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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.02.005 Byline: Robert E. Samuelson (a), Michael D. Smith (b), Richard K. Achterberg (a), John C. Pearl (b) Keywords: Titan; Atmospheres; composition; Ices; IR spectroscopy Abstract: A strong, broad spectral emission feature at 85[degrees] N latitude centered at 221 cm.sup.-1 remains unidentified after candidate ices of H.sub.2O and pure crystalline CH.sub.3CH.sub.2CN are unambiguously ruled out. A much shallower weak emission feature starts at 160 cm.sup.-1 and blends into the strong feature at [approximately equal to]190 cm.sup.-1. This feature is consistent with one formed by an HCN ice cloud composed of a[c]1/25 [mu]m radius particles that resides in the lower stratosphere somewhere below an altitude of 160 km. Titan's stratospheric aerosol appears to have a spectral emission feature at about 148 cm.sup.-1. The aerosol abundance at 85[degrees] N is about a factor 2.2 greater than at 55[degrees] S. Author Affiliation: (a) Department of Astronomy, University of Maryland, College Park, MD 20742, USA (b) NASA/Goddard Space Flight Center, Code 693, Greenbelt, MD 20771, USA Article History: Received 14 June 2006; Revised 19 January 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.01.020 Byline: Louis A. Mayo (a)(b), Robert E. Samuelson (c) Keywords: Titan; Condensates; Stratosphere; Clouds Abstract: Analysis of the 250-560 cm.sup.-1 spectral continuum of Titan's north polar hood just after spring equinox reveals, in addition to the ubiquitous aerosol, a tenuous but relatively uniform cloud of small particles permeating the lower stratosphere at altitudes between 58 and 90 km. Voyager 1 IRIS data suggest the particles are highly scattering, almost certainly condensed organics, with radii between 1 and 5 [mu]m. Mole fractions for the condensed material range between 4x10.sup.-8 and 4x10.sup.-6, depending upon particle size. Vapor pressure arguments imply condensed nitriles near 90 km, the most likely being HCN, with condensed hydrocarbons such as C.sub.2H.sub.6 restricted to regions considerably nearer the tropopause. No direct chemical identification is possible. Negligible methane supersaturation in the troposphere at 67.4[degrees] N latitude, when compared with degrees of supersaturation at other latitudes, hints at precipitation fluxes of north polar stratospheric condensates during the previous northern winter that were perhaps three orders of magnitude greater than those at low latitudes during that time. A scale height of 1.5 times the density scale height above 160 km is reaffirmed for the photochemical aerosol of the north polar hood. There appears to be a depletion of aerosol somewhere below 160 km. An aerosol mole fraction [approximately equal to]8x10.sup.-8 at 160 km is inferred, about 33% greater than the value derived in a previous study. The Cassini CIRS instrument, with its expanded spectral range and higher spectral resolution, should be able to provide highly complementary information for the time period covering most of the northern winter season. Author Affiliation: (a) Raytheon Corporation, Greenbelt, MD 20771, USA (b) Goddard Space Flight Center, Code 630, Greenbelt, MD 20771, USA (c) Department of Astronomy, University of Maryland, College Park, MD 20742, USA Article History: Received 17 August 2004; Revised 13 January 2005

We report high-spectral-resolution ([lambda]/[delta][lambda])t = 800-2300) near-infrared mapping observations of Mars at [L.sub.s] = 130[degrees] (April 1999), which were obtained by drift-scanning the cryogenic long-slit spectrometer at the KPNO 2.2-m telescope across the disk. Data were reformatted into calibrated spectral image cubes (x,y,[lambda]) spanning 2.19 to 4.12 [micro]m, which distinguish atmospheric C[O.sub.2] features, solar lines, and surface and aerosol features. Maps of relative band depth between 3.0 and 3.5 [micro]m trace water ice clouds and show the diurnal evolution of features in the persistent northern summer aphelion cloud belt, which was mapped contemporaneously but at fixed local time by the Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES). Cloud optical depth, particle sizes, and ice aerosol content were estimated using a two-stream, single-layer scattering model, with Mie coefficients derived from recently published ice optical constants, followed by a linear spectral deconvolution process. A comparison of data and model spectra shows evaporating nighttime clouds in the morning followed by afternoon growth of a prominent orographic cloud feature on the west flank of Elysium Mons. Cloud optical depth at 3.2/am evolved to 0.28 [+ or -] 0.13 and ice aerosol column abundance to 0.9 [+ or -] 0.3 pr [micro] in the afternoon. Column abundances as large as 0.17 pr [micro]m were retrieved in nonorographic clouds within the aphelion cloud band around midday. These clouds exhibit a modest decline in optical depth during the afternoon. Results show that ice particle radii from 4 [micro]m exist in both cloud types. However, large particles dominate the spectra, consistent with recent MGS/TES emission phase function measurements of aphelion cloud aerosol properties. Keywords: Mars; Mars atmosphere; infrared observations

We report high-spectral-resolution (λ/δλ = 800–2300) near-infrared mapping observations of Mars at Ls = 130° (April 1999), which were obtained by drift-scanning the cryogenic long-slit spectrometer at the KPNO 2.2-m telescope across the disk. Data were reformatted into calibrated spectral image cubes (x,y,λ) spanning 2.19 to 4.12 μm, which distinguish atmospheric CO2 features, solar lines, and surface and aerosol features. Maps of relative band depth between 3.0 and 3.5 μm trace water ice clouds and show the diurnal evolution of features in the persistent northern summer aphelion cloud belt, which was mapped contemporaneously but at fixed local time by the Mars Global Surveyor Thermal Emission Spectrometer (MGS/TES). Cloud optical depth, particle sizes, and ice aerosol content were estimated using a two-stream, single-layer scattering model, with Mie coefficients derived from recently published ice optical constants, followed by a linear spectral deconvolution process. A comparison of data and model spectra shows evaporating nighttime clouds in the morning followed by afternoon growth of a prominent orographic cloud feature on the west flank of Elysium Mons. Cloud optical depth at 3.2 μm evolved to 0.28 ± 0.13 and ice aerosol column abundance to 0.9 ± 0.3 pr μm in the afternoon. Column abundances as large as 0.17 pr μm were retrieved in nonorographic clouds within the aphelion cloud band around midday. These clouds exhibit a modest decline in optical depth during the afternoon. Results show that ice particle radii from <2 μm to >4 μm exist in both cloud types. However, large particles dominate the spectra, consistent with recent MGS/TES emission phase function measurements of aphelion cloud aerosol properties. [Copyright &y& Elsevier]

Abstract: We utilized aerosol extinction coefficient inferred from Cassini/CIRS spectra in the far and mid infrared region to derive the extinction cross-section near an altitude of 190km at 15°S (from far-IR) and 20°S (from mid-IR). By comparing the extinction cross section that are derived from observations with theoretical calculations for a fractal aggregate of 3000 monomers, each having a radius of 0.05μm, and a fractal dimension of 2, we are able to constrain the refractive index of Titan’s aerosol between 70 and 1500cm−1 (143 and 6.7μm). As the real and imaginary parts of the refractive index are related by the Kramers–Kronig equation, we apply an iterative process to determine the optical constants in the thermal infrared. The resulting spectral dependence of the imaginary index displays several spectral signatures, some of which are also seen for some Titan’s aerosol analogues (tholins) produced in laboratory experiments. We find that Titan’s aerosols are less absorbent than tholins in the thermal infrared. The most prominent emission bands observed in the mid-infrared are due to C vibrations in methyl and methylene groups. It appears that Titan’s aerosols predominantly display vibrations implying carbon and hydrogen atoms and perhaps marginally nitrogen. In the mid infrared, all the aerosol spectral signatures are observed at three additional latitudes (56°S, 5°N and 30°N) and in the 193–274km altitude range, which implies that Titan’s aerosols exhibit the same chemical composition in all investigated latitude and altitude regions. [Copyright &y& Elsevier]

Abstract: We have analyzed the continuum emission of limb spectra acquired by the Cassini/CIRS infrared spectrometer in order to derive information on haze extinction in the 3–0.02mbar range (∼150–350km). We focused on the 600–1420cm−1 spectral range and studied nine different limb observations acquired during the Cassini nominal mission at 55°S, 20°S, 5°N, 30°N, 40°N, 45°N, 55°N, 70°N and 80°N. By means of an inversion algorithm solving the radiative transfer equation, we derived the vertical profiles of haze extinction coefficients from 17 spectral ranges of 20-cm−1 wide at each of the nine latitudes. At a given latitude, all extinction vertical profiles retrieved from various spectral intervals between 600 and 1120 cm−1 display similar vertical slopes implying similar spectral characteristics of the material at all altitudes. We calculated a mean vertical extinction profile for each latitude and derived the ratio of the haze scale height (H haze ) to the pressure scale height (H gas ) as a function of altitude. We inferred H haze /H gas values varying from 0.8 to 2.4. The aerosol scale height varies with altitude and also with latitude. Overall, the haze extinction does not show strong latitudinal variations but, at 1mbar, an increase by a factor of 1.5 is observed at the north pole compared to high southern latitudes. The vertical optical depths at 0.5 and 1.7mbar increase from 55°S to 5°N, remain constant between 5°N and 30°N and display little variation at higher latitudes, except the presence of a slight local maximum at 45°N. The spectral dependence of the haze vertical optical depth is uniform with latitude and displays three main spectral features centered at 630cm−1, 745cm−1 and 1390cm−1, the latter showing a wide tail extending down to ∼1000cm−1. From 600 to 750cm−1, the optical depth increases by a factor of 3 in contrast with the absorbance of laboratory tholins, which is generally constant. We derived the mass mixing ratio profiles of haze at the nine latitudes. Below the 0.4-mbar level all mass mixing ratio profiles increase with height. Above this pressure level, the profiles at 40°N, 45°N, 55°N, at the edge of the polar vortex, display a decrease-with-height whereas the other profiles increase. The global increase with height of the haze mass mixing ratio suggest a source at high altitudes and a sink at low altitudes. An enrichment of haze is observed at 0.1mbar around the equator, which could be due to a more efficient photochemistry because of the strongest insolation there or an accumulation of haze due to a balance between sedimentation and upward vertical drag. [Copyright &y& Elsevier]