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.     

Vertical abundance profiles of hydrocarbons in Titan's atmosphere at 15° S and 80° N retrieved from Cassini/CIRS spectra

Limb spectra recorded by the Composite InfraRed Spectrometer (CIRS) on Cassini provide information on abundance vertical profiles of C₂H₂, C₂H₄, C₂H₆, CH₃C₂H, C₃H₈, C₄H₂, C₆H₆ and HCN, along with the temperature profiles in Titan's atmosphere. We analyzed two sets of spectra, one at 15° S (Tb flyby) and the other one at 80° N (T3 flyby). The spectral range 600-1400 cm⁻¹, recorded at a resolution of 0.5 cm⁻¹, was used to determine molecular abundances and temperatures in the stratosphere in the altitude range 100-460 km for Tb and 170-495 km for T3. Both temperature profiles show a well defined stratopause, at around 310 km (0.07 mbar) and 183 K at 13° S, and 380 km (0.01 mbar) with 207 K at 80° N. Near the north pole, stratospheric temperatures are colder and mesospheric temperatures are warmer than near the equator. C₂H₂, C₂H₆, C₃H₈ and HCN display vertical mixing ratio profiles that increase with height at 15° S and 80° N, consistent with their formation in the upper atmosphere, diffusion downwards and condensation in the lower stratosphere, as expected from photochemical models. The CH₃C₂H and C₄H₂ mixing ratios also increase with height at 15° S. But near the north pole, their profiles present an unexpected minimum around 300 km, observed for the first time thanks to the high vertical resolution of the CIRS limb data. C₂H₄ is the only molecule having a vertical abundance profile that decreases with height at 15° S. At 80° N, it also displays a minimum of its mixing ratio around the 0.1-mbar level. For C₆H₆, an upper limit of 1.1 ppb (in the 0.3-10 mbar range) is derived at 15° S, whereas a constant mixing ratio profile of is inferred near the north pole. At 15° S, the vertical profile of HCN exhibits a steeper gradient than other molecules, which suggests that a sink for this molecule exists in the stratosphere, possibly due to haze formation. All molecules display a more or less pronounced enrichment towards the north pole, probably due, in part, to subsidence of air at the north (winter) pole that brings air enriched in photochemical compounds from the upper atmosphere to lower levels.     

A spatially resolved high spectral resolution study of Neptune’s stratosphere

Using TEXES, the Texas Echelon cross Echelle Spectrograph, mounted on the Gemini North 8-m telescope we have mapped the spatial variation of H₂, CH₄, C₂H₂ and C₂H₆ thermal-infrared emission of Neptune. These high-spectral-resolution, spatially resolved, thermal-infrared observations of Neptune offer a unique glimpse into the state of Neptune’s stratosphere in October 2007, L_S = 275.4° just past Neptune’s southern summer solstice (L_S = 270°). We use observations of the S(1) pure rotational line of molecular hydrogen and a portion of the ν₄ band of methane to retrieve detailed information on Neptune’s stratospheric vertical and meridional thermal structure. We find global-average temperatures of 163.8 ± 0.8, 155.0 ± 0.9, and 123.8 ± 0.8 K at the 7.0 × 10⁻³-, 0.12-, and 2.1-mbar levels with no meridional variations within the errors. We then use the inferred temperatures to model the emission of C₂H₂ and C₂H₆ in order to derive stratospheric volume mixing ratios (hence forth, VMR) as a function of pressure and latitude. There is a subtle meridional variation of the C₂H₂ VMR at the 0.5-mbar level with the peak abundance found at −28° latitude, falling off to the north and south. However, the observations are consistent within error to a meridionally constant C₂H₂ VMR of at 0.5 mbar. We find that the VMR of C₂H₆ at 1-mbar peaks at the equator and falls by a factor of 1.6 at −70° latitude. However, a meridionally constant VMR of at the 1-mbar level for C₂H₆ is also statistically consistent with the retrievals. Temperature predictions from a radiative-seasonal climate model of Neptune that assumes the hydrocarbon abundances inferred in this paper are lower than the measured temperatures by 40 K at 7 × 10⁻³ mbar, 30 K at 0.12 mbar and 25 K at 2.1 mbar. The radiative-seasonal model also predicts meridional temperature variations on the order of 10 K from equator to pole, which are not observed. Assuming higher stratospheric CH₄ abundance at the equator relative to the south pole would bring the meridional trends of the inferred temperatures and radiative-seasonal model into closer agreement.We have also retrieved observations of C₂H₄ emission from Neptune’s stratosphere using TEXES on the NASA Infrared Telescope Facility (IRTF) in June 2003, L_S = 266°. Using the observations from the middle of the planet and an average of the middle three latitude temperature profiles from the 2007 observations (9.5° of L_S later, the seasonal equivalent of 9.5 Earth days within Earth’s seasonal cycle), we infer a C₂H₄ VMR of at 1.5 × 10⁻³ mbar, a value that is 3.25 times that predicted by global-average photochemical models.     

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.     

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).     

Analysis of Cassini/CIRS limb spectra of Titan acquired during the nominal mission: I. Hydrocarbons, nitriles and CO2 vertical mixing ratio profiles

Observations of the Composite InfraRed Spectrometer (CIRS) during the entire nominal Cassini mission (2004-2008) provide us with an accurate global view of composition and temperature in the middle atmosphere of Titan (between 100 and 500 km). We investigated limb spectra acquired at resolution at nine different latitudes between 56°S and 80°N, with a better sampling in the northern hemisphere where molecular abundances and temperature present strong latitudinal variations. From this limb data acquired between February 2005 and May 2008, we retrieved the vertical mixing ratio profiles of C₂H₂, C₂H₄, C₂H₆, C₃H₈, CH₃C₂H, C₄H₂, C₆H₆, HCN, HC₃N and CO₂. We present here for the first time, the latitudinal variations of the C₂H₆, C₃H₈, CO₂, C₂H₄ and C₆H₆ vertical mixing ratios profiles. Some molecules, such as C₂H₆ or C₃H₈ present little variations above their condensation level. The other molecules (except CO₂) show a significant enhancement of their mixing ratios poleward of 50°N. C₂H₄ is the only molecule whose mixing ratio decreases with height at latitudes below 46°N. Regions depleted in C₂H₂, HCN and C₄H₂ are observed around 400 km (0.01 mbar) and 55°N. We also inferred a region enriched in CO₂ located between 30 and 40°N in the 2-0.7 mbar pressure range. At 80°N, almost all molecules studied here present a local minimum of their mixing ratio profiles near 300 km (∼0.07 mbar), which is in contradiction with Global Circulation Models that predict constant-with-height vertical profiles due to subsidence at the north pole.     

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.     

The composition of Titan's stratosphere from Cassini/CIRS mid-infrared spectra

We have analyzed data recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft during the Titan flybys T0-T10 (July 2004-January 2006). The spectra characterize various regions on Titan from 70° S to 70° N with a variety of emission angles. We study the molecular signatures observed in the mid-infrared CIRS detector arrays (FP3 and FP4, covering roughly the 600-1500 cm⁻¹ spectral range with apodized resolutions of 2.54 or 0.53 cm⁻¹). The composite spectrum shows several molecular signatures: hydrocarbons, nitriles and CO₂. A firm detection of benzene (C₆H₆) is provided by CIRS at levels of about 3.5×¹⁰⁻⁹ around 70° N. We have used temperature profiles retrieved from the inversion of the emission observed in the methane ν₄ band at 1304 cm⁻¹ and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere. No longitudinal variations were found for these gases. Little or no change is observed generally in their abundances from the south to the equator. On the other hand, meridional variations retrieved for these trace constituents from the equator to the North ranged from almost zero (no or very little meridional variations) for C₂H₂, C₂H₆, C₃H₈, C₂H₄ and CO₂ to a significant enhancement at high northern (early winter) latitudes for HCN, HC₃N, C₄H₂, C₃H₄ and C₆H₆. For the more important increases in the northern latitudes, the transition occurs roughly between 30 and 50 degrees north latitude, depending on the molecule. Note however that the very high-northern latitude results from tours TB-T10 bear large uncertainties due to few available data and problems with latitude smearing effects. The observed variations are consistent with some, but not all, of the predictions from dynamical-photochemical models. Constraints are set on the vertical distribution of C₂H₂, found to be compatible with 2-D equatorial predictions by global circulation models. The D/H ratio in the methane on Titan has been determined from the CH₃D band at 1156 cm⁻¹ and found to be . Implications of this deuterium enrichment, with respect to the protosolar abundance on the origin of Titan, are discussed. We compare our results with values retrieved by Voyager IRIS observations taken in 1980, as well as with more recent (1997) disk-averaged Infrared Space Observatory (ISO) results and with the latest Cassini-Huygens inferences from other instruments in an attempt to better comprehend the physical phenomena on Titan.     

Jupiter's hydrocarbon polar brightening: Discovery of 3-micron line emission from south polar CH4, C2H2, and C2H6

Jupiter exhibits bright H⁺₃ auroral arcs at 3-4 microns that cool the hot (>1000 K) ionosphere above the ∼¹⁰−7 bar level through the infrared bands of this trace constituent. Below the ¹⁰−7 bar level significant cooling proceeds through infrared active bands of CH₄, C₂H₂, and C₂H₆. We report the discovery of 3-micron line emission from these hydrocarbon species in spectra of the jovian south polar region obtained on April 18 and 20, 2006 (UT) with CGS4 on the United Kingdom Infrared Telescope. Estimated cooling rates through these molecules are 7.5×¹⁰−3, 1.4×¹⁰−3, and , respectively, for a total nearly half that of H⁺₃. We derive a temperature of 450 ± 50 K in the ¹⁰−7-¹⁰−5 bar region from the C₂H₂ lines.     

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.     

Latitudinal variations of HCN, HC3N, and C2N2 in Titan's stratosphere derived from Cassini CIRS data

Mid- and far-infrared spectra from the Composite InfraRed Spectrometer (CIRS) have been used to determine volume mixing ratios of nitriles in Titan's atmosphere. HCN, HC₃N, C₂H₂, and temperature were derived from 2.5 cm⁻¹ spectral resolution mid-IR mapping sequences taken during three flybys, which provide almost complete global coverage of Titan for latitudes south of 60° N. Three 0.5 cm⁻¹ spectral resolution far-IR observations were used to retrieve C₂N₂ and act as a check on the mid-IR results for HCN. Contribution functions peak at around 0.5-5 mbar for temperature and 0.1-10 mbar for the chemical species, well into the stratosphere. The retrieved mixing ratios of HCN, HC₃N, and C₂N₂ show a marked increase in abundance towards the north, whereas C₂H₂ remains relatively constant. Variations with longitude were much smaller and are consistent with high zonal wind speeds. For 90°-20° S the retrieved HCN abundance is fairly constant with a volume mixing ratio of around 1 × 10⁻⁷ at 3 mbar. More northerly latitudes indicate a steady increase, reaching around 4 × 10⁻⁷ at 60° N, where the data coverage stops. This variation is consistent with previous measurements and suggests subsidence over the northern (winter) pole at approximately 2 × 10⁻⁴ m s⁻¹. HC₃N displays a very sharp increase towards the north pole, where it has a mixing ratio of around 4 × 10⁻⁸ at 60° N at the 0.1-mbar level. The difference in gradient for the HCN and HC₃N latitude variations can be explained by HC₃N's much shorter photochemical lifetime, which prevents it from mixing with air at lower latitude. It is also consistent with a polar vortex which inhibits mixing of volatile rich air inside the vortex with that at lower latitudes. Only one observation was far enough north to detect significant amounts of C₂N₂, giving a value of around 9 × 10⁻¹⁰ at 50° N at the 3-mbar level.     

The Titan N/N and C/C isotopic ratios in HCN from Cassini/CIRS

We report the detection of H¹³CN and HC¹⁵N in mid-infrared spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard Cassini, along with the determination of the ¹²C/¹³C and ¹⁴N/¹⁵N isotopic ratios. We analyzed two sets of limb spectra recorded near 13-15° S (Tb flyby) and 83° N (T4 flyby) at 0.5 cm⁻¹ resolution. The spectral range 1210-1310 cm⁻¹ was used to retrieve the temperature profile in the range 145-490 km at 13° S and 165-300 km at 83° N. These two temperature profiles were then incorporated in the atmospheric model to retrieve the abundance profile of H¹²C¹⁴N, H¹³CN and HC¹⁵N from their bands at 713, 706 and 711 cm⁻¹, respectively. The HCN abundance profile was retrieved in the range 90-460 km at 15° S and 165-305 km at 83° N. There is no evidence for vertical variations of the isotopic ratios. Constraining the isotopic abundance profiles to be proportional to the HCN one, we find at 15° S, and at 83° N, two values that are statistically consistent. A combination of these results yields a ¹²C/¹³C value equal to 75±12. This global result, as well as the 15° S one, envelop the value in Titan's methane (82.3±1) [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784] measured at 10° S and is slightly lower than the terrestrial inorganic standard value (89). The ¹⁴N/¹⁵N isotopic ratio is found equal to at 15° S and at 83° N. Combining the two values yields ¹⁴N/¹⁵N = 56 ± 8, which corresponds to an enrichment in ¹⁵N of about 4.9 compared with the terrestrial ratio. These results agree with the values obtained from previous ground-based millimeter observations [Hidayat, T., Marten, A., Bézard, B., Gautier, D., Owen, T., Matthews, H.E., Paubert, G., 1997. Icarus 126, 170-182; Marten, A., Hidayat, T., Biraud, Y., Moreno, R., 2002. Icarus 158, 532-544]. The ¹⁵N/¹⁴N ratio found in HCN is ∼3 times higher than in N₂ [Niemann, H.B., and 17 colleagues, 2005. Nature 438, 779-784], which implies a large fractionation process in the HCN photochemistry.     

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.     

Meridional variations of C2H2 and C2H6 in Jupiter's atmosphere from Cassini CIRS infrared spectra

Hydrocarbons such as acetylene (C₂H₂) and ethane (C₂H₆) are important tracers in Jupiter's atmosphere, constraining our models of the chemical and dynamical processes. However, our knowledge of the vertical and meridional variations of their abundances has remained sparse. During the flyby of the Cassini spacecraft in December 2000, the Composite Infrared Spectrometer (CIRS) instrument was used to map the spatial variation of emissions from 10 to 1400 cm⁻¹ (1000-7 μm). In this paper we analyze a zonally averaged set of CIRS spectra taken at the highest (0.48 cm⁻¹) resolution, firstly to infer atmospheric temperatures in the stratosphere at 0.5-20 mbar via the ν₄ band of CH₄, and in the troposphere at 150-400 mbar, via the H₂ absorption at 600-800 cm⁻¹. Stratospheric temperatures at 5 mbar are generally warmer in the north than the south by 7-8 K, while tropospheric temperatures show no such asymmetry. Both latitudinal temperature profiles however do show a pattern of maxima and minima which are largely anti-correlated between the two levels. We then use the derived temperature profiles to infer the vertical abundances of C₂H₂ and C₂H₆ by modeling tropospheric absorption (∼200 mbar) and stratospheric emission (∼5 mbar) in the C₂H₂ν₅ and C₂H₆ν₉ bands, and also emission of the acetylene (ν₄+ν₅)−ν₄ hotband (∼0.1 mbar). Acetylene shows a distinct north-south asymmetry in the stratosphere, with 5 mbar abundances greatest close to 20° N and decreasing from there towards both poles by a factor of ∼4. At 200 mbar in contrast, acetylene is nearly flat at a level of ∼3×¹⁰⁻⁹. Additionally, the abundance gradient of C₂H₂ between 10 and 0.1 mbar is derived, based on interpolated temperatures at 0.1 mbar, and is found to be positive and uniform with latitude to within errors. Ethane at both 5 and 200 mbar shows increasing VMR towards polar regions of ∼1.75 towards 70° N and ∼2.0 towards 70° S. An explanation for the meridional trends is proposed in terms of a combination of photochemistry and dynamics. Poleward, the decreasing UV flux is predicted to decrease the abundances of C₂H₂ and C₂H₆ by factors of 2.7 and 3.5, respectively, at latitude 70°. However, the lifetime of C₂H₆ in the stratosphere (3×¹⁰¹⁰ s at 5 mbar) is much longer than the dynamical timescale for meridional mixing inferred from Comet SL-9 debris (5-50×¹⁰⁸ s), and therefore the rising abundance towards high latitudes likely indicates that meridional mixing dominates over photochemical effects. For C₂H₂, the opposite occurs, with the relatively short photochemical lifetime (3×¹⁰⁷ s), compared to meridional mixing times, ensuring that the expected photochemical trends are visible.     

An analysis of Pluto occultation light curves using an atmospheric radiative-conductive model

We use a radiative-conductive model to least-squares fit Pluto stellar occultation light curve data. This model predicts atmospheric temperature based on surface temperature, surface pressure, surface radius, and CH₄ and CO mixing ratios, from which model light curves are to be calculated. The model improves upon previous techniques for deriving Pluto’s atmospheric thermal structure from stellar occultation light curves by calculating temperature (as a function of height) caused by heating and cooling by species in Pluto’s atmosphere, instead of a general assumption that temperature follows a power law with height or some other idealized function. We are able to fit for model surface radius, surface pressure, and CH₄ mixing ratio with one of the 2006 datasets and for surface pressure and CH₄ mixing ratio for other datasets from the years 1988, 2002, 2006, and 2008. It was not possible to fit for CO mixing ratio and surface temperature because the light curves are not sensitive to these parameters. We determine that the model surface radius, under the assumption of a stratosphere only (i.e. no troposphere) model in radiative-conductive balance, is . The CH₄ mixing ratio results are more scattered with time and are in the range of 1.8-9.4 × 10⁻³. The surface pressure results show an increasing trend from 1988 to 2002, although it is not as dramatic as the factor of 2 from previous studies.     

The vertical structure of Titan's upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements

Data acquired by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft during its close encounter with Titan on 26 October 2004 reveal the structure of its upper atmosphere. Altitude profiles of N₂, CH₄, and H₂, inferred from INMS measurements, determine the temperature, vertical mixing rate, and escape flux from the upper atmosphere. The mean atmospheric temperature in the region sampled by the INMS is 149±3 K, where the variance is a consequence of local time variations in temperature. The CH₄ mole fraction at 1174 km is 2.71±0.1%. The effects of diffusive separation are clearly seen in the data that we interpret as an eddy diffusion coefficient of , that, along with the measured CH₄ mole fraction, implies a mole fraction in the stratosphere of 2.2±0.2%. The H₂ distribution is affected primarily by upward flow and atmospheric escape. The H₂ mole fraction at 1200 km is 4±1×¹⁰⁻³ and analysis of the altitude profile indicates an upward flux of , referred to the surface. If horizontal variations in temperature and H₂ density are small, this upward flux also represents the escape flux from the atmosphere. The CH₄ density exhibits significant horizontal variations that are likely an indication of dynamical processes in the upper atmosphere.     

Numerical models of Saturn's long-lived anticyclones

New measurements of the dynamical properties of the long-lived Saturn's anticyclonic vortex known as “Brown Spot” (BS), discovered during the Voyager 1 and 2 flybys in 1980-1981 at latitude 43.1° N, and model simulations using the EPIC code, have allowed us to constrain the vertical wind shear and static stability in Saturn's atmosphere (vertically from pressure levels from 10 mbar to 10 bars) at this latitude. BS dynamical parameters from Voyager images include its size as derived from cloud albedo gradient (6100 km East-West times 4300 km North-South), mean tangential velocity ( at 2400 km from center) and mean vorticity (4.0±1.5×¹⁰⁻⁵ s⁻¹), lifetime >1 year, drift velocity relative to Voyager's System III rotation rate, mean meridional atmospheric wind profile at cloud level at its latitude and interactions with nearby vortices (pair orbiting and merging). An extensive set of numerical experiments have been performed to try to reproduce this single vortex properties and its observed mergers with smaller anticyclones by varying the vertical structure of the zonal wind and adjusting the static stability of the lower stratosphere and upper troposphere. Within the context of the EPIC model atmosphere, our simulations indicate that BS's drift velocity, longevity and merging behavior are very sensitive to these two atmospheric properties. The best results at the BS latitude occur for static stability conditions that use a Brunt-Väisäla frequency constant in the upper troposphere (from 0.5 to 10 bar) above 3.2×¹⁰⁻³ s⁻¹ and suggest that the wind speed slightly decays below the visible cloud deck from ∼0.5 to 10 bar at a rate per scale height. Changing the vortex latitude within the band domain introduces latitude oscillations in the vortex but not a significant meridional migration. Simulated mergers always showed orbiting movements with a typical merging time of about three days, very close to the time-span observed in the interaction of real vortices. Although these results are not unique in view of the unknowns of Saturn's deep atmosphere, they serve to constrain realistically its structure for ongoing Cassini observations.     

The effect of using different scale lengths on the production rates of Comet 46P/Wirtanen

The variations in production rates for Comet 46P/Wirtanen for the species H₂O and the parents of C₂ and CN are examined from the point of view of a variety of commonly used scale lengths. The calculations are carried out as a function of heliocentric distance. It is shown that, by using a common set of scale lengths, the results of various investigators can be brought into acceptable accord. The resulting production rates of H₂O and the parents of C₂ and CN versus heliocentric distance are recalculated and plotted versus the heliocentric distance r_H. The curves show reasonable agreement with a slope of ∼r_H⁻⁴. The water production rate near perihelion of 46P/Wirtanen is close to .     

Impact crater formed on sintered snow surface simulating porous icy bodies

To improve the scaling parameter controlling the impact crater formation in the strength regime, we conducted impact experiments on sintered snow targets with the dynamic strength continuously changed from 20 to 200 kPa, and the largest crater size formed on small icy satellites was considered by using the revised scaling parameter. Ice and snow projectiles were impacted on a snow surface with 36% porosity at an impact velocity from 31 m s⁻¹ to 150 m s⁻¹. The snow target was sintered at the temperature from −5 °C to −18 °C, and the snow dynamic strength was changed with the sintering duration at each temperature. We found that the mass ejected from the crater normalized by the projectile mass, π_V, was related to the ratio of the dynamic strength to the impact pressure, , as follows: , where the impact pressure was indicated by P = ρ_tC_0tv_i/2 with the target density of ρ_t, when the impact velocity, v_i, was much smaller than the bulk sound velocity C_0t (typically 1.8 km s⁻¹ in our targets). The ratio of the largest crater diameter to the diameter of the target body, d_max/D, was estimated by calculating the crater diameter at the impact condition for catastrophic disruption and then compared to the observed d_max/D of jovian and saturnian small satellites, in order to discuss the formation condition of these large d_max/D in the strength regime.     

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.