•Analysis of the VIMS solar occultations to characterize Titan’s vertical atmosphere.•Extraction of CH4, CO profiles, detection/identification of additional absorptions.•CH4 stratospheric abundance ...lower than GCMS, CO in agreement with previous results.•Gaseous ethane plays an important role on near-IR spectrum of Titan’s atmosphere.•Detection of aliphatic, aromatic and nitriles absorptions linked to aerosols.
We present an analysis of the VIMS solar occultations dataset, which allows us to extract vertically resolved information on the characteristics of Titan’s atmosphere between ∼100 and 700km with a vertical resolution of ∼10km. After a series of data treatment procedures to correct problems in pointing stability and parasitic light, 4 occultations out of 10 are retained. This sample covers different seasons and latitudes of Titan. The transmittances show clearly the evolution of the haze, with the detection of the detached layer at ∼310km in September 2011 at mid-northern latitudes. Through the inversion of the transmission spectra with a line-by-line radiative transfer code we retrieve the vertical distribution of CH4 and CO mixing ratio. For methane inversion we use its 1.4, 1.7 and 2.3μm bands. The first two bands are always in good agreement and yield an average stratospheric abundance of 1.28±0.08%, after correcting for forward-scattering effects, with no significant differences between the occultations. This is significantly less than the value of 1.48% obtained by the GCMS/Huygens instrument. We find that the 2.3μm band cannot be used for the extraction of methane abundance because it is blended with other absorptions, not included in our atmospheric model. The analysis of the residual spectra after the inversion shows that such additional absorptions are present through a great part of the VIMS wavelength range. We attribute many of these bands, including the one at 2.3μm, to gaseous ethane, whose near-infrared spectrum is not well modeled yet. Ethane also contributes significantly to the strong absorption at 3.2–3.5μm that was previously attributed only to C–H stretching bands from aerosols. Ethane bands may affect the surface windows too, especially at 2.7μm. Other residual bands are generated by stretching modes of C–H, C–C and C–N bonds. In addition to the C–H stretch from aliphatic hydrocarbons at 3.4μm, we detect a strong and narrow absorption at 3.28μm which we tentatively attribute to the presence of PAHs in the stratosphere. C–C and C–N stretching bands are possibly present between 4.3 and 4.5μm. Finally, we obtain the CO mixing ratio between 70 and 170km, through the inversion of its 4.7μm band. The average result of 46±16ppm is in good agreement with previous studies.
Nightside observations of the 1.18‐μm atmospheric window by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) aboard the Venus Express spacecraft were analyzed to measure and map the ...water vapor abundance in the lower atmosphere. Thermal emission in this window originates partly from the surface and partly from the first scale height (0–15 km) of the atmosphere. Constraints on the CO2 continuum absorption, which is the dominant source of gaseous opacity in the window, were obtained from the variation of the 1.185‐μm intensity with surface elevation. An absorption coefficient of 1 ± 0.4 × 10−9 cm−1 amagat−2 best fits the observed variation. We retrieved a water vapor mole fraction of 44 ± 9 ppm from various selections of VIRTIS spectra in the southern hemisphere, in agreement with previous analyses of the nightside emission. This value is somewhat larger than that previously determined at higher altitudes from the 2.3‐ and 1.74‐μm nightside windows, but the error bars still allow a constant with height H2O mole fraction from the surface up to 40 km. Using the intensity ratio in the two wings of the 1.18‐μm window as a proxy, we searched for horizontal variations of the H2O abundance in various VIRTIS observational sequences. We derived stringent upper limits for any possible latitudinal variations on the night side: ±1.5% in the range 60°S–25°N and ±3% for the broader range 80°S–25°N. The lack of detectable latitudinal variations is consistent with a constant with height water profile in the lower atmosphere and probably precludes any strong concentration gradient near the surface.
Detection of Propadiene on Titan Lombardo, Nicholas A; Nixon, Conor A; Greathouse, Thomas K ...
Astrophysical journal. Letters,
08/2019, Volume:
881, Issue:
2
Journal Article
Peer reviewed
Open access
The atmosphere of Titan, the largest moon of Saturn, is rich in organic molecules, and it has been suggested that the moon may serve as an analog for the pre-biotic Earth due to its highly reducing ...chemistry and existence of global hazes. Photochemical models of Titan have predicted the presence of propadiene (historically referred to as allene), CH2CCH2, an isomer of the well-measured propyne (also called methylacetylene) CH3CCH, but its detection has remained elusive due to insufficient spectroscopic knowledge of the molecule. This has recently been remedied with an updated spectral line list. Here we present the first unambiguous detection of the molecule in any astronomical object, observed with the Texas Echelle Cross Echelle Spectrograph on the NASA Infrared Telescope Facility in 2017 July. We model its emission line near 12 m and measure a volume mixing ratio of (6.9 0.8) × 10−10 at 175 km, assuming a vertically increasing abundance profile as predicted in photochemical models. Cassini measurements of propyne made during 2017 April indicate that the abundance ratio of propyne to propadiene is 8.2 1.1 at the same altitude. This initial measurement of the molecule in Titan's stratosphere paves the way toward constraining the amount of atomic hydrogen available on Titan, as well as future mapping of propadiene on Titan from 8 m and larger ground-based observatories, and future detection on other planetary bodies.
We have analyzed spectra recorded between 50 and 650 cm−1 by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft at low and high emission angles to determine simultaneously the ...H2 mole fraction and ortho-to-para ratio in Titan's troposphere. We used constraints from limb spectra between 50 and 900 cm−1 and from in situ measurements by the Huygens probe to characterize the temperature, haze and gaseous absorber profiles. We confirm that the N2-CH4 collision-induced absorption (CIA) coefficients used up to now need to be increased by about 52% at temperatures of 70–85 K. We find that the N2-N2 CIA coefficients are also too low in the N2 band far wing, beyond 110 cm−1, in agreement with recent quantum mechanical calculations. We derived a H2 mole fraction equal to (0.88 ± 0.13) × 10−3, which pertains to the ~1–34 km altitude range probed by the S0(0) and S0(1) lines. This result agrees with a previous determination based only on the H2-N2 dimer transition in the S0(0) line, and with the in situ measurement by the Gas Chromatograph Mass Spectrometer (GCMS) aboard Huygens. It is 3–4 times smaller than the value measured in situ by the Ion Neutral Mass Spectrometer (INMS) of Cassini at 1000–1100 km. The H2 para fraction is close to equilibrium in the 20-km region. CIRS spectra can be fitted assuming ortho-to-para (o-p) H2 thermodynamical equilibrium at all levels or a constant para fraction in the range 0.49–0.53. We have investigated different mechanisms that may operate in Titan's atmosphere to equilibrate the H2 o-p ratio and we have developed a one-dimensional model that solves the continuity equation in presence of such conversion mechanisms. We conclude that exchange with H atoms in the gas phase or magnetic interaction of H2 in a physisorbed state on the surface of aerosols are too slow compared with atmospheric mixing to play a significant role. On the other hand, magnetic interaction of H2 with CH4, and to a lesser extent N2, can operate on a timescale similar to the vertical mixing time in the troposphere. This process is thus likely responsible for the o-p equilibration of H2 in the mid-troposphere implied by CIRS measurements. The model can reproduce the inferred o-p ratio in the 20-km region, assuming low atmospheric mixing in the troposphere down to 15–20 km and conversion rates with CH4 or N2 slightly larger than obtained from an extrapolation of natural ortho-para conversion rate measured in gaseous hydrogen.
•Cassini/CIRS spectra were analyzed to retrieve H2 abundance and ortho-to-para ratio in Titan.•H2 mole fraction is (0.88 ± 0.13) × 10−3 with ortho-to-para ratio close to equilibrium in the mid-troposphere.•A one-dimensional model for the vertical profile of H2 para-fraction was developed.•Equilibration likely occurs through magnetic interaction of H2 with CH4 and N2 in the troposphere.•N2-CH4 and N2-N2 absorption coefficients need to be increased to reproduce CIRS spectra.
► Complete empirical line lists for methane at 80K and 296K between 1.26 and 1.71μm. ► Determination of the lower state energy values. ► Identification of the CH3D and 13CH4 transitions. ► ...Application to Titan ground-based and DISR spectra showing an excellent fit.
Insufficient knowledge of the near infrared spectrum of methane is an important limitation for the analysis of the spectra of Titan and of the outer planetary atmospheres in general. The work reported here is the result of a long-term project aiming to provide astronomers with a line by line list for precise calculations of the methane absorption in the near infrared region. We thus present here our best to date empirical line list between 5854 and 7919cm−1 (1.71–1.26μm) and apply it to Titan, demonstrating its capability to significantly improve planetary spectral analysis.
In recent contributions, we have obtained empirical line lists at room temperature and at 80K (suitable for Titan conditions) from spectra recorded by (i) Differential Absorption Spectroscopy (DAS) in the high energy part of the tetradecad (5854–6195cm−1) and in the icosad (6717–7589cm−1) (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58μm and 1.28μm transparency windows (6165–6750cm−1 and 7541–7919cm−1, respectively). In this work, we construct the global line lists for methane in “natural” isotopic abundance, covering the entire spectral region from 5854 to 7919cm−1. These WKMC (for Wang, Kassi, Mondelain, Campargue) empirical lists include 42,988 and 46,320 lines at 80±3K and 296±3K, respectively and are assembled here with some important improvements:
(i)New spectroscopic parameters for the 5854–6148cm−1 region at 80K which increase significantly the number of observations and lower state energy determinations.(ii)Transitions of 13CH4 and CH3D were systematically identified by comparison with DAS spectra of highly enriched 13CH4 and CH3D, recorded at the same temperatures.(iii)In the 1.58μm transparency window where CH3D lines contribute importantly to the methane spectrum at 80K, the set of CH3D lower state energy values was completed by using recent DAS results for pure CH3D.
The “two temperature method” provided lower state energy values for about 24,000 transitions from the ratios of their line intensities at 80K and 296K. The clear propensity of the derived low J values of 12CH4 and 13CH4 to be integer illustrates the quality of the lower state energy values. The obtained data sets allow us to account for most of the temperature dependence of the absorption over the considered region.
To illustrate the interest of the WKMC line lists for planetary applications, we perform simulations of Titan spectra at different resolutions taken from the ground with instruments such as the FTS at the CFHT and CRIRES at the VLT or by the DISR instrument on board the Huygens probe. The agreement between the simulations and the observations clearly demonstrates an important improvement with respect to previous works.
In this paper we apply a recently released set of methane line parameters (Wang et al., 2011) to the modeling of Titan spectra in the 1.58μm window at both low and high spectral resolution. We first ...compare the methane absorption based on this new set of methane data to that calculated from the methane absorption coefficients derived in situ from DISR/Huygens (Tomasko et al., 2008a; Karkoschka and Tomasko, 2010) and from the band models of Irwin et al. (2006) and Karkoschka and Tomasko (2010). The Irwin et al. (2006) band model clearly underestimates the absorption in the window at temperature–pressure conditions representative of Titan’s troposphere, while the Karkoschka and Tomasko (2010) band model gives an acceptable agreement in the whole window, overestimating the absorption by about 15% in the range 6300–6500cm−1. We also find that the transmittance of Titan’s atmosphere is in excellent agreement with that calculated from the Tomasko et al. (2008a) coefficients after reducing them by about 7%. Synthetic spectra computed with spectral resolutions of 1.2cm−1 (R∼5400) and 0.35cm−1 (R∼18000) are then compared with two high-resolution Earth-based measurements of Titan’s albedo obtained in 1982 and 1993 (with KPNO/FTS and IRTF/CSHELL). The new set of methane line parameters leads to an excellent match of all the CH3D and CH4 absorption features in these spectra, and permits us to derive a ratio of CH3D/CH4=(4.5±1.0)×10−4 – hence a D/H ratio in methane for Titan of (1.13±0.25)×10−4 – and a CO mole fraction of 40±10ppm (from the KPNO/FTS dataset) and 51±7ppm (from the IRTF/CSHELL dataset). We also infer constraints on the far-wing lineshape of methane lines of the 2ν3 band. We finally present two other examples of models of Titan’s spectrum using the new line parameters, one potentially useful for future higher-resolution (R=40,000) observations, another one applicable to the ongoing low-resolution (R∼100) observations by Cassini VIMS. We show that the aerosol model of Tomasko et al. (2008b) produces too much intensity at low phase angle compared to a VIMS spectrum recorded near the Huygens site and we propose a slightly revised model that reproduces this observation.
► First applications to planetology of recently released data on methane. ► In-depth analysis of Titan near-infrared ground-based and space-based spectra with new data on methane. ► New measurements of the D/H ratio and CO abundances in Titan.
Measuring the spatial distribution of chemical compounds in Saturn’s stratosphere is critical to better understand the planet’s photochemistry and dynamics. Here we present an analysis of infrared ...spectra in the range 600–1400
cm
−1 acquired in limb geometry by the Cassini spacecraft between March 2005 and January 2008. We first determine the vertical temperature profiles from 3 to 0.01
hPa, at latitudes ranging from 70°N to 80°S. We infer a similar meridional temperature gradient at 1–2
hPa as in recent previous studies Fletcher, L.N., Irwin, P.G.J., Teanby, N.A., Orton, G.S., Parrish, P.D., de Kok, R., Howett, C., Calcutt, S.B., Bowles, N., Taylor, F.W., 2007. Icarus 189, 457–478; Howett, C.J.A., Irwin, P.G.J., Teanby, N.A., Simon-Miller, A., Calcutt, S.B., Fletcher, L.N., de Kok, R., 2007. Icarus 190, 556–572. We then retrieve the vertical profiles of
C
2
H
6
and
C
2
H
2
from 3 to 0.01
hPa and of
C
3
H
8
around 1
hPa. At 1
hPa, the meridional variation of
C
2
H
2
is found to follow the yearly averaged solar insolation, except for a strong equatorial mole fraction of
8
×
10
-
7
, nearly two times higher than expected. This enhancement in abundance can be explained by the descent of hydrocarbon-rich air, with a vertical wind speed at the equator of
0.25
±
0.1
mm/s at 1
hPa and
0.4
±
0.15
mm/s at 0.1
hPa. The ethane distribution is relatively uniform at 1
hPa, with only a moderate 25% increase from 35°S to 80°S. Propane is found to increase from north to south by a factor of 1.9, suggesting that its lifetime may be shorter than Saturn’s year at 1
hPa. At high altitudes (1
Pa),
C
2
H
2
and
C
2
H
6
abundances depart significantly from the photochemical model predictions of Moses and Greathouse Moses, J.I., Greathouse, T.K., 2005. J. Geophys. Res. 110, 9007, except at high southern latitudes (62, 70 and 80°S) and near the equator. The observed abundances are found strongly depleted in the 20–40°S region and enhanced in the 20–30°N region, the latter coinciding with the ring’s shadow. We favor a dynamical explanation for these anomalies.
•We developed a seasonal-radiative–dynamical model of Titan's atmosphere to investigate temperatures retrieved by Cassini/Huygens at low latitudes.•Radiative relaxation times are found significantly ...shorter than previous estimates.•Radiative heating rates around spring equinox exceed radiative cooling rates at all altitudes, implying adiabatic cooling from upwelling.•Eccentricity of Saturn's orbit is likely responsible for the drop of stratospheric equatorial temperatures since equinox.•Seasonal modulation of the vertical velocity is required to reproduce observations from 2004 to 2016.
We have developed a seasonal radiative–dynamical model of Titan's stratosphere to investigate the temporal variation of temperatures in the 0.2–4 mbar range observed by the Cassini/CIRS spectrometer. The model incorporates gas and aerosol vertical profiles derived from Cassini/CIRS and Huygens/DISR data to calculate the radiative heating and cooling rate profiles as a function of time and latitude. At 20°S in 2007, the heating rate is larger than the cooling rate at all altitudes, and more specifically by 20–35% in the 0.1–5 mbar range. A new calculation of the radiative relaxation time as a function of pressure level is presented, leading to time constants significantly lower than previous estimates. At 6°N around spring equinox, the radiative equilibrium profile is warmer than the observed one at all levels. Adding adiabatic cooling in the energy equation, with a vertical upward velocity profile approximately constant in pressure coordinates below the 0.02-mbar level (corresponding to 0.03–0.05 cm s−1 at 1 mbar), allows us to reproduce the observed profile quite well. The velocity profile above the ∼0.5-mbar level is however affected by uncertainties in the haze density profile. The model shows that the change in insolation due to Saturn's orbital eccentricity is large enough to explain the observed 4-K decrease in equatorial temperatures around 1 mbar between 2009 and 2016. At 30°N and S, the radiative model predicts seasonal variations of temperature much larger than observed. A seasonal modulation of adiabatic cooling/heating is needed to reproduce the temperature variations observed from 2005 to 2016 between 0.2 and 4 mbar. At 1 mbar, the derived vertical velocities vary in the range −0.05 (winter solstice) to 0.16 (summer solstice) cm s−1 at 30°S, −0.01 (winter solstice) to 0.14 (summer solstice) cm s−1 at 30°N, and 0.03–0.07 cm s−1 at the equator.
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