February 16, 2015

Titan Loses its Speckles

Some of the most stunning images of Saturn’s moon Titan are made using a synthetic aperture radar to penetrate the thick atmosphere to see the frigid surface.

By Caleb A. Scharf

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This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

A view onto Ligeia Mare on Titan - a hydrocarbon sea - Left: standard synthetic aperture radar, Right: de-speckled (NASA/JPL-Caltech/ASI)

Some of the most stunning images of Saturn's moon Titan are made using a synthetic aperture radar to penetrate the thick atmosphere to see the frigid surface. But radar images are prone to what's called 'speckle noise'. This is the granular texture that covers the radar maps, and it's caused by the physical roughness of natural surfaces at the same scale as the wavelength of the electromagnetic radiation used. In the case of the Cassini mission's radar that's about 20 millimeters.

As the radar bounces its radiation off Titan's surface, the reflected waves from every little patch on that scale tend to interfere with each other, sometimes constructively, sometimes destructively. The finite resolution of the radar receiver means that the imaging pixels end up with extra noise from all of that wave interference - or speckles.

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Correcting for this is non-trivial since the speckle noise contains information about the surface characteristics. But careful filtering and smoothing of the data can make the images more palatable for human eyes by clarifying where the surface reflectivity really is changing.

And that's what's been done now for a selection of Cassini radar maps - revealing Titan with a visual clarity that is truly stunning (click images to enlarge). The processed data can also help improve three-dimensional mapping of the extraordinary windswept and liquid eroded surface.

Before and after despeckling - each image is about 70 miles across (NASA/JPL-Caltech/ASI)

Areas of varying albedo and topography become much easier to see and interpret. Combined with estimates of altitude these images become tools to evaluate probable cause of many of the features - such as sharp changes in river directions that could correlate with stratified 'bedrock' or specific erosion mechanisms.

A 3-D view of a region of Kraken Mare showing the sharp turns in a 'river' (NASA/JPL-Caltech/ASI)

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