Tuesday, October 5, 2010

High Resolution Imaging of Titan

Cassini VIMS maps of Titan from http://wwwvims.lpl.arizona.edu/

Mapping the surface of Titan is hard.  The smoggy atmosphere scatters most wavelengths of light.  The depth of the atmosphere prevents spacecraft from getting close enough for high resolution images.  Radar offers an alternative to optical imaging, but the equipment necessary is heavy, power hungry, and produces large amounts of data that must be transmitted back to Earth.  Resolution can also be a problem.  The Cassini radar system, which uses the spacecraft's large main antenna, has a maximum resolution of 350 m at closest approach.  The Huygen's landing area with its networks of hills and channels is a bright blur at these resolutions.

A recent presentation at the EPSC2010 conference, offers an alternative.  The Cassini VIMS instrument discovered a clear window in Titan's atmosphere at 5 microns, which is in the infrared region of the spectrum.  The authors propose a camera that would utilize that window to image Titan.  From a spacecraft flying by Titan, 100 images covering 1.25% of the surface could be taken.  Best resolution would be less than 50 m as the spacecraft swoops below ~2300 km above the surface during closest approach. That would provide sufficient detail to study the surface geomorphology in detail.  The channels at the Huygen's landing site, for example, would be seen.  (Cassini's infrared instrument, VIMS, was not optimized for spatial mapping and produces images with a maximum resolution of ~1 km.)

The lead author on the EPSC2010 abstract, Chrstophe Sotin of JPL, is also lead author of an abstract for the upcoming Division of Planetary Studies conference.  The second abstract provides a brief description of the goals for a conceptual mission called JET, a Journey to Enceladus and Titan.  The goals listed are high mass resolution spectroscopy of the material in Enceladus' geysers and Titan upper atmosphere to determine composition, high resolution thermal mapping of the Enceladus' tiger stripes that are the source of the geysers, and imaging of Titan's surface (presumably with the camera described above).

Editorial Thoughts: The proposed Flagship Titan orbiter would have used the 5 micron band to map the entire moon at resolutions of around 50 m.  The abstract summarized above made it clear that the authors are proposing their instrument for a spacecraft that would flyby Titan, presumably while in orbit around Saturn.  The area covered in detail at closest approach would be approximately one million square kilometers, a respectable area.  (France is 547,030 sq. km.)  It's not clear from the abstract how much of that area would be imaged at resolutions of 50 m or less.  By carefully choosing the areas to be imaged to include high priority sites, such an imager should make key contributions to our understanding of Titan's surface and the processes creating it.  While the authors don't discuss it, the imager should be able to image additional areas of Titan at lower resolutions from greater distances.

The imager that had been proposed for the Titan Flagship mission would have been more capable than the camera discussed here.  It too would have imaged the surface at 50 m resolution, but would have used an additional transparent band in the atmosphere to provide color images (at 2.0, 2.7, and 5 μm) that could provide compositional information.  The Flagship instrument also would have provided spectroscopy in the 0.85 –2.4 μm and 4.8–5.8 μm bands at 250 m resolution.  (The lower wavelengths, however, would have been subjected to greater scattering by Titan's atmosphere, reducing resolution.  The 5 micron band would provide the clearest images.)

I am hoping that the Decadal Survey prioritizes a New Frontiers class-Saturn orbiter to study Enceladus and Titan in the coming decade.  Such a mission could carry a small suite of instruments optimized to studying these bodies.  At Titan, a capable mass spectrometer could study the composition of the upper atmosphere while ice penetrating radar could study the near subsurface structure.  (The JET abstract does not mention an ice penetrating radar.)  Both instruments would also be essential for Enceladus studies.  The 5 micron camera presented here might be a Titan-specific instrument, although this could be a channel on a high resolution thermal imager.  The abstract authors don't present any information on their instrument's mass and cost.  The Flagship instrument, however, had a proposed mass of 28 kg, which suggests an instrument that may have costs and mass incompatible with a New Frontiers class mission.  Presumably, this is the reason that the authors of this abstract are proposing a simpler instrument focused only on imaging in one band.  (I use remote sensing in my research; it would be great if this type of instrument could also image in additional wavelengths.  Color images are a great tool for exploring composition with carefully selected wavelengths (as the Landsat imagers demonstrate for Earth studies).)  However, even images in one band would present a big step forward in our ability to study Titan.


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