Magical Xanadu

Surface cloud images show features resembling clouds and a continental area about the size of Australia Image Credit: NASA/JPL

The Huygens spacecraft will descend to a region of Titan called Xanadu–a mythical, magical land originally described in Samuel Taylor Coleridge’s poem, "Kubla Khan".

This Titan image (right and banner), reveal the bright ‘continent-sized’ terrain known as Xanadu.

The image was acquired with the narrow angle camera through a spectral filter centered at 938 nm, a wavelength region at which Titan’s surface can be most easily detected. One sees the surface at higher contrast than in previously-released Titan images due to a lower phase angle (sun-Titan-Cassini angle), a viewing geometry which minimizes scattering by the haze. The image shows details about 10 times smaller than can be seen from the Earth.

Surface materials with different brightnesses (or albedos) rather than topographic shading are being seen. The image has been calibrated and slightly contrast-enhanced. Further processing to reduce atmospheric blurring and optimize the mapping of surface features will be released soon. The origin and geography of Xanada (erosion of highlands, volcanism, impact cratering, etc.) remain mysteries at this range. Bright features near the south pole (bottom) are clouds. On Tuesday Cassini acquired images from about 100 times closer range and higher resolution, showing features in the left-central portion of this image.

False color image from the VIMS infrared instrument tuned to the narrow atmospheric window that enables peering through thick cloud cover to the surface. Image Credit: JPL/Space Science Institute

This detailed view contrasts with the high altitude haze and perhaps cloud layers that are visible in the left image acquired on October 24 as the Cassini spacecraft neared its first close encounter with Titan. The image was acquired at a distance of about one million kilometers in a near ultraviolet filter that is sensitive to scattering by small particles.

The Sun preferentially illuminates the southern hemisphere at this time, and the northern day-night terminator is visible at the upper boundary. The well-known global detached haze layer, hundreds of kilometers above Titan’s surface, is visible as a thin bright ring around the entire planet produced by photochemical reactions. At the northern high-latitude edge of the image, additional striations are visible, caused by particulates that are at high enough altitude to be illuminated by the Sun near the horizon even though the north polar region surface below is in darkness. These striations may simply be caused by wave perturbations propagating through the detached haze, or they may be evidence of additional regional haze or cloud layers not present at other latitudes. It’s currently winter in Titan’s northern hemisphere, so high northern latitudes are not illuminated, resulting in the map’s upper limit at roughly 45 degrees North latitude.

Based on previous observations, it is anticipated that during the flyby today the size of the smallest visible surface features will be approximately five times larger than the image scale. Thus as shown in the banner map, the smallest visible features within the region bounded by the red curve should be ~ 1 to 1.2 km across. The yellow X marks the predicted landing site for the Huygens probe, the target of the camera’s highest-resolution mosaic. Images of this site that will be taken near closest approach may have higher resolution than indicated here. Features a few hundred meters across may be discernible, depending on the effect the relative motion between the Titan surface and the spacecraft has on the quality of the images.

As the English romantic poet Coleridge described Xanadu in the 1700’s:

In Xanadu did Kubla Khan
A stately pleasure-dome decree:
A savage place! as holy and enchanted
As e’er beneath a waning moon was haunted

Related Web Pages

Saturn Edition, Astrobiology Magaz.
Saturn’s Rings in UV
Cassini Closes In on Saturn

Saturn– JPL Cassini Main Page
Lord of the Rings
Space Science Institute, Imaging Team Boulder, Colorado
Saturn: The Closest Pass
Prebiotic Laboratory
Planet Wannabe
Where is Cassini Now?