Icy particles containing a variety of Fe compounds are present in the upper atmospheres of planets such as the Earth and Saturn. In order to explore the role of ice sublimation and energetic ion bombardment in releasing Fe species into the gas phase, Fe-dosed ice films were prepared under UHV conditions in the laboratory. Temperature-programmed desorption studies of Fe/H2O films revealed that no Fe atoms or Fe-containing species co-desorbed along with the H2O molecules. This implies that when noctilucent ice cloud particles sublimate in the terrestrial mesosphere, the metallic species embedded in them will coalesce to form residual particles. Sputtering of the Fe-ice films by energetic Ar+ ions was shown to be an efficient mechanism for releasing Fe into the gas phase, with a yield of 0.08 (Ar+ energy=600 eV). Extrapolating with a semi-empirical sputtering model to the conditions of a proton aurora indicates that sputtering by energetic protons (>100 keV) should also be efficient. However, the proton flux in even an intense aurora will be too low for the resulting injection of Fe species into the gas phase to compete with that from meteoric ablation. In contrast, sputtering of the icy particles in the main rings of Saturn by energetic O+ ions may be the source of recently observed Fe+ in the Saturnian magnetosphere. Electron sputtering (9.5 keV) produced no detectable Fe atoms or Fe-containing species. Finally, it was observed that Fe(OH)2 was produced when Fe was dosed onto an ice film at 140 K (but not at 95 K). Electronic structure theory shows that the reaction which forms this hydroxide from adsorbed Fe has a large barrier of about 0.7 eV, from which we conclude that the reaction requires both translationally hot Fe atoms and mobile H2O molecules on the ice surface.
Analogues have been developed and characterised for both interplanetary dust and meteoric smoke particles. These include amorphous materials with elemental compositions similar to the olivine mineral solid solution series, a variety of iron oxides, undifferentiated meteorites (chondrites) and minerals which can be considered good terrestrial proxies to some phases present in meteorites. The products have been subjected to a suite of analytical techniques to demonstrate their suitability as analogues for the target materials.
Polar mesospheric clouds form in the summer high latitude mesopause region and are primarily comprised of H2O ice, forming at temperatures below 150 K. Average summertime temperatures in the polar mesosphere (78°N) are approximately 125 K and can be driven lower than 100 K by gravity waves. Under these extreme temperature conditions and given the relative mesospheric concentrations of CO2 and H2O (~360 ppmv and ~10 ppmv, respectively) it has been hypothesised that CO2 molecules could become trapped within amorphous mesospheric ice particles, possibly making a significant contribution to the total condensed volume. Studies of CO2 trapping in co-deposited gas mixtures of increasing CO2:H2O ratio (deposited at 98 K) were analysed via temperature programmed desorption. CO2 trapping was found to be negligible when the H2O flux to the surface was reduced to 4.8×1013 molecules cm?2 s?1. This corresponds to an average of 0.4 H2O molecules depositing on an adsorbed CO2 molecule and thereby trapping it in amorphous ice. Extrapolating the experimental data to mesospheric conditions shows that a mesospheric temperature of 100 K would be required (at a maximum mesospheric H2O concentration of 10 ppmv) in order to trap CO2 in the ice particles. Given the rarity of this temperature being reached in the mesosphere, this process would be an unlikely occurrence.
The kinetics of heterogeneous HO2 uptake onto meteoric smoke particles (MSPs) has been studied in the laboratory using analogues of MSP aerosol entrained into a flow tube. The uptake coefficient, ³, was determined on synthetic amorphous olivine (MgFeSiO4) to be (6.9 ± 1.2) × 10?2 at a relative humidity (RH) of 10%. On forsterite (Mg2SiO4), ³ = (4.3 ± 0.4) × 10?3 at RH = 11.6% and (7.3 ± 0.4) × 10?2 at RH = 9.9% on fayalite (Fe2SiO4). These results indicate that Fe plays a more important mechanistic role than Mg in the removal of HO2 from the gas phase. Electronic structure calculations show that Fe atoms exposed at the particle surface provide a catalytic site where HO2 is converted to H2O2 via an Eley-Rideal mechanism, but this does not occur on exposed surface Mg atoms. The impact of this heterogeneous process in the middle atmosphere was then investigated using a whole atmosphere chemistry-climate model which incorporates a microphysical treatment of MSPs. Using a global MSP production rate from meteoric ablation of 44 t/day, heterogeneous uptake (with ³ = 0.2) on MSPs significantly alters the HOx budget in the nighttime polar vortex. This impact is highly latitude dependent and thus could not be confirmed using currently available satellite measurements of HO2, which are largely unavailable at latitudes greater than 70°.
The uptake and potential reactivity of metal atoms on water ice can be an important process in planetary atmospheres and on icy bodies in the interplanetary and interstellar medium. For instance, metal atom uptake affects the gas-phase chemistry of the Earth's mesosphere, and has been proposed to influence the agglomeration of matter into planets in protoplanetary disks. In this study the fate of Mg and K atoms incorporated into water-ice films, prepared under ultra-high vacuum conditions at temperatures of 110?140 K, was investigated. Temperature-programmed desorption experiments reveal that Mg- and K-containing species do not co-desorb when the ice sublimates, demonstrating that uptake on ice particles causes irreversible removal of the metals from the gas phase. This implies that uptake on ice particles in terrestrial polar mesospheric clouds accelerates the formation of large meteoric smoke particles (e1 nm radius above 80 km) following sublimation of the ice. Energetic sputtering of metal-dosed ice layers by 500 eV Ar+ and Kr+ ions shows that whereas K reacts on (or within) the ice surface to form KOH, adsorbed Mg atoms are chemically inert. These experimental results are consistent with electronic structure calculations of the metals bound to an ice surface, where theoretical adsorption energies on ice are calculated to be ?68 kJ mol?1 for K, ?91 kJ mol?1 for Mg, and ?306 kJ mol?1 for Fe. K can also insert into a surface H2O to produce KOH and a dangling H atom, in a reaction that is slightly exothermic.
The heterogeneous oxidation of CO by O2 on olivine, Fe sulfate and Fe oxide particles was studied using a flow tube apparatus between 300 and 680 K. These particles were chosen as possible analogues of unablated cosmic dust and meteoric smoke in Venus? atmosphere. On olivine and Fe oxides, the rate of CO oxidation to CO2 only becomes significant above 450 K. For iron sulfates, CO2 production was not observed until these dust analogues had decomposed into iron oxides at
Temperature-programmed desorption and reflection-absorption infrared spectroscopy have
been used to explore the interaction of oxygen (O2), nitrogen (N2), carbon monoxide (CO) and
water (H2O) with an amorphous silica film as a demonstration of the detailed characterization
of the silicate surfaces that might be present in the interstellar medium. The simple diatomic
adsorbates are found to wet the silica surface and exhibit first-order desorption kinetics in
the regime up to monolayer coverage. Beyond that, they exhibit zero-order kinetics as might
be expected for sublimation of bulk solids. Water, in contrast, does not wet the silica surface
and exhibits zero-order desorption kinetics at all coverages consistent with the formation of
an islanded structure. Kinetic parameters for use in astrophysical modelling were obtained
by inversion of the experimental data at sub-monolayer coverages and by comparison with
models in the multilayer regime. Spectroscopic studies in the sub-monolayer regime show that
the C?O stretching mode is at around 2137 cm?1 (5.43 ¼m), a position consistent with a linear
surface?CO interaction, and is inhomogenously broadened as resulting from the heterogeneity
of the surface. These studies also reveal, for the first time, direct evidence for the thermal
activation of diffusion, and hence de-wetting, of H2O on the silica surface. Astrophysical
implications of these findings could account for a part of the missing oxygen budget in dense
interstellar clouds, and suggest that studies of the sub-monolayer adsorption of these simple
molecules might be a useful probe of surface chemistry on more complex silicate materials.
A low-temperature flow tube and ultra-high vacuum apparatus were used to explore the uptake and heterogeneous chemistry of acetylene (C2H2) on cosmic dust analogues over the temperature range encountered in Titan's atmosphere below 600 km. The uptake coefficient, ³, was measured at 181 K to be (1.6 ± 0.4) × 10-4, (1.9 ± 0.4) × 10?4 and (1.5 ± 0.4) × 10?4 for the uptake of C2H2 on Mg2SiO4, MgFeSiO4 and Fe2SiO4, respectively, indicating that ³ is independent of Mg or Fe active sites. The uptake of C2H2 was also measured on SiO2 and SiC as analogues for meteoric smoke particles in Titan's atmosphere, but was found to be below the detection limit (³
The uptake of HNO3, H2O, NO2 and NO was studied on meteoric smoke particle analogues using a low-pressure Knudsen cell operating at 295 K. The analogues used were olivine (MgFeSiO4) and a haematite/goethite (Fe2O3/FeO(OH)) mixture synthesised by the sol?gel process. For uptake on MgFeSiO4, the following uptake coefficients were obtained: ³(HNO3)=(1.8±0.3)×10?3, ³(H2O)=(4.0±1.3)×10?3, ³(NO2)=(5.7±0.2)×10?4 and ³(NO)
Although several research groups have studied the formation of H2 on interstellar dust grains using
surface science techniques, few have explored the formation of more complex molecules. A small
number of these reactions produce molecules that remain on the surface of interstellar dust grains
and, over time, lead to the formation of icy mantles. The most abundant of these species within the
ice is H2O and is of particular interest as the observed molecular abundance cannot be accounted
for using gas-phase chemistry alone. This article provides a brief introduction to the astronomical
implications and motivations behind this research and the requirement for a new dual atomic beam
ultrahigh vacuum (UHV) system. Further details of the apparatus design, characterisation, and
calibration of the system are provided along with preliminary data from atomic O and O2 beam
dosing on bare silica substrate and subsequent temperature programmed desorption measurements.
The results obtained in this ongoing research may enable more chemically accurate surface formation
mechanisms to be deduced for this and other species before simulating the kinetic data under