NASA's Cassini Saturn orbiter may be nearing the end of its 20-year mission, but it's still sending back a few surprises. On July 25, the unmanned probe passed within 607 mi (976 km) of Saturn's largest moon, Titan, and sent back highly-detailed radar images showing long, linear, undulating dunes made of hydrocarbon sands that can help shed new light on Titan's winds.

According to NASA, linear dunes encircle most of Titan's equator and are made of hydrocarbon grains formed by the moon's atmosphere. They're dynamic structures that are deflected around obstacles the same way as on Earth and their patterns can help scientists not only learn more about the local winds, but also the composition of the dunes and the topography of the landscape.

Meanwhile, Cassini sent back images of the Xanadu annex – an area named after Titan's larger Xanadu region, which was first seen vaguely in 1994 by NASA's Hubble Space Telescope. NASA says that the Xanadu annex has the same mountainous terrain as Xanadu, but that this wasn't confirmed until Cassini's close pass because something yet to be determined is masking light wavelengths outside the microwave radar spectrum.

Synthetic-aperture radar (SAR) image obtained by NASA's Cassini spacecraft on July 25, 2016(Credit: NASA/JPL-Caltech/ASI/Universite Paris-Diderot)

Another mystery is that the mountains in Xanadu and the Xanadu annex cover large areas, while on the rest of Titan they only appear in small, isolated patches.

"These mountainous areas appear to be the oldest terrains on Titan, probably remnants of the icy crust before it was covered by organic sediments from the atmosphere," says Rosaly Lopes, a Cassini radar team member at the Jet Propulsion Laboratory (JPL) in Pasadena, California. "Hiking in these rugged landscapes would likely be similar to hiking in the Badlands of South Dakota."

Launched in 1997, Cassini has been in orbit around Saturn since 2004. The July 25 flyby was the probe's 122nd of Titan and the last time it will pass over the moon's southern polar region. The remaining four flybys will be over the methane lakes of the north pole and in April of next year, the spacecraft will start a series of 22 close orbits of Saturn that will go between the planet and its famous rings.

Radar image of the Shangri-La Sand Sea on Titan from NASA's Cassini spacecraft(Credit: NASA/JPL-Caltech/ASI/Universite Paris-Diderot)

Then, on September 15, 2017, Cassini will make a controlled plunge into Saturn's atmosphere, where it will burn up to prevent possible bio-contamination of the planet's moons.

The video below shows Cassini making a close encounter with Titan using its radar vision and zooming in on the Huygens lander.

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