Atomic oxygen distribution in the Venus mesosphere from observations of O2 infrared airglow by VIRTIS-Venus Express

This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the infrared atmospheric system of O₂ at 1.27 μm. The emission is produced by three-body recombination of oxygen atoms created by photodissociation of CO₂ on the dayside. It is consistently bright so that emission limb profiles can be extracted from the images. The vertical distribution of O₂() may be derived following Abel inversion of the radiance limb profiles. Assuming photochemical equilibrium, it is combined with the CO₂ vertical distribution to determine the atomic oxygen density. The uncertainties on the O density caused by the Abel inversion reach a few percent at the peak, increasing to about 50% near 120 km. We first analyze a case when the CO₂ density was derived from a stellar occultation observed with the SPICAV spectrometer simultaneously with an image of the O₂ limb airglow. In other cases, an average CO₂ profile deduced from a series of ultraviolet stellar occultations is used to derive the O profile, leading to uncertainties on the O density less than 30%. It is found that the maximum O density is generally located between 94 and 115 km with a mean value of 104 km. It ranges from less than 1×¹⁰¹¹ to about 5×¹⁰¹¹ cm⁻³ with a global mean of 2.2×¹⁰¹¹ cm⁻³. These values are in reasonable agreement with the VIRA midnight oxygen profile. The vertical O distribution is generally in good agreement with the oxygen profile calculated with a one-dimensional chemical-diffusive model. No statistical latitudinal dependence of the altitude of the oxygen peak is observed, but the maximum O density tends to decrease with increasing northern latitudes. The latitudinal distribution at a given time exhibits large variations in the O density profile and its vertical structure. The vertical oxygen distribution frequently shows multiple peaks possibly caused by waves or variations in the structure of turbulent transport. It is concluded that the O₂ infrared night airglow is a powerful tool to map the distribution of atomic oxygen in the mesosphere between 90 and 115 km and improve future Venus reference atmosphere models.     

Detection of CH3D on Titan

We report the detection of ¹³CH₃D in Titan's stratosphere from Cassini/CIRS infrared spectra near 8.7 μm. Fitting simultaneously the ν₆ bands of both ¹³CH₃D and ¹²CH₃D and the ν₄ band of CH₄, we derive a D/H ratio equal to and a ¹²C/¹³C ratio in deuterated methane of , consistent with that measured in normal methane.     

Detection of C2HD and the D/H ratio on Titan

We report here the first detection of mono-deuterated acetylene (acetylene-d1, C₂HD) in Titan's atmosphere from the presence of two of its emission bands at 678 and 519 cm⁻¹ as observed in CIRS spectral averages of nadir and limb observations taken between July 2004 and mid-2007. By using new laboratory spectra for this molecule, we were able to derive its abundance at different locations over Titan's disk. We find the C₂HD value () to be roughly constant with latitude from the South to about 45° N and then to increase slightly in the North, as is the case for C₂H₂. Fitting the 678 cm⁻¹ν₅ band simultaneously with the nearby C₂H₂ 729 cm⁻¹ν₅ band, allows us to infer a D/H ratio in acetylene on Titan with an average of the modal values of 2.09±0.45×¹⁰⁻⁴ from the nadir observations, the uncertainties being mainly due to the vertical profile used for the fit of the acetylene band. Although still subject to significant uncertainty, this D/H ratio appears to be significantly larger than the one derived in methane from the CH₃D band (upper limit of 1.5×¹⁰⁻⁴; Bézard, B., Nixon, C.A., Kleiner, I., Jennings, D.E., 2007. Icarus, 191, 397-400; Coustenis, A., Achterberg, R., Conrath, B., Jennings, D., Marten, A., Gautier, D., Bjoraker, G., Nixon, C., Romani, P., Carlson, R., Flasar, M., Samuelson, R.E., Teanby, N., Irwin, P., Bézard, B., Orton, G., Kunde, V., Abbas, M., Courtin, R., Fouchet, Th., Hubert, A., Lellouch, E., Mondellini, J., Taylor, F.W., Vinatier, S., 2007. Icarus 189, 35-62). From the analysis of limb data we infer D/H values of (at 54° S), (at 15° S), (at 54° N) and (at 80° N), which average to a mean value of 1.63±0.27×¹⁰⁻⁴.     

Prebiotic organic synthesis in early Earth and Mars atmospheres: Laboratory experiments with quantitative determination of products formed in a cold plasma flow reactor

The goal of this study was to explore prebiotic chemistry in a range of plausible early Earth and Mars atmospheres. To achieve this laboratory continuous flow plasma irradiation experiments were performed on N₂/H₂/CO/CO₂ gas mixtures chosen to represent mildly reducing early Earth and Mars atmospheres derived from a secondary volcanic outgassing of volatiles in chemical equilibrium with magmas near present day oxidation state. Under mildly reducing conditions (91.79% N₂, 5.89% H₂, 2.21% CO, and 0.11% CO₂), simple nitriles are produced in the gas phase with yield (G in molecules per 100 eV), for the key prebiotic marker molecule HCN at G∼1×¹⁰⁻³ (0.1 nmol J⁻¹). In this atmosphere localized HCN concentrations possibly could approach the 10⁻² M needed for HCN oligomerization. Yields under mildly oxidizing conditions (45.5% N₂, 0.1% H₂, 27.2% CO, 27.2% CO₂) are significantly less as expected, with HCN at G∼3×¹⁰⁻⁵ (). Yields in a Triton atmosphere which can be plausibly extrapolated to represent what might be produced in trace CH₄ conditions (99.9% N₂, 0.1% CH₄) are significant with HCN at G∼1×¹⁰⁻² (1 nmol J⁻¹) and tholins produced. Recently higher methane abundance atmospheres have been examined for their greenhouse warming potential, and higher abundance hydrogen atmospheres have been proposed based on a low early Earth exosphere temperature. A reducing (64.04% N₂, 28.8% H₂, 3.60% CO₂, and 3.56% CH₄), representing a high CH₄ and H₂ abundance early Earth atmosphere had HCN yields of G∼5×¹⁰⁻³ (0.5 nmol J⁻¹). Tholins generated in high methane hydrogen gas mixtures is much less than in a similar mixture without hydrogen. The same mixture with the oxidizing component CO₂ removed (66.43% N₂, 29.88% H₂, 0% CO₂, and 3.69% CH₄) had HCN yields of G∼1×¹⁰⁻³ (0.1 nmol J⁻¹) but more significant tholin yields.     

Airglow on Mars: Some model expectations for the OH Meinel bands and the O2 IR atmospheric band

This work presents model calculations of the diurnal airglow emissions from the OH Meinel bands and the O₂ IR atmospheric band in the neutral atmosphere of Mars. A time-dependent photochemical model of the lower atmosphere below 80 km has been developed for this purpose. Special emphasis is placed on the nightglow emissions because of their potential to characterize the atomic oxygen profile in the 50-80 km region. Unlike on Earth, the OH Meinel emission rates are very sensitive to the details of the vibrational relaxation pathway. In the sudden death and collisional cascade limits, the maximum OH Meinel column intensities for emissions originating from a fixed upper vibrational level are calculated to be about 300 R, for transitions v^′=9→v^′?8, and 15,000 R, for transitions v^′=1→v^′=0, respectively. During the daytime the 1.27 μm emission from O₂(), primarily formed from ozone photodissociation, is of the order of MegaRayleighs (MR). Due to the long radiative lifetime of O₂(), a luminescent remnant of the dayglow extends to the dark side for about two hours. At night, excited molecular oxygen is expected to be produced through the three body reaction O + O + CO₂. The column emission of this nighttime component of the airglow is estimated to amount to 25 kR. Both nightglow emissions, from the OH Meinel bands and the O₂ IR atmospheric band, overlap in the 50-80 km region. Photodissociation of CO₂ in the upper atmosphere and the subsequent transport of the atomic oxygen produced to the emitting layer are revealed as key factors in the nightglow emissions from these systems. The Mars 5 upper constraint for the product [H][O₃] is revised on the basis of more recent values for the emission probabilities and collisional deactivation coefficients.     

N2 escape rates from Pluto's atmosphere

Hydrodynamic escape of N₂ molecules from Pluto's atmosphere is calculated under the assumption of a high density, slow outflow expansion driven by solar EUV heating by N₂ absorption, near-IR and UV heating by CH₄ absorption, and CO cooling by rotational line emission as a function of solar activity. At 30 AU, the N₂ escape rate varies from in the absence of heating, but driven by an upward thermal heat conduction flux from the stratosphere, for lower boundary temperatures varying from 70-100 K. With solar heating varying from solar minimum to solar maximum conditions and a calculated lower boundary temperature, 88.2 K, the N₂ escape rate range is , respectively. LTE rotational line emission by CO reduces the net solar heat input by at most 35% and plays a minor role in lowering the calculated escape rates, but ensures that the lower boundary temperature can be calculated by radiative equilibrium with near-IR CH₄ heating. While an upward thermal conduction heat flux at the lower boundary plays a fundamental role in the absence of heating, with solar heating it is downward at solar minimum, and is, at most, 13% of the integrated net heating rate over the range of solar activity. For the arrival of the New Horizons spacecraft at Pluto in July 2015, predictions are lower boundary temperature, T₀∼81 K, and N₂ escape rate , and peak thermospheric temperature ∼103 K at 1890 km, based on expected solar medium conditions.     

Titan's hydrodynamically escaping atmosphere: Escape rates and the structure of the exobase region

In Strobel [Strobel, D.F., 2008. Icarus, 193, 588-594] a mass loss rate from Titan's upper atmosphere, , was calculated for a single constituent, N₂ atmosphere by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating due to CH₄ absorption. It was estimated, but not proven, that the hydrodynamic mass loss is essentially CH₄ and H₂ escape. Here the individual conservation of momentum equations for the three major components of the upper atmosphere (N₂, CH₄, H₂) are solved in the low Mach number limit and compared with Cassini Ion Neutral Mass Spectrometer (INMS) measurements to demonstrate that light gases (CH₄, H₂) preferentially escape over the heavy gas (N₂). The lightest gas (H₂) escapes with a flux 99% of its limiting flux, whereas CH₄ is restricted to ?75% of its limiting flux because there is insufficient solar power to support escape at the limiting rate. The respective calculated H₂ and CH₄ escape rates are 9.2×¹⁰²⁷ and 1.7×¹⁰²⁷ s−1, for a total of . From the calculated densities, mean free paths of N₂, CH₄, H₂, and macroscopic length scales, an extended region above the classic exobase is inferred where frequent collisions are still occurring and thermal heat conduction can deliver power to lift the escaping gas out of the gravitational potential well. In this region rapid acceleration of CH₄ outflow occurs. With the thermal structure of Titan's thermosphere inferred from INMS data by Müller-Wodarg et al. [Müller-Wodarg, I.C.F., Yelle, R.V., Cui, J., Waite Jr., J.H., 2008. J. Geophys. Res. 113, doi:10.1029/2007JE003033. E10005], in combination with calculated temperature profiles that include sputter induced plasma heating at the exobase, it is concluded that on average that the integrated, globally average, orbit-averaged, plasma heating rate during the Cassini epoch does not exceed ().     

Titan's hydrodynamically escaping atmosphere

The upper atmosphere of Titan is currently losing mass at a rate , by hydrodynamic escape as a high density, slow outward expansion driven principally by solar UV heating by CH₄ absorption. The hydrodynamic mass loss is essentially CH₄ and H₂ escape. Their combined escape rates are restricted by power limitations from attaining their limiting rates (and limiting fluxes). Hence they must exhibit gravitational diffusive separation in the upper atmosphere with increasing mixing ratios to eventually become major constituents in the exosphere. A theoretical model with solar EUV heating by N₂ absorption balanced by HCN rotational line cooling in the upper thermosphere yields densities and temperatures consistent with the Huygens Atmospheric Science Investigation (HASI) data [Fulchignoni, M., and 42 colleagues, 2005. Nature 438, 785-791], with a peak temperature of ∼185-190 K between 3500-3550 km. This model implies hydrodynamic escape rates of and , or some other combination with a higher H₂ escape flux, much closer to its limiting value, at the expense of a slightly lower CH₄ escape rate. Nonthermal escape processes are not required to account for the loss rates of CH₄ and H₂, inferred by the Cassini Ion Neutral Mass Spectrometer (INMS) measurements [Yelle, R.V., Borggren, N., de la Haye, V., Kasprzak, W.T., Niemann, H.B., Müller-Wodarg, I., Waite Jr., J.H., 2006. Icarus 182, 567-576].     

A sensitive search for nitric oxide in the lower atmospheres of Venus and Mars: Detection on Venus and upper limit for Mars

High-resolution spectra of Venus and Mars at the NO fundamental band at 5.3 μm with resolving power ν/δν=76,000 were acquired using the TEXES spectrograph at NASA IRTF on Mauna Kea, Hawaii. The observed spectrum of Venus covered three NO lines of the P-branch. One of the lines is strongly contaminated, and the other two lines reveal NO in the lower atmosphere at a detection level of 9 sigma. A simple photochemical model for NO and N at 50-112 km was coupled with a radiative transfer code to simulate the observed equivalent widths of the NO and some CO₂ lines. The derived NO mixing ratio is 5.5±1.5 ppb below 60 km and its flux is . Predissociation of NO at the (0-0) 191 nm and (1-0) 183 nm bands of the δ-system and the reaction with N are the only important loss processes for NO in the lower atmosphere of Venus. The photochemical impact of the measured NO abundance is significant and should be taken into account in photochemical modeling of the Venus atmosphere. Lightning is the only known source of NO in the lower atmosphere of Venus, and the detection of NO is a convincing and independent proof of lightning on Venus. The required flux of NO is corrected for the production of NO and N by the cosmic ray ionization and corresponds to the lightning energy deposition of . For a flash energy on Venus similar to that on the Earth (∼¹⁰⁹ J), the global flashing rate is ∼90 s⁻¹ and ∼6 km⁻² y⁻¹ which is in reasonable agreement with the existing optical observations. The observed spectrum of Mars covered three NO lines of the R-branch. Two of these lines are contaminated by CO₂ lines, and the line at 1900.076 cm⁻¹ is clean and shows some excess over the continuum. Some photochemical reactions may result in a significant excitation of NO (v=1) in the lowest 20 km on Mars. However, quenching of NO (v=1) by CO₂ is very effective below 40 km. Excitation of NO (v=1) in the collisions with atomic oxygen is weak because of the low temperature in the martian atmosphere, and we do not see any explanation of a possible emission of NO at 5.3 μm. Therefore the data are treated as the lack of absorption with a 2 sigma upper limit of 1.7 ppb to the NO abundance in the lower atmosphere of Mars. This limit is above the predictions of photochemical models by a factor of 3.     

Gravitational tidal waves in Titan's upper atmosphere

Tidal waves driven by Titan's orbital eccentricity through the time-dependent component of Saturn's gravitational potential attain nonlinear, saturation amplitudes (|T^′|>10 K, , and ) in the upper atmosphere (?500 km) due to the approximate exponential growth as the inverse square root of pressure. The gravitational tides, with vertical wavelengths of ∼100-150 km above 500 km altitude, carry energy fluxes sufficient in magnitude to affect the energy balance of the upper atmosphere with heating rates in the altitude range of 500-900 km.     

The C/C isotopic ratio in Titan hydrocarbons from Cassini/CIRS infrared spectra

We have analyzed infrared spectra of Titan recorded by the Cassini Composite Infrared Spectrometer (CIRS) to measure the isotopic ratio ¹²C/¹³C in each of three chemical species in Titan's stratosphere: CH₄, C₂H₂ and C₂H₆. This is the first measurement of ¹²C/¹³C in any C₂ molecule on Titan, and the first measurement of ¹²CH₄/¹³CH₄ (non-deuterated) on Titan by remote sensing. Our spectra cover five widely-spaced latitudes, 65° S to 71° N and we have searched for both latitude variability of ¹²C/¹³C within a given species, and also for differences between the ¹²C/¹³C in the three gases. For CH₄ alone, we find (1-σ), essentially in agreement with the ¹²CH₄/¹³CH₄ measured by the Huygens Gas Chromatograph/Mass Spectrometer instrument (GCMS) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784]: 82.3±1.0, and also with measured values in H¹³CN and ¹³CH₃D by CIRS at lower precision [Bézard, B., Nixon, C., Kleiner, I., Jennings, D., 2007. Icarus 191, 397-400; Vinatier, S., Bézard, B., Nixon, C., 2007. Icarus 191, 712-721]. For the C₂ species, we find in C₂H₂ and 89.8±7.3 in C₂H₆, a possible trend of increasingly value with molecular mass, although these values are both compatible with the Huygens GCMS value to within error bars. There are no convincing trends in latitude. Combining all fifteen measurements, we obtain a value of , also compatible with GCMS. Therefore, the evidence is mounting that ¹²C/¹³C is some 8% lower on Titan than on the Earth (88.9, inorganic standard), and lower than typical for the outer planets (88±7 [Sada, P.V., McCabe, G.H., Bjoraker, G.L., Jennings, D.E., Reuter, D.C., 1996. Astrophys. J. 472, 903-907]). There is no current model for this enrichment, and we discuss several mechanisms that may be at work.     

Extension of atmospheric dust loading to high altitudes during the 2001 Mars dust storm: MGS TES limb observations

Mars Global Surveyor (MGS) visible (solarband bolometer) and thermal infrared (IR) spectral limb observations from the Thermal Emission Spectrometer (TES) support quantitative profile retrievals for dust opacity and particle sizes during the 2001 global dust event on Mars. The current analysis considers the behavior of dust lifted to altitudes above 30 km during the course of this storm; in terms of dust vertical mixing, particle sizes, and global distribution. TES global maps of visible (solarband) limb brightness at 60 km altitude indicate a global-scale, seasonally evolving (over 190-240° solar longitudes, L_S) longitudinal corridor of vertically extended dust loading (which may be associated with a retrograde propagating, wavenumber 1 Rossby wave). Spherical radiative transfer analysis of selected limb profiles for TES visible and thermal IR radiances provide quantitative vertical profiles of dust opacity, indicating regional conditions of altitude-increasing dust mixing ratios. Observed infrared spectral dependences and visible-to-infrared opacity ratios of dust scattering over 30-60 km altitudes indicate particle sizes characteristic of lower altitudes (cross-section weighted effective radius, ), during conditions of significant dust transport to these altitudes. Conditions of reduced dust loading at 30-60 km altitudes present smaller dust particle sizes . These observations suggest rapid meridional transport at 30-80 km altitudes, with substantial longitudinal variation, of dust lifted to these altitudes over southern hemisphere atmospheric regions characterized by extraordinary (m/s) vertical advection velocities. By L_S=230° dust loading above 50 km altitudes decreased markedly at southern latitudes, with a high altitude (60-80 km) haze of fine (likely) water ice particles appearing over 10°S-40°N latitudes.     

Meridional variations of temperature, C2H2 and C2H6 abundances in Saturn's stratosphere at southern summer solstice

Measurements of the vertical and latitudinal variations of temperature and C₂H₂ and C₂H₆ abundances in the stratosphere of Saturn can be used as stringent constraints on seasonal climate models, photochemical models, and dynamics. The summertime photochemical loss timescale for C₂H₆ in Saturn's middle and lower stratosphere (∼40-10,000 years, depending on altitude and latitude) is much greater than the atmospheric transport timescale; ethane observations may therefore be used to trace stratospheric dynamics. The shorter chemical lifetime for C₂H₂ (∼1-7 years depending on altitude and latitude) makes the acetylene abundance less sensitive to transport effects and more sensitive to insolation and seasonal effects. To obtain information on the temperature and hydrocarbon abundance distributions in Saturn's stratosphere, high-resolution spectral observations were obtained on September 13-14, 2002 UT at NASA's IRTF using the mid-infrared TEXES grating spectrograph. At the time of the observations, Saturn was at a LS≈270°, corresponding to Saturn's southern summer solstice. The observed spectra exhibit a strong increase in the strength of methane emission at 1230 cm⁻¹ with increasing southern latitude. Line-by-line radiative transfer calculations indicate that a temperature increase in the stratosphere of ≈10 K from the equator to the south pole between 10 and 0.01 mbar is implied. Similar observations of acetylene and ethane were also recorded. We find the 1.16 mbar mixing ratio of C₂H₂ at −1° and −83° planetocentric latitude to be and , respectively. The C₂H₂ mixing ratio at 0.12 mbar is found to be at −1° planetocentric latitude and at −83° planetocentric latitude. The 2.3 mbar mixing ratio of C₂H₆ inferred from the data is and at −1° and −83° planetocentric latitude, respectively. Further observations, creating a time baseline, will be required to completely resolve the question of how much the latitudinal variations of C₂H₂ and C₂H₆ are affected by seasonal forcing and/or stratospheric circulation.     

Interaction between eddies and mean flow in Jupiter's atmosphere: Analysis of Cassini imaging data

Beebe et al. [Beebe, R.F., et al., 1980. Geophys. Res. Lett. 17, 1-4] and Ingersoll et al. [Ingersoll, A.P., et al., 1981. J. Geophys. Res. 86, 8733-8743] used images from Voyagers 1 and 2 to analyze the interaction between zonal winds and eddies in Jupiter's atmosphere. They reported a high positive correlation between Jupiter's eddy momentum flux, , and the variation of zonal velocity with latitude, . This correlation implied a surprisingly high rate of conversion of energy from eddies to zonal flow: , a value more than 10% of Jupiter's thermal flux emission. However, Sromovsky et al. [Sromovsky, L.A., et al., 1982. J. Atmos. Sci. 39, 1413-1432] argued that possible biases in the analysis could have caused an artificially high correlation. In addition, significant differences in the derived eddy flux between datasets put into question the robustness of any one result. We return to this long-standing puzzle using images of Jupiter from the Cassini flyby of December 2000. Our method is similar to previous analyses, but utilizes an automatic feature tracker instead of the human eye. The number of velocity vectors used in this analysis is over 200,000, compared to the 14,000 vectors used by Ingersoll et al. We also find a positive correlation between and and derive a global average power per unit mass, , ranging from . Utilizing Ingersoll et al.'s estimate of the mass per unit area involved in the transport, this would imply a rate of energy conversion of . We discuss the implications of this result and employ several tests to demonstrate its robustness.     

New Horizons Alice ultraviolet observations of a stellar occultation by Jupiter’s atmosphere

The Alice ultraviolet spectrograph onboard the New Horizons spacecraft observed two occultations of the bright star χ Ophiucus by Jupiter’s atmosphere on February 22 and 23, 2007 during the approach phase of the Jupiter flyby. The ingress occultation probed the atmosphere at 32°N latitude near the dawn terminator, while egress probed 18°N latitude near the dusk terminator. A detailed analysis of both the ingress and egress occultations, including the effects of molecular hydrogen, methane, acetylene, ethylene, and ethane absorptions in the far ultraviolet (FUV), constrains the eddy diffusion coefficient at the homopause level to be  cm² s⁻¹, consistent with Voyager measurements and other analyses (Festou, M.C., Atreya, S.K., Donahue, T.M., Sandel, B.R., Shemansky, D.E., Broadfoot, A.L. [1981]. J. Geophys. Res. 86, 5717-5725; Vervack Jr., R.J., Sandel, B.R., Gladstone, G.R., McConnell, J.C., Parkinson, C.D. [1995]. Icarus 114, 163-173; Yelle, R.V., Young, L.A., Vervack Jr., R.J., Young, R., Pfister, L., Sandel, B.R. [1996]. J. Geophys. Res. 101 (E1), 2149-2162). However, the actual derived pressure level of the methane homopause for both occultations differs from that derived by [Festou et al., 1981] and [Yelle et al., 1996] from the Voyager ultraviolet occultations, suggesting possible changes in the strength of atmospheric mixing with time. We find that at 32°N latitude, the methane concentration is  cm⁻³ at 70,397 km, the methane concentration is  cm⁻³ at 70,383 km, the acetylene concentration is  cm⁻³ at 70,364 km, and the ethane concentration is  cm⁻³ at 70,360 km. At 18°N latitude, the methane concentration is  cm⁻³ at 71,345 km, the methane concentration is  cm⁻³ at 71,332 km, the acetylene concentration is cm⁻³ at 71,318 km, and the ethane concentration is  cm⁻³ at 71,315 km. We also find that the H₂ occultation light curve is best reproduced if the atmosphere remains cold in the microbar region such that the base of the thermosphere is located at a lower pressure level than that determined by in situ instruments aboard the Galileo probe (Seiff, A., Kirk, D.B., Knight, T.C.D., Young, R.E., Mihalov, J.D., Young, L.A., Milos, F.S., Schubert, G., Blanchard, R.C., Atkinson, D. [1998]. J. Geophys. Res. 103 (E10), 22857-22889) - the Sieff et al. temperature profile leads to too much absorption from H₂ at high altitudes. However, this result is highly model dependent and non-unique. The observations and analysis help constrain photochemical models of Jupiter’s atmosphere.     

Observations of C2H6 and C2H2 in the stratosphere of Saturn

We have performed high-resolution spectral observations at mid-infrared wavelengths of C₂H₆ (12.16 μm), and C₂H₂ (13.45 μm) on Saturn. These emission features probe the stratosphere of the planet and provide information on the hydrocarbon photochemical processes taking place in that region of the atmosphere. The observations were performed using our cryogenic echelle spectrometer Celeste, in conjunction with the McMath-Pierce 1.5-m solar telescope in November and December 1994. We used Voyager IRIS CH₄ observations (7.67 μm) to derive a temperature profile on the saturnian atmosphere for the region of the stratosphere. This profile was then used in conjunction with height-dependent volume mixing ratios of each hydrocarbon to determine global abundances for ethane and acetylene. Our ground-based measurements indicate abundances of for C₂H₆ (1.0 mbar pressure level), and for C₂H₂ (1.6 mbar pressure level). We also derived new mixing ratios from the Voyager mid-latitude IRIS observations; 8.6±0.9×¹⁰⁻⁶ for C₂H₆ (0.1-3.0 mbar pressure level), and 1.6±0.2×¹⁰⁻⁷ for C₂H₂ (2.0 mbar pressure level).     

Titan's corona: The contribution of exothermic chemistry

The contribution of exothermic ion and neutral chemistry to Titan's corona is studied. The production rates for fast neutrals N₂, CH₄, H, H₂, ³CH₂, CH₃, C₂H₄, C₂H₅, C₂H₆, N(⁴S), NH, and HCN are determined using a coupled ion and neutral model of Titan's upper atmosphere. After production, the formation of the suprathermal particles is modeled using a two-stream simulation, as they travel simultaneously through a thermal mixture of N₂, CH₄, and H₂. The resulting suprathermal fluxes, hot density profiles, and energy distributions are compared to the N₂ and CH₄ INMS exospheric data presented in [De La Haye, V., Waite Jr., J.H., Johnson, R.E., Yelle, R.V., Cravens, T.E., Luhmann, J.G., Kasprzak, W.T., Gell, D.A., Magee, B., Leblanc, F., Michael, M., Jurac, S., Robertson, I.P., 2007. J. Geophys. Res., doi:10.1029/2006JA012222, in press], and are found insufficient for producing the suprathermal populations measured. Global losses of nitrogen atoms and carbon atoms in all forms due to exothermic chemistry are estimated to be and .     

The Venus nightglow: Ground-based observations and chemical mechanisms

In 1999, observations of the Venus nightglow with the Keck I telescope showed that the 5577 Å oxygen green line was a significant feature, comparable in intensity to the terrestrial green line. Subsequent measurements have been carried out at the Apache Point Observatory (APO) and again at Keck I, confirming the presence of the line with substantially varying intensity. The Herzberg II emission intensity, from the O₂(c-X) transition, was found to have an intensity near 3 kR in one APO run, comparable to the value found on all previous measurements. Thus, of the three oxygen features seen at Venus—the green line, the Herzberg II emission system, and the 1.27-μ 0-0 band of the IR atmospheric system—the first is quite variable, the second is relatively constant, while the third also shows large variations. The reaction between O₂(, v=0) and CO is considered as a possible mechanism to explain green line production and its variability, as well as the variability of the 1.27-μ emission and the stability of the CO₂ atmosphere. This reaction may catalyze CO₂ recombination some five orders of magnitude faster than the slow three-body O + CO reaction.     

Methane and its isotopologues on Saturn from Cassini/CIRS observations

High spectral resolution observations from the Cassini Composite Infrared Spectrometer [Flasar, F.M., and 44 colleagues, 2004. Space Sci. Rev. 115, 169-297] are analysed to derive new estimates for the mole fractions of CH₄, CH₃D and ¹³CH₄ of (4.7±0.2)×¹⁰⁻³, (3.0±0.2)×¹⁰⁻⁷ and (5.1±0.2)×¹⁰⁻⁵ respectively. The mole fractions show no hemispherical asymmetries or latitudinal variability. The analysis combines data from the far-IR methane rotational lines and the mid-IR features of methane and its isotopologues, using both the correlated-k retrieval algorithm of Irwin et al. [Irwin, P., and 9 colleagues, 2008. J. Quant. Spectrosc. Radiat. Trans. 109, 1136-1150] and a line-by-line approach to evaluate the reliability of the retrieved quantities. C/H was found to be enhanced by 10.9±0.5 times the solar composition of Grevesse et al. [Grevesse, N., Asplund, M., Sauval, A., 2007. Space Sci. Rev. 130 (1), 105-114], 2.25±0.55 times larger than the enrichment on Jupiter, and supporting the increasing fractional core mass with distance from the Sun predicted by the core accretion model of planetary formation. A comparison of the jovian and saturnian C/N, C/S and C/P ratios suggests different reservoirs of the trapped volatiles in a primordial solar nebula whose composition varies with distance from the Sun. This is supported by our derived D/H ratio in methane of (1.6±0.2)×¹⁰⁻⁵, which appears to be smaller than the jovian value of Lellouch et al. [Lellouch, E., Bézard, B., Fouchet, T., Feuchtgruber, H., Encrenaz, T., de Graauw, T., 2001. Astron. Astrophys. 370, 610-622]. Mid-IR emission features provided an estimate of , which is consistent with both the terrestrial ratio and jovian ratio, suggesting that carbon was accreted from a shared reservoir for all of the planets.     

Impact of atmospheric gravity waves on the jovian ionosphere

We investigate the effects of atmospheric gravity waves on the vertical and horizontal structure of the ionosphere of Jupiter. The presented non-linear, two-dimensional model of the jovian ionosphere allows for spatially and temporally varying neutral wind and temperature fields and tracks the time evolution of six ionospheric species, , and . An analytical approach is used to validate the model results for linear, small-amplitude waves and to elucidate the mechanisms that leads to perturbations in the density of the main ion species, H⁺ and . We demonstrate that the long-lived H⁺ ions are perturbed directly by wave dynamics whereas short-lived ions such as are perturbed by chemical interactions with other perturbed ion species. The model is then applied using larger gravity wave amplitudes consistent with observations. Atmospheric gravity waves propagating at high altitudes create layers of enhanced electron density similar to the system of layers observed during the J0-ingress radio occultation of the Galileo spacecraft. Our best fit to the J0-ingress observation is achieved using an 82 min period forcing wave with horizontal and vertical wavelengths of 500 km and 60 km respectively, and peaks at 510 km above the 1 bar pressure level. We further investigate the effects of the wave-induced ion flux on the background ionospheric structure and demonstrate that in the presence of a gravity wave the background density profiles of the H⁺ and ions are significantly modified. We also find that the column density of has variations that can exceed 10% as the wave propagates.