M. Hartung

Titan's bright spots: Multiband spectroscopic measurement of surface diversity and hazes

[1]xa0Spatially resolved near-infrared spectra of Titan, which simultaneously cover two CH4 absorption bands (from 1.45 to 2.45 μm), have been obtained using the SINFONI integral-field spectrometer with adaptive optics at the Very Large Telescope (VLT) of the European Southern Observatory. We observe the enhancement in 2 μm surface albedo of the “5 μm bright spot” at 80°W and 24°S, and find an analogously bright region at 2 μm near 88°W and 6°S. Surface albedos are recovered at ∼60 mas (375 km) resolution in both 1.5 and 2.0 μm windows using a two-stream, plane-parallel, radiative transfer model to fit the observed spectra. The surface albedos near the center of the disk range from 8 to 15% at 1.5 μm and are generally ∼2% lower at 2.0 μm. Vertical (altitude) profiles of aerosol extinction that are consistent with the Huygens/DISR measurements are used to model these observations, and we retrieve latitudinal trends in both stratospheric and tropospheric aerosol extinction. On 28 February 2005 UT, the stratospheric aerosol extinction is measured to increase linearly at a rate of 0.65 ± 0.05% per degree latitude from 40°S to 60°N. Meanwhile, the tropospheric haze near the south pole is confined to southern latitudes above 40°S and is enhanced in extinction by a factor of ∼1.7 relative to the extinction measured at 10°S, the latitude where the Huygens probe landed.

Observations of a stationary mid-latitude cloud system on Titan

Abstract We report the observation of a cloud system on Titan that remained localized near 40°S latitude and 60°W longitude for at least 34xa0h. Ground-based observations obtained with the SINFONI imaging spectrograph at the Very Large Telescope over four consecutive nights recorded the lifetime and altitude of the unresolved cloud system. Concomitant measurements made by Cassini/VIMS over 3xa0h resolved changes in the altitude and opacity of individual regions within the system during this time. Clouds are measured from 13 to 37xa0km altitude with optical depths per pixel ranging from τ = 0.13 to 7. Short timescale rise times are consistent with previous measurements of the evolution of mid-latitude clouds; however the long timescale localization of the cloud structure is unexplained. We speculate about the role of mesoscale circulation in relation to cloud formation.

Evidence for condensed-phase methane enhancement over Xanadu on Titan

Abstract We present evidence for condensed-phase methane precipitation near Xanadu using nine nights of observations from the SINFONI integral-field spectrograph at the Very Large Telescope and imaging analysis with empirical surface subtraction. Radiative transfer models are used to support the imaging technique by simulating the spectrometer datacubes and testing for variations in both the surface reflectivity spectrum and atmospheric opacity. We use the models and observations together to argue against artifacts that may arise in the image analysis. High phase angle observations from Cassini/VIMS are used to test against surface scattering artifacts that may be confused with sources of atmospheric opacity. Although changes in the surface reflectivity spectrum can reproduce observations from a particular viewing geometry on a given night, multiple observations are best modeled by condensed-phase methane opacity near the surface. These observations and modeling indicate that the condensed-phase methane opacity observed with this technique occurs predominantly near Xanadu and is most likely due to precipitation.

Titan: Atmospheric and surface features as observed with Nasmyth Adaptive Optics System Near-Infrared Imager and Spectrograph at the time of the Huygens mission

[1]xa0At the time of the Huygens probe descent in Titans atmosphere, on 14 January 2005, many ground-based telescopes were pointed toward Saturns satellite. Here, we describe the data collected on 15 and 16 January with Nasmyth Adaptive Optics System Near-Infrared Imager and Spectrograph (NACO) at the Very Large Telescope. We acquired adaptive optics images in the near-infrared in several NACO modes, 18 then 41 hours after Huygens landing. A variety of different filters, a cryogenic tunable Fabry-Perot interferometer, and a simultaneous differential imager were used. All these data allowed us to construct a diagnostic of the appearance of Titans atmosphere and surface at the time of the Huygens probes descent. We describe the north-south and east-west asymmetries characterizing the atmosphere, while reporting on more active phenomena, like cloud activity. The surface of Titan around the Huygens probes landing site is imaged with a 320-km spatial resolution, through three methane windows at 1.28, 1.6, and 2.0 μm; it shows features similar to those previously published in near-infrared maps, with a spectral behavior compatible with methane and water ices.

Molecular Hydrogen Outflows in the Central Arcseconds of the T Tauri System

We present new, near-infrared, adaptive optics observations of the enigmatic pre-main-sequence object T Tauri, using broadband filter imagery, long-slit spectroscopy, and Fabry-Perot imaging spectra. The broadband filter images spatially resolve the three stellar components of T Tau in the H, Ks, and L photometric bands. We clearly detect T Tau Sb in the J band, and place upper limits on the J brightness of the infrared companion, T Tau Sa. The K-band spectrum of the T Tau S binary also resolves both components, and confirms strong Brγ emission in both stars and photospheric features in Sb. We also report 2.058 μm helium recombination radiation associated with T Tau Sb. The Fabry-Perot channel map centered on the v = 1-0 S(1) line of molecular hydrogen shows a number of spatially extended structures, including a loop of emission north of the stars that is also visible in continuum frames taken at ±900 km s-1 from the line center. The continuum-subtracted H2 image displays bow shock structures associated with the southeast-northwest and east-west outflows. The east-west jet system also shows bright, oblique shocks lining the flow channel. The part of the long-slit spectrum that overlaps this channel wall displays H2 line ratios consistent with shock heating. This region also corresponds to the brightest knot of UV fluorescent emission reported by Saucedo and coworkers. The Fabry-Perot images unambiguously identify T Tau S binary as the source of the east-west outflow, although we cannot yet determine which of the two components produces the jet.

Limits to the abundance of surface CO2 ice on Titan

[1]xa0In the context of the Cassini/Huygens mission, we performed supporting ground-based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO2 ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO2 possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 μm window of Titans atmosphere and (2) the width of these bands matches the band pass of the Fabry-Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO2 ice on the surface of Titan at sub-Earth longitudes 284°W and 307°W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO2 ice does not exceed 7% or 14% for bright or dark surface regions, respectively.

Correction to “Titan's bright spots: Multiband spectroscopic measurement of surface diversity and hazes”

[1] Spatially resolved near-infrared spectra of Titan, which simultaneously cover two CH4 absorption bands (from 1.45 to 2.45 mm), have been obtained using the SINFONI integral-field spectrometer with adaptive optics at the Very Large Telescope (VLT) of the European Southern Observatory. We observe the enhancement in 2 mm surface albedo of the ‘‘5 mm bright spot’’ at 80 W and 24 S, and find an analogously bright region at 2 mm near 88 W and 6 S. Surface albedos are recovered at 60 mas (375 km) resolution in both 1.5 and 2.0 mm windows using a two-stream, plane-parallel, radiative transfer model to fit the observed spectra. The surface albedos near the center of the disk range from 8 to 15% at 1.5 mm and are generally 2% lower at 2.0 mm. Vertical (altitude) profiles of aerosol extinction that are consistent with the Huygens/DISR measurements are used to model these observations, and we retrieve latitudinal trends in both stratospheric and tropospheric aerosol extinction. On 28 February 2005 UT, the stratospheric aerosol extinction is measured to increase linearly at a rate of 0.65 ± 0.05% per degree latitude from 40 S to 60 N. Meanwhile, the tropospheric haze near the south pole is confined to southern latitudes above 40 S and is enhanced in extinction by a factor of 1.7 relative to the extinction measured at 10 S, the latitude where the Huygens probe landed.

Solar System Objects with the NACO Fabry-Perot and SINFONI

3-D spectroscopy assisted by adaptive optics (AO), provides an extremely eficient way to study the surface or atmospheric composition of planetary bodies. AO supported near-infrared instruments such as NACO and SINFONI – which provide both the high-angular resolution required for spatially resolving the disks of small solar system bodies and the spectral resolution adequate for compositional studies – open new windows of investigation to planetary scientists. Since their installation in UT4 (Yepun) at ESO/VLT, these instruments have been used to map the surface and atmospheric composition of small angular-size bodies such as Vesta and Titan, to study the spatial compositional variation of Pluto’s moon Charon and to obtain spectra of faint bodies such as the trans-neptunian objects. Focusing on these targets, we demonstrate and compare the capabilities of the NACO Fabry-Perot (FP) imager (12) and the new integral-field spectrograph SINFONI (6; 10). The prospects for 3-D spectroscopy applied to the understanding of our solar system are discussed.