P21D-01 INVITED
Titan's atmospheric composition: from Voyager to Cassini and beyond
Titan's atmosphere was revealed by the Voyager missions in the 80s. The trace composition was in particular inferred from infrared spectra by the V1/IRIS Spectrometer. ISO gave us an opportunity to further explore this exciting milieu in 1997 (Coustenis et al., 1998; 2003) and brought the discovery of new molecules : H2O and C6H6. Our understanding of Titan's atmospheric chemical composition has recently been enhanced by the data returned by the Cassini instruments. Spectra recorded by the Composite Infrared Spectrometer (CIRS) aboard the Cassini spacecraft have been processed from the Titan flybys spanning three years now since SOI (Flasar et al., 2005; Teanby et al., 2006, Vinatier et al., 2006; Nixon et al., 2006; Coustenis et al., 2007). The spectra characterize various regions on Titan from 85°S to 80°N with a variety of emission angles. We have studied the emission observed in the CIRS detector arrays (covering the 10-1500 cm-1 spectral range with apodized resolutions of 2.54 or 0.53 cm-1). We have used temperature profiles retrieved from the inversion of the emission observed in the methane band at 1304 cm-1 and a line-by-line radiative transfer code to infer the abundances of the trace constituents and some of their isotopes in Titan's stratosphere (Coustenis et al., 2007a). The composite spectra show several signatures of previously identified molecules: hydrocarbons, nitriles, H2O and CO2. Besides these well-known trace species, a firm detection of benzene (C6H6) is provided by CIRS at 674 cm-1 and allows for the study of its latitudinal variations. No longitudinal variations were found for any of the gases. Information is retrieved on the meridional variations of the trace constituents and tied to predictions by dynamical-photochemical models (Hourdin et al., 2004; Lavvas et al., 2007). Molecules showing a significant enhancement at northern latitudes are the nitriles (HC3N, HCN) and the complex hydrocarbons (C4H2, C3H4). The D/H ratio on Titan was also determined from the CH3D band at 8.6 micron and found to be about 1.2 ± 0.2 × 10-4. We have also identified the presence of C2HD at 678 cm-1 for the first time (Coustenis et al., 2007b, in preparation). Constraints are also set on the vertical distribution of C2H2. However successful, the Cassini-Huygens mission has brought new enquiries that can only be answered by future missions to Titan. Such a mission, a collaboration between ESA and NASA in the spirit of Cassini, was recently proposed by the TANDEM Consortium in response to ESA's Cosmic Vision Call. References : Coustenis et al., 1989, Icarus 80, 54; Coustenis et al., 1998, A & A 336, L85-L89; Coustenis et al., 2003, Icarus 161, 383; Coustenis et al., 2007a, Icarus 1889, 35-62; Flasar et al., 2005, Science 308, 975 ; Hourdin et al., 2004, J. Geophys. Res. 109, E1205; Nixon et al., 2007, Icarus, in press; Lavvas et al., 2007, Plan. Space Sci., in press; Teanby et al., 2006, Icarus 181, 243; Vinatier et al., 2006, Icarus, 188, 120.
P21D-02
The Meridional and Vertical Structure of Titan's Atmosphere from Cassini Radio Occultations
The four radio occultations during the Cassini prime mission have provided soundings in northern winter at eight mid- and high-latitude locations: 74 S, 69 S, 53 S, 34 S, 33 S, 31 S, 53 N, and 73 N. In the lower troposphere, the meridional contrast in temperature is muted, and temperatures in the lowest two kilometers differ by only 1.5 K, except for the northernmost sounding, which is colder by another ~1-1.5 K. Near 30 S, the lapse rates of the retrieved temperature profiles near the surface are nearly adiabatic, but at higher latitudes the profiles are more stably stratified in both hemispheres. At high northern latitudes the profiles exhibit stable inversions above the surface. We discuss the effect of the methane distribution on the retrieved temperatures and interpret the observed thermal structure in terms of the atmosphere's radiative response and surface-atmospheric exchange.
P21D-03
Photochemical Origin of Nitrogen on Titan and Enceladus
Saturn's largest moon, Titan, and one of its smallest, Enceladus, contain nitrogen (N2) in their atmospheres. N2 comprises 95% of the volume of Titan's air, whereas it is about ~4% in the Enceladus' plume environment. At Titan, gravitational escape of N2 is relatively slow. However, the gas is subject to rapid escape from the smaller moon. This implies that any nitrogen present in Enceladus' environment must be constantly replenished. The Huygens GCMS data at Titan show that the abundance of primordial argon (36Ar) is several factors of ten below that expected if nitrogen accreted as N2. This implies that N2 was converted from nitrogen-bearing compounds —primarily ammonia (NH3), in the dense and relatively warm subnebula of Saturn. NH3 could then be dissociated back into N2 in Titan's past by (a) photochemistry (Atreya et al., 1978), (b) shock induced chemistry (Jones and Lewis, 1987; McKay, et al., 1988), and (c) thermal dissociation. Mechanism (b) does not seem plausible in view of water-ammonia chemistry—which prevents N2 formation—and untenable amounts of H2 resulting from the dissociation of NH3 and CH4. Photochemistry is capable of producing 5-8 bars of nitrogen—an amount needed originally to explain the current 1.5 bars after accounting for escape—in 17-27 Myr (Adams 2006, Wilson 2002, Atreya 1986, Atreya et al., 1978). In this talk we discuss details of this likely process. Photochemical production of N2 is not viable at Enceladus because of the long time constants of the process and the exospheric type atmospheric densities. On the other hand, mechanism (c), first invoked by Matson et al. (2007a) could work. It was suggested that dissociation of ammonia in the interior of Enceladus at temperatures in excess of 650K could produce the N2 detected in the moon's environment. In a companion paper (Matson et al., 2007b) we examine the feasibility of such a mechanism at Titan also to assess the contribution of thermal dissociation of NH3 to Titan's primordial nitrogen. References: Adams EY, thesis, U. Michigan, 2006. Atreya SK et al., Evolution of a Nitrogen Atmosphere on Titan, Science 201, 611-613, 1978. Atreya SK, Atmospheres and Ionospheres of the Outer Planets and their Satellites, Springer-Verlag, New York- Berlin,188-190,1986. Jones and Lewis, Icarus 72, 381-393, 1987. Matson DL et al., Icarus 187, 569-573, 2007a. Matson et al., Endogenic Origin of Titan's N2, Fall AGU Meeting 2007b. Wilson EH, thesis, U. Michigan, 2002. McKay et al., Nature 332, 520-522, 1988. http://www.umich.edu/~atreya
P21D-04
Endogenic Origin of Titan's N2
The composition of Titan's atmosphere measured by the Huygens probe has been interpreted as indicating that nitrogen was not a primordial ingredient; the abundance of non-radiogenic argon being so small relative to molecular nitrogen that very little of the either could have accreted in Titan. If Titan formed 2.5 to 5.0 My after the calcium-aluminum inclusions (CAIs) were created (as suggested for Iapetus, Castillo-Rogez et al., Icarus 190, 179-202, 2007) then differentiation occurred early and a stable core formed. During differentiation, heat from short-lived radioisotope decay and gravitational energy enabled rapid serpentinization of most of the silicate phase . Water and chemical reactants were trapped by the hydrated silicate that accumulates into the core. After a few hundred My, temperatures become high enough for ammonia decomposition to take place, producing molecular nitrogen and hydrogen. This process can be further aided by the catalytic action of metal and clay minerals. Released molecular hydrogen can engage in reactions involving organic material, C, CO and CO2 to produce primarily methane (Atreya et al., Planet. Space Sci. 54, 1177-1187, 2006). Some of the molecular nitrogen is conjectured to have reached the surface but the details of the process are not known. A high-pressure ice layer is expected to have formed a barrier against the upward transfer of material from the core to the ocean. This barrier would have become more efficient with time as it was thickening, The increasing temperatures in the core eventually induce silicate dehydration. We believe that during this stage volatiles and organics could have been released from the core, and the associated burst of hot upwelling material could have destabilized the high- pressure layer. The presence in the atmosphere of 40Ar (as a result of 40K decay) indicates that gases from the core reach the surface. The isotopic fractionation of nitrogen (14N/15N) in the atmosphere as measured by Huygens indicates that the loss of nitrogen to space may be happening over a long time, although an early, massive escape is a definite possibility. Since it is difficult for Ar to escape from the atmosphere to space, the under-abundance of 40Ar in the atmosphere compared to the amount expected to have accreted suggests that gases are still present in the interior of Titan. Thus, current venting from the interior might include N2, CH4, in addition to Ar. The near-terrestrial 12C/13C vs. non-terrestrial 14N/15N suggests a different evolutionary history for methane than nitrogen on Titan, however, and is being investigated. In a companion paper (Atreya et al., 2007, Fall AGU Meeting) we discuss the formation of Titan's nitrogen atmosphere by the solar UV photolysis of ammonia. This work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.
P21D-05
Titan Airglow Spectra from Cassini UVIS
We present the first UV airglow observations of Titan's atmosphere by the Ultraviolet Imaging Spectrograph (UVIS) on Cassini. The known UV emissions of Titan have been examined with higher spectral resolution (0.4 nm FWHM) by the Cassini UVIS than in the past by the Voyager Ultraviolet Spectrometer (UVS) (3.0 nm FWHM). The UVIS observations, confirming Voyager UVS results, have shown that molecular nitrogen is the major constituent of the upper atmosphere of Titan. Using one spectral channel in the EUV from 56--118 nm and one in the FUV from 112--191 nm, the UVIS observed the disk on 13 December, 2004. The combined EUV and FUV spectral region is a probe of the stratosphere-mesosphere-thermosphere-exosphere region from about 300--2000 km. The EUV spectrum consists of three band systems of N2 (b {1}Πu, b' {1}Σu+, c{4'} {1}Σ{u+}--X {1}Σ{g+}), while the FUV spectrum consists of one N2 (a {1}Πg--X {1}Σ{g+}). The UVIS observations reveal that the c{4'}(0)--X(0) vibrational band near 95.8 nm is suppressed, and that N \sc{i} multiplets near 95.32 and 96.45 nm are present instead. Magnetospheric particle excitation is weak on this orbit, since the nightside EUV spectrum shows no observable N2 emission features and only H Lyman-β. The absence of significant darkside emission demonstrates that nitrogen emissions are predominantly excited on this orbit by photoelectrons near 900 km in the thermosphere. Above 145 nm most of the observed signal is due to sunlight reflected by N2 in the Titan mesosphere-stratosphere and modified by aerosol and hydrocarbon absorption. Mixing ratios of C2H2, C4H2, C2H4 and tholins have been derived from the reflected sunlight emission between 145 and 190 nm, using a Rayleigh scattering model. Assuming that the energy deposition at these wavelengths occurs near 300 km, these mixing ratios are in good agreement with recent photochemical models and previous Voyager observations in the IR. We also present the first geometric albedo measurement of Titan from 150--190 nm.
P21D-06
Dissociation of Methane Clathrate Hydrates and Cryovolcanism on Titan: Experimental Constraints.
In absence of any replenishment mechanism, the present-day atmospheric methane on Titan should be removed in a few tens of millions of years owing to UV-driven active photochemistry. Remote sensing observations by Cassini (Sotin et al. 2005, Lopes et al. 2007) and numerical models (Tobie et al. 2006) suggest that methane may be released during cryovolcanic events, from the dissociation of a subsurface clathrate reservoir. However, processes leading to clathrate dissociation at depth remain poorly understood, mainly because methane clathrates are very stable structures within the icy crust of Titan. High pressure - low temperature experiments have been conducted in order to investigate the variability of methane clathrate stability in Titan's conditions, using a sapphire-anvil cell cooled within a nitrogen cryostat. Three main factors may influence the stability of clathrates at depth: 1) the presence of ammonia in the icy crust and in the putative liquid layer; 2) the low amount of methane within the ice - possibly lower than the stochiometric value of the filled-cages clathrates structure; and 3) the antagonism between N2 and CH4 in the clathrate cages. In this work, a synthesis of the last three years of experimental and theoretical investigations of the ammonia effect on methane clathrates" stability will be proposed. In addition, it will be shown that experimental results concerning the effect of nitrogen and of the methane concentration relative to water are in good agreement with thermodynamic models. A low methane concentration decreases the dissociation temperature of structure I clathrates by more than 20-30 degrees, while the methane - nitrogen competition yields a shift that could reach 15 degrees. A combination of these two factors and the ammonia inhibition of methane clathrates is taken into account for providing new constraints on cryovolcanic processes within Titan.
P21D-07 INVITED
Titan's Spin State from Cassini SAR Data: Evidence for an Internal Ocean
Nineteen areas on Titan's surface have been imaged with Cassini SAR on two separate flybys with intervals from 2 months to 2 years. We have used the apparent misregistration of features between separate flybys (which is 10-30 km) to construct a refined model of Titan's spin state, estimating six parameters: pole right ascension and declination, spin rate, and these quantities' first time derivatives. Because we have only observed Titan for 2-3 years, our dataset is unlikely to be sensitive to higher order derivatives. We have studied the uncertainty and degree of correlation of the model parameters, and have also searched the parameter space to eliminate the possibility of more than one solution. Our model spin state differs significantly from both the zero-inclination synchronous model and from any other plausible Cassini state. The previously estimated pole location and spin rate used by the IAU and the Cassini mission definitely cannot account for the observed misregistration. Because our imaging resolution is between 300 m and 1 km, we are very sensitive to the pole location and spin rate. Our estimated corrections to the pole and spin rate exceed their corresponding standard errors by factors of 40 and 4, respectively. We examined 150 different features in 19 different twice-observed regions. Applying our pole correction reduces the feature misregistration from tens of km to 3-4 km. Applying the spin rate and derivative corrections further reduces the misregistration to 1-2 km. We propose that our result reflects coupling between atmospheric angular momentum changes and an internal water ocean, for two reasons. First, astrodynamical theory predicts that if Titan is in a dynamically relaxed Cassini state there is a relationship between the moment of inertia factor C/MR2 and the obliquity of a few tenths of a degree. Our results (from two independent analyses of the overlaps) show an appreciable deviation from the expected range of states: either Titan suffered a recent dynamical excitation, or the theory does not hold because the surface is decoupled from the deep interior. We cannot identify an evident source of a recent excitation, so we favor the latter. Second, much as the Earth's length-of-day changes by ~1 ms over a year, seasonal changes in Titan's atmospheric angular momentum (Tokano and Neubauer, 2005) will manifest themselves in a change in surface rotation rate. The change in rate is ~10x higher, amounting to some hundreds of seconds, when the surface is decoupled from the interior by a water-ammonia ocean. Our preliminary rotation solutions indicate a present- day spin rate offset of several tenths of a degree per year that may be accelerating. The spin rate and its rate of change suggest that significant atmospheric changes are occurring and that Titan has an internal ocean. The research described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
P21D-08
The Determination Of Titan's Rotational State From Cassini SAR Images
SAR images acquired by the spacecraft Cassini in overlapping strips have been used to determine the vectorial angular velocity of Titan. The method entails the tracking of surface landmarks at different times (and mean anomalies). Cassini radar observations have provided so far 14 high resolution image pairs of the same portion of Titan surface, spanning a period from 2004 to 2007. Each image is referenced both in an inertial frame and in the IAU, Titan-centric, body-fixed reference frame. This referencing is quite precise, as the position of Cassini relative to Titan is known with an accuracy smaller than 100 m during each flyby. The IAU body-fixed frame assumes a spin axis different from the actual one. Therefore, in this putative frame a landmark appears at different geographic coordinates in the two observations. By correlating the two images of the same surface region, one gets a two-dimensional vector, which retains information about the true spin axis. This vector provides the magnitude and direction of the displacement to be applied to a reference point of each image in order to produce maximum correlation. The correlation results therefore in a new Titan-centric, inertial referencing of the images, R(t1) and R(t2). The spin axis s is then obtained by requiring that [R(t2) - R(t1)] ∙ s = 0 for each overlapping image pairs. Due to experimental errors (dominated by image correlation errors and inaccuracies in the spacecraft orbit relative to Titan) the left hand sides cannot be simultaneously zeroed and the spin axis must be determined by means of a least square procedure. The magnitude of the angular velocity is then derived from the angle between the vectors R(t1) and R(t2) and the known time difference between the two observations. Our analysis indicates that the Titan pole coordinates are consistent with the occupancy of the fourth Cassini state. The uncertainties are obtained assuming a realistic error of 250 m in the reconstruction of the inertially- referenced vectors. Titan's rotational state is therefore more complex than expected. If Titan were a rigid body in a Cassini state (with an icy crust anchored to the mantle), one could use theoretical arguments to derive the moment of inertia from the obliquity and the second degree gravity field. However, the new findings suggest that those theoretical arguments cannot be straightforwardly applied.