider planets accreting 1 to 12 Jupiter masses at distances between 1 and 20 AU to a Sun-like star. Accretion disks around Jupiter-mass planets closer than ~4.5 AU to Sun-like stars do not feature H2O ice lines, but the most massive super-Jovians can form icy satellites as close as ~3 AU to Sun-like stars. Super-Jovian planets forming beyond ~5 AU can host Mars-mass moons. We study a broad range of disk parameters for planets at 5.2 AU and find that the H2O ice lines are universally between ~15 and 30 Jupiter radii when the last generation of moons is forming. If the abundant population of super-Jovian planets at ~1 AU formed in situ, then they should lack giant icy moons because their disks did not host H2O ice in the final stages of accretion. In the more likely case that these planets migrated to their current locations from beyond a few AU, they might be orbited by large, H2O-rich moons. In this case, Mars-mass ocean moons might be common in the stellar habitable zones. Future exomoon searches can provide powerful constraints on the formation and migration history of giant exoplanets.

First extreme and far ultraviolet spectrum of a Comet Nucleus: Results from 67P/Churyumov-Gerasimenko

We used the Alice spectrograph onboard the Rosetta comet orbiter spacecraft to observe the surface of Comet 67P/Churyumov-Gerasimenko in the extreme and far ultraviolet (EUV/FUV) from 700 to 2050Å in mid-August 2014. These observations were before significant EUV/FUV coma signatures were observed by Alice. The resulting coadded spectrum has high signal to noise and reveals: (1) a very FUV-dark surface with (2) a blue spectral slope and (3) no evidence of significant H2O ice absorption in the FUV. We fit the measured reflectance spectrum with a model including 99.5% tholins, 0.5% H2O-ice, and a neutral darkening agent. Since we could not find any natural material with sufficiently low EUV/FUV reflectance, we interpret the low I/F as evidence of a fluffy, light-trapping surface. We interpret the blue spectral slope as consistent with a surface consisting primarily of tholins, though it may alternatively be the result of Rayleigh scattering by fine particles in the regolith.

Ferroelectricity in high-density H2O ice.

The origin of longstanding anomalies in experimental studies of the dense solid phases of H2O ices VII, VIII, and X is examined using a combination of first-principles theoretical methods. We find that a ferroelectric variant of ice VIII is energetically competitive with the established antiferroelectric form under pressure. The existence of domains of the ferroelectric form within anti-ferroelectric ice can explain previously observed splittings in x-ray diffraction data. The ferroelectric form is stabilized by density and is accompanied by the onset of spontaneous polarization. The presence of local electric fields triggers the preferential parallel orientation of the water molecules in the structure, which could be stabilized in bulk using new high-pressure techniques.

Revealing sub-μm and μm-scale textures in H2O ice at megabar pressures by time-domain Brillouin scattering.

The time-domain Brillouin scattering technique, also known as picosecond ultrasonic interferometry, allows monitoring of the propagation of coherent acoustic pulses, having lengths ranging from nanometres to fractions of a micrometre, in samples with dimension of less than a micrometre to tens of micrometres. In this study, we applied this technique to depth-profiling of a polycrystalline aggregate of ice compressed in a diamond anvil cell to megabar pressures. The method allowed examination of the characteristic dimensions of ice texturing in the direction normal to the diamond anvil surfaces with sub-micrometre spatial resolution via time-resolved measurements of the propagation velocity of the acoustic pulses travelling in the compressed sample. The achieved imaging of ice in depth and in one of the lateral directions indicates the feasibility of three-dimensional imaging and quantitative characterisation of the acoustical, optical and acousto-optical properties of transparent polycrystalline aggregates in a diamond anvil cell with tens of nanometres in-depth resolution and a lateral spatial resolution controlled by pump laser pulses focusing, which could approach hundreds of nanometres.

Asteroid (90) Antiope: Another icy member of the Themis family?

Many members of the Themis family show evidence of hydration in the form of oxidized iron in phyllosilicates (Florczak, M. et al. [1999]. Astron. Astrophys. Suppl. Ser. 134, 463–471), and OH-bearing minerals (Takir, D., Emery, J.P. [2012]. Icarus 219, 641–654). The largest member, (24) Themis, has H2O ice covering its surface (Campins, H. et al. [2010]. Nature 464, 1320–1321; Rivkin, A.S., Emery, J.P. [2010]. Nature 464, 1322–1323). We have investigated the second largest Themis-family asteroid, (90) Antiope, which Castillo-Rogez and Schmidt (Castillo-Rogez, J.C., Schmidt, B.E. [2010]. Geophys. Res. Lett. 37, L10202) predict to have a composition that includes water ice and organics. We obtained 2–4-μm spectroscopy of (90) Antiope in 2006 and 2008, and we find an absorption in the 3-μm region clearly present in our 2008 spectrum and likely in our 2006 spectrum. Both spectra have rounded, bowl-shaped absorptions consistent with those ascribed to water ice as in the spectrum of Asteroid (24) Themis. We also present and compare Spitzer 8–12-μm mid-infrared spectra of (24) Themis and (90) Antiope. We find that (90) Antiope is lacking a “fairy castle” dusty surface, which is in contrast to (24) Themis, other Themis family members (Licandro, J. et al. [2012]. Astron. Astrophys. 537, A73), and Jupiter Trojans (e.g. Emery, J.P., Cruikshank, D.P., Van Cleve, J. [2006]. Icarus 182, 496–512). We conclude that the surface structure of (90) Antiope is most similar to Cybele Asteroid (121) Hermione (Hargrove, K.D. et al. [2012]. Icarus 221, 453–455).

CO DIFFUSION INTO AMORPHOUS H2O ICES

The mobility of atoms, molecules and radicals in icy grain mantles regulate ice restructuring, desorption, and chemistry in astrophysical environments. Interstellar ices are dominated by H2O, and diffusion on external and internal (pore) surfaces of H2O-rich ices is therefore a key process to constrain. This study aims to quantify the diffusion kinetics and barrier of the abundant ice constituent CO into H2O dominated ices at low temperatures (15-23 K), by measuring the mixing rate of initially layered H2O(:CO2)/CO ices. The mixed fraction of CO as a function of time is determined by monitoring the shape of the infrared CO stretching band. Mixing is observed at all investigated temperatures on minute time scales, and can be ascribed to CO diffusion in H2O ice pores. The diffusion coefficient and final mixed fraction depend on ice temperature, porosity, thickness and composition. The experiments are analyzed by applying Fick's diffusion equation under the assumption that mixing is due to CO diffusion into an immobile H2O ice. The extracted energy barrier for CO diffusion into amorphous H2O ice is ~160 K. This is effectively a surface diffusion barrier. The derived barrier is low compared to current surface diffusion barriers in use in astrochemical models. Its adoption may significantly change the expected timescales for different ice processes in interstellar environments.

THE COMPOSITION OF INTERSTELLAR GRAINS TOWARD ζ OPHIUCHI: CONSTRAINING THE ELEMENTAL BUDGET NEAR THE DIFFUSE-DENSE CLOUD TRANSITION

We investigate the composition of interstellar grains along the line of sight toward Zeta Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H2O ice mantles. However, tentative evidence for thick H2O ice mantles on large (a ~ 2.8 microns) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of elemental Mg and Si along the line of sight are present in amorphous silicate grains, while a substantial fraction of elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ~156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.