Silicon negative ion chemistry in the atmosphere-in situ and laboratory measurements

The results of a rocket-borne mass spectrometer measurement indicate that large concentrations of negative ions exist above the bottom of the atmospheric atomic oxygen layer. A large majority of these ions have a mass greater than 100 amu. In addition, an ion at mass 76 was observed with concentrations too large to be CO₄^?. In order to explain these features, a number of reactions involving silicon oxide negative ions have been measured in a flowing afterglow system. The ion SiO₃^? is produced by reaction of O₃^?, and CO₃^?, with SiO. The SiO₃^? ion is extremely stable and does not react measurably with NO, NO₂, CO, CO₂, O₃ or O. Since meteoroid ablation produces a large silicon input into the atmosphere, it appears possible that the ions observed at mass 76 may be SiO₃^?. Possible production mechanisms for this ion as well as the heavy ions are discussed.     

Negative ion composition and sulfuric acid vapour in the upper stratosphere

The nature of negative ions in the altitude region 42–45 km has been investigated by means of a balloon borne mass spectrometer. Apart from the NO₃⁻ and HSO₄⁻ clusters, ions with different cores, which can be identified as CO₃⁻, HCO₃⁻, Cl⁻ and ClO₃⁻ were observed. The spectra have been used to estimate the sulfuric acid number density at 45.2 and 42.3 km altitude.     

Upper stratosphere negative ion composition measurements and inferred trace gas abundances

In situ composition measurements of atmospheric negative ions were made at 40.8 km altitude using a balloon-borne mass spectrometer with large mass range and improved mass resolution. The data obtained show marked differences compared to previous data obtained mostly around or below 33 km.It appears that these differences are mostly due to a higher atmospheric temperature, a lower nitric acid vapour abundance and a larger HSO₃-vapour abundance prevailing at the higher altitude.A particularly striking feature is the relatively large fractional abundance of HSO₃-containing cluster ions.Another interesting result is that nitric acid vapour abundances can be inferred from the negative ion composition data with better accuracy than is possible for lower altitudes. The reason being that collisional ion dissociation occurring during ion sampling is less disturbing.The inferred nitric acid vapour abundance for 40.8 km altitude is consistent with current 2-dimensional model calculations.     

The first height measurements of the negative ion composition of the stratosphere

For the first time, height profiles of the stratospheric negative ion composition are presented. The results are from two nights of balloon borne mass spectrometers and cover an altitude range from 23.8 to 38.9 km. Below approx. 30km, NO^?₃ · mHNO₃ ions are dominant. These are replaced by HSO₄^? · nH₂SO₄ · oHNO₃ ions above this height. There are indications that the most abundant ions above 32 km have masses greater than 280 atomic mass units (amu), the instruments' mass range. The fractional ion count rates as a function of altitude are presented and their significance for neutral trace gas analysis and ion sampling is discussed.     

Combined mass spectrometric composition measurements of positive and negative ions in the lower ionosphere—I. Positive ions

Recent improvements in rocket-borne mass spectrometer technology have made it possible to measure lower ionospheric ions with greater sensitivity and to extend the measurements to lower heights. The improvements made to the instrument and positive ion results from a flight of this instrument will be reported here. In addition to the previously known ions, such as NO⁺(H₂O)_n and H⁺(H₂O)_n, new ion species were found. The total fractional count rate of these ions was found to be constant with height indicating an upper altitude source. Possible identifications of these ions are proposed along with possible production mechanisms.     

Combined mass spectrometric composition measurements of positive and negative ions in the lower ionosphere—II. Negative ions

Negative ion composition measurements of the lower ionosphere made simultaneously with positive ion measurements show some interesting features not previously observed. The higher sensitivity of this instrument allowed for detection of numerous previously undetected species. A layer of heavy ions (> 100 a.m.u.) was observed to exist between 80 and 90 km. The properties of this layer seem to indicate a meteoric source. The heavy ions are seen to exist down to the lowermost heights measured and their abundance increases below 58 km indicating a stratospheric source. Besides these ions, numerous new light ions were also seen. Their abundance level and sources are discussed.     

First model of negative ion composition in the troposphere

A chemical model of negative ions in the troposhere (0–15 km) is presented. This model is an extension of the negative ion composition model in the lower stratosphere (Kawamoto and Ogawa, 1984, Planet. Space Sci. 32, 1223) with some modifications. The computed result shows that the predominant ions are NO₃⁻HNO₃H₂O below 10km and NO⁻₃(HNO₃)₂ above 10km, and that the fractional abundance of cluster ions having a HSO⁻₄ core increases with height below 12km and decreases with height above it. The ions having CO⁻₃ cores are at most 2% in fractional abundance. The other kinds of negative ions are far smaller in fractional abundance than the NO⁻₃, HSO⁻₄ and CO⁻₃ core ions. The result is compared with the two mass spectrometric observed results (Heitmann and Arnold, 1983, Nature, Lond. 306, 747; Perkins and Eisele, 1984, J. geophys. Res. 89, 9649). The problems on the tropospheric negative ions which arose are discussed.     

Thermal structure of Saturn from Voyager infrared measurements: Implications for atmospheric dynamics

Saturn atmospheric temperatures at the 150-mbar level retrieved from Voyager IRIS measurements indicate the presence of small-scale meridional gradients which are approximately symmetric with respect to the equator, but are superposed on a large-scale hemispheric thermal asymmetry. Under the assumption that the retrieved values at this atmospheric level represent kinetic temperatures on a constant pressure surface, it is suggested that the small-scale structure is produced by a meriodional circulation associated with the dissipative decay of the zonal winds with height, while the hemispheric asymmetry represents a thermal response to the seasonally varying insolation. The small-scale gradients are correlated with zonal winds derived from Voyager images at mid and high latitudes through the thermal wind relation; the calculated thermal wind shears suggest a decay with height of the jet system toward a state of uniform eatward flow. The existence of the approximately symmetric zonal winds and associated temperature gradients in the presence of a large-scale seasonal thermal response suggests that the jet system is driven at depths substantially below the levels where seasonally modulated insolation is important ($\text{p?0.5}\text{bar}$).     

First composition measurements of positive ions in the upper troposphere

First mass-spectrometric composition measurements of atmospheric ions between 3250 and 11700 m altitude are reported. They reveal the presence of very massive cluster ions, the majority of which cannot be attributed to a single hydrated ion family like, for example H⁺(H₂O)_n. The observed fraction of very massive ions increases with decreasing altitude. Masses as large as about 540 amu were observed at 8200 m altitude. Implications of the observations for ion and nucleation processes are discussed.     

Observations of energetic water-group ions at comet giacobini-zinner: Implications for ion acceleration processes

Observations of energetic water-group pick-up ions made by the EPAS instrument during the ICE fly-by of comet P/Giacobini-Zinner are investigated for evidence concerning the processes which accelerate the ions from initial pick-up energies of around 10 keV up to energies of a few hundred kilo-electronvolts. The form of the ion spectrum in the ion rest frame is first investigated and compared with theoretical suggestions that exponential energy distributions might be produced by either first or second order Fermi acceleration in the cometary environment. It is shown that such distributions do not fit the data at all well, but that rather (over the EPAS ion bulk rest frame energy range of ∼30 to <300 keV) the observed distribution functions closely approximate an exponential in ion speed. The higher energy (> 50 keV) data also fit a power law distribution very well, but at lower energies the data tend to show a flattening below the power law form. It is also shown that the observed spectra are much softer than those calculated by Ip and Axford (1986, Planet. Space Sci. 34, 1061) for the G-Z ion pick-up region upstream of the bow shock, indicating that if these distributions are indeed due to second order Fermi acceleration as they propose, then the ion mean free path must be rather larger than used in their calculations (i.e. rather larger than 5 water-group ion gyroradii). Overall, it is concluded that at the present stage of theoretical development it is premature to draw firm conclusions about acceleration mechanisms from studies of the ion spectrum alone. However, from the general spectral analysis it is also possible to investigate the variations of ion intensity and spectral hardness which take place during the comet encounter, and to gain an indication of the degree of isotropy of the ion distribution in the rest frame of the flow. We find that a ∼.5 × 10⁴km-wide region (∼ 35 min along the spacecraft trajectory) of rapid ion intensification and spectral hardening occurs immediately upstream from the turbulent mass-loaded region, suggestive of a first order Fermi process in which ions are successively reflected between the outer layer of the slowed, turbulent region and waves in the faster upstream flow. It is shown that within the turbulent region the ion distribution becomes rapidly isotropized in its own bulk rest frame, indicative of a sudden reduction in ion mean free path. Additional processes (possibly second order Fermi acceleration) must also occur in order to account for the further modest intensifications of the energetic ion fluxes observed within the turbulent mass-loaded region, as well as the presence of ions in the outer pick-up region which have energies well in excess of the local pick-up energy.     

Korteweg-de Vries equation for ion acoustic soliton with negative ions in the presence of nonextensive electrons

Korteweg-de Vries (KdV) equation for electrostatic ion acoustic wave in a three component plasma containing positive and negative ions along with the nonextensive electrons is derived. Fast and slow ion acoustic modes which propagate with different velocities are excited. The effects of variation of quantities like q (nonextensive parameter), Q (mass ratio of positive to negative ion), μ (electron to positive ion number density ratio), θ _i (positive ion to electron temperature ratio) and θ _n (negative ion to electron temperature ratio) have been presented for fast and slow ion acoustic modes. Both compressive and rarefactive solitons are observed. It is found that the solitary excitations strongly depend on the mass and density ratios of the positive and negative ions as well as on nonextensive electron parameter.     

Mass spectrometric measurements of fractional ion abundances in the stratosphere—Positive ions

Measured fractional abundances for stratospheric positive ions are reported for the first time. The measurements which were obtained from balloon-borne ion mass spectrometer experiments relied on recent simulation studies of electric field induced cluster ion dissociation conducted at our laboratory.The ion abundance data provide strong support for identifications of the observed ions as H⁺(H₂O)_n and Hx⁺x_L(H₂O)_m proposed previously. Moreover, it is found that x most likely cannot be identified as NaOH or MgOH which implies that gaseous metal compounds do not exist in the middle stratosphere in significant abundances.Implications of the present findings for the composition and chemistry of stratospheric ions as well as for stratospheric aerosols are discussed.     

Mass spectrometric measurements of fractional ion abundances in the stratosphere—Negative ions

Fractional abundances of stratospheric negative ions are for the first time explicitly reported. The measurements made by balloon-borne ion mass spectrometers also rely on recent studies of electric field induced collisional dissociation of negative cluster ions conducted at our laboratory. These indicate that the negative ion composition measurements around 36 km conducted by our group do not suffer from any significant dissociation. The new composition data support ion identifications NO₃^?(HNO₃)_b and HSO₄^?(H₂SO₄)_c(HNO₃)_d and the underlying ion reactions propo previously. Moreover, it is found that HSO₄^?(H₂SO₄)_g-ions appear to be particularly stable and that H₂SO₄-association is very fast. Implications of the ion composition data for ion processes are discussed.     

Laboratory measurements of the microwave opacity and vapor pressure of sulfuric acid vapor under simulated conditions for the middle atmosphere of Venus

Microwave absorption observed in the 35- to 48-km-altitude region of the Venus atmosphere has been attributed to the presence of gaseous sulfuric acid (H₂SO₄) in that region. This has motivated the laboratory measurement of the microwave absorption at 13.4- and 3.6-cm wavelengths from gaseous H₂SO₄ in a CO₂ atmosphere under simulated conditions for that region. As part of the same experiments, upper limits on the saturation vapor pressure of gaseous H₂SO₄ have also been determined. The measurements for microwave absorption have been made in the 1- to 6-atm pressure range, with temperatures in the 500 to 575°K range. Using a theoretically derived temperature dependence, the best-fit expression for absorption from gaseous H₂SO₄ in a CO₂ atmosphere at the 13.4-cm wavelength is $\text{9.0 × 10}{}^9\text{q(P)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{T}{}^{\mathrm{?3}}\text{(}\text{dB km}{}^{\mathrm{?1}}\text{),}\text{where}\text{q}$ is the H₂SO₄ number mixing ratio, P is the pressure in atmospheres, and T is the temperature in degrees Kelvins. The best-fit expression for absorption at the 3.6-cm wavelength is 4.52 × 10¹⁰q(P)^0.85T^?3 (dB km^?1). The inferred H₂SO₄ vapor pressure above liquid H₂SO₄ corresponds to ln p = 8.84 ? 7220/t where p is the H₂SO₄ vapor pressure (in atm) and T is the temperature in degrees Kelvins. These results suggest that abundances of gaseous H₂SO₄ on the order of 15 to 30 ppm could account for the microwave absorption observed by radio occultation experiments at 13.3- and 3.6-cm wavelengths. They also suggest that such abundances would correspond to saturation vapor pressure existing at or above the 46- to 48-km range, which correlates with the observed cloud base. It is suggested that future measurements of absorption in the 1- to 3-cm wavelength range will provide additional tools for monitoring variations in H₂SO₄ abundance via radio occultation and radio astronomical observations.     

Photochemistry of the stratosphere of Venus: Implications for atmospheric evolution

The photochemistry of the stratosphere of Venus was modeled using an updated and expanded chemical scheme, combined with the results of recent observations and laboratory studies. We examined three models, with H₂ mixing ratio equal to 2 × 10^?5, 5 × 10^?7, and 1 × 10^?13, respectively. All models satisfactorily account for the observations of CO, O₂, O₂(¹Δ), and SO₂ in the stratosphere, but only the last one may be able to account for the diurnal behavior of mesospheric CO and the uv albedo. Oxygen, derived from CO₂ photolysis, is primarily consumed by CO₂ recombination and oxidation of SO₂ to H₂SO₄. Photolysis of HCl in the upper stratosphere provides a major source of odd hydrogen and free chlorine radicals, essential for the catalytic oxidation of CO. Oxidation of SO₂ by O occurs in the lower stratosphere. In the high-H₂ model (model A) the OO bond is broken mainly by S + O₂ and SO + HO₂. In the low-H₂ models additional reactions for breaking the OO bond must be invoked: NO + HO₂ in model B and ClCO + O₂ in model C. It is shown that lightning in the lower atmosphere could provide as much as 30 ppb of NO_x in the stratosphere. Our modeling reveals a number of intriguing similarities, previously unsuspected, between the chemistry of the stratosphere of Venus and that of the Earth. Photochemistry may have played a major role in the evolution of the atmosphere. The current atmosphere, as described by our preferred model, is characterized by an extreme deficiency of hydrogen species, having probably lost the equivalent of 10²–10³ times the present hydrogen content.     

Distribution of hydrate on Europa: Further evidence for sulfuric acid hydrate

Sulfuric acid hydrate has been proposed as an important species on Europa's surface, the acid being produced by radiolysis of surficial sulfur compounds. We investigated the spectral properties of disordered and crystalline forms of sulfuric acid and suggest that the hydration properties of Europa's hypothesized sulfuric acid lie between two end members: liquid sulfuric acid and its higher crystalline hydrates. The spectra of these end members are similar except for spectral shifts at the band edges. We measured the optical constants of sulfuric acid octahydrate and used these with simple radiative transfer calculations to fit Europa spectra obtained by Galileo's Near Infrared Mapping Spectrometer (NIMS). The global distribution of the hydrate that we associate here with hydrated sulfuric acid shows a strong trailing-side enhancement with a maximum fractional hydrate abundance of 90% by volume, corresponding to a sulfur atom to water molecule ratio of 10%. The hydrate concentration spatially correlates with the ultraviolet and visible absorption of the surface and with the sulfur dioxide concentration. The asymmetric global distribution is consistent with Iogenic plasma ion implantation as the source of the sulfur, possibly modified by electron irradiation and sputtering effects. The variegated distribution also correlates with geologic forms. A high spatial resolution image shows resolved lineae with less hydrate appearing within the lineae than in nearby crustal material. The low concentration of hydrated material in these lineae argues against their conveying sulfurous material to the surface from the putative ocean.     

The ultraviolet reflectance of Enceladus: Implications for surface composition

The reflectance of Saturn’s moon Enceladus has been measured at far ultraviolet (FUV) wavelengths (115-190 nm) by Cassini’s Ultraviolet Imaging Spectrograph (UVIS). At visible and near infrared (VNIR) wavelengths Enceladus’ reflectance spectrum is very bright, consistent with a surface composed primarily of H₂O ice. At FUV wavelengths, however, Enceladus is surprisingly dark - darker than would be expected for pure water ice. Previous analyses have focused on the VNIR spectrum, comparing it to pure water ice (Cruikshank, D.P., Owen, T.C., Dalle Ore, C., Geballe, T.R., Roush, T.L., de Bergh, C., Sandford, S.A., Poulet, F., Benedix, G.K., Emery, J.P. [2005] Icarus, 175, 268-283) or pure water ice plus a small amount of NH₃ (Emery, J.P., Burr, D.M., Cruikshank, D.P., Brown, R.H., Dalton, J.B. [2005] Astron. Astrophys., 435, 353-362) or NH₃ hydrate (Verbiscer, A.J., Peterson, D.E., Skrutskie, M.F., Cushing, M., Helfenstein, P., Nelson, M.J., Smith, J.D., Wilson, J.C. [2006] Icarus, 182, 211-223). We compare Enceladus’ FUV spectrum to existing laboratory measurements of the reflectance spectra of candidate species, and to spectral models. We find that the low FUV reflectance of Enceladus can be explained by the presence of a small amount of NH₃ and a small amount of a tholin in addition to H₂O ice on the surface. The presence of these three species (H₂O, NH₃, and a tholin) appears to satisfy not only the low FUV reflectance and spectral shape, but also the middle-ultraviolet to visible wavelength brightness and spectral shape. We expect that ammonia in the Enceladus plume is transported across the surface to provide a global coating.     

Mass spectrometer measurements of the positive ion composition in the D- and E-regions of the ionosphere

On 14 December 1971, during the maximum of the Geminid Meteor Shower, the positive ion composition was measured in the D- and E-regions above Sardinia. The payload was launched at 12:11 UT, and measurements were made between 68.5 and 152 km altitude. A magnetic sector type mass spectrometer with dual collector and a liquid helium cryopump was used. The instrument covered the mass range from 11 to 73 AMU and had a resolution at the 1 % level of M/ΔM = 60.In the E-region two distinct metal ion layers were observed, centred at 95 and 119 km, respectively. In the lower layer Fe⁺ and Mg⁺ were the most abundant metal ions, and in the upper layer Si⁺ was dominant. Si⁺ ions were conspicuously absent in the lower layer (Si⁺/Mg⁺ < 2 × 10⁻³). This particular behaviour of Si could be due to the inability of atomic oxygen to reduce SiO, whereas in the upper layer Si⁺ions might be formed directly by the charge rearrangement reaction SiO + O⁺ → Si⁺+ O₂. In addition, Na⁺, Al⁺, K⁺, Ca⁺, Ti⁺, Cr⁺, Ni⁺ and Co⁺ were also identified. The metal oxide ions AlO⁺ and SiO⁺ were detected, and probably also MgO⁺ and SiOH⁺. The concentrations of NO⁺ and O₂⁺ show a deep minimum at the maximum of the lower metal ion layer. A very high neutral metal density of 6 × 10⁷ cm⁻³ would be required to explain this feature as resulting from charge transfer reactions between the molecular and metal ions Such a high metal density is in contradiction to direct measurements and to cosmic dust influx rates. The isotopic ratios of Mg⁺, Si⁺, and of the major isotopes of Fe⁺ and Ni⁺ were measured, some of them with an accuracy of a few per cent (²⁵Mg⁺/²⁴Mg⁺ = 0.124 ± 0.006; ²⁶Mg⁺/²⁴Mg⁺ = 0.139 ± 0.008; ²⁹Si⁺/²⁸Si⁺ = 0.050 ± 0.004; ⁵⁴Fe⁺/⁵⁶Fe⁺ = 0.069 ± 0.005; ⁵⁷Fe⁺/⁵⁶Fe⁺ = 0.029 ± 0.004; ⁶⁰Ni⁺/⁵⁸Ni⁺ = 0.31 ± 0.12). The isotopic ratios agree within the experimental errors with the corresponding terrestrial ratios, thus giving evidence that these elements have the same isotopic composition in the Geminid meteors as in the Earth's crust, in chrondrites, and in lunar material.In the D-region the ions Na⁺H₂O, Na⁺(H₂O)₂, NaO⁺ and NaOH⁺ were tentatively identified. Below 95 km altitude the relative abundances of the ions 32⁺, 33⁺ and 34⁺ deviate from the values expected for molecular oxygen isotopes. Their abundances can not be explained by the presence of S-ions only, and we conclude that HO₂⁺ and H₂O₂⁺ are present.The ion density profiles of the major D-region constituents show some remarkable deviations from typical D-region conditions. These deviations are related to the winter anomaly in ionospheric absorption observed over Spain during the launch day, and our data represent the first ion composition measurements during such conditions. In particular, H⁺(H₂O)₂ is the major ion only up to 77 km, and at 80 km altitude the NO⁺ concentration exceeds the total water cluster ion density by almost two orders of magnitude. An increase of the mesospheric NO, O₃ and O concentrations as well as of the O/H₂O ratio could explain the observed ion profiles. The low NO⁺/O₂⁺ ratios of approximately unity measured in the E-region are in agreement with a strong downward transport of NO and/or O into the mesosphere during the launch day. A simple four-ion model was used to interpret our D-region data. The calculated neutral NO concentration increases from about 2 × 10⁷ cm⁻³ at 85 km to 5 × 10⁷ cm⁻³ at 80 km. In addition, evidence for an increased O₂⁺ production rate above 83 km was found, probably due to an enhanced O₃ concentration. We conclude that our data strongly support vertical transport of minor neutral consituents as cause of the winter anomaly.     

Attitude and angular rates of planetary probes during atmospheric descent: Implications for imaging

Attitude dynamics data from planetary missions are reviewed to obtain a zeroth-order expectation on the tilts and angular rates to be expected on atmospheric probes during descent: these rates are a strong driver on descent imager design. While recent Mars missions have been equipped with capable inertial measurements, attitude measurements for missions to other planetary bodies are rather limited but some angular motion estimates can be derived from accelerometer, Doppler or other data. It is found that robust camera designs should tolerate motions of the order of 20-40°/s, encountered by Mars Pathfinder, Pioneer Venus, Venera and the high speed part of the Huygens descent on Titan. Under good conditions, parachute-stabilized probes can experience rates of 1-5°/s, seen by the Mars Exploration Rovers and Viking, Galileo at Jupiter, and the slow speed parts of the Huygens descent. In the lowest 20 km of the descent on Titan, the Huygens probe was within 2° of vertical over 95% of the time. Some factors influencing these motions are discussed.