Mountains on Titan observed by Cassini Radar
Introduction
The Cassini spacecraft, with its ensemble of instruments designed to peer through the hydrocarbon haze enshrouding Saturn's largest moon, Titan, has revealed a diverse array of geological features on Titan's surface. The Cassini Titan Radar Mapper (hereafter, Cassini Radar) has observed dunes (Lorenz et al., 2006a, Lorenz et al., 2006b; Elachi et al., 2006a, Radebaugh et al., 2007), lakes (Stofan et al., 2007), cryovolcanic features (Elachi et al., 2005, Elachi et al., 2006a; Lopes et al., 2007), river-like channels (Lorenz et al., 2007a, Lunine et al., 2007), and degraded impact craters (Elachi et al., 2006a, Stofan et al., 2006, Wood et al., 2005, Lorenz et al., 2007b). The Cassini Radar has also discovered features of relatively high topography that we refer to as mountains (Radebaugh et al., 2006), and whose topography we analyze here.
The dominant topographic forms on the only two moons of comparable size and mass to Titan, Ganymede and Callisto, are impact craters and Ganymede's grooved terrain. Hence, just prior to Cassini it was assumed that most of the topography would be dominated by impact structures, so that mountains and mountain ranges would be central peaks and rims of craters, with low-lying impact crater basins perhaps filled with liquid methane that had periodically rained out of the atmosphere (Lorenz and Lunine, 2005, Lorenz and Mitton, 2002; etc). Cassini has revealed that impact structures are sparse (Porco et al., 2005; Elachi et al., 2005, Elachi et al., 2006a; Stofan et al., 2006, Lorenz et al., 2007b). Instead, it appears that Earth-like processes, perhaps with a complex “methanological” cycle that includes rainout, fluvial erosion (Tomasko et al., 2005, Lorenz et al., 2007a) and reevaporation, have shaped Titan's surface into a much more Earth-like landscape than was expected. Overall, Titan's topography is subdued, with amplitudes of a few hundred meters in the small fraction of the surface covered by altimetry (Elachi et al., 2005, Elachi et al., 2006a; Callahan et al., 2006), and stereo image pairs from Huygens (Tomasko et al., 2005). Interestingly, topography of this amplitude was predicted for Titan's surface based on atmospheric gravity wave analysis (Friedson, 1994). We desire to know what forces shaped Titan's presumed-icy crust and led to the surface topography we see. In addition to the geological and geophysical interest, we also note that future exploration of Titan may involve aerial platforms such as balloons (Tokano and Lorenz, 2006) and thus the characterization of mountain area, distribution and height is important in defining the topographic hazards to such vehicles.
We use data from Cassini Radar operating at 2.17 cm wavelength in synthetic aperture radar (SAR) mode (Elachi et al., 2005) to study features that reveal topography in the form of closely paired bright and dark image areas, indicative of radar illumination across a sharp topographical boundary. Images have range and azimuth resolutions of 300 to 1500 m, although the image products we analyze are oversampled to obtain 175 m pixel−1 (256 pixels per degree) ground sample distance. Current radar swath coverage of Titan is shown in Fig. 1.
Available techniques for estimating topographic relief include altimetry, stereogrammetry, and shape-from-shading. As discussed, radar altimeter data cover only a limited area of Titan, to date in areas for which high-resolution imaging is not available, hindering interpretation of the results. The resolution of optical images is inadequate to assess the low relief found so far on Titan by stereo methods, but the Cassini Radar will return overlapping images of a few areas of Titan with excellent stereo geometry. At the time this is written, however, such observations are just beginning. Fortunately, the radar images already obtained are well suited to detecting topographic relief from its shading effects and making quantitative estimates by shape-from-shading (or radarclinometry). The Radar is far better suited than optical remote sensing instruments on the Orbiter for this purpose, because absorption and scattering of light in Titan's thick, hazy atmosphere leads to diffuse surface illumination which makes contrasts in the optical images too weak for photoclinometry. Indeed, no appreciable shadows are cast, and evidence for topographic shading in the optical images is ambiguous. (By operating below the bulk of the atmospheric haze, the DISR instrument on the Huygens probe was able to provide high resolution topography via stereo pairs created as it drifted past features, but over a very limited patch of terrain only; Tomasko et al., 2005.) Thus, we rely on the imposed shading created by the radar illumination process to reveal topography, and quantify relief with models, based on theory and instrument design, that take account of the relationship between radar brightness and slope.
In this paper, we use data obtained during Cassini's T3 flyby in February 2005, the T8 flyby in October 2005, and isolated data from the T13 flyby in April 2006 to find the heights, morphologies, and locations of mountains within these regions on Titan. Few discrete mountain features were seen in the TA data (October 2004) and in the T7 swath (September 2005). Extensive ranges of mountains were seen in the T13 swath (May 2006) within the Xanadu region, where we have measured just a few features thus far, and some isolated mountains are seen in other regions on Titan in later swaths. Since new radar swaths are continually being obtained, and new sets of mountains are being observed, this paper should be considered a discovery paper, containing data for those select regions mentioned above along with preliminary discussions of possible formation mechanisms.
Section snippets
Features and study areas
The features under study in the Radar swaths T3, T8, and T13 are small regions (usually under 20 km across) that are elevated above the surrounding terrain. In passing, we note the application of the term ‘mountains’ to certain, low-amplitude features found on the Earth's US desert southwest (see Section 4.2) and follow that usage here. Although the Titan topography may be modest compared with the largest mountains on Io, Mars, or Earth, when compared to the very subdued topography we observe
Heights and slopes
As an initial survey of mountainous features seen in the radar swaths from T3 and T8, we measured heights and slopes of the most prominent features of high topography in the study areas using the technique of radarclinometry, or shape-from-shading.
Mountain formation scenarios
The mountains on Titan are presumably made up of water ice, which is expected from cosmochemical abundance considerations and water's ubiquity in the Saturn system (Cruikshank et al., 2005). Titan's Voyager-measured density indicates that water ice makes up about 60% of Titan's mass (Sohl et al., 1995). This exists in a shell of water, with liquid overlain by a solid ice crust, that in turn overlies a silicate core (Lunine and Stevenson, 1987, Sohl et al., 1995). Groundbased spectroscopy (e.g.,
Volume considerations and erosion rates to obtain mountain ages
We can use simple volume considerations to estimate the total volume of a mountain block and from this obtain an approximate age of the feature. Mountains in our study areas are typically surrounded by radar-bright, diffuse blankets of material, often to several tens of kilometers from the mountain base. These are materials that have perhaps been eroded off the mountains and then deposited in a gently sloping mantle that becomes thinner with greater radial distance. The blankets do not appear
Distribution on Titan's surface and implications for global processes
Mountains of various morphologies and associations have been observed on the of Titan's surface imaged so far at better than 1 km pixel−1 resolution. Regions containing mountains within the T3 and T8 swaths amount to about 0.5% of Titan's surface, and if we include the mountainous regions in swaths up to T20 (October 2006), mountains cover over 1% of Titan's surface (these numbers refer to regions delineated in Fig. 4, and thus include both mountains and flat plains between blocks and
Acknowledgements
We thank Richard Ghail and an anonymous reviewer for their careful reading of and useful commenting on the manuscript. The authors are supported at least in part by the Cassini mission. We gratefully acknowledge those who designed, developed and operate the Cassini mission, which is a joint endeavor of the National Aeronautics and Space Administration (NASA), the European Space Agency (ESA), and the Italian Space Agency (ASI) and is managed by JPL/Caltech under a contract with NASA.
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