Phosphine photochemistry in the atmosphere of Saturn

A model is presented for the photochemistry of PH₃ in the upper troposphere and lower stratosphere of Saturn that includes the effects of coupling with NH₃ and hydrocarbon photochemistry, specifically the C₂H₂ catalyzed photodissociation of CH₄. PH₃ is rapidly depleted with altitude (scale height ～35 km) in the upper troposphere when K～10⁴cm²sec^?1; an upper limit for K at the tropopause is estimated at ～10⁵cm²sec^?1. If there is no gas phase P₂H₄ because of sublimation, P₂ and P₄ formation is unlikely unless the rate of the spin-forbidden recombination reaction PH + H₂ + M → PH₃ + M is exceedingly slow. An upper limit P₄ column density of ～2×10¹⁵cm^?2 is estimated in the limit of no recombination. If sublimation does not remove all gas phase P₂H₄, P₂ and P₄ may be produced in potentially larger quantities, although they would be restricted almost entirely to the lowest levels of our model, where T?100°K. Potentially observable amounts of the organophosphorus compounds CH₃P₂H₂ and HCP are predicted, with column densities of >10¹⁷ cm^?2 and production rates of ～2×10⁸cm^?2sec^?1. The possible importance of electronically excited states of PH_x and additional PH₃/hydrocarbon photochemical coupling paths are also considered.     

An estimate of the PH3, CH3D,and GeH4 Abundances on Jupiter from the Voyager IRIS data at 4.5 μm

The abundances of PH₃, CH₃D, and GeH₄ are derived from the 2100- to 2250-cm^?1 region of the Voyager 1 IRIS spectra. No evidence is seen for large-scale variations of the phosphine abundance over Jovian latitudes between ?30 and +30°. In the atmospheric regions corresponding to 170–200°K, the derived PH₃/H₂ value is (4.5 ± 1.5) × 10^?7 or 0.75 ± 0.25 times the solar value. This result, compared with other PH₃ determinations at 10 μm, suggests than the PH₃/H₂ ratio on Jupiter decreases with atmospheric pressure. In the 200–250°K region, we derive, within a factor of 2, CH₃D/H₂ and GeH₄/H₂ ratios of 2.0 × 10^?7 and 1.0 × 10^?9, respectively. Assuming a C/H value of 1.0 × 10^?3, as derived from Voyager, our CH₃D/H₂ ratio implies a D/H ratio of 1.8 × 10^?5, in reasonable agreement with the interstellar medium value.     

Phosphine on Jupiter and Saturn from Cassini/CIRS

The global distribution of phosphine (PH₃) on Jupiter and Saturn is derived using 2.5 cm⁻¹ spectral resolution Cassini/CIRS observations. We extend the preliminary PH₃ analyses on the gas giants [Irwin, P.G.J., and 6 colleagues, 2004. Icarus 172, 37-49; Fletcher, L.N., and 9 colleagues, 2007a. Icarus 188, 72-88] by (a) incorporating a wider range of Cassini/CIRS datasets and by considering a broader spectral range; (b) direct incorporation of thermal infrared opacities due to tropospheric aerosols and (c) using a common retrieval algorithm and spectroscopic line database to allow direct comparison between these two gas giants.The results suggest striking similarities between the tropospheric dynamics in the 100-1000 mbar regions of the giant planets: both demonstrate enhanced PH₃ at the equator, depletion over neighbouring equatorial belts and mid-latitude belt/zone structures. Saturn's polar PH₃ shows depletion within the hot cyclonic polar vortices. Jovian aerosol distributions are consistent with previous independent studies, and on Saturn we demonstrate that CIRS spectra are most consistent with a haze in the 100-400 mbar range with a mean optical depth of 0.1 at 10 μm. Unlike Jupiter, Saturn's tropospheric haze shows a hemispherical asymmetry, being more opaque in the southern summer hemisphere than in the north. Thermal-IR haze opacity is not enhanced at Saturn's equator as it is on Jupiter.Small-scale perturbations to the mean PH₃ abundance are discussed both in terms of a model of meridional overturning and parameterisation as eddy mixing. The large-scale structure of the PH₃ distributions is likely to be related to changes in the photochemical lifetimes and the shielding due to aerosol opacities. On Saturn, the enhanced summer opacity results in shielding and extended photochemical lifetimes for PH₃, permitting elevated PH₃ levels over Saturn's summer hemisphere.     

The meridional phosphine distribution in Saturn's upper troposphere from Cassini/CIRS observations

The Cassini Composite Infrared Spectrometer (CIRS) has been used to derive the vertical and meridional variation of temperature and phosphine (PH₃) abundance in Saturn's upper troposphere. PH₃ has a significant effect on the measured radiances in the thermal infrared and between May 2004 and September 2005 CIRS recorded thousands of spectra in both the far (10-600 cm⁻¹) and mid (600-1400 cm⁻¹) infrared, at a variety of latitudes covering the southern hemisphere. Low spectral resolution (15 cm⁻¹) data has been used to constrain the temperature structure of the troposphere between 100 and 500 mbar. The vertical distributions of phosphine and ammonia were retrieved from far-infrared spectra at the highest spectral resolution (0.5 cm⁻¹), and lower resolution (2.5 cm⁻¹) mid-infrared data were used to map the meridional variation in the abundance of phosphine in the 250-500 mbar range. Temperature variations at the 250 mbar level are shown to occur on the same scale as the prograde and retrograde jets in Saturn's atmosphere [Porco, C.C., and 34 colleagues, 2005. Science 307, 1243-1247]. The PH₃ abundance at 250 mbar is found to be enhanced at the equator when compared with mid-latitudes. At mid latitudes we see anti-correlation between temperature and PH₃ abundance at 250 mbar, phosphine being enhanced at 45° S and depleted at 25 and 55° S. The vertical distribution is markedly different polewards of 60-65° S, with depleted PH₃ at 500 mbar but a slower decline in abundance with altitude when compared with the mid-latitudes. This variation is similar to the variations of cloud and aerosol parameters observed in the visible and near infrared, and may indicate the subsidence of tropospheric air at polar latitudes, coupled with a diminished sunlight penetration depth reducing the rate of PH₃ photolysis in the polar region.     

Laboratory simulations of PH3 photolysis in the atmospheres of Jupiter and Saturn

Photolysis of NH₃PH₃ mixtures (11 Torr) at 175°K resulted in the same initial rate of P₂H₄ formation as when the 11 Torr of pure PH₃ was photolyzed. A higher yield of P₂H₄ is obtained at 175°K than at 298°K because some of the P₂H₄ condenses on the cell wall at 175°K and is not subject to further reaction. Some reaction of P₂H₄ is taking place as observed by the decrease in its yield and on the formation of red phosphorus on extended photolysis of PH₃ at 175°K. No NH₂PH₂ or (PN)_x were detected as photoproducts as indicated by the absence of change in the UV spectral properties of the P₂H₄ and red phosphorus fraction, respectively, when NH₃ is present. Although the pathway for PH₃ decomposition is changed, the outcome of the photochemical process is essentially the same in the absence or presence of NH₃. The formation of P₂H₄ and red phosphorus was not inhibited by small amounts of C₂H₄ and C₂H₂, so the low levels of hydrocarbons on Jupiter and Saturn will not have a significant effect on the course of PH₃ photolysis. The ratio of products of PH₃ photolysis are only slightly affected by the wavelength of light used. Use of xenon lamp, with a continuous emission in the ultraviolet where P₂H₄ absorbs, results in only a modest decrease in the yield of P₂H₄ and a modest increase in the rate of formation of red phosphorus as compared to the rates observed with a 206.2-nm light source. The quantum yield for P₂H₄ formation is pressure independent in the 0.5–11 Torr range. This quantum yield is not affected by lowering the temperature to 157°K or by the addition of 100 Torr of H₂. It is concluded that photolysis of PH₃ to P₂H₄ and the subsequent conversion of P₂H₄ to red phosphorus are likely procses on Jupiter and Saturn and that particles of P₂H₄ condense in the atmospheres of these planets. The conversion of some of the P₂H₄ to red phosphorus may take place on Jupiter.     

Variability of phosphine on Jupiter from 5-μm spectroscopy

Spectra of Jupiter recorded in the 1900- to 2300-cm^?1 range at the IRTF in Hawaii, July 1982, provide tentative evidence for variability of the Jovian atmosphere between zones and belts. It is concluded from analysis of the ν₁and ν₃ bands of PH₃ that there is a possible enhancement of the PH₃/H₂ ratio in the belts when compared to the zones. There is an apparent reduction of the PH₃ abundance between the IRIS Voyager 1 determinations and these spectra, implying temporal or spatial variability of PH₃ on Jupiter. Interpretation of this variability in the troposphere could involve both dynamical and thermochemical processes.     

The gas composition of jupiter derived from 5-μm airborne spectroscopic observations

The atmospheric transmission window between 1850 and 2250 cm⁻¹ in Jupiter's atmosphere was observed at a spectral resolution of 0.5 cm⁻¹ from the Kuiper Airborne Observatory. The mole fractions of NH₃, PH₃, CH₄, CH₃D, CO, and GeH₄ were derived for the 1- to 6-bar portion of Jupiter's troposphere using a spectrum synthesis program. Knowledge of the abundances of these gases below the visible clouds is necessary to calculate the global inventory of nitrogen, phosphorus, carbon, and deuterium, which, in turn, may constrain models of Jupiter's formation. The N/H ratio is 1.5 ± 0.2 times the value for the Sun's photosphere. The P/H ratio for the 5-bar level is between 1.0 and 1.6 times the solar abundance. The weak ν₃−ν₄ hot band of CH₄ was detected for the first time on Jupiter, thus providing a deep atmospheric value for C/H of 3.6 ± 1.2 times solar. The Jovian deuterium abundance is comparable to that measured in the interstellar medium (D/H = 1.2 ± 0.5) × 10⁻⁵. CO appears to be well mixed with a mole fraction of (1.0 ± 0.3) × 10⁻⁹. Multiple absorption features confirm that GeH₄ is present on Jupiter with a mole fraction of (7.0_−2.0^+4.0) × 10⁻¹⁰. The observed abundances of CO, GeH₄, and PH₃ are consistent with models of convective transport from Jupiter's deep atmosphere.     

Saturn’s tropospheric composition and clouds from Cassini/VIMS 4.6-5.1 μm nightside spectroscopy

The latitudinal variation of Saturn’s tropospheric composition (NH₃, PH₃ and AsH₃) and aerosol properties (cloud altitudes and opacities) are derived from Cassini/VIMS 4.6-5.1 μm thermal emission spectroscopy on the planet’s nightside (April 22, 2006). The gaseous and aerosol distributions are used to trace atmospheric circulation and chemistry within and below Saturn’s cloud decks (in the 1- to 4-bar region). Extensive testing of VIMS spectral models is used to assess and minimise the effects of degeneracies between retrieved variables and sensitivity to the choice of aerosol properties. Best fits indicate cloud opacity in two regimes: (a) a compact cloud deck centred in the 2.5-2.8 bar region, symmetric between the northern and southern hemispheres, with small-scale opacity variations responsible for numerous narrow light/dark axisymmetric lanes; and (b) a hemispherically asymmetric population of aerosols at pressures less than 1.4 bar (whose exact altitude and vertical structure is not constrained by nightside spectra) which is 1.5-2.0× more opaque in the summer hemisphere than in the north and shows an equatorial maximum between ±10° (planetocentric).Saturn’s NH₃ spatial variability shows significant enhancement by vertical advection within ±5° of the equator and in axisymmetric bands at 23-25°S and 42-47°N. The latter is consistent with extratropical upwelling in a dark band on the poleward side of the prograde jet at 41°N (planetocentric). PH₃ dominates the morphology of the VIMS spectrum, and high-altitude PH₃ at p < 1.3 bar has an equatorial maximum and a mid-latitude asymmetry (elevated in the summer hemisphere), whereas deep PH₃ is latitudinally-uniform with off-equatorial maxima near ±10°. The spatial distribution of AsH₃ shows similar off-equatorial maxima at ±7° with a global abundance of 2-3 ppb. VIMS appears to be sensitive to both (i) an upper tropospheric circulation (sensed by NH₃ and upper-tropospheric PH₃ and hazes) and (ii) a lower tropospheric circulation (sensed by deep PH₃, AsH₃ and the lower cloud deck).     

The role of phosphorus in the upper atmosphere of Jupiter

The reaction of elemental phosphorus and H atoms to form PH₃ was observed and should be a major factor in the recycling of PH₃ in the stratosphere of Jupiter. The formation of PH₃ in this manner should predominate at high altitudes where, due to the very low temperatures, reactions that require higher activation energies than these atom reactions cannot occur. At lower altitudes, in the troposphere, the rapid formation of H atoms from the strong absorption of light by NH₃ will contribute to phosphine production also in this same manner. Recent experiments have also shown that elemental phosphorus reacts readily with aqueous ammonia to form PH₃. This reaction may also be important in the recycling of PH₃ in the upper troposphere of Jupiter if water-ammonia clouds, as had been previously thought, exist. Considerations of the coloration of the Great Red Spot have been made based upon the nature of the phosphorus obtained by decomposition of phosphine.     

Formation and photochemistry of Methylamine in Jupiter's atmosphere

The formation of methylamine (CH₃NH₂) in the upper troposphere and lower stratosphere of Jupiter is investigated. Translationally hot hydrogen atoms are produced in the photolysis of ammonia, phosphine, and acetylene which react with methane to produce methyl (CH₃) radicals; the latter recombine with NH₂ to form CH₃NH₂. Also, methane is catalytically dissociated to CH₃ + H by the species C₂ and C₂H produced in the photolysis of acetylene. It is shown that the combined production of CH₃NH₂ and subsequent photolysis to HCN is unlikely to account for the HCN observed near Jupiter's tropopause. Recombination of NH₂ and C₂H₅N followed by photolysis to HCN is the preferred path. Production of C₂H₆ by these two processes is negligible in comparison to the downward flux of C₂H₆ from the Lyman α photolysis region of CH₄. An upper limit column density on CH₃PH₂ is estimated to be ～10¹³ cm^?2 as compared to 10¹⁵ cm^?2 for CH₃NH₂. Hot H atoms account for a negligible fraction of the total ortho-para conversion by the reaction H + H₂     

A determination of the composition of the Saturnian stratosphere using the IUE

We reduced ultraviolet spectra of Saturn from the IUE satellite to produce a geometric albedo of the planet from 1500 to 3000 Å. By matching computer models to the albedo we determined a chemical composition consistent with the data. This model includes C₂H₂ and C₂H₆ with mixing ratios and distributions of (9 ± 3) × 10^?8 in the top 20 mbar of the atmosphere with none below for C₂H₂ and (6 ± 1) × 10^?6 also in the top 20 mbar with none below for C₂H₆. The C₂H₂ and C₂H₆ distributions and the C₂H₆ mixing ratio are taken directly from the Voyager IRIS model [R. Courtin et al., Bull. Amer. Astron. Soc.13, 722 (1981), and private communication]. The Voyager IRIS model also includes PH₃, which is not consistent with the uv albedo from 1800 to 2400 Å. Our model requires a previously unidentified absorber to explain the albedo near 1600 Å. After considering several candidates, we find that the best fit to the data is obtained with H₂O, having a column density of (6 ± 1) × 10^?3 cm-am.     

Laboratory measurements of the Ka-band (7.5 mm to 9.2 mm) opacity of phosphine (PH3) and ammonia (NH3) under simulated conditions for the Cassini-Saturn encounter

Recently, a model for the centimeter-wavelength opacity of PH₃ under conditions characteristic of the outer planets was developed by Hoffman et al. (2001, PhD thesis), based on centimeter wavelength laboratory measurements. New laboratory measurements have been conducted which show that this model is also accurate at low pressures and temperatures, and at millimeter wavelengths such as will be employed in Cassini Ka-band (9.3 mm) radio occultation studies. The opacity of PH₃ in a hydrogen/helium (H₂/He) atmosphere has been measured at frequencies in the Ka-band region at 32.7 GHz (9.2 mm), 35.6 GHz (8.4 mm), 37.7 GHz (8.0 mm), and 39.9 GHz (7.5 mm) at pressures of 0.5, 1, and 2 bar and at temperatures of 295, 209, and 188 K. Additionally, new high-precision laboratory measurements of the opacity of NH₃ in an H₂/He atmosphere have been conducted under the same temperature and pressure conditions described for PH₃. These new measurements better constrain the NH₃ opacity model supporting use of a Ben-Reuven lineshape model. These measurements will also elucidate the interpretation of millimeter wavelength observations conducted with the NRAO/VLA at 43 GHz (7 mm).     

Observations of the J = 10 manifold of the pure rotational band of phospine of Saturn

Saturn was observed in the vicinity of the J = 10 manifold of the pure rotational band of phosphine on 1984 July 10 and 12 from NASA's Kuiper Airborne Observatory with the facility farinfrared cooled grating spectrometer. On each night observations of the full disk plus rings were made at 4 to 6 discrete wavelengths which selectively sampled the manifold and the adjacent continuum. The previously reported detection of this manifold is confirmed. After substraction of the flux due to the rings, the data are compared with disk-averaged models of Saturn. It is found that PH₃ must be strongly depleted above the thermal inversion (～ 70 mbar). The best fitting models consistent with other observational constraints indicate that PH₃ is significantly depleted at even deeper atmospheric levels (≦500 mbar), implying an eddy diffusion coefficient for Saturn of ～ 10⁴ cm² sec^?1.     

Spectral reflectance change and luminescence of selected salts during 2–10 KeV proton bombardment: Implications for Io

Radiation damage and luminescence, caused by magnetospheric charged particles, have been suggested by several authors as mechanisms for explaining some of the peculiar spectral/albedo features of Io. We have pursued this possibility by measuring the uv-visual spectral reflectance and luminescent efficiency of several proposed Io surface constituents during 2 to 10-keV proton irradiation at room temperature and at low temperature (120 < T < 140°K). The spectral reflectance of NaCl and KCl during proton irradiation exhibits the well-known F-center absorption bands at 4580 and 5560 Å. Na₂SO₄ shows a generalized darkening which increases toward longer wavelengths. NaNO₃ shows a spectral reflectance change indicative of the partial alteration of NaNo₃ to NaNo₂. NaNO₂ shows no change. The luminescent efficiencies of NaCl and KCl are ～10^?4 at 300°K and increase by one-half order of magnitude at ～130°K. The efficiencies of K₂CO₃, Na₂CO₃, Na₂SO₄, and NaNO₃ are 10^?4, 10^?4, 10^?5 and 10^?6, respectively, at 300°K and they all decrease by one-half order of magnitude at ～130°K. These results indicate that magnetospheric proton irradiation of Io could cause spectral features in its observed ultraviolet and visible reflection spectrum if salts such as those studied here are present on its surface. However, because the magnitude of these spectral effects is dependent on competing factors such as surface temperature, incident particle energy flux, solar bleaching effects, and trace element abundance, we are unable at this time to make a quantitative estimate of the strength of these spectral effects on Io. The luminescent efficiencies of pure samples that we have studied in the laboratory suggest that charged-particle induced luminescence from Io's surface might be observable by a spacecraft such as Voyager when viewing Io's dark side.     

Phosphine absorption in the 5-μm window of Jupiter

Since the original suggestion by Gillett et al. (1969) it has generally been assumed that the region of partial transparency near 5 μm in Jupiter's atmosphere (the 5-μm window) is bounded by the v₄ NH₃ at 6.1 μm and the v₃ CH₄ band at 3.3 μm. New measurements of Jupiter and of laboratory phosphine (PH₃) samples show that PH₃ is a significant contributor to the continuum opacity in the window and in fact defines its short-wavelength limit. This has important implications for the use of 5-mu;m observations as a means to probe the deep atmospheric structure of Jupiter. The abundance of PH₃ which results from a comparison of Jovian and laboratory spectra is about 3 to 5 cm-am. This is five to eight times less than that found by Larson et al. [Astrophys. J. (1977) 211, 972–979] in the same spectral region, but is in good agreement with the result of Tokunaga et al. [Astrophys. J. (1979) 232, 603–615] from 10-μm observations.     

High-resolution 3-μm spectra of Jupiter: Latitudinal spectral variations influenced by molecules, clouds, and haze

We present latitudinally-resolved high-resolution (R = 37,000) pole-to-pole spectra of Jupiter in various narrow longitudinal ranges, in spectral intervals covering roughly half of the spectral range 2.86-3.53 μm. We have analyzed the data with the aid of synthetic spectra generated from a model jovian atmosphere that included lines of CH₄, CH₃D, NH₃, C₂H₂, C₂H₆, PH₃, and HCN, as well as clouds and haze. Numerous spectral features of many of these molecular species are present and are individually identified for the first time, as are many lines of and a few unidentified spectral features. In both polar regions the 2.86-3.10-μm continuum is more than 10 times weaker than in spectra at lower latitudes, implying that in this wavelength range the single-scattering albedos of polar haze particles are very low. In contrast, the 3.24-3.53 μm the weak polar and equatorial continua are of comparable intensity. We derive vertical distributions of NH₃, C₂H₂ and C₂H₆, and find that the mixing ratios of NH₃ and C₂H₆ show little variation between equatorial and polar regions. However, the mixing ratios of C₂H₂ in the northern and southern polar regions are ∼6 and ∼3 times, respectively, less than those in the equatorial regions. The derived mixing ratio curves of C₂H₂ and C₂H₆ extend up to the 10⁻⁶ bar level, a significantly higher altitude than most previous results in the literature. Further ground-based observations covering other longitudes are needed to test if these mixing ratios are representative values for the equatorial and polar regions.     

Martian isotopic ratios and upper limits for possible minor constituents as derived from Mariner 9 infrared spectrometer data

The Mariner 9 infrared spectrometer obtained data over a large part of Mars for almost a year beginning late in 1971. Mars' infrared emission spectrum was measured from 200 to 2000 cm^?1 with an apodized resolution of 2.4 cm^?1. No significant deviation from terrestrial ratios of carbon (¹²C/¹³C) or oxygen (¹⁶O/¹⁸O; ¹⁶O/¹⁷O) isotopes was observed on Mars. The ¹²C/¹³C isotopic ratio was found to be terrestrial with an uncertainty of 15%. Upper limits have been calculated for several minor constituents. With an effective noise equivalent radiance of 1.2 × 10^?9 W cm^?2 sr^?1/cm^?1, new upper limits in centimeter-atmospheres of 2 × 10^?5 for C₂H₂, 4 × 10^?3 for C₂H₄, 3 × 10^?3 for C₂H₆, 2 × 10^?4 for CH₄, 1 × 10^?3 for N₂O, 1 × 10^?4 for NO₂, 4 × 10^?5 for NH₃, 1 × 10^?3 for PH₃, 7 × 10^?4 for SO₂, and 1 × 10^?4 for OCS have been derived.     

Potential energy curves and dissociation energies of NbO,SiC, CP,PH+, SiF+, and NH+

The potential energy curves for the electronic ground states of astrophysically important NbO, SiC, CP, PH⁺, SiF⁺, and NH⁺ molecules are constructed by the RKRV method. The dissociation energies are determined by curve-fitting techniques using the five-parameter Hulburt-Hirschfelder function. The estimated dissociation energies are 7.86±0.16, 3.66±0.09, 5.12±0.12, 3.08±0.09, 6.46±0.14, and 3.02±0.09 eV for NbO, SiC, CP, PH⁺, SiF⁺, and NH⁺, respectively. The estimatedD ₀ values are in reasonably good agreement with literature values. If we utilizeD ₀ values of PH⁺, SiF⁺, and NH⁺, ionization potentials for PH, SiF, and NH are derived. The ionization potentials are 10.12, 7.13, and 13.66 eV, respectively, for PH, SiF, and NH. Dissociation energies for the above molecules are also estimated by use of the Birge-Sponer extrapolation and Hildenbrand and Murad methods.     

Absorption and photodissociation in the Schumann-Runge bands of molecular oxygen in the terrestrial atmosphere

The penetration in the terrestrial atmosphere of solar radiation corresponding to the spectral range of the Schumann-Runge bands of molecular oxygen is analyzed between 1750 and 2050 Å. The variation of the absorption cross section with temperature is taken into account and it is shown that average O₂ absorption cross sections cannot lead to correct photodissociation coefficients. Reduction factors are defined in order to simplify the computation of the molecular oxygen photodissociation and to permit a simple determination of the photodissociation coefficients of any minor constituent with smoothly varying absorption cross section. Examples are given for O₂, H₂O, CO₂, N₂O, HNO₃ and H₂O₂. Numerical approximations are developed for three types of spectral subdivisions: Schumann-Runge band intervals, 500 cm^?1 and 10 Å intervals. The approximations are valid from the lower thermosphere down to the stratosphere and they can be applied for a wide range of atmospheric models and solar zenith distances.