P44A-01 INVITED
Exploring the Surface of Titan with Cassini-Huygens
Over the past year, the Cassini-Huygens mission has returned a wealth of data about the surface of Saturn's satellite Titan. Cassini's Imaging Science Subsystem (ISS), RADAR, and Visual and Infrared Mapping Spectrometer (VIMS), and Huygens' Descent Imaging Spectral Radiometer (DISR) have revealed an intriguing surface that is at once familiar and alien. Although water-ice and liquid hydrocarbons play the roles that rock and water play on Earth, the surface appears to have been worked by a wide variety of processes resulting in a seemingly Earth-like balance of fluvial, aeolian, and volcanic features, with relatively few impact craters. There seem to be at least two classes of surface material: dark areas (at visible-IR wavelengths) that are spectrally consistent with contaminated water ice, and brighter areas of unknown composition which show greater variations. The expected bodies of liquids have yet to be definitively identified; however, circumstantial evidence for liquids having acted upon the surface in Titan's past is abundant, primarily in the form of channels, and possible ponds or lakes, which have been observed by multiple instruments. Other features suggest that wind redistributes some surface materials, most likely the detritus of the complex atmospheric chemistry, creating diffuse IR-bright deposits and long, narrow, radar-dark (2.2-cm) stripes, all of which trend generally east-west. Only two impact structures have been identified to date, although several other suspiciously circular features have been documented. A variety of the morphologies observed bear strong resemblances to volcanic structures. A number of other features remain mysterious and further co-analysis of these data sets, as well as the anticipated acquisition of more data, will be needed to fully understand the nature of Titan's surface, the albedo variations observed at different wavelengths, and the processes that have acted upon it (and may continue to). In addition to the Huygens descent, the Cassini orbiter has observed Titan on six close passes to date, and two more are planned for Fall 2005 which will increase both the spatial coverage, at various wavelengths, and the time base over which observations have been made as northern-hemisphere spring approaches. We will present observations of Titan's surface acquired by the complementary suite of instruments on board Cassini-Huygens, the combination of which is proving essential to interpreting Titan's geology.
P44A-02
Geologic Features on Titan's Surface as Revealed by the Cassini Titan Radar Mapper
The Cassini Titan Radar Mapper is one of the prime investigations to explore Titan's surface from orbit. Because of its almost opaque atmosphere, microwave remote sensing contributes uniquely to that investigation. The Titan Radar Mapper operates as a passive radiometer, scatterometer, altimeter, and synthetic aperture radar (SAR). We review the diversity of geologic features revealed using SAR during four fly-bys (Ta: October 2004, T3: February 2005, T7: September 2005, and T8: October 2005) and their context. Early SAR images from Ta and T3 reveal that Titan is very geologically complex (see Elachi et al., 2005, Science 13, 970-4). A variety of landforms and surface units were characterized morphologically and mapped, based on brightness variations, general planform shape and texture. Significant differences were seen in the geology between the Ta swath (centered at ~ 50N, 80W) and the T3 swath (centered at ~ 30N, 70W). The units in the Ta swath appear relatively young and no impact craters could be unambiguously identified. A variety of features which we argue to be cryovolcanic in origin were seen, including extensive flows, paterae, and a circular feature (Ganesa Macula) interpreted as a volcanic dome. We interpret radar-bright braided and sinuous channels and associated deposits to be fluvial in origin. Five distinct units were mapped in Ta, including a dark mottled unit that may represent the presence of surface liquids. The T3 swath displayed many of the same units seen in Ta, except for cryovolcanic features which are absent or indistinct. Among the new features in T3 are a large impact (440 km diameter) basin, a smaller (80 km diameter) crater, and dark lineated streaks, nicknamed "cat scratches" that are thought to be aeolian in origin. The dominant unit in T3 is a bright mottled unit that may contain ubiquitous small (less than 10 km across) topographic features. Groups of material that appear to be hills are more common in the T3 data than Ta. Based on the first two swaths (Ta and T3) we expect significant variations in the types and distribution of geologic features in the T7 and T8 data. The T8 swath will cover the landing site of the Huygens probe, providing a larger geologic context for the high-resolution near-infrared images obtained during the descent of the Huygens probe.
P44A-03
Titan's Surface as Seen From Combined Cassini RADAR Scatterometry and Radiometry
The Cassini Titan Radar Mapper has played a significant role in investigating the surface of Titan. Each of the RADAR's four operational modes individually contributes essential information about an area of coverage. When taken together, the different radar data types reveal a more complete story, providing complementary information on the physical and electrical properties of the surface. This paper studies overlapping data collected by the RADAR's Scatterometer and Radiometer over the seven close Titan flybys to-date (Ta-Inbound and Ta-Outbound, October 2004; T3, February 2005; T4, March 2005; T7, September 2005; T8-Inbound and T8-Outbound, October 2005) and attempts to reconcile the implications of the two. The scatterometer uses the central antenna beam at 13.78 GHz (2.17 cm-$\lambda$) in its active, real-aperture mode to produce regional-scale backscatter images across large areas of the surface. While the instrument awaits the same linear (SL) polarization echoes, the Radiometer is passively listening, measuring the brightness of thermal emission at the same polarization from the same footprint area on Titan's surface. For each observation, we separately model the backscatter and brightness temperature measurements, and form surface maps from the residuals after correcting for angular effects. We compare these relative surface maps side-by-side and find that most observed features are common to both and are anti-correlated (i.e. radar bright regions are radiometrically cold, radar dark regions are radiometrically warm). To model the observed backscatter, we fit the data to traditional scattering models: a Hagfors' or Gaussian specular term plus a cosine term that accounts for the diffuse, volume scattering within Titan's surface. The best fit model suggests values for the dielectric constant and rms surface slope of the observed area. In modeling the observed brightness temperatures, we assume Titan to be a perfectly emitting blackbody sphere covered with a uniform layer of unknown material and use Fresnel's equations to calculate the power transmitted through the layer and emitted at the surface. We then determine the dielectric constant whose model best-fits the data. The results from the backscatter and emissivity models appear to be in agreement, suggesting dielectric constants that vary between 1 and 2 over the areas observed. Over the course of the six scatterometer observations, there have been several instances of overlap; the Ta-Outbound and T3 passes intersected in a region bounded by -35° to +50° N and 0° to 10° W, and Ta-Inbound and T8-Inbound will intersect in a region bounded by -17° to 0° N and 100° to 225° W. By observing overlapping regions with varying angular and polarization coverage, we are able to further constrain inferences made about surface structure and composition.
P44A-04
Titan's surface from combined SAR and radiometry using the Cassini RADAR
The Cassini Titan Radar Mapper has proven to be valuable for the investigations of Titan's surface. Titan's Synthetic Aperture RADAR (SAR) and radiometry of four flybys (Ta: October 2004; T3: February 2005; T7: September 2005; T8: October 2005) are reviewed in this paper with emphasis on the correlation and comparison of surface features and physical characteristics. The radiometry data used are acquired during the SAR active mode using the 5 antenna beams at 13.78 GHz ( 2.17 cm). The observations are then modeled to derive brightness temperature measurements and form surface maps covering the same areal extent of the SAR swath to allow comparison of radiometry and SAR surface features. The response of surface features to SAR and radiometry in the Ta and T3 fly-bys has demonstrated the value of characterizing their physical properties and surface characterisctics. We examine the most recent data sets from T7 and T8 fly-bys in this context. In Ta and T3 the correlation between SAR-bright and radiometric cold regions, SAR-dark and radiometric warm regions has been observed extensively in association with diverse geological features, which suggests that the correlation is not caused by a specific geologic process but characterizes the constituent material and surface properties of the features. The correlation between SAR and radiometry is limited by the radiometry footprint resolution at closest approach, which imposes a limit to the detection and correlation of surface features smaller than 6 km azimuth resolution. Further there is a large-scale variability in calibration among the five radiometer beams caused by sidelobes that must be accounted for to obtain relative brightness variations. However, a general comparison of surface characteristics among swaths is possible.
P44A-05
The Cassini VIMS Titan Photometric Control Network: Relevance to Understanding Titan's Surface and Atmosphere
The Cassini Saturn orbiter has completed six close flybys of Titan since July 2004. The Visual and Infrared Mapping Spectrometer on the spacecraft is able to image Titan's surface at discrete wavelength bands in the spectral range 0.4-5.2 microns where methane, the principal absorbing gas in the atmosphere, is transparent. Using data from the first five Titan flybys we established a preliminary network of 48 photometric control points on Titan's surface which we have used to make preliminary inferences about the normal reflectance of Titan's surface units. These control points are divided evenly into about four overall reflectance classes which span the maximum and minimum of the observed surface reflectance (Nelson et al, 2005). We have increased the size of this network as previously unseen regions of the surface are observed at each successive flyby. Our objective is to observe each control point repeatedly at each flyby at different incident, emission and phase angle. This network will provide a basis on which the photometric function of the surface can be studied in conjunction with the scattering properties of Titan's atmospheric haze. We have also used this network to search for changes in the reflectance of the surface units that may not be related to angular scattering properties but to surface or atmospheric activity. We have already identified clearly at least one regional change in reflectance, spectrum and size which may be related to cloud activity or to surface changes that have occurred from one Titan flyby to another This work performed at JPL under contract with NASA Nelson et al, 2005, submitted to Planetary and Space Science
P44A-06
Titan Surface Composition at the Huygens Landing Site from DISR Spectra
The Descent Imager/Spectral Radiometer (DISR) aboard the Huygens probe recorded complete visible and near infrared spectra (480-1600 nm) of the surface of Titan near the landing site during the last tens of meters of the descent and from a few tens of centimetres after landing. The surface spectrum has a relatively low albedo, peaking around 0.18 at 830 nm, a red slope in the visible range, a quasi-linear decrease of the reflectivity from 830 to 1420 nm, and displays a broad absorption near 1540 nm. The featureless blue slope of this spectrum above 830 nm is very unusual and has no known equivalent on any other object in the Solar System. In addition no published laboratory data of ices or organic materials displays such a spectral behaviour. Laboratory studies and numerical simulations of the DISR spectrum have been performed to try to understand the nature and composition of the materials constituting the surface of Titan. In particular reflectance spectra over the solar spectrum (300-4000 nm) of several types of tholins and organics have been recorded with the LPG spectro-gonio radiometer. Numerical simulations of the surface reflectance spectrum have been performed in order to constrain the optical properties of the different components (ices, organics, etc) and to infer their composition. We will present the current state of analysis of this spectrum.
P44A-07
Resurfacing of Titan by Ammonia-Water Cryomagma
The Cassini Titan Radar Mapper observed several large features interpreted as cryovolcanic during the October 26, 2004, pass at high northern latitudes. These included a dome (~180 km in diameter) similar to Venusian "pancake" domes, two caldera-like features with associated flows tens of km long, and a large flow field ~24,000 km$^2$ in area. No impact craters were seen on this region of Titan, implying a young surface. Circumstantial evidence from mass spectrometric measurements indicates the past and possibly present-day existence of ammonia within Titan. We show that ammonia-water mixtures can erupt from a subsurface ocean on Titan through the ice shell, leading to cryovolcanism. Cryovolcanic processes are related to bottom crevasse formation in an ice shell floating on an ammonia-water ocean, transport of ammonia-water pockets to the base of the stagnant lid by convective motions in the ice, refreezing of chambers of ammonia-water, and diurnal tidal pumping of ammonia-water liquid to the surface. We determine the tidal dissipation rates of mechanical energy in the bottom crevasses required to damp the orbital eccentricity of Titan and we estimate the maximum number of fractures in the ice shell. Rather than suggesting steady-state volcanism over the history of the solar system, the cryovolcanic features could have been associated with a late (~4.2 Gyr) onset of convection in a cooling shell, and hence a recent episode of methane outgassing.
P44A-08
Wind-induced Seasonal Angular Momentum Exchange at Titan's Surface and its Influence on Titan's Length-of-day
Titan's substantial obliquity and the global extent of the Hadley circulation give rise to a seasonal variation in the mean zonal wind speed and direction in the geostrophic lower troposphere. Consequently, a substantial amount of angular momentum is exchanged between the surface and atmosphere. Titan's rotational rate is more sensitive to atmospherically induced seasonal variation than the Earth's rotational rate because of Titan's slow rotational rate and dense atmosphere. The seasonal wind-induced length-of-day (LOD) variation of Titan calculated using the global wind profile predicted by a Titan general circulation model (GCM) amounts to 30 s in the absence of a deep subsurface ocean or 390 s in the presence of a deep ocean. This effect should give rise to longitudinal offsets of surface landmarks of the order of 100 km compared to predicted positions based on a constant rotation rate, and may be detectable by Cassini imaging.