New technique gives Cassini a clearer view of Titan
Titan is Saturn's largest moon and the only natural satellite known to have a dense atmosphere. Because Titan is less massive than Earth, its gravity doesn’t hold on to its gaseous envelope as tightly, so the atmosphere extends to an altitude ten times higher than Earth's, nearly 600 km (370 miles) into space.
The atmosphere is about 95 per cent nitrogen and about 5 per cent methane, with small amounts of other carbon-rich compounds. High in the atmosphere, these methane and nitrogen molecules are split apart by the Sun's rays and recombine to form a variety of organic molecules, creating a thick, orange-coloured haze that obscures the moon's surface from view.
Side-by-side comparisons of Cassini Synthetic Aperture Radar (SAR) view and one using the new technique. The view is a mosaic of Ligeia Mare, one of the large hydrocarbons seas on Titan. Image credit: NASA/JPL-Caltech/ASI
During its ten year mission to Saturn, Cassini has made 109 seperate flybys of Titan, and the spacecraft's radar instrument has peered through the enveloping smog to map almost half of the giant moon's surface. However, the radar images have a characteristic grainy appearance due to electronic noise. This "speckle noise" can make it difficult for scientists to interpret small-scale features or identify changes in images of the same area taken at different times.
The new technique, referred to by its developers as "despeckling," uses an algorithm to modify the noise, resulting in clearer views that can be easier for researchers to interpret.
Four examples for comparison. The three leftmost image pairs show Ligea Mare. The rightmost pair shows a valley network along Jingpo Lacus, one of Titan's larger northern lakes. Each thumbnail represents an area 112 km (70 miles) wide. Image credit: NASA/JPL-Caltech/ASI
"Noise in the images gave me headaches," said Antoine Lucas, who now works at the astrophysics division of France's nuclear center (CEA). Lucas found that a team near Paris was working on a "de-noising" algorithm, and he began working with them to adapt their model to the Cassini radar data.
"My headaches were gone, and more importantly, we were able to go further in our understanding of Titan's surface using the new technique," Lucas said in a statement.
"This is an amazing technique, and Antoine has done a great job of showing that we can trust it not to put features into the images that aren't really there" said Randy Kirk, a Cassini radar team member from the U.S. Geologic Survey in Flagstaff, Arizona.
Kirk said the radar team is going to have to prioritize which images are the most important to applying the technique. "It takes a lot of computation, and at the moment quite a bit of 'fine-tuning' to get the best results with each new image, so for now we'll likely be despeckling only the most important—or most puzzling—images," he added.
Side-by-side comparisons of a Cassini SAR view, at left, and one using the new technique at right. The terrain is in the flow region Leilah Fluctus. With the speckle noise suppressed cone-shaped features near lower right stand out, which could be produced by the action of rivers or floods. The image shows an area about 80 km (50 miles) wide. Image credit: NASA/JPL-Caltech/ASI
The new technique can be used to produce 3-D digital elevation maps of Titan's surface with greatly improved quality. Clearer views of river channels, lake shorelines and windswept dunes, mean researchers can perform more precise analyses of processes shaping Titan's surface.
And Lucas suspects that the speckle noise itself, when analyzed separately, may hold information about properties of the surface and subsurface.
"This new technique provides a fresh look at the data, which helps us better understand the original images," said Stephen Wall, deputy team lead of Cassini's radar team, which is based at NASA's Jet Propulsion Laboratory.