► 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.
The composition of Venus' lower atmosphere, beneath the clouds, has been investigated through both in situ measurements and remote‐sensing observations. In 1978 the mass spectrometers and gas ...chromatographs aboard the Venera 11–12 landers and Pioneer Venus Large probe returned unique information on the abundances of carbon dioxide, nitrogen, noble gases, and other minor species. Radio occultations from the Pioneer and Magellan spacecraft provided measurements of the vertical profile of sulfuric acid vapor. In the 1980s and 1990s the discovery of near‐infrared emission from the night side of Venus, through narrow spectral windows in the 0.9–2.5 micron region, opened a new opportunity of probing the lower atmosphere and surface. The abundances of water vapor, halides, carbon monoxide, sulfur dioxide, and carbonyl sulfide were determined, as well as the deuterium‐to‐hydrogen ratio. The vertical profile of water vapor in the 0‐ to 45‐km‐altitude range and the CO and OCS mean vertical gradients in the 30‐ to 40‐km region have been determined from various analyses. In addition, evidence for latitudinal variations of these two species was found from Galileo and recent ground‐based nightside observations. Conflicting results have been reported on the SO2 abundance below 35 km and on the H2O profile below 20 km. The whole set of available measurements provides important constraints on the chemistry and dynamics of Venus' lower atmosphere. However, key information is still missing, including the abundances of CO and sulfur compounds near the surface, composition maps at high latitudes, and possible small‐scale and temporal variability of trace compounds.
We present a composite spectrum of Trojan asteroid 624 Hektor, 0.3–3.6 μm, and models computed for the full wavelength range with the Hapke scattering theory. The data show that there is no ...discernible 3-μm absorption band. Such a band would indicate the presence of OH
− or H
2O-bearing silicate minerals, or macromolecular carbon-rich organic material of the kind seen on the low-albedo hemisphere of Saturn's satellite Iapetus. The absence of spectral structure is itself indicative of the absence of the nitrogen-rich tholins (which show a distinctive absorption band attributed to N–H).
The successful models in this study all incorporate magnesium-rich pyroxene (Mg, Fe SiO
3), which satisfactorily matches the red color of Hektor. Pyroxene is a mafic mineral common in terrestrial and lunar lavas, and is also identified in Main Belt asteroid spectra. An upper limit to the amount of crystalline H
2O ice (30-μm grains) in the surface layer of Hektor accessible to near-infrared remote sensing observations is 3 wt%. The upper limit for serpentine, as a representative of hydrous silicates, is much less stringent, at 40%, based on the shape of the spectral region around 3 μm. Thus, the spectrum at 3 μm does not preclude the presence of a few weight percent of volatile material in the uppermost surface layer of Hektor. Below this “optical” surface that our observations probe, any amount of H
2O ice and other volatile-rich materials might exist. All of the models we calculated require a very low-albedo, neutral color material to achieve the low geometric albedo that matches Hektor; we use elemental carbon. If elemental carbon is present on Hektor, it could be of organic or inorganic origin. By analogy, other D-type asteroids could achieve their red color, low albedo, and apparent absence of phyllosilicates from compositions similar to the models presented here. Our models appear to demonstrate that organic solids are not required to match the red color and low albedos of D-type asteroids.
This paper presents the analysis of near-infrared observations of the icy surface of Triton, recorded on 1995 September 7, with the cooled grating spectrometer CGS4 at the United Kingdom Infrared ...Telescope (Mauna Kea, HI). This analysis was performed in two steps. The first step consisted of identifying the molecules composing Triton's surface by comparing the observations with laboratory transmission spectra (direct spectral analysis); this also gives information on the physical state of the components. Most of the bands in Triton's spectrum were assigned to specific vibration bands of the CH4, N2, CO, and CO2 molecules previously discovered. A detailed comparison of the frequencies of the CH4 bands confidently indicated that this molecule exists in a diluted state in solid β-N2. Three new bands peaking at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm, respectively) were also observed. Laboratory experiments have shown that C2H6 isolated in solid N2 fits well the second band, but this would imply the appearance of unobserved bands and thus rules out this assignment. However, C2H6 may exist in another physical state, and more experiments are necessary. No plausible candidate was found for these three bands when comparing with the spectra of nine molecules (C2H2, C2H4, C3H8, NH3, SO2, HC3N, CH3OH, NO, NO2).
In view of the results of D. P. Cruikshank et al. (1993, Science261, 742; in preparation), the work presented here leads to two possible representations of the surface of Triton. First, a two-region surface composed of a N2:CH4:CO terrain, N2:CH4:CO consisting of a solid solution in which N2 is the dominant molecule, and of a H2O+CO2 terrain, composed of a mixture of pure crystalline H2O and CO2 grains. The second representation is a three-region surface composed of a N2:CH4:CO terrain and two geographically separated H2O and CO2 terrains.
The second step of the analysis consisted of using a bidirectionnal reflectance model (S. Douté and B. Schmitt 1998, J. Geophys. Res. Planets103, 31367). The modeling first confirms the direct spectral analysis in that CH4 is diluted in solid β-N2, giving a high degree of confidence to the conclusion that the N2:CH4:CO terrain is in fact a solid solution. It also provides numerical information on this terrain, namely the size of the grains, the geographical abundance, and the CH4 and CO concentrations. The large grain size (around 10 cm) would mean that the texture of this terrain is a compact crystalline solid rather than granular, which is in agreement with calculations from J. Eluszkiewicz (1991, J. Geophys. Res.96, 19,217). In addition, an accurate modeling of the N2 band could suggest that the temperature is greater or equal to 35.6 K.
Although undistinguishable in the spectra, a maximum of 10% surface area of pure CH4 ice can be present at the surface of Triton, thus explaining the high atmospheric CH4 abundance observed by Voyager 2.
Finally, the modeling showed that none of the two- or three-region representations was able to fit simultaneously the K and H regions of the spectrum of Triton. The origin of this misfit is not yet elucidated, but an instrumental effect is suspected. Some questions about the physical state of the H2O and CO2 molecules are thus raised, but unfortunately observational constraints are missing. New near-infrared observations could partly provide these missing constraints, and would be important for detecting new molecules on Triton's surface. Such new data would be especially useful to identify the three bands at 5717, 5943, and 6480 cm−1 (1.749, 1.683, and 1.543 μm).
Context. Triton possesses a thin atmosphere, primarily composed of nitrogen, sustained by the sublimation of surface ices. Aims. We aim at determining the composition of Triton's atmosphere to ...constrain the nature of surface-atmosphere interactions. Methods. We perform high-resolution spectroscopic observations in the 2.32–2.37 μm range, using CRIRES at the VLT. Results. From this first spectroscopic detection of Triton's atmosphere in the infrared, we report (i) the first observation of gaseous methane since its discovery in the ultraviolet by Voyager in 1989; and (ii) the first ever detection of gaseous CO in the satellite. The CO atmospheric abundance is remarkably similar to its surface abundance, and appears to be controlled by a thin, CO-enriched, surface veneer resulting from seasonal transport and/or atmospheric escape. The CH4 partial pressure is several times higher than inferred by Voyager. This confirms that Triton's atmosphere is seasonally variable and is best interpreted by the warming of CH4-rich icy grains as Triton passed southern summer solstice in 2000. The presence of CO in Triton's atmosphere also affects its temperature, photochemistry, and ionospheric composition. An improved upper limit on CO in Pluto's atmosphere is also reported.
Context. Pluto possesses a thin atmosphere, primarily composed of nitrogen, in which the detection of methane has been reported. Aims. The goal is to constrain essential but so far unknown parameters ...of Pluto's atmosphere, such as the surface pressure, lower atmosphere thermal stucture, and methane mixing ratio. Methods. We use high-resolution spectroscopic observations of gaseous methane and a novel analysis of occultation lightcurves. Results. We show that (i) Pluto's surface pressure is currently in the 6.5–24 μbar range, (ii) the methane mixing ratio is 0.5±0.1%, adequate to explain Pluto's inverted thermal structure and ~100 K upper atmosphere temperature, and (iii) a troposphere is not required by our data, but if present, it has a depth of at most 17 km, i.e. less than one pressure scale height; in this case methane is supersaturated in most of it. The atmospheric and bulk surface abundances of methane are strikingly similar, a possible consequence of a CH4-rich top surface layer.
Context. The study of the surface properties of Centaurs and trans-Neptunian objects (TNOs) provides essential information about the early conditions and evolution of the outer Solar System. Due to ...the faintness of most of these distant and icy bodies, photometry currently constitutes the best technique to survey a statistically significant number of them. Aims. Our aim is to investigate color properties of a large sample of minor bodies of the outer Solar System, and set their taxonomic classification. Methods. We carried out visible and near-infrared photometry of Centaurs and TNOs, making use, respectively, of the FORS2 and ISAAC instruments at the Very Large Telescope (European Southern Observatory). Using G-mode analysis, we derived taxonomic classifications according to the Barucci et al. (CITEa, AJ, 130, 1291) system. Results. We report photometric observations of 31 objects, 10 of them have their colors reported for the first time ever. 28 Centaurs and TNOs have been assigned to a taxon. Conclusions. We combined the entire sample of 38 objects taxonomically classified in the framework of our programme (28 objects from this work; 10 objects from DeMeo et al. CITE, A&A, 493, 283) with previously classified TNOs and Centaurs, looking for correlations between taxonomy and dynamics. We compared our photometric results to literature data, finding hints of heterogeneity for the surfaces of 4 objects.
Deuterium on Mars has been detected by the resolution of several Doppler-shifted lines of HDO near 3.7 micrometers in the planet's spectrum. The ratio of deuterium to hydrogen is (9 $\pm $ 4) $\times ...$ 10$^{-4}$; the abundance of H$_{2}$O was derived from lines near 1.1 micrometers. This ratio is enriched on Mars over the telluric value by a factor of 6 $\pm $ 3. The enrichment implies that hydrogen escaped more rapidly from Mars in the past than it does now, consistent with a dense and warm ancient atmosphere on the planet.
The first high-resolution spectroscopic observations of the night side of Venus obtained in two narrow spectral windows centered at 1.74 and 2.3 microns, where the nightside is anomalously bright, ...are reported. Absorption features from CO2, CO, H2O, HDO, HCl, HF, and COS are detected, and there are a number of unidentified features. A preliminary analysis indicates that the observed radiation is thermal emission from atmospheric layers in the eight-bar pressure region for the 2.3 micron window and even deeper at 1.7 micron. The derived CO and H2O abundances agree with in situ Pioneer measurements in the deep troposphere and are consistent with the high deuterium enrichment inferred from Pioneer data. The first measurements of HCl and HF below the clouds are reported along with the first firm detection of COS.
► Revised methane line data give very good fit to Uranus H-band spectra. ► Reflectivity of particles in main cloud deck is found to decrease with wavelength from 1.4 to 1.6μm. ► Uranus’ CH3D/CH4 ...ratio is determined to be 2.9-0.5+0.9×10-4. ► New line data allow differentiation between methane humidity and cloud top height. ► Methane humidity is found to be high in the tropics, but to decrease rapidly polewards of 45°N and S.
New line data describing the absorption of CH4 and CH3D from 1.26 to 1.71μm (Campargue, A., Wang, L., Mondelain, D., Kassi, S., Bézard, B., Lellouch, E., Coustenis, A., de Bergh, C., Hirtzig, M., Drossart, P. 2012. Icarus 219, 110–128), building upon previous papers by Campargue et al. (Campargue, A., Wang, L., Kassi, S., Masat, M., Votava, O. 2010. J. Quant. Spectrosc. Radiat. Transfer 111, 1141–1151; Wang, L., Kassi, S., Campargue, A. 2010. J. Quant. Spectrosc. Radiat. Transfer 111, 1130–1140; Wang, L., Kassi, S., Liu, A.W., Hu, S.M., Campargue, A. 2011. J. Quant. Spectrosc. Radiat. Transfer 112, 937–951)) have been applied to the analysis of Gemini-N/NIFS observations of Uranus made in 2010 and compared with earlier disc-averaged observations made by KPNO/FTS in 1982. The new line data are found to improve greatly the fit to the observed spectra and present a huge advance over previous methane absorption tables by allowing us to determine the CH3D/CH4 ratio and also start to break the degeneracy between methane abundance and cloud top height. The best fits are obtained if the cloud particles in the main cloud deck at the 2–3bar level become less scattering with wavelength across the 1.4–1.6μm region and we have modelled this variation here by varying the extinction cross-section and single-scattering albedo of the particles.
Applying the new line data to the NIFS spectra of Uranus, we determine a new estimate of the CH3D/CH4 ratio of 2.9-0.5+0.9×10-4, which is consistent with the estimate of de Bergh et al. (de Bergh, C., Lutz, B.L., Owen, T., Brault, J., Chauville, J. 1986. Astrophys. J. 311, 501–510) of 3.6-2.8+3.6×10-4, made by fitting a disc-averaged KPNO/FTS spectrum measured in 1982, but much better constrained. The NIFS observations made in 2010 have been disc-averaged and compared with the 1982 KPNO/FTS spectrum and found to be in excellent agreement.
Using k-tables fitted to the new line data, the central meridian observations of Uranus’ H-band spectrum (1.49–1.64μm) made by Gemini-N/NIFS in 2010 have been reanalyzed. The use of the new methane absorption coefficients and the modified scattering properties of the cloud particles in the main cloud deck appears to break the degeneracy between cloud height and methane abundance immediately above it in this spectral region and we find that both vary with latitude across Uranus’ disc. Overall, we find that the main cloud deck becomes higher, but thinner from equator to poles, with a local maximum in cloud top height in the circumpolar zones at 45°N and 45°S. At the same time, using the ‘D’ temperature pressure profile of Lindal et al. (Lindal, G.F., Lyons, J.R., Sweetnam, D.N., Eshleman, V.R., Hinson, D.P. 1987. J. Geophys. Res. 92, 14987–15001) and a deep methane abundance of 1.6% (Baines, K.H., Mickelson, M.E., Larson, L.E., Ferguson, D.W. 1995. Icarus 144, 328–340) we find that the relative humidity of methane is high near the equator (∼60%) and decreases sharply towards the poles, except near the circumpolar zone at 45°N, which has brightened steadily since 2007, and where there is a local maximum in methane relative humidity. In tests conducted with the warmer ‘F1’ profile of Sromovsky et al. (2011) we find a similar variation of methane abundance above the main cloud, although for this warmer temperature profile this abundance is dependent mostly on the fitted deep methane mole fraction.
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