Titan's surface before Cassini
Introduction
Beginning in 2004 the Cassini–Huygens mission is expected to return data from Titan far superior than any that can be obtained from Earth-based observatories. It is therefore timely to review present understanding of Titan's surface, summarizing the many developments in the six years since our last overview (Lorenz and Lunine, 1997). For further background, we refer readers to that work, earlier reviews (Lunine, 1993) and two books on Titan that have been published within that time period (Coustenis and Taylor, 1999, Lorenz and Mitton, 2002). The goal of the paper is to selectively review the most recent literature and summarize pre-Cassini understanding of Titan's surface, including implications of surface models for the volatile budget, crater distribution, and co-evolution of the surface with the atmosphere. We also preview observations that are planned for Cassini–Huygens, and what might be contemplated for future missions to Titan.
The fundamental context for Titan remains as before: as a giant satellite in the outer solar system, it is presumably an ice-rock body with some component of volatile and/or organic material added. The short photochemical lifetime of methane in Titan's atmosphere suggests that there must be a source or reservoir of that compound either on or below Titan's surface: there must also be a sink for the ethane and other higher organics produced by the photolysis of the methane. Thus Titan's atmospheric history is connected with the surface and interior in a profound way.
While the bulk of observational and theoretical work in the last decade or so has continued to emphasize the atmosphere, a number of theoretical investigations have been made pertaining to the surface, which will be reviewed in this paper. Especially in the past five years, improved radar and adaptive optics observations of Titan from the ground have yielded important constraints on the surface. It is notable also that Voyager data continues to be re-analyzed to yield new information about Titan's surface and near-surface environment. The Cassini–Huygens mission, if successful, should refocus studies of Titan toward understanding how the surface and atmosphere interact with each other.
The improving availability and capability of large telescopes with adaptive optics systems able to compensate for the Earth's atmosphere has brought striking improvement in the last 5 years or so in the number of occasions and wavelengths at which spatially resolved measurements of Titan have been made. At the time of our last review, only the first Titan map, using the Hubble Space Telescope (HST), had been published (Smith et al., 1996). Since then, several other maps have been produced and several other groups have presented individual images.
Most recently, with Saturn now visible from the refurbished Arecibo (305 m) and Green Bank (100 m) radio telescopes, new groundbased radar data have become available which provides a tantalizing perspective on the possible constitution of the surface, but raises further questions about Titan and the Saturnian system generally.
Section snippets
HST imaging
The most widely cited Titan map (see Fig. 1), namely that by Smith et al. (1996), used the Wide-Field Planetary Camera 2 (WFPC-2) and a wide F850LP filter that primarily sampled the 940 nm “window” (wavelength region of low methane absorption) on Titan. Additional maps were also made at 673 nm (red—see later) and at 1040 nm, although the signal-to-noise in the latter was poor as this is the upper end of CCD sensitivity.
The NICMOS camera (with different detectors) on HST made better maps at this
Organics—tholin
In the past few years new measurements of the optical properties of Titan tholin have been made (Ramirez et al., 2002, Tran et al., 2003). These measurements are of a material broadly similar to that made and studied by Khare et al. (1984) which have been used in essentially all spectral studies of Titan. However, the new material may represent a closer simulation of Titan aerosols, because of the gas composition and energy sources used, and preservation against oxygen exposure. The tholins
Remarks on Titan's hydrological cycle
It has been speculated since the Voyager encounter that Titan may have a hydrological cycle using methane as a working fluid. There has been evidence in recent years for cloud activity on Titan, in the altitude range at which methane clouds would be expected (e.g. Roe et al., 2002) HST imaging (Lemmon et al., 2002) indicated a large cloud feature covering about 10% of Titan's disk in 1995. Griffith et al. (1998) deduced from disk-integrated spectroscopy that a 10% cloud lay at altitudes of 15–20
On Titan's volatile inventory
Determining the amount of methane that has cycled through the surface–atmosphere system of Titan is a major goal of the Cassini mission. The undersaturation of methane in Titan's lower atmosphere must be largely due to vapor pressure suppression by involatile solutes like ethane—otherwise significant evaporation would occur (Flasar, 1983), which would require a latent heat flux that radiative transfer models (McKay et al., 1989) suggest is not available. Photochemical models imply the formation
Constraints on topography
It should be noted that the measured radius of Titan is in fact that at the equator. The occultations took place at (ingress) and (egress) (Lindal et al., 1983). The former location, assuming Titan rotates synchronously, is on the eastern edge of the bright feature. The latter is in the middle of the dark region. The ingress and egress profiles were very well matched, and cut-off occurred (presumably due to occultation by Titan's surface) at Titanocentric radii of .
Impact cratering
Titan presents what may be a unique set of planetary conditions for impact cratering, in the sense that the local gravity and planetary curvature resembles Ganymede and Callisto, while the crustal composition and lithospheric structure may be significantly different, owing to the different thermal history among these satellites and the likely incorporation of ammonia only in the interior of Titan. The atmospheric interactions on Titan—in particular the effect of the atmosphere on the expansion
Cassini prognosis
Studying Titan's surface is a primary goal of the Cassini–Huygens mission, and a principal objective of the RADAR instrument on the orbiter and the Surface Science Package (SSP) and Descent Imager/Spectral Radiometer (DISR) on the probe. The Imaging Science Subsystem (ISS) and in particular the Visual and Infrared Mapping Spectrometer (VIMS) are expected to make major contributions towards understanding Titan's surface. We will not attempt a review of the capabilities of these instruments,
The James Webb Space Telescope (JWST) and large ground-based telescopes
Holes will remain in our knowledge when the Cassini–Huygens mission ends in 2008. First, geometric constraints associated with the fixed-pallet placement of instruments on the Cassini orbiter (a cost compromise) dictate that each of the 45 close flybys will be devoted to only a subset of the instrument techniques. In the end, radar and near-infrared studies will cover only 20% of the surface at their best spatial resolutions of hundreds of meters. Second, the VIMS near-infrared spectrometer has
Conclusions
We are poised at the beginning of a new adventure in planetary science—the revelation of a large and essentially unknown surface in contact with a dense and volatile-rich atmosphere. Despite the progress of recent years, our impression of the composition of Titan's surface remains largely unchanged—a water ice surface covered with, or mixed with, darker materials such as heavy hydrocarbons and possibly silicates (the latter hinted at in spectroscopic data from Coustenis et al. (1997)). However,
Acknowledgements
We acknowledge the support of the Cassini project. Roger Clark is thanked for discussion of VIMS capabilities. Athena Coustenis and Peter Smith improved the paper with their reviews.
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