► 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.
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.
Surface Ices and the Atmospheric Composition of Pluto Owen, Tobias C.; Roush, Ted L.; Cruikshank, Dale P. ...
Science (American Association for the Advancement of Science),
08/1993, Letnik:
261, Številka:
5122
Journal Article
Recenzirano
Observations of the 1.4- to 2.4-micrometer spectrum of Pluto reveal absorptions of carbon monoxide and nitrogen ices and confirm the presence of solid methane. Frozen nitrogen is more abundant than ...the other two ices by a factor of about 50; gaseous nitrogen must therefore be the major atmospheric constituent. The absence of carbon dioxide absorptions is one of several differences between the spectra of Pluto and Triton in this region. Both worlds carry information about the composition of the solar nebula and the processes by which icy planetesimals formed.
Aims. We investigate color properties and define or check taxonomic classifications of objects observed in our survey. Methods. All observations were performed between October 2006 and September 2007 ...at the European Southern Observatory 8 m Very Large Telescope, UT1 and UT2 at the Paranal Observatory in Chile. For visible photometry, we used the FORS1 instrument, and for near-infrared, ISAAC. Taxonomic classifications from the Barucci system were assigned using G-mode analysis. Results. We present photometric observations of 23 TNOs and Centaurs, nine of which have never been previously observed. Eighteen of these objects were assigned taxonomic classifications: six BB, four BR, two RR, and six that are given two or more categories due to insufficient data. Three objects that had been previously observed and classified, changed classes most likely due to surface variation: 26375 (1999 DE9), 28978 (Ixion), and 32532 (Thereus). Two objects, 47932 (2000 GN171) and 54598 (Bienor) had absolute magnitude values that were significantly different from previously published results, attributed to extreme lightcurve amplitudes.
Aims. The aim of this paper is to investigate the surface composition of the Transneptunian Object (TNO) Orcus. Methods. High quality observations have been carried out with the new instrument ...SINFONI at the Very Large Telescope (VLT) of ESO. Results. Crystalline water ice, and possibly ammonia ice, have been found from spectroscopic observations of the TNO Orcus between 1.4 and 2.4 mum. Conclusions. The existence of such ices on the surface of Orcus may indicate a renewal mechanism on the surface and geological activity. The presence of ammonia on the surface of Orcus, if confirmed, could have important implications for the composition of the primitive solar nebula and the formation of the TNO population.
Context.Spectroscopic observations of numerous trans-Neptunian objects (TNOs), considered to be among the most pristine objects of the solar system, have revealed the presence of several kinds of ...surface ices. The high-sensitivity spectra that can be measured for the brightest objects also provide constraints on the physical properties of the surface (e.g. ice phase, temperature). Aims.We observed one of the largest and brightest TNOs, 136108 (2003 EL61), to determine its surface composition properties and to constrain its surface properties. Methods.We obtained new visible spectra with EMMI on the ESO-NTT and near-infrared spectra with the new 3D spectrograph SINFONI at the ESO-VLT. Our analysis consists of radiative transfer modelling to constrain composition and surface properties and to identify the precise minimum of the 1.65 micron band to constrain the surface temperature. Results.The observations reveal a surface essentially composed of water ice. An absorption feature at 1.65 micron clearly indicates the presence of ice in the crystalline phase. Spectral modelling suggests that a small fraction of the surface ice is in the amorphous state. We also derive the temperature of the crystalline ice at the surface.
Ices on the Surface of Triton Cruikshank, Dale P.; Roush, Ted L.; Owen, Tobias C. ...
Science (American Association for the Advancement of Science),
08/1993, Letnik:
261, Številka:
5122
Journal Article
Recenzirano
The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially ...segregated from the other more volatile ices, covering about 10 percent of Triton's surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.
Aims.The surface composition of the largest TNO, the dwarf planet 136199 Eris, is studied and compared to Pluto's. Methods.High signal-to-noise visible and near-infrared reflectance spectra were ...obtained at the TNG and ESO-VLT observatories. The nature and properties of the compounds present on the surface of Eris are investigated by applying Hapke and Shkuratov radiative transfer models to our spectra. Results.The surface of Eris can be modeled using two areas of distinct composition: about 50% appears to be covered with pure methane ice, while the rest of its surface would be made of an intimate mixture of methane, nitrogen and water ices, and ice tholin. The use of nitrogen in our model is shown to improve significantly the data fit, in particular for high surface albedo values. The icy grains are found to be large, from sub-mm to a few tens of mm in size.
Aims. We present new visible and near-infrared photometric measurements and near-infrared spectroscopic measurements for three outer solar system small bodies, the Jupiter-coupled object (52872) ...Okyrhoe and the TNOs (90482) Orcus and (73480) 2002 PN34. Methods. We analyzed their surface compositions by modeling their spectra in the visible and near-infrared wavelength ranges. We then compared this new data with previous measurements of Okyrhoe and Orcus to search for heterogeneity on their surfaces. All observations were performed at the European Southern Observatory 8 m Very Large Telescope, UT1 and UT4 at the Paranal Observatory in Chile. Results. We find varying amounts of H2O ice among these bodies, Okyrhoe shows no trace of it in our spectrum, 73480 has small amounts, and Orcus has large quantities. While we do clearly see for Orcus that a significant fraction of the H2O ice is in crystalline form from the 1.65-μm feature, we cannot detect the 2.21-μm feature supposedly due to ammonia hydrate, because of the low signal-to-noise of the data. We also do not see any indication of ices more volatile than H2O, such as CH4 or CO2, in the spectrum, so we limit their presence to no more than about 5% based on the data presented here and on high-quality data from Barucci et al. (2008, A&A, 479, L13).
Aims. A second large observational program was started to improve our knowledge of the physical properties of Centaurs and trans-Neptunian objects, TNOs. The program is being executed in the ...facilities at the Cerro Paranal of the European Southern Observatory, ESO, in Chile. Methods. Visible spectroscopy was carried out using FORS1 at the UT2 of the VLT telescope. We computed the spectral slope, S, for each object, and we searched for the possible existence of weak absorption features. We compared our data with those from the literature. Results. We present spectra for 21 objects, 7 of them with no previously reported spectra. Our sample includes 5 Centaurs, 5 resonant objects, 8 classical TNOs, 2 detached objects, and 1 scattered disk object. There are significant differences between the distribution of spectral slopes for Centaurs and classical TNOs. The data presented here generally agree with previous published data. One exception is 60 558 Echeclus, whose spectral slope is considerably smaller than previous measurements. Another interesting object is 47 932 (2000 GN sub(171)), which does not show evidence of a proposed feature at 0.7 mum. Most of the spectra are linear and featureless; however, the well-known bands of solid methane are detected in 136 199 Eris spectra and some weak features are detected for a few other objects. The most interesting case is that of 2003 AZ sub(84), which has a feature at about 0.65 mum, maybe due to aqueous alteration.
PDF
