P11D-01 INVITED
Titan's Surface at the end of the Cassini-Huygens Prime Mission
Titan exhibits ample surface and crustal processes including lakes and seas, fluvial erosive features, indirect for subsurface reservoirs of liquid, and rainfall. Together these constitute strong evidence for a multi- composition hydrological system, composed mostly of methane and ethane as well as trace amounts of other alkanes. Estimates of the volume of liquid methane required in streams and rainfall to produce erosional features suggest that these could be relatively recent, perhaps periodically renewed as the overall climate cycles between dry and wet epochs. The end state of the longer-term chemical processing of methane in the upper atmosphere is expressed on the surface in the form of deposits of solid organics (acetylene, and other hydrocarbons and nitriles) organized into dunes, and ethane (in the lakes). The long-term evolution of the methane cycle may have involved episodic resupply of methane to the surface or gradual depletion of a larger surface reservoir of methane, but in either case removal of large amounts of ethane from the surface remains a puzzle. The tectonic style of Titan is less well understood, in part because of the likely burial of features by the dunes and other organic deposits. Poorly organized mountains, hills, and chains of mountains do not show an obvious pattern, with the exception of the concentration of such features on the large "continent"-like feature Xanadu. It is not even clear what the ages of most features are on the surface, though the crater density has been interpreted in the published literature to be consistent with an overall surface age between 0.2-1 gigayear. This, in turn, requires interior models that maintain a thin crust until relatively recent times (~ 1 gigayear ago), after which the crust has thickened. The inference from Titan's rotation state of a subcrustal, presumably water, ocean is consistent with a number of published thermal evolution models of the satellite. Some evidence for mobilization of water in the form of cryovolcanism exists. Continued mapping of the surface in the Cassini Equinox and possible additional extended missions will help address some of these problems, but inevitably a Titan orbiter along with in-situ probes will be needed to pursue a deeper understanding of this intriguing world.
P11D-02
Titan's Shape from Cassini Radar Altimeter and SAR Monopulse Observations
We have estimated global elevations on Titan using a combination of nadir-looking altimetry and SAR monopulse measurements acquired by the Cassini spacecraft radar on multiple Titan encounters. These data correspond to a set of one-dimensional tracks spread over much of the moon's surface, but with fewer observations in the southern hemisphere than in the north. We fit the measured points with spheres, biaxial ellipsoids, and a set of spherical harmonic functions to produce global elevation estimates. The best-fit sphere has radius 2574.95 km, but the sphere center is displaced 270 m southward so that the north polar radius is about 500 m less than the south polar radius, referenced to Titan's barycenter. A biaxial ellipsoid fit yields a polar radius of 2574.20 km and an equatorial radius of 2575.08, so that the moon appears to be slightly oblate. In this case the ellipse seems to be displaced northward by about 140 m, so both poles are still low compared to the equator but in this solution the north may be a bit further from the barycenter than the south pole. Rotation of the solution from an ideal oblate ellipsoid is negligible (<4 deg). The spherical expansion approach also gives a north pole radius several hundred meters less than the equatorial radius, and suggests that the south pole radius is comparable to the northern value, but it has larger uncertainty. For example, a 6th order fit yields north, equatorial, and south pole radii of 2574.35, 2574.85, and 2574.27 km, respectively, although these values are dependent on the selection of a parameter for a smoothing constraint. The paucity of data from the far south lead to greater uncertainty in the south polar radius and at this time it is not clear whether the south pole is closer to Titan's barycenter than is the north pole. However, the solutions for north polar radius and mean equatorial radius are robust over our analysis cases and are more trustworthy. Our early error analysis implies uncertainties in all of these values of perhaps +/-200 m. Both of our data types are limited in accuracy by knowledge of the spacecraft attitude, which affects the incidence angle of the observations. These data are consistent with the observation that liquid lakes are seen predominantly near Titan's north pole, which all of our measurements show to be depressed as compared with the equatorial bulge. The presence of the lakes may be linked with a subsurface methane table level dependent on Titan's geoid, so that lakes might be expected to appear on the surface in the lowest lying regions. Lakes seem to be less extensive at the south pole, so it will be crucial to determine the south polar radius accurately if we are to be able to distinguish among competing hypotheses for their formation.
P11D-03
Proximity of Titan's spin pole to a Cassini state
The rotational state of Titan remains somewhat puzzling, despite recent
observational progress by the Cassini mission. It has long been known that
Titan's orbit pole precesses about Saturn's spin pole, with a period of
~700 years. Tidal dissipation is expected to damp Titan's spin
pole to a Cassini state, in which it would precess about the orbit pole
while remaining coplanar with the orbit pole and Saturn's spin pole, much
like the Moon does with the Earth. If Titan were known to exactly occupy
such a state, then measurements of its obliquity and degree two gravity
field would provide constraints on the polar moment of inertia.
Analysis of radar returns from the surface of Titan yields an obliquity of
0.3 degree [1]. Analysis of Doppler data from two flybys of Titan yield
gravity field coefficients J2 = (27.22 + 0.18) 10-6, and C2,2
= (11.16 + 0.04) 10-6 [2]. The polar moment of inertia value implied by
these parameter values, assuming occupation of a Cassini state, is C = 0.55
M R2 [3], which is greater than the homogeneous value. This argument provides
independent evidence that Titan has a subsurface fluid layer which
mechanically decouples the surface from the deeper interior, as has been
separately argued [4] on the basis of a small departure from synchronous
rotation.
The Titan spin pole apparently lies slightly out of the plane defined by the
orbit pole and Saturn's spin pole [1], which suggests that it is not in a
pure Cassini state. For bodies with non-uniform orbital precession rates,
like the Galilean satellites of Jupiter [5], the coplanarity constraint only
applies on a mode-by-mode basis. However, in the case of Titan, the other
satellites of Saturn make only small perturbations to the orbit plane.
The presumed cause of the observed departure from synchronous rotation is a
torque applied by zonal winds in the atmosphere of Titan [6]. This wind is
driven by solar heating and is thus modulated over Saturn's orbital period,
with a substantial semi-annual (14.8 year period) variation. Because of
Titan's non-zero obliquity, the periodic atmospheric torque will also
perturb the spin pole orientation, and could be the cause of the angular
departure from a conventional Cassini state. However, the semi-annual
forcing is quite far from the spin pole precession period (of 700 years) and
will thus yield a relatively small amplitude response.
An alternative suggestion is that the wobble period of the spin pole is in a 2:1
resonance with the orbit pole precession period [7]. If that resonance were
occupied, then the spin pole would be forced to precess about the Cassini
state configuration, with an amplitude governed by the rate of tidal
dissipation. However, that requires a polar moment of inertia value of 0.355
M R2, which though plausible for Titan as a whole, is quite far from
the value suggested by the observed obliquity.
References
[1] B.W. Stiles, et al. Astron. J., 135, 1669-1680, 2008; [2] L. Iess et al.
NASA/CP-2007-214158, 2007; [3] B.G. Bills, F.Nimmo, Icarus, 196, 293-297,
2008;[4] R.D. Lorenz et al. Science, 319, 1649-1651, 2008; [5] B.G. Bills,
Icarus, 175, 233-247, 2005; [6] T. Tokano, N. Neubauer, GRL, 32, L24203,
2005; [7] B. Noyelles, Celest. Mech. Dyn. Astr. 101, 13-30, 2008.
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P11D-04
Spectroscopic Evidence for Titan Surface Materials
We have been systematically searching for spectroscopic evidence to identify surface materials remotely using data from the Cassini Visual and IR Mapping Spectrometer (VIMS) [McCord et al., Pl. and Sp. Sci., 54, 2006; McCord et al., Icarus, 194, 2008]. The Titan atmosphere allows viewing of the surface only in a few narrow spectral windows in the near IR due to methane absorptions, and atmospheric particulate scattering affects even these windows. We so far have found one weak absorption near 4.92 μm for a few regions (e.g. Tui Regio and Hotei Regio) that also have anomalous brightness in the 5 microns and contrast across the 2.8-μm windows. This evidence seems consistent with at least two materials, fine- grained CO2 frost and cyanoacetylene, HC3N, and is not consistent with some other materials proposed for Titan's surface. Further, we are able to model and map the entire Titan surface viewed by VIMS using all methane windows and spectral unmixing analysis methods, with only a few spectral end-member components, which include water ice, CO2 frost, a model of aerosol scattering and an unknown material bright at 2 microns. A neutral dark material may also be present. We continue this effort with each new VIMS data set and summarize here on our latest findings.
P11D-05
Cryovolcanism on Titan
Remote sensing observations yield evidence for cryovolcanism on Titan, and evolutionary models support (but do not require) the presence of an ammonia-water subsurface ocean. The impetus for invoking ammonia as a constituent in an internal ocean and cryovolcanic magma comes from two factors. First, ammonia-water liquid has a lower freezing temperature than pure liquid water, enabling cryovolcanism under the low- temperature conditions prevalent in the outer Solar System. Second, pure water is negatively buoyant with respect to pure water ice, which discourages eruption from the subsurface ocean to the surface. In contrast, the addition of ammonia to the water decreases its density, hence lessening this problem of negative buoyancy. A marginally positive buoyant ammonia-water mixture might allow effusive eruptions from a subsurface ocean. If the subsurface ocean were positively buoyant, all the ammonia would have been erupted very early in Titan's history. Contrary to this scenario, Cassini-Huygens has so far observed neither a global abundance nor a complete dearth of cryovolcanic features. Further, an ancient cryovolcanic epoch cannot explain the relative youth of Titan's surface. Crucial to invoking ammonia-water resurfacing as the source of the apparently recent geological activity is not how to make ammonia-water volcanism work (because the near neutral buoyancy of the ammonia-water mixture encourages an explanation), but rather how to prevent eruption from occurring so easily that cryovolcanic activity is over early on. Although cryovolcanism by ammonia-water has been proposed as a resurfacing process on Titan, few models have specifically dealt with the problem of how to transport ammonia-water liquid onto the surface. We proposed a model of cryovolcanism that involve cracking at the base of the ice shell and formation of ammonia-water pockets in the ice. While the ammonia-water pockets cannot easily become neutral buoyant and promote effusive eruptions, large scale tectonics stress (due to tides, non-synchronous rotation, satellite volume changes, and/or topography) may all promote resurfacing at localized times and spaces. Thermal convection in the ice-I shell can play an important role in ensuring recent cryovolcanism activity on Titan. Ammonia-water pockets trapped in the ice shell provides a possible mechanism for explaining episodic cryovolcanism. Our model has several advantages over more simplistic ones. Because of the relative inefficiency of trapping liquid in the shell and transporting it to the surface, our mechanism makes volcanism a marginal process. In this way we can explain why Titan did not lose all its ammonia into cryovolcanic flows early in Solar System history as would happen were ammonia-water liquid to be positively buoyant, hence making cryovolcanism too "easy". At the same time, our mechanism allows cryovolcanism to be an important process on regional scales: ammonia should be present at the surface and hence detectable so long as it is not buried by subsequent sedimentation of organic aerosols. Finally, because we posit that the cryovolcanic liquid comes from localized pockets rather than directly from the ocean, our scenario also allows the ocean to remain dilute in ammonia, hence much denser than the overlying ice and mechanically stable over the history of the Solar System.
P11D-06
Titan: Callisto With Weather?
Instead of being endogenically active, Titan's interior may be cold and dead. Those landforms on Titan that are unambiguously identifiable can all be explained by exogenic processes (aeolian, fluvial, impact cratering, and mass wasting). At the scale of available imaging data, the surface is dominated by vast dune ergs and by fluvial erosion, transportation, and deposition. The sparse distribution of recognizable impact craters (themselves exogenic) is consistent with the presence of aeolian and fluvial activity sufficient to cover and or erode smaller craters, leaving only large ones. Previous suggestions of endogenically produced landforms have been, without exception, inconclusively identified. Features suggested to be cryovolcanic flows may be debris flows and other mass movements, facilitated by hydrocarbon-fluidized unconsolidated materials. Ganesa Macula has been suggested as a putative cryovolcanic dome, but it may simply be an impact structure that contains radar-dark dune or mass-wasted materials. Mountains, which are heavily modified by fluvial and mass wasting processes, could have formed as the scarps of large impact features and/or by slow contraction due to global cooling and freezing of an internal ammonia-water ocean, rather than by endogenically powered orogeny. A cold and inactive interior is consistent with an internal ammonia-water ocean, which has a peritectic temperature of 173K, easily obtained in Titan by radioactive decay alone in the absence of tidal heating. Titan's orbital eccentricity should have damped if its interior is warm and dissipative; instead, its high eccentricity can be ancient if the interior is assumed to be cold and non-dissipative. Indeed, it has been suggested that Titan may be non-hydrostatic, consistent with a thick ice shell and a cold and rigid interior. We suggest that the satellite most akin to Titan may be Callisto. Like Callisto, which may have formed relatively slowly in the outer circumjovian accretion disk, Titan might have accreted relatively cold. Without being in a forced resonance, Titan's interior may have never undergone significant tidal heating. Analogous to Callisto's tenuous CO2 atmosphere, believed to be generated by sublimation of interior ices, interior clathrated methane within Titan may slowly diffuse outward from the cold interior, rather than the atmosphere being replenished by cryovolcanism. The hypothesis that Titan is "Callisto with weather" -- with geological processes that are principally exogenic -- can be tested through geophysical and thermal modeling, and by modeling the evolution of landscapes that are shaped by exogenic processes alone.
P11D-07
Titan's global lake distribution and implied hydrocarbon hydrology from Cassini SAR imagery and topography
Synthetic Aperture Radar (SAR) images of Titan's polar regions reveal quasi-circular to complex features which are interpreted to be liquid hydrocarbon lakes. The global distribution and relative topography of observed lake features is used to study methane transport in Titan's hydrologic cycle. At the end of Cassini's primary mission, the SAR dataset covers ~34% of the surface (~27% at 350 m/pixel or better) and indicates multiple lake morphologies which are correlated across the observed polar region. Lake frequency in the north is an order of magnitude higher than in the south, suggesting a fundamental difference in methane transport and storage. Northern lakes vary from < 10 to more than 100,000 km2 and are limited to latitudes above 55N where 60% of the surface has been observed, as opposed to 19% below 55S. Filled lakes take up ~10% of the observed area in the north and ~1% in the south. Southern features are dominated by Ontario Lacus (18000 km2), observed by the Imaging Science Subsystem, which is ~20 times larger than the sum of all southern lakes observed by the radar to date. The location and character of hydrologic features can be used to constrain parameters associated with methane evaporation, runoff, and subsurface transport in Titan's hydrologic cycle. Lakes are expected to be a mixture of primarily methane, ethane, and nitrogen. Methane will evaporate in Titan's atmosphere while ethane and nitrogen are comparatively stable. All three liquids can interact with a porous regolith through subsurface flow. Sartopo, topographic information derived from overlapping beams patterns in the Cassini RADAR, is used to compare observed lake morphologies to expected subsurface transport directions. As predicted, the large seas found between 100E and 140E appear to be the lowest point in the region. The north polar region is the most topographically varying terrain observed on Titan to date. Relative elevations are correlated to observed lake classifications and show that lakes do not lie on a global equipotential surface. While most areas show the expected relationship of empty above filled lakes, some areas have shown empty lakes topographically below filled lakes in a state of hydrologic disequilibrium. Additional radar passes, obtained during Cassini's extended mission, will provide an opportunity to observe lake level change in these areas and place limits on the permeability of the local regolith.
P11D-08
Titan's global asymmetry in lake distribution and implications
Titan's global hydrocarbon cycle includes lakes and seas discovered and mapped by the Cassini Orbiter. Radar data show that poleward of about 60°N the lakes and seas are interpreted to be in a range of states, from filled to partially filled, to empty. In the south, observational radar coverage is significantly sparser, but the one pass obtained indicates a significant paucity in lakes relative to the north, which will be tested by further observations. Hemispheric differences where also apparent in telescopic observations. The asymmetry in lake distribution has been suggested to be due to a number of factors including 1) seasonal variations, 2) a topographic asymmetry, and 3) a seasonal asymmetry. With fewer empty lakes seen in the south and the basins' expected persistence for durations longer than the 29.7 year seasonal period , seasonal variations now seem an unlikely explanation for the asymmetry. A topographic, or subsurface lithologic asymmetry could be responsible for the precipitation, drainage, and infiltration rates, but large asymmetries have not been identified so far, using initial data of direct altimetry and SAR-Topo techniques from the Cassini Radar. Still, this remains a possibility. The observed asymmetry in lake distribution may be due to the asymmetry in Titan's seasons due the properties of Saturn's orbit. Saturn's obliquity is 26.7° (Titan's inclination and obliquity are 0.33° and 0.6°, respectively), its longitude of perihelion passage is Ls,p=279°.46 (near northern winter solstice) and orbital eccentricity is e=0.054. Hence southern summers are shorter and more intense than their northern counterparts and the asymmetry in insolation is about 12 percent (or twice that peak-to-peak). In this scenario, an enhancement in insolation driven evaporation, precipitation, or both would account for the unequal accumulation of hydrocarbon lakes. The candidate liquids, methane and ethane, have a volatility difference of a factor of nearly 104 at the relevant 92 K polar temperature, and a correspondingly shorter transport timescale. Methane, therefore, may be moved seasonally between the polar reservoirs while ethane's distribution would reflect a longer-term insolation history. However, depending on the rate with which the system may respond to changes, this asymmetry would disappear and reverse with Saturn's precession of perihelion passage, eccentricity variations, and position of spin axis. The period of Saturn's perihelion precession is roughly 60 kyr (currently -19.4889 arcsec/yr), and the other orbital parameter variations will further modulate the signal. If the volatile transport timescales are sufficiently fast and the lake distribution indeed records the forced seasonal asymmetry, then the lake regions on Titan, poleward of ~60° in both hemispheres, have a surface age younger than this orbital timescale. The upper limit on the age is very low, especially when considering the identification of several (~5) craters on the surface. These craters reside in the low latitudes and indicate a surface age of between 200 Myr and 1 Gyr.
P11D-09
A Three-Dimensional View of Titan's Surface Features from Cassini RADAR Stereogrammetry
As of the end of its four-year Prime Mission, Cassini has obtained 300-1500 m resolution synthetic aperture radar images of the surface of Titan during 19 flybys. The elongated image swaths overlap extensively, and ~2% of the surface has now been imaged two or more times. The majority of image pairs have different viewing directions, and thus contain stereo parallax that encodes information about Titan's surface relief over distances of ~1 km and greater. As we have previously reported, the first step toward extracting quantitative topographic information was the development of rigorous "sensor models" that allowed the stereo systems previously used at the USGS and JPL to map Venus with Magellan images to be used for Titan mapping. The second major step toward extensive topomapping of Titan has been the reprocessing of the RADAR images based on an improved model of the satellite's rotation. Whereas the original images (except for a few pairs obtained at similar orbital phase, some of which we have mapped previously) were offset by as much as 30 km, the new versions align much better. The remaining misalignments, typically <1 km, can be removed by a least-squares adjustment of the spacecraft trajectories before mapping, which also ensures that the stereo digital topographic models (DTMs) are made consistent with altimetry and SAR topography profiles. The useful stereo coverage now available includes a much larger portion of Titan's north polar lake country than we previously presented, a continuous traverse of high resolution data from the lakes to mid-southern latitudes, and widely distributed smaller areas. A remaining challenge is that many pairs of images are illuminated from opposite sides or from near-perpendicular directions, which can make image matching more difficult. We find that the high-contrast polarizing display of the stereo workstation at USGS provides a much clearer view of these unfavorably illuminated pairs than (for example) anaglyphs, and lets us supplement automatic image matching with interactive measurements where the former fails. We are collecting DTMs of all usable image pairs and will present the most interesting results. Examples of geologic questions that may be addressed are: What is the relation between Ganesa and surrounding features? Is it a dome or shield? Can the height of Titan's dunes be measured, and what is the relief of the bright "islands" that appear to divert the dunes? How high are the mountains of Xanadu and what gradients drive the channels between them? What are the relative and absolute height relations between seas and lakes of different types, and what does this tell us about the "hydro(carbono)logic" cycle of precipitation, evaporation, and surface and subsurface fluid flow?