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Cassini mission completes 100th fly of Titan
BY STEPHEN CLARK
ASTRONOMY NOW

Posted: March 7, 2014


NASA's Cassini spacecraft, in the final months of its first decade spiraling around Saturn, zipped past the moon Titan on Thursday for the 100th time, continuing an unbroken string of flybys of one of the most Earth-like worlds in the known universe.


Shrouded in an orange haze, Titan is backdropped by Saturn and its famous rings in this image from Cassini. Photo credit: NASA/JPL-Caltech/Space Science Institute
 
The robotic spacecraft was supposed to fly within 933 miles, or 1,500 kilometers, of Titan to study the gravity field and internal structure of the solar system's second-largest moon.

Titan is enveloped in an orange haze and thick nitrogen-rich atmosphere producing methane rainfall. With temperatures dipping as low as minus 290 degrees Fahrenheit (minus 179 degrees Celsius), the moon harbors rivers and lakes of liquid ethane and methane. Blocks of rock-hard ice dot the landscape.

Saturn's largest moon resembles how Earth might have looked as life first sprang up.

"Titan is a laboratory for how simple life got started on Earth," said Linda Spilker, Cassini's project scientist at the Jet Propulsion Laboratory in Pasadena, Calif.

Cassini's sojourn around Saturn began in 2004 after a seven-year cruise from Earth. The nuclear-powered probe's mission was extended twice until September 2017 as Saturn's northern hemisphere transitions from spring to summer.


Artist's concept of Cassini at Saturn. Photo credit: NASA/JPL-Caltech/Space Science Institute
 
"The next few years will be some of the most exciting times for Titan weather," Spilker said during a Google+ Hangout in December.

More intense storms or hurricane-like cyclones could pop up in Titan's northern hemisphere as it receives more sunlight with the onset of summer, she said.

The approaching summer also allows Titan's northern lakes and seas to shine brighter in near-infrared, giving scientists better data on their composition. Scientists also hope to resolve waves on Titan's hydrocarbon lakes.

Such surface features are invisible to the eye, but specialized instruments aboard Cassini can peer through Titan's clouds in other wavelengths.

Cassini is currently in the solstice mission, and the mission's extension to 2017 will allow the probe to follow a complete seasonal cycle at Saturn -- from northern winter to northern summer.

During Thursday's flyby, scientists planned to analyze radio signals beamed down from Cassini to assess the presence of a global ocean of liquid water hidden underneath Titan's icy crust.

Researchers announced in 2012 they discovered Titan likely concealed volumes of liquid water between the moon's outer shell and rocky interior. They made the conclusion after proving the strong tug of Saturn's gravity created bigger tidal bulges than predicted if the moon was made entirely of solid rock.


Ligeia Mare, shown here in an artistically enhanced image from NASA's Cassini mission, is the second largest known body of liquid on Saturn's moon Titan. It is filled with liquid hydrocarbons, such as ethane and methane, and is one of the many seas and lakes that bejewel Titan's north polar region. Photo credit: NASA/JPL-Caltech/ASI/Cornell
 
"The astrobiological potential for Titan is two-fold," Spilker said in a press release. "Could a unique form of methane-based life exist in Titan's liquid lakes and seas? With a global ocean of liquid water beneath its icy crust, could life exist in Titan's subsurface ocean?"

Cassini's radio science experiment works by detecting subtle changes in communications signals from the spacecraft picked up by ground stations. Scientists can learn about the materials the signal passes through by observing the behavior of the radio waves once they arrive on Earth.

The radio investigation will help measure short-period changes in Titan's gravity field caused by Saturn's tidal forces, determine the shape of the moon and fluctuations in Titan's gravity, and examine the mechanics of ice flows on the moon's surface, according to NASA.

Thursday's encounter was the third of 11 close approaches between Cassini and Titan this year. Officials will devote other encounters to observe Titan's interaction with the solar wind, map its lakes and terrain, and profile the moon's atmosphere, which is rich in organic molecules.


Artist's concept of the hypothesized internal structure of Titan. Photo credit: NASA/A. Tavani
 
"Methane is not only in the atmosphere, but probably in the crust," said Jonathan Lunine, a scientist on the Cassini mission at Cornell University in Ithaca, N.Y. "It's a hint there are organics not only in Titan's air and on the surface, but even in the deep interior, where liquid water exists as well. Organics are the building blocks of life, and if they are in contact with liquid water, there could be a chance of finding some form of life."

Scientists employ Cassini's sensor suite to look at many of Saturn's other moons, such as Enceladus, which is covered in an icy shell with plumes of vapors shooting into space from fissures near the moon's south pole.

Running low on fuel, the $3.2 billion Cassini mission is due to end in September 2017 with a guided crash into Saturn to cleanly dispose of the spacecraft and its toxic components, ensuring Saturn's moons and their potential biospheres are preserved for future research.

Follow Stephen Clark on Twitter: @StephenClark1.

The Planets
From tiny Mercury to distant Neptune and Pluto, The Planets profiles each of the Solar System's members in depth, featuring the latest imagery from space missions. The tallest mountains, the deepest canyons, the strongest winds, raging atmospheric storms, terrain studded with craters and vast worlds of ice are just some of the sights you'll see on this 100-page tour of the planets.
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Hubble Reborn
Hubble Reborn takes the reader on a journey through the Universe with spectacular full-colour pictures of galaxies, nebulae, planets and stars as seen through Hubble's eyes, along the way telling the dramatic story of the space telescope, including interviews with key scientists and astronauts.
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3D Universe
Witness the most awesome sights of the Universe as they were meant to be seen in this 100-page extravaganza of planets, galaxies and star-scapes, all in 3D!
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